Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system

Journal of Experimental Marine Biology and Ecology 289 (2003) 223 – 262 www.elsevier.com/locate/jembe

Pilot trophic model for subantarctic water over the Southern Plateau, New Zealand: a low biomass, high transfer efficiency system Janet M. Bradford-Grieve a,*, P. Keith Probert b, Scott D. Nodder a, David Thompson a, Julie Hall c, Stuart Hanchet d, Philip Boyd e, John Zeldis f, Allan N. Baker f, Hugh A. Best g, Niall Broekhuizen c, Simon Childerhouse f, Malcolm Clark a, Mark Hadfield a, Karl Safi c, Ian Wilkinson g a

National Institute of Water and Atmospheric Research, P.O. Box 14901, Kilbirnie, Wellington, New Zealand b Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand c NIWA Hamilton, P.O. Box 11115, Hamilton, New Zealand d NIWA Nelson, P.O. Box 893, Nelson, New Zealand e NIWA Centre for Chemical and Physical Oceanography, Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand f NIWA Christchurch, P.O. Box 8602, Christchurch, New Zealand g Department of Conservation, P.O. Box 10420, Wellington, New Zealand Received 21 June 2002; received in revised form 9 January 2003; accepted 22 January 2003

Abstract The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially significant species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km2 with average depth of 615 m. The region is noted for its relatively low levels of phytoplankton biomass and primary production that is iron-limited. In order to evaluate the implications of these attributes for the functioning of this ecosystem a steady-state, 19-compartment model was constructed using Ecopath with Ecosim software of Christensen et al. [www.ecopath.org]. The system is driven by primary production that is primarily governed by the supply of iron and light. The total system biomass of 6.28 g C m
2 is very low compared with systems so far modelled with a total system throughput of 1136 g C m
2 year
1. In the model, the Southern Plateau retains 69% of the biomass in the pelagic system and 99% of total production. Although fish are caught demersally,

* Corresponding author. Tel.: +64-4-386-0300; fax: +64-4-386-2153. E-mail address: [email protected] (J.M. Bradford-Grieve). 0022-0981/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0981(03)00045-5

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most of their food is part of production in the pelagic system. Top predators represent about 0.3% of total biomass and account for about 0.24 g C m
2 year
1 of food consumed made up of birds 0.058 g C m
2 year
1, seals 0.041 g C m
2 year
1, and toothed 0.094 g C m
2 year
1 and baleen whales 0.051 g C m
2 year
1. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food eaten by non-mesopelagic fish. Mean transfer efficiencies between trophic levels II and IV of 23% are at the high end of the range reported in the literature. In the model, adult fish production is almost completely accounted for by the fisheries take (32%), consumption by seals (7%), toothed whales (21%), other adult fish (13%), and squid (20%). Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively. The gross efficiency of the fishery is 0.018% (catch/primary production). Although not all data come from direct knowledge of this system, the model reflects its general characteristics, namely a low primary production system dominated by the microbial loop, low sedimentation to the seafloor, high transfer efficiencies, a long food web and supporting high-level predators. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Ecosystem structure; Fish production; New Zealand; Subantarctic; Transfer efficiency; Trophic model

1. Introduction The Southern Plateau subantarctic region, southeast of New Zealand, is an important feeding area for birds, seals and fish, and a fishing ground for commercially important species. The Southern Plateau is a major morphometric feature, covering approximately 433,620 km2 and includes the Campbell Plateau, the Bounty Plateau, Pukaki Rise, Campbell Rise, and the Auckland Shelf (Carter et al., 1997). The Plateau rises above sea level at the Auckland, Campbell, Bounty, and Antipodes Islands. Water depths range from 0 –500 m on the rises to 1500 m at the plateau edge (Summerhayes, 1967a,b; Krause and Cullen, 1970). The Southern Plateau region is noted for its relatively low levels of phytoplankton biomass (chlorophyll a) observed from satellite measurements (Murphy et al., 2001) and field observations and low levels of primary production despite there being plenty of phytoplankton nutrients such as nitrates (Bradford-Grieve et al., 1997). Recent studies in subantarctic waters show that some of the reasons for these observations are the very low levels of dissolved iron and the way this interacts with the availability of light and silicate (Boyd et al., 1999). The most obvious impact of iron-limited growth is the dominance of the phytoplankton by small forms. Therefore, the food web in this region is relatively long. The length of the food web implies that only a small fraction of the low phytoplankton production reaches the highest trophic levels of large carnivores. Nevertheless, the subantarctic region over the Southern Plateau is of special interest to fishers who catch several species of fish there, and to conservationists because there are large populations of seabirds and populations of seals and sea lions most of which feed in the region. Contrary to this expectation of low efficiency, this study shows the system is highly efficient. The aim of the present paper is to identify and quantify the elements of the oceanic food web that support the birds, mammals and fish of the Southern Plateau, and to

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answer questions such as: What are the implications of the ‘‘microbial’’ system for the large animals, fish, and birds that depend on production from this subantarctic ecosystem? What are the differences between this system and other types of ecosystems? What are the priorities for information acquisition to obtain a more reliable picture of this ecosystem? To model the Bounty/Campbell ecosystem, the Ecopath software was used (Christensen et al., 2000). Ecopath has two elements, one that describes the production term and another for the energy balance for each group. Ecopath bases the model on an assumption of mass balance over an arbitrary period, usually a year.

2. Materials and methods 2.1. The study area The area of the Southern Plateau was measured in plan view using digital bathymetric data and a Geographic Information System to produce closed polygons representing the various isobaths. The areas between each isobath were calculated and summed. This plateau is considered to be the area contained by the 1000-m isobath, excluding the islands, excluding the South Island at the base of the continental slope, and including the Pukaki Saddle shallower than about 1500 m (Fig. 1). This area is 433,620 km2 of which 9% is 0– 250 m, 18% is 250 – 500 m, 41% is 500 –750 m, and 32% is 750 –1000 m and with an average depth of about 615 m. Salinities and temperatures are nearly uniform over the Campbell Plateau and have values characteristic of Subantarctic Mode Water in the Southwest Pacific Basin (34.35 – 34.40 and 7 jC, respectively) (Morris et al., 2001). Morris et al. (2001) show that winter surface mixed layers are 200 – 300 m deep and that there is weak thermal stratification over the top 500 m across the central plateau region. The Campbell Plateau is a relatively quiescent region and is bounded almost on all sides by major ocean fronts. Northwest of the plateau, subantarctic water is bounded by the Southland Front which is part of the Subtropical Front (e.g. Heath, 1981) which passes northeastward through the Snares Trough separating the Campbell Plateau from the Snares Shelf immediately south of New Zealand. Around the southern and eastern flank of the Campbell Plateau, subantarctic water is separated from less saline Circumpolar flows in the Southwest Pacific Basin by the Subantarctic Front (Fig. 1). Water flows generally from west to east around the plateau and follows the bathymetry into cyclonic flow around the Bounty Trough; there is evidence of weak flows on the plateau, among them an anticyclonic flow around the Pukaki Rise (Morris et al., 2001). Macronutrients in subantarctic water over the Campbell Plateau are in good supply (Levitus et al., 1993; Bradford-Grieve et al., 1997) although NO3 is in excess relative to DRSi (Zentara and Kamykowski, 1981). Low dissolved iron concentrations are observed in upper subantarctic water south of 45jS southeast of Tasmania and southeast of New Zealand (Sedwick et al., 1997; Boyd et al., 1999). Recent phytoplankton physiological experiments report iron stress as well as light limitation in spring phytoplankton populations (Boyd et al., 1999). It appears that the low micronutrient status of this region

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Fig. 1. Map of the region showing the Southern Plateau (Bounty/Campbell Plateau) and the water masses and fronts in relation to the Plateau.

ensures that the ecosystem behaves like a low-nutrient system despite an excess of nitrate (Bradford-Grieve et al., 1999). Thus the food web is characterised by low chlorophyll concentrations and small phytoplankton cells. In addition, there is low seasonality in chlorophyll concentrations, but higher seasonality in phytoplankton and microzooplankton carbon biomass (Bradford-Grieve et al., 1999). Bradford-Grieve et al. (1999) also report a large contribution to total community biomass by bacteria and winter bacterial production to be greater than primary production in the surface 100 m. In terms of the food web classification of Legendre and Rassoulzadegan (1995), it appears that, in winter, a ‘‘microbial’’ web or even ‘‘microbial loop’’ system existed because microbial rather than autotrophic processes dominated (Bradford-Grieve et al., 1999). The spring food web is classified as ‘‘microbial’’ as phytoplankton made a larger contribution to the creation of living organic particles than in winter, but material was still being directed to small heterotrophs such that microzooplankton biomass was greater than that of mesozooplankton. There is a very low sedimentation of particles from this ‘‘microbial’’ system (Nodder and Northcote, 2001; H. Neil, personal communication).

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2.2. Modelling approach The modelling approach of Ecopath with Ecosim is given by Christensen et al. (2000). In this model, a system of simultaneous linear equations can be solved using standard matrix algebra: B1 PB1 EE1
B1 QB1 DC11
B2 QB2 DC21
. . .
Bn QBn DCn1
EX1 ¼ 0 B2 PB2 EE2
B1 QB1 DC12
B2 QB2 DC22
. . .
Bn QBn DCn2
EX2 ¼ 0 ... Bn PBn EEn
B1 QB1 DC1n
B2 QB2 DC2n
. . .
Bn QBn DCjn
EXn ¼ 0 where Bn is the biomass of element (compartment or box) n, PBn is the production/ biomass ratio, QBn is the consumption/biomass ratio, DCjn is the fraction of prey (n) in the average diet of predator j, EEn is the ecotrophic efficiency of n, and EXn is the export of n. Ecopath sets up a system with as many linear equations as there are groups in the system and it solves the set for one of the following parameters for each group: biomass (B); production/biomass ratio (P/B); consumption/biomass ratio (Q/B); ecotrophic efficiency (EE) = fraction of production that is used in the system, i.e. either passed up the food web, used for biomass accumulation, migration or export. As well as three of the above parameters, the following parameters must also be entered for all groups: catches (and discards); net migration rate (here set to 0); biomass accumulation rate (here set to 0); assimilation rate; diet compositions; fate of detritus. Energy balance within a compartment is ensured using the equation: consumption = production + respiration + unassimilated food. 2.3. Model groups (compartments) From our knowledge of the functioning of the early food web and the diet of high-level predators, we define the food web of the Bounty Campbell Plateau as having the following functional compartments. All available information on biomass, catch, P/B ratios, and consumption rates (Q/B) have been assembled from our own research data, the literature, and catch statistics. Then species of similar sizes, diets, and consumption rates, etc., were aggregated within a compartment (Fig. 2). The resulting 19-compartment model consists of (1) birds, (2) seals and sea lions (3) toothed whales, (4) fish (adult), (5) fish (juvenile) (6) baleen whales, (7) squid, (8) mesopelagic fish, (9) macrozooplankton, (10) macrobenthos, (11) mesozooplankton, (12) ciliates, (13) heterotrophic flagellates, (14) meiobenthos, (15) bacteria (water column), (16) bacteria (sediment), (17) phytoplankton, (18) detritus water (column), and (19) detritus (benthic). Biomasses that were not already available in terms of carbon were converted to carbon using the following relationships. Birds and mammals: 10% of wet weight is assumed

