Primary production and standing crop of phytoplankton along the ice-edge in the Weddell Sea

Deep-Sea Research, Vol. 28A, No. 9, pp. 1017 to 1032, 1981. Printed in Great Britain.

0198~149/81/091017-16 $02.00/0 © 1981 Pergamon Press Ltd.

Primary production and standing crop of phytoplankton along the ice-edge in the Weddell Sea SAYED Z. EL-SAYED* a n d SATORU TAGUCHIJf

(Received 14 October 1980; in revisedform 23 January 1981; accepted 6 March 1981) Abstract---Chlorophyll a, phaeopigments, primary production, nannoplankton, inorganic nutrients, and physical measurements were made in the upper water column at 17 stations occupied in the Weddell Sea in February and March 1977. The most conspicuous feature is the marked contrast between the low standing crop and primary productivity of the northern and central regions, compared with the much more productive shelf waters at the head of the Weddell Sea. Chlorophyll a in the euphotic zone was 4.36 + 1.75 mg m-2 for the former regions and 31.6 + 9.5 mg m-2 for the southern stations. Production in the water columns of the southern stations ( 0 . 4 1 + 0 . 2 3 g C m -2 day -1) was approximately four times that at the northern-central ones (0.104 + 0.092 g C m-2 day-1). Based on plant carbon: chlorophyll a ratio of 30 + 10 estimated in the present study, an average specific growth rate (la) of 0.71 was calculated. The nutrient concentrations showed an inverse distribution compared with those of chlorophyll a and primary production: higher nutrient concentrations were recorded at the northern stations than at the southern waters. Phosphate : nitrate : silicate ratios in the water column suggested the importance of nitrate for phytoplankton production. A significant relationship (P < 0.001) between chlorophyll a concentration and day-zooplankton biomass and a significantly higher ratio (P < 0.06) of night-to-day catch of zooplankton in the northern-central regions than in the southern region were found, This suggests that zooplankton do not need to migrate vertically in the southern regions due to abundance of food supply, whereas in the northern-central regions zooplankton must migrate upwards during the night to consume available food produced through the photosynthetic process. The study demonstrates that water column stability, grazing, and proximity to land masses are the most significant factors controlling phytoplankton production in the Weddell Sea.

INTRODUCTION

THEINTERESTshown in recent years in the study of the Weddell Sea stems from the fact that it is an important region for the formation of Antarctic Bottom Water, which has profound influence on the deep circulation of the World Ocean (DEACON, 1937; WOSX, 1939; CARMACKand FOSTER,1975; WARREN, 1979). Investigations by GILL (1973), DEACONand FOSXER(1977), FOSTERand MIDDL~TON(1979), and DEACON(1979) have contributed much to our understanding of the physical oceanography of that sea. Our knowledge of the biological oceanography of the Weddell Sea, by comparison, is meagre. Until recently our knowledge of the standing crop and primary production of the Weddell Sea was based largely on the observations made in the early 1960's by EL-SAYEDand MANDELLI(1965) and in the course of the International Weddell Sea Oceanographic Expedition (IWSOE) in 1968 (EL-SAYED,1970, 1971a). * Department of Oceanography, Texas A & M University, College Station, Texas 77843, U.S.A. t Hawaii Institute of Marine Biology, University of Hawaii at Manoa, P.O. Box 1346, Coconut Island, Kaneohe, Hawaii 96744, U.S.A. 1017

1018

SAY[I)Z EL-SAYEDand SaTORUI"A{;UCHI

The extensive studies on the distribution and concentrations of Euphausia superha carried out in the South Atlantic and the Scotia Sea (MARR, 1962; MACKINTOSH, 1972: MAKAROV, 1974, 1975; VLADIMIRSKAYA,t975), point to the significance of the Weddell Sea as a spawning area for krill, and to the importance of the cyclonic circulation of that sea to the life history of krill. MAKAROV(1972) suggested that the populations in the Weddell Sea form a self-sustaining stock maintained by the clockwise circulation. The SCAR*/SCORtflABO++/ACMRR§ Working Party on Krilt Biology has recently called attention to the need for oceanographic data on the rates of movement of water masses of the Weddell Sea and on primary production rates to allow assessment of rates of immigration of krill in different regions (MAUCHLINE, 1979). Such data, it was pointed out, would allow production of dynamic maps and models of krill circumpolar distribution. In the present study we are interested in relating the krill's basic food supply to the physical and chemical conditions prevailing during a late austral summer cruise. Data on the standing crop and species composition of the ice algae collected in ice cores during this cruise have been published by ACKLEY,BUCK and TAGUCH! (1979). MATERIALS AND METHODS Seventeen stations were occupied during the cruise of the USCGC Burton Island (16 February to 6 March, 1977) along a more or less northwest-southeast transect between Elephant Island and the Filchner Ice Shelf (Fig. 1). Three stations, 14, 15, and 16 were in shallow water ( < 500 m) along that shelf. Stations 17 and 18 were occupied near Stas 9 and 2, respectively, toward the end of the cruise to determine if any temporal changes in the physical, chemical, and biological parameters had taken place within an interval of one to two weeks (Table 1). Stations 5, 7, 8, 9, 10, and 11 were close to the edge of the pack ice, but in open and much deeper waters (> 2,000 m) than the southern stations. Water samples were collected at eight depths with 10-/PVC Niskin samplers shortly after measurements of submarine light penetration were made using a Secchi disc. The depths correspond to 100, 50, 25, 12, 6, 1, 0.i, and 0.01Yo of surface irradiance. The depths were calculated using an extinction coefficient determined by dividing 1.7 by the Secchi disc reading. At Stas 7, l 3, and 17, seven light levels were chosen (100, 45, 21, 10, 5, 3, and 1.5°o) in order to match the neutral density filters used in the simulated in situ productivity experiments. Temperature was measured by means of reversing thermometers and salinity by an inductive salinometer (Plessey Environmental System Model 6230). Water samples for nannoplankton chlorophyll a measurements were fractionated by passing the water first through a 20-1am Nytex screen and then through an HA Millipore" filter (0.45-1am pore size). The same amount of water was filtered directly through a Whatman G F / C glass filter for total chlorophyll measurements. Rates of carbon assimilation were estimated using the '4C isotope uptake technique of STEEMANNNIELSEN (1952) as modified by STRICKLANDand PARSONS(1972). Water samples in four light bottles and one dark bottle (Pyrex boiling flask, 125 ml) were inoculated with 4 laCi NaH~*CO3 for both in situ and simulated in situ experiments. The simulated in situ