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Fig. 2. Trophic model for the Southern Plateau, New Zealand. The box size is proportional to the square root of the compartment biomass. All flows are in g C m
2.

to be carbon; fish and squid: 10% of wet weight is carbon assuming that carbon content is about 50% of dry weight and that dry weight is about 20% of wet weight (Vinogradov, 1953); macro- and mesozooplankton: log(WW) =
1.537 + 0.852log(C)

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Table 1 Prey – predator matrix used in model Prey/predator (1) Birds (2) Seals, etc. (3) Toothed whales (4) Fish (adult) (5) Fish (juvenile) (6) Baleen whales (7) Squid (8) Mesopel. fish (9) Macrozoopl. (10) Macrobenthos (11) Mesozoopl. (12) Ciliates (13) Het. flagellates (14) Meiobenthos (15) Bacteria (col.) (16) Bacteria (sed.) (17) Phytoplankton (18) Detritus (col.) (19) Detritus (sed.) Sum

1

2

3

4

0.05

0.25 0.3 0.015 0.040

5

6

7

8

9

10

11

12

13

14

15 16

0.06 0.07

0.30 0.30 0.7 0.015 0.10 0.31 0.45 0.155 0.40 0.07 0.34 0.615 0.40 0.50 0.70 0.38 0.05 0.160 0.05 0.60 0.10 0.62 0.55 0.15 0.20 0.15 0.55 0.60 0.10 0.30 0.20 0.20 0.60 0.80 0.30 0.10 0.40 0.70 1.0 1.00 1.00 1.0 1.00

1.00 1.00 1.00 1.0

1.00 1.00 1.00

1.0 1.00 1.00 1.0 1.0

Numbers represent biomass fractions of food ingested.

and log(DW) = 0.499 + 0.991log(C), where WW = wet weight, DW = dry weight, and C = carbon (Weibe, 1988). 2.4. Input data The origin of input data is given in Appendix A and is summarised in Tables 1– 3. 2.5. Balancing the model Initially ecotrophic efficiencies (the fraction of total production consumed by predators or exported from the system) of a number of early food web compartments were >1. Part Table 2 Average annual catch (wet tonnes) by species from stocks inhabiting the Campbell Plateau (data from Annala et al., 1999) Species

Range

Average

N

Hake Hoki

1803 – 3956 10,000 – 34,000 subantarctic 76,000 – 165,000 west coast SI 935 – 7510 200 – 3850 2528 – 5645 13.7 – 76.2

2999 (area 1) 19,300 subantarctic 109,667 west coast SI 4946 (area 6) 1158 4457 30,117

9 9 9 9 6 4 9

Ling Orange roughy Oreos Southern blue whiting

N = number of years averaged. SI = South Island.

230

Catch (1) Birds (2) Seals (3) Toothed whales (4) Fish (adult) (5) Fish (juvenile) (6) Baleen whales (7) Squid (8) Mesopelagic fish (9) Macrozooplankton (10) Macrobenthos (11) Mesozooplankton (12) Ciliates (13) Heterotr. flagellates (14) Meiobenthos (15) Bacteria (column) (16) Bacteria (sediment) (17) Phytoplankton (18) Detritus (column) (19) Detritus (sediment)

Trophic Biomass level

0.00000105 5.3 0.00000066 5.7 5.9 0.0153 4.8 4.5 5.0 0.0042 5.0 4.5 3.6 3.5 3.5 2.8 2.3 3.3 2.0 2.0 1.0 1.0 1.0

P/B

Q/B

EE

P/Q

Net R efficiency

0.00158 0.30 36.5 0.002 0.008 0.010 0.0009 0.22 46.0 0.04 0.005 0.006 0.0085 0.06 11.0 0.00 0.005 0.007 0.4517 0.30 2.60 0.935 0.115 0.141 0.086 1.00 3.00 0.945 0.333 0.417 0.004 0.038 12.8 0.00 0.003 0.004 0.0204 8.00 22.0 0.992 0.364 0.455 0.285 1.00 16.0 0.95 0.063 0.104 0.311 10.0 33.0 0.95 0.303 0.505 0.25 1.00 2.86 0.895 0.350 0.583 1.058 20.00 57.14 0.836 0.350 0.538 0.167 88.00 247.89 0.927 0.355 0.507 0.307 292 829.55 0.932 0.352 0.503 0.1 10.00 32.26 0.859 0.310 0.443 0.598 87.40 383.33 0.975 0.228 0.289 1.5 2.508 8.36 0.80 0.300 0.375 1.253 248 0.769 572 0.963 433 0.931

A

R/A

P/R

R/B

Flow to detritus

0.046 0.046 0.990 0.010 28.900 0.012 0.033 0.033 0.994 0.006 36.580 0.008 0.074 0.075 0.993 0.007 8.740 0.019 0.805 0.940 0.856 0.169 1.760 0.244 0.120 0.206 0.583 0.714 1.400 0.056 0.041 0.041 0.996 0.004 10.202 0.010 0.196 0.359 0.545 0.833 9.600 0.091 2.449 2.734 0.896 0.116 8.600 1.837 3.046 6.154 0.495 1.020 9.800 4.258 0.179 0.429 0.417 1.400 0.714 0.312 18.137 39.297 0.462 1.167 17.143 24.625 14.282 28.978 0.493 1.029 85.521 13.485 88.625 178.269 0.497 1.011 288.682 82.288 1.258 2.258 0.557 0.795 12.581 1.108 128.829 183.094 0.711 0.406 215.433 49.470 6.270 10.032 0.625 0.600 4.180 3.261 61.409 8.781 0.000

Catch, biomass (B), production (P), respiration (R), assimilation (A), and flow to detritus in terms of g C m
2 year
1. P/B, Q/B, R/B year
1. EE, P/Q, net efficiency (P/A), R/A, and P/R are dimensionless.

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Table 3 Input parameters and calculated parameters (in bold) for the Southern Plateau

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of this problem appeared to be that phytoplankton production relative to biomass (P/B), as calculated from a limited set of original data, was not great enough to support the biomass that immediately depended on it, as defined in the model. Therefore we chose a value for primary production that was defined in proportion to bacterial production based on the conclusions of Ducklow (2000). Detritus derived from intertidal primary producers (macroalgae) was not taken into account in the model although it will be important locally around the islands on the Southern Plateau. Macroalgae can make a direct input to the deep sediment (Reichardt, 1987) but of most interest is the production of fine particulate detrital material and DOC into the water column (Mazure and Field, 1980 and references therein). Kelp beds may be highly productive with their productivity per unit area often being greater than the phytoplankton by a factor of 10 (Mann, 1973). The particles remain suspended in the water for a time and are acted on by bacteria (Laycock, 1974). Mazure and Field (1980) estimate that the total amount of carbon available to suspension-feeders from kelp plants in the Benguela current kelp beds may be in the order of 1200– 1600 g C m
2 year
1. It is likely that subantarctic algae do not produce as much detritus as this, but as yet no data are available on which to evaluate their contribution to this system. Initially we did not believe that annual P/B for sediment bacteria was as low as experimental data suggested and chose a literature-derived value of 11. The model produced a very low ecotrophic efficiency for bacteria so it was decided to let the model calculate P/B by assuming an ecotrophic efficiency of 0.8 and P/Q of 0.3. The subsequently calculated annual P/B for sediment bacteria of 2.5 is nearer the experimentally derived P/B of 1 than the literature value of 11. Slight changes in the diet of some compartments (Table 1) reduced the ecotrophic efficiencies of their prey to < 1. Initially there was not enough material flowing to the detritus compartments. Changes in the ratio of unassimilated food/food consumption corrected this. Final balancing was achieved by choosing relatively high production/consumption ratios for ciliates, heterotrophic flagellates, and meiobenthos (Table 3).

3. Results and discussion 3.1. Evaluation of data A summary of all the final input data and the calculated parameters is found in Table 3. Although we have detailed data available for subantarctic water just north of the Plateau, we do not know if these data are representative of the Plateau itself. Also, a number of the parameters were estimated from literature values. The absence of data on production/biomass ratios and quantitative food consumption in relation to biomass for commercial fish species on the Plateau was particularly conspicuous. Our knowledge of annual and interannual variability on the Plateau is very limited therefore our estimation of annual averages has to be considered approximate. The primary production figure used in the model is probably overestimated. Boyd et al. (unpublished manuscript) compare in situ primary productivity measurements and

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primary production estimated from SeaWiFS data and the algorithm of Behrenfeld and Falkowski (1997). This method was modified for New Zealand waters using sea surface temperature data from AVHRR and local optical spectral attenuation data. This technique showed a robust relationship between the measured and estimated data and gave an annual production of 142.5 g C m
2 year
1 for a box which takes in most, but not all, of the Southern Plateau region (Murphy et al., 2001). This means that the localised high chlorophyll a in the vicinity of the Auckland Islands (>1 mg m
3) and the calculated column integrated primary production of 2– 2.5 g C m
2 day
1 are not included in the above estimate of average annual primary production for the Southern Plateau. Thus, although average annual primary production is likely to be greater than 142.5 g C m
2 year
1, it is probably not as great as the 265 g C m
2 year
1 use in the model. The model was balanced by using what may be unrealistically high P/Q ratios for some compartments (ciliates: 0.355; heterotrophic flagellates: 0.352; macrobenthos: 0.35; meiobenthos: 0.31). Total daily consumption/biomass for microzooplankton is estimated from a limited set of dilution experiments at 10 m for microzooplankton grazing on < 200 Am phytoplankton, ciliates grazing on heterotrophic flagellates, and heterotrophic flagellates grazing on bacteria (J.H., unpublished data). These data were converted to an annual basis and the P/Q ratio determined using a P/B of 220. The P/Q values average 0.242 for ciliates and heterotrophic flagellates combined. This is less than the 0.35 used in the model for ciliates and heterotrophic flagellates but may be justified on the grounds that the above experimental estimate is from water at 10 m and probably does not reflect the average situation down the water column. Experimental data on the iron-limited system of the North Pacific suggests that this particular nutrient limitation impacts a number of ecological parameters. P/Q for heterotrophic bacteria (Tortell et al., 1966) was 0.12 –0.36 in iron-deficient conditions and for heterotrophic marine microflagellates (Chase and Price, 1997) was 0.18. In the model, P/Q for water column bacteria (0.228) fits within these values and the range of experimentally derived values, whereas P/Q for microzooplankton is less than the P/Q used here (0.352). Total consumption by microzooplankton in the NE Pacific of bacteria is greater than that on phytoplankton (58% and 26% of the total ration, respectively) (Rivkin et al., 1999). Data from Hall et al. (1999) and Bradford-Grieve et al. (1997) suggest that bacteria and picophytoplankton each make up about 50% of the total ration of heterotrophic flagellates. Nevertheless, the model could be balanced only with 20% of heterotrophic flagellates diet being made up of bacteria. The reason for this may be located in the possible underestimation of water column bacterial biomass. It was established that bacterial P/B and B are more important than the substrate for bacterial growth in determining the availability of bacteria to heterotrophic flagellates. That is, importing detritus from macroalgae does not make a difference to the carrying capacity of the pelagic system as defined. For some parameters we have direct measurements. For example, the input of carbon sedimented to the seafloor (1.23 g C m
2 year
1) is less than the 8.8 g C m
2 year
1 that the model sends to the seafloor to fuel the activities of the bacteria, meiobenthos, and macrobenthos. This could be a result of overestimating their biomasses through using data

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from the south side of Chatham Rise. Further data are needed to decide whether the current choice of parameter values for the Southern Plateau region is a serious flaw in the model. There was a discrepancy between measured annual P/B for sediment bacteria of 1 and literature values of about 11. The model calculates that P/B for sediment bacteria of 2.5 is all that is necessary to satisfy the demand of the benthic system, implying that the measured P/B for sediment bacteria is not as unrealistic as initially thought. Many of the other parameter estimates appear to be reasonable. The gross efficiencies (GE or P/Q) invertebrates are between 0.3 and 0.36 are at the high end of the range reported in the literature (e.g. Parsons et al., 1984 [K1 (as %) in Table 25]). The P/Q for mesopelagic fish appears to be very low and is the result of the particular choice of P/B and Q/B for this compartment. The production/respiration ratios (P/R) are more or less as expected for those groups of organisms where there are data (Humphrey, 1979). In other groups, P/R seems high at about 1 but this may not be surprising given the variability in Humphrey’s (1979) data. It is possible that in this model not enough energy from food consumption is set aside for respiration and that P/B and/or the proportion of food that is unassimilated are too high in a number of groups. There is evidence that this tightly coupled ecosystem may not always be able to support all animals at optimal food consumption and growth. Since we are assuming optimum growth in selecting production and food consumption parameters, this may be the reason that in the model there is not enough energy for respiration. Evidence for this is seen in the data on growth of the 1991 year class of southern blue whiting (Fig. 3). Larvae of this species survived in particularly large numbers in 1991. Subsequently, they grew slower than some previous year classes, suggesting that, in this model, lower P/B and Q/ B would have to have been assigned to reflect the true state of affairs. The low respiration/biomass for mesozooplankton may be partly accounted for by the fact that one of the large calanoid copepods in this system (Neocalanus tonsus) spends from late January/February until September below 500 m where it enters a type of diapause (Ohman, 1987). The model calculates that there is 0.31 g C m
2 of macrozooplankton. This is converted to 0.99 g DW m
2 using the relationship given by Weibe (1988). This concentration is within the range of biomass values given by Pakhomov et al. (1994, 1996) for subantarctic open water of 0.01 –4.4 g DW m
2 and around islands of 0.01 –

Fig. 3. Weight at age by cohort for 1987 and 1991 year classes of southern blue whiting (S.H., unpublished data).

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2.86 g DW m
2. The biomass of mesopelagic fish, calculated by the model (0.29 g C m
2) for the Southern Plateau, is 30% more than the micronekton biomass of 0.22 g C m
2 on the mid-slope region off southern Tasmania, Australia (Williams and Koslow, 1997), north of the Subtropical Front. 3.2. Biomass, consumption, and production Omitting the large predators (birds, seals, whales), the pelagic domain contains 69% of total living biomass and benthic compartments (including 0.07 g C m
2 demersally feeding fish (16% of biomass)) contain 31%. The benthic compartments account for food intake of 16.67 g C m
2 year
1 which is < 3% of total non-detrital consumption. Heterotrophic flagellates had the greatest food intake (255 g C m
2 year
1) followed by water column bacteria (229 g C m
2 year
1), mesozooplankton (60 g C m
2 year
1), and ciliates (41 g C m
2 year
1). In our model, bacteria (sediment plus water column) utilise 52 g C m
2 year
1 of detritus of which 94% is derived from water column processes. Twenty six percent of water column detritus is derived directly from phytoplankton. The fish compartments (adult fish, juvenile fish) represent about 9% of total biomass and account for about 1.43 g C m
2 year
1 of food intake. Fish and squid catches are at the trophic levels of 4.8 and 5.0, respectively (Table 3). According to model results, a significant proportion (56%) of primary production and water column heterotrophic production is not directly used but enters the detritus pool that is used by water column and sediment bacteria. The Southern Plateau retains 69% of the biomass in the pelagic system but 99% of total production. Although fish are caught demersally, most of their food is part of production in the pelagic system. The small proportion of fish depending on the benthos for food appears to agree with conclusions of other bathyal habitats. Haedrich and Merrett (1992) determine that only 13% of fish species in the Porcupine Seabight depend on benthos for food, and a number of other studies show that slope fish species feed on the mesopelagic fauna (Mauchline and Gordon, 1984, 1986, 1991; Sedberry and Musick, 1978; Marshall, 1979; Gordon and Mauchline, 1990). Blaber and Bulman (1987) also show that fish of the eastern Tasmanian slope (420 – 550 m) depend on mesopelagic fish resources. Top predators represent about 0.26% of total biomass and account for about 0.24 g C m
2 year
1 of food consumed made up of birds 0.058 g C m
2 year
1, seals 0.041 g C m
2 year
1, and toothed 0.094 g C m
2 year
1 and baleen whales 0.051 g C m
2 year
1. This amounts to 105,803 tonnes carbon over the whole of the Southern Plateau and is about 17% of the total amount of food (621,399 tonnes of carbon) eaten by fish (non-mesopelagic). The fact that fisheries discards have been directed to ‘‘detritus’’ in the model gives an erroneous picture of the impact of fishing on the food of some sea birds that tend to congregate around fishing vessels. Nevertheless, of the estimated >10 million sea birds that live in the area (D.T., unpublished data), only a small fraction benefits from the presence of fishing vessels. Therefore, the omission of discards from the food of seabirds is considered to be immaterial to the functioning of the ecosystem as a whole and birds in particular.

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Fig. 4. Partitioning of adult fish production amongst those compartments consuming it.

The partitioning of total adult fish production in the model among those compartments utilising it (Fig. 4) shows that fisheries take the greatest proportion (32%). The remaining proportions are 7% consumed by seals, 21% by toothed whales, 13% by other adult fish, 20% by squid, with 7% dying of other causes (give the steady state assumptions). These proportions do not necessarily absolutely represent the Southern Plateau system but give a picture of what might be occurring. It would be instructive to know the fate of production of the most important fish species, but this would require the collection of data specific to New Zealand’s fish species and that biomasses, production, and consumption information on the major predators would have to be estimated more accurately. 3.3. Transfer efficiency Ecosystem components can be grouped into discrete trophic levels and transfer efficiencies estimated (Lindeman, 1942). Transfer efficiency is the fraction of total flows at each trophic level (throughput) that are either exported or transferred to another trophic level through consumption. Transfer efficiencies are greater at the beginning of the food web compared with higher trophic levels because of intrinsic differences in metabolic characteristics of organisms at different levels in the food web (e.g. R/A and P/R in Table 3). Transfer efficiencies in the Southern Plateau ecosystem (Table 4) are at the high end of the range reported in the literature (e.g. Parsons et al., 1984). They are higher than the

Table 4 Transfer efficiencies (the proportion of energy transferred from one trophic level to the next) for each trophic level Source/trophic level

I

II

III

IV

V

VI

VII

VIII

IX

Producer Detritus All flows

– – –

25.8 22.3 23.9

22.8 24.5 23.7

19.2 23.6 21.5

14.4 19.2 17.2

8.3 14.9 12.6

4.9 8.4 7.0

5.0 4.6

1.5

Proportion of total flow originating from detritus: 0.51. Transfer efficiencies (calculated as geometric mean for TL II – IV): from primary producers, 22.4%; from detritus, 23.5%; total, 23.0%.

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upper end of average values (8 –15%) reported by Christensen and Pauly (1993), Wolff (1994), and Wolff et al. (1996). 3.4. Mixed trophic impacts Direct and indirect interactions within the food web can be evaluated by changing the biomass of a group and assessing the effect this change will have on other groups in the system assuming the trophic structure remains the same (Fig. 5). The mixed trophic impact (MTI) (scaled
1 to + 1) for each group is calculated using the approach in Ecopath with Ecosim (Christensen et al., 2000) which is based on the work of Ulanowicz and Puccia (1990). In the model, a small increase in the biomass of top predators (birds, seals, toothed whales, adult fish) all have a negative impact on the biomass of their preferred prey, but it is the adult fish that have the greatest impact on their prey (apart from macrozooplankton) and also negatively impact birds and seals as they are directly competing for resources. All compartments have a negative effect on themselves probably reflecting competition for resources. An increase in phytoplankton biomass positively impacts most other compartments apart from itself and water column bacteria which are the prey of heterotrophic flagellates whose biomass is increased by the increase in phytoplankton biomass. An increase in the biomass of mesozooplankton has the greatest number of positive impacts on other compartments ranging from small to moderate. Of these impacts, it appears that the increase in macrozooplankton biomass has the greatest (nevertheless small) impact on adult fish biomass. An increase in adult fish has a large negative impact on birds, seals, presumably because of competition for food. The greatest impact is noted on bird and seal biomass if the fishing fleet is increased and the resulting accidental bycatch and competition of the fisheries for the food of birds and seals, as portrayed in the model, is increased. It is also noted that an increase in the fishing fleet has a positive impact on mesopelagic fish and macrobenthos that make up the food of fish. These changes cannot be interpreted as predictions as the model, as used, does not accommodate changes in diet composition that would result. Also, if we were to identify the exact diets of birds, seals, and individual species of fish in the fishery in more detail, we might find that there is not the degree of competition for the same food that the model currently implies. 3.5. Summary statistics The total throughput (the sum of all flows: consumption, exports, respiratory flows, and flows to detritus) at each trophic level is 1136 g C m
2 year
1 (Table 5), which is low and similar to throughput within the Golfo Dulce (Wolff et al., 1996). About 97% of throughput is achieved from trophic levels I to III: 50% from levels I to II plus 38% from levels II – III. About 54% of the total is due to consumption, < 0.1% is exported (sedimentation and fishery), 22% flows into detritus, and 23% is respired (Fig. 6). The total primary production/respiration (P/R) ratio calculated by the model is 1.004 (Table 5), which is similar to estimates obtained from in situ primary production and plankton respiration data (P/R = 0.9 –1.2, see Wolff et al., 1996). That is, the Southern

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Fig. 5. Mixed trophic impacts (MTIi, j = DCi, j
FCi, j) on the Southern Plateau (where DC is diet composition, FC is a host composition term giving the proportion of the predation on j that is due to i as predator). Direct and indirect impacts of a small increase in the biomass of each of the impacting groups (vertical axis) on every other group (horizontal axis) are shown. Positive impacts are shown above the line, negative below. Impacts are relative and comparable between groups.

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Table 5 Summary statistics for the Southern Plateau ecosystem model Parameter

Value

Units g g g g g g

C C C C C C

Comment
2

m m
2 m
2 m
2 m
2 m
2


1

Sum of all consumption Sum of all exports Sum of all respiration flows Sum of all flows into detritus Total system throughput Sum of all production

610 0.971 264 251 1136 451

Fishery’s mean trophic level Its gross efficiency (catch/primary production)

4.48 0.000181

Calculated total net primary production Total primary production/total respiration Total biomass/total throughput Total biomass (excluding detritus) Total catches

265 1.004 0.005 6.219 0.048

g C m
2 year
1

Throughput cycled (excluding detritus) Finn’s cycling index Average path length

35.28 19.75 5.79

g C m
2 year
1 %

year year
1 year
1 year
1 year
1 year
1

sediment and fishery

g C m
2 g C m
2 year
1

of total throughput

Plateau lies within the range of open ocean systems where productivity and respiration tends to be balanced (P/R = 1 Christensen and Pauly, 1993) and where primary production is comparatively low. The system biomass of 6.219 g C m
2 is very low compared with the 41 trophic models presented by Christensen and Pauly (1993) and Wolff (1994), and even lower than in a tropical fjord-like embayment (Wolff et al., 1996). An ecosystem picture emerges in which energy fluxes, and to a lesser extent biomass, is concentrated in the pelagic environment. The mean level of fisheries (4.5) is similar to that found in the tropical Golfo Dulce (Costa Rica) (5.3) and reflects the fact that the Southern

Fig. 6. Partitioning of throughput in the system among consumption by predators, exports, flows to detritus, and respiration.

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Plateau fishery targets predatory fish feeding mainly on prey dependant on the pelagic system, which is itself oligotrophic in nature because of iron limitation and dominated by microbial processes. The very low gross efficiency of the fishery (catch/primary productivity) (0.018%) (Table 5) shows most of the system’s production, which occurs in the pelagic part of the system, is not harvested in this particular part of the New Zealand EEZ but is going to sustain the system as a whole. Another feature of this type of system should be kept in mind. Fish production should become increasingly sensitive to variations in primary production as primary production decreases (Iverson, 1990). Iverson calculates that F 20% variation in annual phytoplankton production could result in F 24% variation of fish production at 250 g C m
2 year
1 primary production, whereas at 50 g C m
2 year
1 primary production, there would be a F 74% variation in fish production. He concludes that ‘‘the effects of interannual variations in phytoplankton production on fish production should result in more difficult fisheries management in environments with low phytoplankton production values’’. The truth of this generalisation needs to be further investigated for the Southern Plateau as this region plays an important part in the life cycle of hoki. It is possible that the condition of hoki adults that go to breed off the west coast of South Island may be directly impacted by variations in primary productivity or competition for food among elements of the ecosystem over the Plateau and, as noted above, there is recent evidence of food limitation of the growth of southern blue whiting in this system. By definition, the Southern Plateau system is high on the ‘‘maturity’’ scale as we have assumed steady state and there is no biomass accumulation, i.e. P/R = 1.0 (Odum, 1971). A high mean total transfer efficiency of 23% exceeds that of any of the systems reported by Christensen and Pauly (1993). Han (1997) considers that total system standing stock (TSS)/total system throughput (TST) is the best measure of system structure and state. In the Southern Plateau system, this ratio is 0.006 year
1 which is even lower than the Golfo Dulce (Wolff et al., 1996) and suggests this ecosystem, as modelled, is highly complex compared with other systems that have been modelled (Table 6). The degree of energy cycling (assumed to increase as systems ‘‘mature’’ (Odum, 1971)) compared with the 41 systems reported by Christensen and Pauly (1993) shows that Southern Plateau is intermediate to high (Finn’s cycling index (FCI) = 19.75%; average path length = 5.79). Nevertheless, these conclusions may be partly an artefact of the detail with which the zooplankton and microbial system has been described in the present model, as other Ecopath models referred to here, divide the fish compartment but tend to aggregate the zooplankton and microbial compartments. 3.6. Sensitivity analysis The sensitivity analysis routine was used to explore the sensitivity of the model to the input parameters used in the model. Input parameters are varied in the routine independently in steps of 10% from
50% to + 50% of the mean estimates used as input to the model. Altering input parameters of a group often has large effects on the output parameters of that group. As expected, changing biomass of most groups, usually by
20% or + 30%, had a >20% impact on ecotrophic efficiencies (EEs) of the same groups (except toothed and baleen whales, mesopelagic fish, macrozoopolankton). Changing P/B

240

Ecosystem

TSS (g C m
2)

TST (g C m
2 year
1)

TSS/TST (year
1)

FCI (%)

Average path length

Fishery mean trophic level

Gross efficiency fishery (%)

Average transfer efficiency levels II – IV

Reference

Southern Plateau, NZ Golfo Dulce, Costa Rica Tongoy Bay, Chile Venuzuela shelf Weddell Sea, Antarctica South Benguela, 1989 South Benguela, 1995 – 1999

6.22 10.43 236.3 122.10 16.9 36.0 56.6

1136 1405 20,835 7621 261 2332.7 2081.8

0.006 0.007 0.011 0.016 0.065 0.015 0.027

19.8 18.9 10.1 1.6 18.8 – –

5.79 3.37 4.91 – 3.5 3.19 2.78

4.84

0.02

23.0

3.6 – – 3.48 3.66

0.89 – – – 0.04

13.0 – – 10.6 27.4

This study Wolff et al., 1996 Wolff, 1994 Mendoza, 1993 Jarre´-Teichmann et al., 1997 Jarre´-Teichmann et al., 1998 Jarre´-Teichmann et al., 1998

TSS = total system standing stock (biomass); TST = total system throughput; FCI = Finn’s cycling index; gross efficiency of fishery = catch/primary production.

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Table 6 Ecosystem maturity statistics

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of most groups by
20% or + 30% had a >20% effect on the EEs of the same groups (except for toothed and baleen whales). The largest impacts of one group on another are seen when relatively low percentage changes ( F 20%) in input parameters of adult and juvenile fish, squid, mesopelagic fish, macrozooplankton, macrobenthos, ciliates, heterotrophic flagellates, and bacteria are made. Among the groups impacted by >20%, apart from themselves, are mesopelagic fish, macrobenthos, macrozooplankton, ciliates, and phytoplankton which are the most sensitive groups that are impacted by a 20% or 30% change in specific parameters of the impacting group. 3.7. Limitations and strengths of the approach How representative of the Southern Plateau ecosystem is this model? At best it can be seen as preliminary as a number of the parameters were estimated from literature values or were derived from data collected north of the study region. The fact that the model could be balanced only by using relatively high P/Q ratios may be a true characteristic of the subantarctic system. For example, Ohman and Runge (1994) hinted that a calanoid copepod (Calanus finmarchicus) may need to consume less food when feeding on microzooplankon compared with the same species feeding on phytoplankton. All of the gaps in our knowledge of the Southern Plateau ecosystem, identified in the discussion and in Appendix A, suggest priorities for further work to quantify these parameters for the Southern Plateau region especially in the area of obtaining production, food, and respiration characteristics for New Zealand’s major fisheries species. The assumption of steady state for this system may have caused some of the difficulties in balancing this model. There is interannual variability in surface chlorophyll a in the Southern Plateau region (Murphy et al., 2001). Therefore, there is likely to be similar interannual variability in primary production, perhaps evidenced by variable recruitment to the southern blue whiting fishery (Hanchet et al., 1998) and less than optimal growth of the 1991 year class of southern blue whiting that survived in particularly large numbers (Fig. 3). Because the data used here have been collected in different years, we may be getting only a general picture. Nevertheless, we are confident the model reflects the general characteristics of this system. That is, being a low primary production system dominated by the microbial loop and with low sedimentation to the seafloor, it is a system with high transfer efficiencies that supports a range of trophic levels and high-level predators.

Acknowledgements This study was funded by the Foundation for Research Science and Technology, New Zealand, Contract No. CO1214. We thank Dr. Helen Neil, Dr. Rosie Hurst, and Marieke van Kooten, all of NIWA, and Dr. W. Balzer, University of Bremen, Germany, for providing as yet unpublished data that is acknowledged in the text. The authors are also grateful to Dr. Lynne Shannon, of Marine and Coastal Management, South Africa, for sharing her knowledge and experience of this type of modelling. [RW]

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Appendix A . Input data A.1 . Birds Biomass data were collated on the populations of the 11 most common seabird species (Diomedea antipodensis, Diomedea gibsoni, Diomedea epomophora, Thalassarche impavida, Thalassarche steadi, Thalassarche salvini, Puffinus griseus, Procellaria aequinoctalis, Pachyptila desolata, Pterodroma lessonii, Eudyptes sclateri) that breed on the Bounty/Campbell Plateau (Robertson and Bell, 1984; Gales, 1998; Taylor, 2000), their wet body weights (Marchant and Higgins, 1990), months of residence in the area (Marchant and Higgins, 1990), and breeding success (data cited in Marchant and Higgins, 1990). It is assumed that the minor species make a negligible contribution to seabird biomass. It is also assumed that the populations of smaller sea birds breeding in the Bounty/Campbell Plateau region also feed there. We do not have the data to decide how reasonable these assumptions are. It is known that some large species feed outside the region (e.g. Walker, 1995; Waugh et al., 2000). We assume that 10% of wet weight is carbon, based on the data for fish (Vinogradov, 1953). Thus the mean annual biomass of seabirds is assumed to be 0.00158 g C m
2. Production to biomass ratios were calculated by using available data on chick maximum weight, number of months to attain this weight, breeding success, number of breeding pairs, and total population biomass. An upper bound to annual P/B was calculated from: [(no. of breeding pairs  no. of eggs laid  maximum chick weight)/total biomass of population]/(no. of months to maximum weight if over 12 months/12). A lower bound to annual P/B was calculated from: [(no. of breeding pairs  no. of eggs laid  breeding success  maximum chick weight)/total biomass of population]/(no. of months to maximum weight if over 12 months/12). A P/B of 0.30 year
1 is used although Crawford et al. (1991) use 0.2 year
1 for southwest African seabirds and Wolff (1994) uses 0.07 year
1 for northern Chile seabirds. Annual consumption to biomass ratios were calculated from available data on food requirements in the literature (e.g. Woehler and Green, 1992) and by calculating the standard metabolic rate, multiplying it by 2.5, and converting to units of carbon (Schneider and Hunt, 1982). A Q/B of 36.5 year
1 is used in the model. Consumption may also be calculated from the relationship of Nagy (1987): C = 0.495M0.704, where C is daily consumption in dry grams and M is body mass in grams. A Q/B of 62 year
1 was used by Wolff (1994) for northern Chile sea birds, therefore further work probably needs to be done on the food requirements of New Zealand sea birds. The diet of seabirds is estimated to be composed of squid, mesopelagic fish, and macrozooplankton, and juvenile fish. Cherel et al. (1999) show that albatrosses during chick rearing prey on southern blue whiting juveniles that are 4 –5 months old. Birds have been incidentally killed by fisheries activities on the Southern Plateau. Bartle (1991) estimated that 1140 white-capped albatrosses were killed during the 1990 southern squid trawl fishery. The main agent of these deaths was the net monitor cable the use of which was banned in 1992. Grey petrels have also been killed as part of the ling long-line fishery on the Pukaki Rise. Estimates of bird deaths due to fishing activities have been made recently for the Auckland Island squid trawl fishery at 38 birds (c.v. = 30%)

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(Baird, 1999) and 82 birds were killed in the squid fisheries for 1999– 2000 (Buller’s, Campbell, and white-capped albatrosses, grey and white-chinned petrels, sooty shearwater, cape pigeon and diving pigeon); about 4% or trawls capture seabirds (Baird, in press). In the absence of consistently collected data, annual fishing related mortalities of birds are estimated here to be between 4560 kg WW of white-capped albatrosses or 38 kg WW of, for example, grey petrels (or 456 or 3.8 kg C). On a per square meter basis over the whole of the Southern Plateau, these mortalities are 1.05  10
6 or 8.76  10
9 g C m
2 year
1. In the model, we use the greater value to estimate the potential impact of fishing on the sea bird populations. A.2 . Seals and sea lions The New Zealand fur seal (Arctocephalus forsteri) is found on Auckland, Campbell, Bounty, and Antipodes Islands on the Southern Plateau where they are non-migratory stocks of a total of about 9600 individuals (Wilson, 1974; Crawley and Warneke, 1979). Adult males weigh 120– 185 kg and adult females 40– 70 kg (Crawley and Warneke, 1979). Assuming a 50:50 sex ratio and using a median weight for males and females, then the average annual wet biomass is 998.4 tonnes. Assuming that these mammals have the same carbon content as fish (10% of wet weight), the total carbon biomass is 99.8 tonnes or 0.0002 g C m
2. This species eats barracouta, cephalopods, and small fish (Street, 1964). Although southern elephant seals (Mirounga leonina) have been seen in the New Zealand region, it is assumed that they are not a regular part of the Southern Plateau ecosystem. Their P/B and Q/B ratios are given by Laws (1984, Table III). Hooker’s sea lion (Phocarctos hookeri) is New Zealand’s only endemic seal. Males weigh about 350 kg and females 110 kg (Gales, 1995), and the total population on the Auckland and Campbell Islands is probably about 12,500 (Gales and Fletcher, 1999), although in a recent disease event during January and February 1998, approximately 60% of the sea lion pups and an unknown number of adults died over a 30-day period on Enderby and Dundas Islands (Baker, 1999). Assuming a 50:50 sex ratio and using a median weight for males and females, the average annual wet biomass is 2875 tonnes. Assuming that these mammals have the same carbon content as fish (10% of wet weight), the total carbon biomass is 288 tonnes or 0.00066 g C m
2. This species appears to feed on the bottom and eats cephalopods, crustaceans, and fish and has been know to eat penguins (Gales and Mattlin, 1997; Childerhouse et al., 2001). The total average annual seal biomass is therefore estimated to be 0.0009 g C m
2. Using the data of (Laws, 1984, Table VIII) and assuming that mortality equals production in a stable population, then the average P/B for Antarctic seals is about 0.22 year
1 which is used here. Annual food consumption in relation to biomass (Q/B) averages 46 year
1 assuming a daily food intake of 10% of body weight, the upper food intake for captive animals (Laws 1984); this value is used in the model. Similar values (54 and 47 year
1) are obtained for fur and Hooker’s seals, respectively, if the equation of Nagy (1987) for all eutherian mammals is used: C = 0.235M0.822, where C is the daily food consumed in g DW and M is the wet mass of the animal in g. Jarre-Teichmann et al. (1998) estimate that Cape fur seals have a Q/B ratio of 19.3 year
1 and a P/B ratio of

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0.95 year
1 although the former seems too low and the latter too high. The diet of seals and sea lions is assumed to be composed of squid, and mesopelagic and large fish (Street, 1964). In the Bounty/Campbell Plateau region, Hooker’s sea lion are caught incidentally around the Auckland Islands in the southern squid trawl fishery and New Zealand fur seal are caught around the Bounty Islands in the southern blue whiting fishery (Baird, 1996, 1997, 1999; Baird et al., 1999; Doonan, 1999). The estimated numbers caught in each fishery have been averaged over the years that estimates have been made based on observer data. Therefore an average of 71 Hooker’s sea lions were caught annually from 1988 to 1999 and 119 New Zealand fur seals were caught annually from 1993 to 1998. This export represents 6.6  10
6 g C m
2. A.3 . Toothed whales and dolphins There are a number of species of toothed whales and dolphins known to have the Southern Plateau in their living range (e.g. Gaskin, 1982; Baker, 1990): Arnoux’s beaked whale (Berardius arnuxii), southern bottlenose whale (Hyperoodon planifrons), hourglass dolphin (Lagenorhynchus cruciger), Andrew’s beaked whale (Mesoplodon bowdoini), straptoothed beaked whale (Mesoplodon layardii), spectacled porpoise (Phocoena dioptrica), goosebeak whale (Ziphius cavirostris), southern rightwhale dolphin (Lissodelphus peronii), and sperm whale (Physeter macrocephalus). Estimates have been made of beaked whales (mostly southern bottlenosed whale), but also including Arnoux’s beaked whale (450,000 – 800,000 individuals), the hourglass dolphin (100,000 – 200,000 individuals) south of 50jS (although these species are distributed north of this latitude so these are underestimates), and sperm whale south of 30j (128,000 – 290,000 individuals) (Kasamatsu and Joyce, 1995). Sperm whales are known to migrate, so their average annual biomass in the Southern Plateau region is estimated in the following paragraph. The Southern Plateau extends over 8j of latitude and 16j of longitude. Humpback whales migrate southwards in spring at an average speed of 15j per month (Brown and Lockyer, 1984). If we assume that all whales migrate at the same speed through the region, then the Southern Plateau would be passed in 16 days for a one-way journey and 32 days for a return journey. Thus, the average annual biomass is calculated based on 32 days residence. If we assume that whales are evenly distributed with longitude in the Southern Ocean and that the proportion of the total population that migrates through the Southern Plateau region is proportional to the longitudinal spread of the plateau relative to the total extent of the Southern Ocean, then we can calculate an average annual whale biomass that passes through the Southern Plateau region (16j as a proportion of 320j, the estimated longitudinal extent of the Southern Ocean) for 32 days by multiplying total whale biomass by 0.0044. Therefore the annual average biomass of sperm whales is estimated to be 29,604 tonnes wet weight or 2960 tonnes C or 0.0068 g C m
2. The biomass of the other species in the Southern Plateau region are not included. The surveys of odontocete whales carried out between 1976 and 1988 in spring and summer in Antarctic waters show that species known to occur occasionally over the plateau have

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their main distributions south of 50jS in summer (Kasamatsu and Joyce, 1995). Little is known of any migration patterns of these species. Therefore, we decided to increase the average annual biomass of sperm whales by 25%, pending better population estimates of other species known to occur in the Southern Plateau region. This gives a grand total biomass of 0.0085 g C m
2. P/B is calculated from the relationship of Banse and Mosher (1980): log(P/B) = loga + blogMs, where Ms is in kcal, loga = 1.11, b =
0.33. [Note that K. Banse (personal communication) indicated that there is an error in his Table 2 (Banse and Mosher, 1980): the column labelled a should be labelled loga.] Whale wet weight is converted approximately to kcal by assuming that like fish, there are 1000 cal g wet mass
1 (Schindler et al., 1993). Therefore P/B is 0.042, 0.073, 0.088, and 0.288 year
1 for an average sperm whale, Arnoux’s beaked whale, southern bottlenosed whale, and hourglass dolphin, respectively. Therefore, we use a weighted average of 0.06 year
1. Daily food requirements of sperm whales are calculated to be about 3% of body weight (Brown and Lockyer, 1984) although in other species daily food consumption can vary from 4% to 14% of body weight. Another method for calculating daily rations, based on feeding rates of cetaceans in captivity (see Sigurjo´nsson and Vı´kingsson, 1998), is I = 0.42M0.67, where I is the ingestion rate (kg day
1) and M is body weight in kg; this equation gives similar values. Here we assume that the food requirements dolphins is 4% of body weight. The Q/B ratio of sperm whales is therefore assumed to be 11.0 and 14.6 year
1 for hourglass dolphins. The diet of beaked whales and dolphins is composed of fish (including demersal fish) and squid (including giant squid) (Brown and Lockyer, 1984; Berzin, 1972). A.4 . Fish (adult) The total (doorspread) biomass of fish caught on the Campbell Plateau estimated from Tangaroa and Amatal Explorer surveys from 1989 to 1996 has been very consistent at about 200,000 wet tonnes (Hurst and Schofield, 1995; O’Driscoll and Bagley, 2001). Equivalent biomass estimates from two Shinkai Maru surveys during the early 1980s were about 600,000 tonnes (Hatanaka et al., 1989). The difference appears to reflect differences in fishing power between the two vessels. The catchability coefficient ( q), which can be used to scale the estimate up to actual biomass, of most species, is unknown although modelling suggests that for hoki it is about 0.1– 0.15 for the Tangaroa, and 0.25 for the Shinkai Maru. If a catchability of 0.1 is applied to all species, we get a total standing stock of all species of about 2.5 million wet tonnes. This 2.5 million tonnes is made up of 1.8% squid, 9.8% benthic feeders, and 88.4% pelagic feeders (based on proportions in Hurst and Schofield, 1995). The main species of fish that have been consistently caught commercially through the period of data collection on the Bounty/Campbell Plateau are hoki (Macruronus novaezelandiae), southern blue whiting (Micromesistius australis), hake (Merluccius australis), ling (Genypterus blacodes), orange roughy (Hoplostethus atlanticus), javelin fish and rattails (e.g. Hurst and Schofield, 1995), although more than 78 species are found on the plateau (Anderson et al., 1998). Of these, hake, hoki, ling, and southern

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blue whiting are among the quota species for which a Total Allowable Commercial Catch has been set. Hoki is the most important fish in the Southern Plateau fishery. The adult fish that live on the Southern Plateau are part of the Western stock. These fish migrate to the west coast of South Island to spawn and juvenile fish 2 – 4 years are found on the Chatham Rise throughout the year (Annala et al., 2000). There is strong circumstantial evidence that hoki recruit to the Southern Plateau from the Chatham Rise although the size and timing of this recruitment is not known. For the purposes of this model, we assume that there is a fairly steady biomass on the Southern Plateau that is substantially altered only during the breading season (June to mid-September). In order to estimate the annual average biomass, we take into account the fact that many adult hoki migrate out of the area for 3 months (R. Hurst, personal communication) of the year to breed. Estimations of the total biomass of adult fish (recruited fish z 55 cm) on the Bounty/ Campbell Plateau have been made in November – December 1991, 1992, and 1993 (Chatterton and Hanchet, 1994; Ingerson et al., 1995; Ingerson and Hanchet, 1995). The mean total doorspread hoki biomass for 1991– 1993 on the Bounty/Campbell Plateau (data from strata 1– 5 have been removed) is 77,467 tonnes. 99.8% of this is mature fish z 55 cm. We assume that hoki biomass is 28.8% of the non-squid biomass of 2.455 million tonnes (see 300 – 1000 m summer biomass 1990; Hurst and Schofield, 1995). About 707,040 tonnes of hoki lives on the Bounty/Campbell Plateau in summer. 705,626 tonnes (99.8%) are adult fish and 70% of this migrates to the west coast, South Island, for 3 months in winter (Livingston et al., 1997). Therefore the average annual total fish biomass is assumed to be 2,331,516 wet tonnes or 233,152 tonnes C or 0.5377 g C m
2. In the next section, we calculate the proportion that represents juvenile fish (0.086 g C m
2). This is subtracted from the total fish biomass to give the nonjuvenile biomass of 0.4517 g C m
2. The annual catch is estimated from data on Quota Management species (Annala et al., 1999). The average annual catch is about 172,700 wet tonnes (including catches of hoki on the west coast of South Island where adults went to breed) to which we add an estimated 10% to account for the bycatch species which gives about 189,970 wet tonnes or 18,997 tonnes C or 0.0438 g C m
2. To estimate total landings, the discards (see below) have been subtracted to give 0.0430 g C m
2. It appears that a large proportion of fish biomass on the Bounty/Campbell Plateau depends on planktonic food resources. The diet of 7 of the main commercial fish species was analysed by Clark (1985). He found that hoki, southern blue whiting, and javelin fish were plankton (water column) feeders, feeding mainly on natant decapods, euphausiids, amphipods, and some mesopelagic fish. Other dietary information collected during trawl surveys suggests that arrow squid, hoki, and southern blue whiting are pelagic or semipelagic feeders with squid feeding on euphausiids, fish, and other squid and ling feed mainly on other fish and are thought to bottom feeders (Hatanaka et al., 1989). Using the relative biomass data given by Hurst and Schofield (1995, Table 7), minus the squid, and the classification of feeding types given above, then 6– 11% of fish biomass (15 major species, rattails, and other quota species) is thought to depend on a benthic source of food. The diet of hoki on the Campbell Plateau can be divided into macrozooplankton (16%), mesopelagic fish (23%), fish (52%), and Cephalopoda (9%),

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although this can vary considerably with season and location (Clark, 1985, Table 1). Similarly the diet of southern blue whiting on the Campbell Plateau is 71% macrozooplankton and 14% mesopelagic fish, 11% fish, and 4% squid. The diet of javelin fish is composed of 38% macrozooplankton, 8% mesopelagic fish, 5% fish, 46% Cephalopoda, and 3% macrobenthos. Ling eat 4% macrozooplankton, 12% macrobenthos, and 83% fish, some of which are benthic feeders. Silverside ate mainly macrozooplankton— salps (84.7%) and macrobenthos (7.4%). The group of fish that have more benthic organisms in their diets do not feed exclusively on benthic organisms. Smooth rattail ate 17% macrozooplankton, 33% macrobenthos, and 50% fish. Small-scaled notothenid eat 27% macrozooplankton, 33% macrobenthos, and 39% fish. Therefore adult fish are assumed to eat adult fish, juvenile fish, squid, mesopelagic fish, macrozooplankton, and macrobenthos. The annual food consumption/biomass is known only for orange roughy. Orange roughy adults consume 1.15% of body weight per day and juveniles 0.91% of body weight (Bulman and Koslow, 1992). Therefore Q/B is 4.2 year
1 for adults or 3.2 year
1 for juveniles. Consumption/biomass is therefore calculated from the empirical relationship given by Christensen et al. (2000): log(Q/B) = 7.964
0.204logWl
1.965T V + 0.083A + 0.532h + 0.398d, where Wl is asymptotic weight of fish (g), T V = 1000/mean habitat temperature of fish (K), A is the aspect ratio of the caudal fin = h2/s (h is height of tail and s is surface area of tail), and h and d are a dummy variables expressing food type (0 for carnivores and detrivores or 1 for herbivores). Southern blue whiting maximum weight (Wl) of a 60-cm fish is calculated from weight (g) = a  length (cm)b, where a = 0.00515 for females and 0.00407 for males, and b = 3.092 for females and 3.152 for males. Therefore Wl is 1600 g for females and males. T V = 1000/(7 + 273.15) = 3.57, A = h2/ s = 1.42. Therefore Q/B is 2.60 year
1. The above equation does not apply to eel-like fish (e.g. hoki, ling). Southern blue whiting is an intermediate-sized fish so its Q/B is used here for the whole fish compartment. Annual production/biomass ratios (P/B) for fish is calculated from the equations given by Banse and Mosher (1980): P/B = aMsb or log(P/B) = loga + blogMs, where loga = 0.44 and b =
0.26 when Ms is in kcal. The equation for fish is P/B = 2.75M
0.26. loga = 0.38 when Ms is in g wet weight (Haedrich and Merrett, 1992) or P/B = 2.4M
0.26. Using this latter relationship, P/B for 970 g hoki is 0.40 year
1, for a 90 g hoki is 0.74 year
1, for 350 g ling is 0.52 year
1, and for 860 g hake is 0.41 year
1. Alternatively, by assuming, in a steady state population, the rate of all mortality (natural and fishing) is equal to production rate then from population estimates and catch data (Annala et al., 1999), P/B is estimated at 0.32 year
1 for southern blue whiting and 0.36 year
1 for hoki. P/B for the large fish compartment is set at 0.30 year
1. Orange roughy P/B ratio has been calculated to be 0.15 (Pankhurst and Conroy, 1987). A.5 . Discarded fish Discarded fish are all fish, both target and non-target species that are returned to the sea as a result of economic, legal, or personal consideration (Anderson et al., 2000). Factory waste could also be added here, but, because most vessels do not now discard their waste, this is not included. The calculation of average annual discards is made by using discard

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ratios and the total catch of the target species (Anderson et al., 2000; Clark et al., 2000). Discard ratios have been worked out for arrow squid (0.0171), ling (0.182), southern blue whiting (0.0159), orange roughy (0.0366), hoki (0.0544), and oreos (0.0286). A.6 . Fish (juvenile) Juvenile fish are placed in a separate compartment. The proportion of hoki juveniles < 59 cm total length was on average about 2% by weight across of the total hoki population in 1991 – 1993 (Livingston and Schofield, 1996). The proportion of southern blue whiting juveniles varies widely from dominating the populations as they did in 1993 to forming 14% of the total population in 1995 (Hanchet et al., 1998). These two species dominate the populations on the Southern Plateau and are fished in a proportion of approximately 2:1 (southern blue whiting/hoki). Therefore we assume that the biomass of juvenile fish in the total population is 0.086 g C m
2. Young fish about 1– 2 years old have a considerably larger P/B (>0.70) than adult fish (using relationships given in Appendix A.4). P/B of 1 is used. Q/B of 3 is used although a Q/B of 19 has been reported for North Atlantic cod of 10 cm (Daan, 1973). We assume juvenile fish have a diet of macrozooplankton and mesozooplankton. A.7 . Baleen whales The populations assumed to be migrating through the Southern Plateau region are mainly Eubalaena australis (right whale), Balaenoptera acutorostrata (minke whale), Balaenoptera musculus (blue whale), Balaenoptera physalus (fin whale), Balaenoptera borealis (sei whale), and Megaptera novaeangliae (humpback whale). All six species are thought to breed in tropical, subtropical, or warm temperature waters in winter and feed in polar or cold temperate waters in summer, with spring and autumn migrations between the two regions (Brown and Lockyer, 1984). Although it is generally assumed that whales do not feed on their migration from the Antarctic to the tropics, several species have been directly observed to feed in the New Zealand region (A.B., unpublished data) if food is available. The average annual whale biomass of what passes through the Southern Plateau region is calculated by the same method used for toothed whales. Therefore the average annual biomass of baleen whales on the Southern Plateau is 401,170  0.044 wet tonnes = 17,651 tonnes or 1765 tonnes C (assuming that carbon is 10% of wet weight) or 0.0040 g C m
2. Three to four percent of body weight is consumed daily (Brown and Lockyer, 1984 and references therein) although very young whales have been observed to take up to 13% of body weight per day. This gives a Q/B of 12.8. We assume that baleen whales eat macrozooplankton, mesozooplankton, and mesopelagic fish. Production/biomass ratios (P/ B) for some mammals is calculated from the equations in Banse and Mosher (1980) as for toothed whales. Therefore an 80-tonne blue whale (80  106 kcal) has P/B of 0.032 year
1, a 30-tonne humpback whale (30  106 kcal) has P/B of 0.044 year
1, a 45-tonne fin whale (45  106 kcal) has P/B of 0.038 year
1, and a 10-tonne minke whale (10  106 kcal) has P/B of 0.036 year
1. Sakshaug et al. (1994) also estimate minke whales to have a of P/B of 0.035. P/B for the whale compartment was set at 0.038 year
1.

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A.8 . Squid The most common squids taken on the Bounty/Campbell Plateau are the warty squid (Moroteuthis spp.) and arrow squid (Nototodarus sloani). Giant squid (Architeuthis) are also found in waters 300 – 600 m deep in this region (Fo¨rch, 1998) but their biomass is unknown. Because giant squid have not been included in squid biomass, the biomass of this compartment is probably underestimated. A commercial fishery is based on Nototodarus sloani. Robertson et al. (1978) record it as part of the mesopelagic fauna. Squid biomass appears to be about 1.8% of the ‘‘all species’’ biomass (see Hurst and Schofield, 1995, Table 7). Therefore the average annual wet weight biomass of squid is estimated to be 45,000 wet tonnes. The carbon content of a squid from the Chatham Rise is assumed to be 20% of wet weight. This is derived from an assumption that ash-free dry weight is equivalent to the carbon content. Arrow squid dry weight is 22.5% of wet weight and ash is 6.2% of dry weight (Vlieg, 1988). Vinogradov (1953) gives similar data for dry weight of Cephalopoda ranging from 13% to 30% of wet weight and ash is 0.9 – 2.4% of wet weight. If we assume that ash-free dry weight is equivalent to the carbon content, then the average annual biomass of squid is 9000 tonnes C or 0.0204 g C m
2. The annual catch of squid in the Bounty/Campbell Plateau region was obtained for the years 1992/1993 to 1997/1998 from the Ministry of Fisheries (Trawl Catch, Effort and Processing Return data and Catch, Effort and Landing Return data). Groomed data on Quota Management species were compared with the data (Annala et al., 1999) and substitutions made where there was substantial disagreement. The average annual catch is about 18,000 wet tonnes. Converted to units of carbon, average annual catch is 0.0042 g C m
2. P/B ratios for gonatid squid in the Bering Sea are estimated to be 6.7 (Radchenko, 1992), for Sthenoteuthis pteropus in the tropical Atlantic to be 8.0– 8.5 (Laptikhovskij, 1995), and for captive Illex illecebrosus measured to be 2.9– 9.1 at 7 jC (Hirtle et al., 1981). Therefore annual production to biomass ratio for squid on the Bounty/Campbell Plateau is assumed to be about 8 year
1 from this work. O’Dor et al. (1980) point out that growth rates of I. illecebrocsus from field data are well below those for captive animals, indicating that food supply of the natural population is an important limiting factor. The daily ration of Loligo pealei ranges from 3.2% to 5.8% of body weight per day (Vinogradov and Noskov, 1979), which represents a Q/B of 11.7 –21.2 year
1. The mean daily ration of I. illecebrosus is 5.2% (Hirtle et al., 1981). Here we assume that Q/B is 22 which is necessary if we assume a food conversion rate similar to the highest for I. illecebrosus (25 –36%) (Hirtle et al., 1981). Food of squid appears to be composed of macrozooplankton (crustaceans), fish, and other squid (Vinogradov and Noskov, 1979). Here we assume that the diet is made up of adult fish, juvenile fish, squid, mesopelagic fish, and macrozooplankton (Mattlin and Colman, 1988; Hatanaka et al., 1989). A.9 . Mesopelagic fish The mesopelagic fauna south of the Subtropical Front is assessed from the work of Robertson et al. (1978). The most common mesopelagic fish taken in subantarctic water were the myctophids Protomyctophum normani and Gymnoscopelus procerus. Eight other mesopelagic species were relatively common: the goniostomatid Cyclothone

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pseudopallida, and the myctophids Electrona carlsbergi, Electrona subaspera, Electrona paucirastra, Hygophum hanseni, Symbolophorus ?boops, Lampanyctodes hectoris, and Lampichthys procerus. The sternoptychid Maurolicus australis was a minor constituent of the subantarctic fauna but was extremely common around shallow areas of the Chatham Rise. We do not have data on the biomass of mesopelagic fish in the Southern Plateau region therefore the model calculated this parameter. P/B ratio for mesopelagic fish is assumed to be about 1. This compares well with data for M. muelleri for which the growth parameters are very well known (P/B = 1.15) (Ikeda 1996). Similar P/B ratios (0.87 – 1.38) are given by (Childress et al., 1980) for mesopelagic fishes off California. Consumption/biomass ratios have been estimated to be 10.6 year
1 from the agespecific daily rations of 0 – 1.8 year old M. muelleri with a lifetime average of 2.9% or a Q/ B of 10.6 year
1. Age-specific net growth efficiency of 0 –1.8 year old M. muelleri has a lifetime Q/B average of 16.7 year
1. Pakhomov et al. (1996) estimated similar Q/B for subantarctic myctophid species. Here we use 16 year
1. The diet of M. muelleri is described by Ikeda et al. (1994) and includes a wide variety of mesozooplankton species, especially copepods. Therefore we assumed that mesopelagic fish eat mesozooplankton and macrozooplankton. The biomass of mesopelagic fish is calculated by the model, assuming that ecotrophic efficiency is 0.95. A.10 . Macrozooplankton The mesopelagic fauna south of the Subtropical Front is determined from the work of Robertson et al. (1978). Macrozooplankton are assumed to be mainly Euphausiacea, although Decapoda and Amphipoda, are also included; Cephalopoda are treated in a separate group (see above). The biomass of macrozooplankton is unknown in the Southern Plateau region. Subantarctic open water macrozooplankton biomass ranges from 0.012 to 4.4 g DW m
2 and subantarctic around islands ranges from 0.007 to 2.86 g DW m
2 (Pakhomov et al., 1994). To calculate macroplankton biomass, we assume that their ecotrophic efficiency is 0.95. Production/biomass ratios are taken from the literature. Euphausia lucens has P/B = 10.14 – 16.01 year
1 (Stuart and Pillar, 1988) which is high relative to that of Nematoscelis megalops (5– 6 year
1) (Lindley, 1982). Cartes and Maynou (1998) use P/B ranging from 1.24 to 4.75 for euphausiids and 8.05 for peracarids. Here we use P/B = 10 year
1 because of lower food availability and colder temperatures. Consumption to biomass ratios have been estimated to be 1.205% DW/WW (or about 9% WW/WW) for the mesopelagic shrimp Pasiphaea multidentata (Q/B is therefore about 33 year
1) to 0.061% DW/WW for the crab Geryon longipes, with mean values of 0.364% DW/WW (Q/B 10.7 year
1) on the middle slope and 0.524% DW/WW (Q/B 15 year
1) on the lower slope (Cartes and Maynou, 1998). Stuart and Pillar (1990) show that E. lucens is an omnivore that ingests on a carbon-specific basis 15– 60% phytoplankton, the remainder being mainly small copepods. Five to fourteen percent of body C day
1 was ingested by adults and Q/B ranged from 17 to 51 year
1. Q/B of 33 year
1 was used. The diet of macrozooplankton (euphausiids) may include phytoplankton, microzooplankton, and mesozooplankton with copepods dominating the diet

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(Barange et al., 1991). Therefore we assume that macrozooplankton eat, phytoplankton, microzooplankton, and mesozooplankton. A.11. Macrobenthos Macrofauna biomass on the Bounty/Campbell Plateau is estimated at 0.25 g C m
2 from data collected at 750 m on the southern flank of Chatham Rise (S.D.N. et al., unpublished data). A P/B ratio for macrofauna can be estimated from the relationship given by Brey and Gerdes (1998) showing an exponential increase of annual community P/B with water temperature. If a mean bottom temperature of 6.5 jC for 600 m is assumed (Livingston and Schofield, 1993; Schofield and Livingston, 1994), and the regression equation of Brey and Gerdes is applied, an annual P/B of 1.00 is obtained. A P/B ratio of 1.83 is used by Cartes and Maynou (1998) for polychaetes. We assume that production/consumption is 0.35. The macrobenthos are mainly deposit feeders, for example, on Chatham Rise, and average of 65 – 71% of polychaetes were deposit feeder (Probert et al., 1996; P.K.P., unpublished data). Since polychaetes dominate the macrofaunal, their trophic structure should be a reasonable representation for the macrofauna as a whole. We have assumed that the macrobenthos is fuelled largely by the sediment bacteria but also feed on other benthos although benthic crustaceans have been shown to also eat euphausiids (Cartes and Maynou, 1998). In subantarctic water, 7– 22 Ag Chl m
2 day
1 reaches 550 m (Nodder and Gall, 1998) but this is a very small quantity and is ignored as a source of food for the macrobenthos. Therefore we assume that the macrobenthos eats macrozooplankton, macrobenthos, meiobenthos, and bacteria. A.12 . Mesozooplankton The average annual biomass (wet weight and as carbon) of mesozooplankton is calculated using data collected in 1993 (Bradford-Grieve et al., 1998), and historical data collated by Bradford (1980). These data have been adjusted for the average depth of the water column over the Plateau. Mean annual carbon biomass 0 –615 m from these data is 1.058 g C m
2. The production/biomass ratio for mesozooplankton for low productivity water is about 12 (Shushkina et al., 1998). This may be compared with P/B of a subtropical copepod Acrocalanus inermis which was measured by Kimmerer (1983) and varied from 0.07 to 0.36 day
1 and 0.2 day
1 (Vidal, 1980). Baird and Ulanowicz (1989) estimated an average P/B ratio of 0.37 day
1 over an entire year in Chesapeake Bay, an enclosed coastal system. Secondary production is not continuous in subantarctic water because primary production is very low in winter (Bradford-Grieve et al., 1997). Therefore P/B is estimated by assuming that secondary production occurs over only 6 months of the year and daily P/B is 0.11. We assume that P/B for mesozooplankton is about 20 year
1. Food intake has been determined experimentally (see Parsons et al., 1984) and ranges from 10% to 20% of body weight per day for large crustaceans to 40 – 60% per day for small crustaceans. Paracalanus may eat 1.5 Ag N Ag body N
1 day
1 (Checkley, 1980) although their specific ingestion of C was 3.6 day
1 when feeding on N-deficient Thalassiosira. For large copepods such as C. finmarchicus, Ohman and Runge (1994) showed that, in the lower estuary region of the Gulf of St Lawrence, total food was ingested (diatoms

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dominant) at the rate of 42 – 48% of body C day
1 and in the open gulf total food was ingested (dominated by aloricate ciliates) at a rate of up to 4% of body C day
1. At all these stations the copepods were laying eggs although the authors consider the possibility that these copepods might not have been in equilibrium with the food supply. The implication appears to be that protozoa may be a much better food source that autotrophic food particles. It was assumed that P/Q is 0.35. We assume that the mesozooplankton feed on phytoplankton, microzooplankton, and mesozooplankton (Bradford-Grieve et al., 1998; Zeldis et al., 2002). Assimilation of copepods is assumed to be 0.3 for animals that are feeding on microzooplankton (Pavlovskaya and Zesenko, 1985). A.13 . Ciliates The average annual biomass of ciliates (as carbon) is calculated using data collected in a number of months (Bradford-Grieve et al., 1998; Hall et al., 1999; J.H., unpublished data). Integrations are made to 100 m and the assumption is made that there are no ciliates below 100 m if there are no measurements below this depth. Ciliate carbon biomass was calculated using a factor 0.19 pg C Am
3 (Putt and Stoecker, 1989). The average annual ciliate biomass is 0.167 g C m
2. Mean daily P/B of ciliates is 0.24 (n = 5) calculated from dilution grazing experiments (J. H., unpublished data). These data are from subantarctic waters in August and January– February; there was little difference in P/B between the two periods. A ciliate production rate of 0.3 day
1 (110 year
1) is near the mean of estimates from a number of studies tabulated by Kiørboe (1998) although growth rates of up to 0.9 day
1 have been measured (Verity et al., 1993). Also, in the subarctic Pacific, ciliate production of 0.10 day
1 is given by Landry et al. (1993) although this may be too low if predators were not fully excluded from incubations. We therefore use an annual P/B of 88. We assume that production/consumption is 0.36. The proportions in which ciliates consume their food (phytoplankton and heterotrophic flagellates) can only be estimated although we know that ciliates consume 70% of the biomass of heterotrophic flagellates and autotrophic biomass per day (J.H., unpublished data). A.14 . Heterotrophic flagellates The average annual biomass of heterotrophic flagellates (as carbon) is calculated using data collected in a number of months (Bradford-Grieve et al., 1998; Hall et al., 1999; J.H., unpublished data). Integrations are made to 100 m and the assumption is made that there are no heterotrophic flagellates below 100 m if there are no measurements below this depth. Heterotrophic flagellate carbon biomass was calculated using calculated cell volumes (Chang and Gall, 1998). The average annual heterotrophic flagellate biomass is 0.307 g C m
2. Mean daily P/B of heterotrophic flagellates is 0.80 (n = 10) (292 year
1) calculated from dilution grazing experiments (J.H., personal communication). These data are from subantarctic waters in August and January– February; there was little difference in P/B between the two periods. Growth rates of heterotrophic microflagellates of >2 day
1 have been measured when conditions are not limited by iron (Chase and Price, 1997) but are

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< 1 day
1 at the low prey Fe/C of 9 Amol mol
1 observed in the open subarctic Pacific (see Tortell et al., 1966). In low iron growth conditions, carbon-specific growth of microflagellates was 0.7 – 1.6 day
1. The lower end of these growth rates is similar to the growth rates calculated for subantarctic waters from dilution grazing experiments. We assume that production/consumption is 0.35. The proportions in which heterotrophic flagellates consume their food (bacteria and phytoplankton) can only be estimated. We know that heterotrophic flagellates consume 4.4% of picophytoplankton biomass and 2.4% of bacterial biomass per day (Safi and Hall, 1999; J.H. unpublished data). Assimilation efficiency ((ingestion
excretion)/ingestion) of heterotrophic flagellates in low iron conditions is 0.84 (Chase and Price, 1997) although we initially use 0.70. A.15 . Meiobenthos Meiofaunal biomass for the Bounty/Campbell Plateau at slope depths is estimated from some nearby measurements (S.D.N. et al., unpublished data) and values from the literature. Meiofaunal biomass integrated to 5 cm at 750 –1000 m depth just north of the plateau ranges from 0.07 to 0.09 g C m
2. These data are somewhat lower than those derived from regression equations for the temperate North Atlantic which indicate values of 0.35– 0.40 g C m
2 for a depth of 600 m (Tietjen, 1992; Soltwedel, 2000). On the basis of various regressions of density vs. depth (mainly North Atlantic), the total meiofaunal density at 600 m may be calculated to be f 1400 individuals 10 cm
2. A mean weight per individual nematode (the dominant taxon) of 0.2 Ag dry weight, or 280 Ag dry weight 10 cm
2, converts the number of individuals to 1.12 g wet weight m
2, or 0.112 g C m
2, assuming dry weight to be 25% of wet weight and carbon to be 40% of dry weight (Feller and Warwick, 1988). These data are of a similar order of magnitude therefore we assume an average annual meiofaunal biomass of 0.1 g C m
2. Annual P/B ratios of meiofauna vary a lot, but 10 is often taken as an average value (Feller and Warwick, 1988). The prime source of food for the meiobenthos is assumed to be bacteria with some contribution from other meiobenthos. Annual production/consumption was assumed to be 0.31. A.16 . Bacteria (water column) The average annual biomass of bacteria (as carbon) is calculated using data collected just north of the region (Bradford-Grieve et al., 1998; Smith and Hall, 1997; J.H., unpublished data) and is estimated at 0.598 g C m
2 using the carbon conversion factor of Fukuda et al. (1998). This value is likely to be an underestimate as we have not taken into account the likelihood that bacterial biomass is greater around the islands due to the additional input of detritus from macroalgae as well as from the water column system because of greater phytoplankton biomass and consequent trophic flows. The annual production/biomass ratio for bacteria is estimated to be 87.4 year
1. Shushkina et al. (1998) estimate bacterial P/B to be 92 based on the analysis of for low productivity waters whereas Sorokin (1999, Table 2.2) gives P/B of 0.5 day
1 for eutrophic coastal habitats; 0.6 day
1 in mesotrophic temperate seas; and 1.2 day
1 in oligotrophic tropical seas which seem extremely high. Estimates of production/consumption (P/Q) where obtained

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by Tortell et al. (1966) for iron-limited water in the North Pacific. These estimates were 0.12– 0.18, although we have used 0.23. A.17 . Bacteria (sediment) We do not have direct measurements of bacterial biomass for the Bounty/Campbell Plateau. Data from the surface 3 cm are available at slope depths on the southern flank of Chatham Rise (S.D.N. et al., unpublished data). In order to compare these data with those from other studies where sediment bacteria are integrated to greater depths, we use the data at one station in summer which indicate that biomass at 0– 9 cm is 3.6– 4.5 times the biomass at 0– 3 cm and bacterial production at 0 –9 cm was 1.2 –1.5 times the production at 0 –3 cm. These data are very variable and because we suspect that the Chatham Rise sediment is strongly influenced by the Subtropical Front sitting above it, we have decided to use a biomass estimate from the regression of Deming and Yager (1992) of about 1.5 g C m
2 (to a sediment depth of 15 cm). Annual bacterial production (0 –3 cm) ranges from 0.18 to 0.35 g C m
2 year
1 for stations 750– 1000 m and these same stations have an annual bacterial P/B ratio of about 1 (S.D.N. et al., unpublished data). These values are considerably lower than the average bacterial production of 16.9 g C m
2 year
1 reported by Kemp (1994) for slope sediments ( < 2000 m). Alongi (1990) found bacterial productivity (surface 0– 5 mm sediment) to decrease significantly with depth (695 – 4350 m) and, from the regression given, one gets a figure for 600 m depth of 34.7 g C m
2 year
1. For the Celtic Sea (135 – 1680 m), Poremba and Hoppe (1995) measured microbial activity in the upper 10 cm of sediments. Activity was always very much higher in the top 0 – 1 cm of sediment. They give specific growth rates for their station at 571 m: bacterial P/B in the top 0– 1 cm was 0.0298 day
1 or 10.9 year
1. Alongi (1990) also gives specific growth rates for bathyal and abyssal stations. They vary widely from 0.001 to 0.12 day
1 (0.37 –43.8 year
1) but decrease with water depth. Sorokin (1999) gives annual P/Bs of 14.6 and 7.3 off Japan. Using these latter values of biomass and production, one gets average annual P/B ratios of around 11– 12. Because of this uncertainty, P/B is calculated by the model by assuming that ecotrophic efficiency is 0.80. A growth efficiency (P/Q) of 0.3 (Kirchman, 2000) is assumed here. A.18 . Phytoplankton Average annual biomass is estimated to be 0.895 g C m
2 from Bradford-Grieve et al. (1999). This value does not take into account the often heightened chlorophyll concentrations around the islands and over the Pukaki Rise (Murphy et al., 2001). We therefore increase annual average phytoplankton biomass by a factor of 1.2 to give 1.07 g C m
2. Daily primary production of 28 –62 mg C m
2 day
1 in winter (June) and of 230– 271 mg C m
2 day
1 in early spring (October) in SAW was measured in 1993 (BradfordGrieve et al., 1997), although in October, 1997, primary production in SAW (390 – 680 mg C m
2 day
1) was greater than in 1993 (Boyd et al., 1999). As it is difficult to be sure of the annual average primary production in the region from these data, we investigated estimates from satellite data. Annual primary production, estimated from the data presented by Moore and Abbott (2000) for the Subantarctic Water Ring, is about 80 g

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255

C m
2 year
1. Therefore, these data give a P/B of about 89 and 118 year
1, respectively. P/B of 89 year
1 is very low compared with annual P/B = 218 given by Shushkina et al. (1998) for low productivity water. It is noted by Ducklow (2000) that: ‘‘bacterial production in the water column is maintained in a remarkably constant ratio to primary production, averaging about 0.15 – 0.20 across oligotrophic and oceanic HNLC and upwelling and blooming systems.’’ For The Southern Plateau system, however, the ratio is considerably higher at 0.66 (taking bacterial and primary production as 53 and 80 g C m
2 year
1, respectively). Our bacterial biomass and production estimates are typical. This suggests that annual primary production over the Southern Plateau might be nearer to 265 g C m
2 year
1 and annual P/B might be about 248. Nevertheless, Southern Plateau phytoplankton production thus estimated is much lower than the range of values given by Ducklow in his Table 5 (465 – 1548 mg C m
2 day
1). A.19 . Detritus and dissolved organic carbon (water column) Annual average integrated particulate carbon (PC) for the water column is estimated from data collected in 1993 (spring and winter) (McCarter and Hall, 1996a,b). Where only particulate nitrogen (PN) data were available, particulate carbon was estimated assuming that PC/PN is 6. Data were integrated down to 615 m. Detritus was estimated by subtracting phytoplankton, bacteria, and microzooplankton biomass from the PC data. Antarctic concentrations of dissolved organic carbon (DOC) in spring surface water of the Atlantic sector of about 0.46 –0.66 mg C l
1 or g C m
3 (Kaehler et al., 1997) and 0.52 –0.84 g C m
3 in October in subantarctic water east of New Zealand (M. van Kooten, personal communication). In February – March in the Atlantic sector of the Antarctic, Romankevich and Ljutsarev (1990) record surface concentrations of DOC >1.2 mg C l
1. Therefore we assume that annual average concentrations of DOC on the Southern Plateau of 0.9 g C m
3 and that these concentrations occur to 615 m to give an integrated concentration of 554 g C m
2. The sum of the particulate detrital carbon and DOC is 17.6 + 554 = 572 g C m
2. A.20 . Detritus and dissolved organic carbon (sediment) Average annual organic carbon content of the subantarctic sediment on the Pukaki Rise is estimated from Carter et al. (1999). At 28 cm below surface, the total organic carbon content of the sediment is 0.28% of the dry weight of sediment. The dry density of sediment at 3.10 m below the surface is 1.031 g cm
3. Therefore assuming the properties of the sediment about the sampling depth are similar, 1 m2 of sea floor down to 15 cm depth is 150,000 cm3 or 154,650 g. Therefore, the carbon content of 0.28% of dry sediment is 433 g m
2. This result may be compared with data from 450 m on the south side of Chatham Rise. At this station, the surface 5 cm of sediment has 0.6% total organic carbon. The dry density of the sediment at this depth is about 1.65 g cm
3. One square meter of sea floor down to 15 cm depth is 150,000 cm3 or 247,500 g. Therefore, the carbon content of 0.28% of dry sediment is 693 g m
2. The actual average figure for the Plateau is probably

256

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somewhere between although we have chosen the lower value since the water column input to the sediment is known to be low (H. Neil, personal communication). Annual average detritus supplied to the sediment has been measured by Neil on the southern flank of Pukaki Rise at 50j15.64VS, 171j28.35VE to be 1.23 g C m
2 year
1. We assume pore-water has similar concentrations of DOC as are found at Station M271-96 at 806 m SW of Britain at about 48jN in summer (Otto and Balzer, 1998) and the porosity of the sediment ranged from 0.66 to 0.47 (W. Balzer, personal communication). The total DOC down to 15 cm is 0.43 g C m
2; carbon in this form is assumed to be included in the Total Organic Carbon content of the sediment of 433 g C m
2. A.21 . Assimilation efficiency The portion of food consumed and not assimilated was taken as 0.2 (the default for Ecopath) except for mesopelagic fish, macrozooplankton, macrobenthos, mesozooplankton, ciliates, heterotrophic flagellates, meiobenthos (set at 0.4, 0.4, 0.4, 0.35, 0.3, 0.3, 0.3), respectively, mainly to ensure there is enough substrate for bacteria in the model.

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