* ScientificCommitteeon Antarctic Research (SCAR). t ScientificCommitteeon Oceanic Research (SCOR). { InternationalAssociationof BiologicalOceanography(IABO). § AdvisoryCommitteeon Marine Resources Research of FAO (ACMRRL

Primary production in the Weddell Sea

1019

Fig. 1. Cruise track and stations occupied during February and March 1977. Extent of summer ice is shown ( - - - ); bathymetry in meters.

Table 1. Position of the stations occupied, date, extent of ice cover, and depth of 1% light level during USCGC Burton Island cruise in the Weddell Sea from 16 February to 6 March, 1977

Station 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Position 63°57'S 63°56'S 64°58'S 65°23'S 66°45'S 67°33'S 69°21'S 70°15'S 71°25'S 73°07'S 74°38'S 75°39'S 77°45'S 76°55'S 76°21YS 70°40'S 64°01'S

53°12'W 52°03'W 52°26'W 51°28'W 50°30'W 48°57'W 46°46'W 45°37'W 43°00'W 43°44'W 38°55'W 34°27'W 35°10'W 34°04'W 28°40'W 46°10'W 52°0YW

Date

Ice coverage* (oktas)

Depth of 1% light level (m)

16 Feb. 17 Feb. 18 Feb. 19 Feb. 20 Feb. 21 Feb. 22 Feb. 23 Feb. 24 Feb. 25 Feb. 26 Feb. 27 Feb. 28 Feb. I March 1 March 4 March 6 March

1 3 2 0 1 0 7 2 3 0 0 6 7 7 0 0 0

55 58 63 55 68 63 63 38 53 20 23 16 23 20 23 50 76

* Ice coverage is described by 8 levels, thus 8 oktas rekrs to 100% coverage and 0 okta to 0% coverage (i.e., open water).

1020

SAYED Z. EI.-SAYEDand SATORU ]-A(iU(H!

experiments were carried out in a deck incubator consisting of a long clear plastic tube. inside which the light and darkened bottles (wrapped in black nylon screening to match the levels of light intensities) were placed. The bottles were incubated at the ambient temperature of the seawater and natural light. The experiments were started around 1000 h and terminated about 1700 h (local time), The radioactivity of the filters was assayed twice for 10 min using a liquid-scintillation counter (Packard Model 3550). Quench correlation was made using the channel ratio method (HERBERt, 1965). The daily primary production was calculated by the method described in EL-SAVEDand TAGUCHI(1977), in which the 1~C uptake during the incubation was multiplied by a ratio of the daily incident radiation to the incident radiation during incubation. Samples for determining nutrient concentrations were filtered through GF/F glass fiber filters. Immediately following filtration, the filtrate was frozen at -2ff~C. Nutrients were later determined by the following automated methods: nitrate, phosphate, and silica (ATLAS, HAGER, GORDON and PARK, 1971; STRICKLANDand PARSONS, 1972), using a 4channel Technicon AutoAnalyzer ~ system. Water samples for particulate organic carbon and nitrogen were filtered through precombusted Whatman GF/C glass fiber filters. The filterate was kept in a glass ampoule for dissolved organic carbon analysis. Particulate organic carbon and nitrogen were determined using a Hewlett Packard CHN analyzer model F & M 185 (HIROTAand SZYPER, 1975). Dissolved organic carbon was determined with an infrared analyzer (STRICKLAND and PARSONS, 1972). Vertical hauls were made with 0.5-m nets (333-~tm mesh size) from 200 m to the surface. The nets (open area : mouth ratio of 4.8) were provided with flow meters and the samples were fixed with 5°~i formalin. Wet weight of the preserved samples was determined with a microbalance. RESULTS

Physical variables Most of the temperatures recorded during the cruise were below -1.0~'C (average = - 1.2°C) (Fig. 2A). Upper waters at the northern stations (between Stas 2 and 7) were relatively warmer than at the southern stations. Temperature in the upper 100 m was almost uniform at most of the stations except at Stas 4 to 8, where colder water with temperature below - 1.75°C was observed between 70 and 120 m ; this water probably was carried from the south along the shelf break and at the stations occupied along the continental shelf (Stas 14 to 16). The latter stations exhibited the lowest temperatures (below - 1.75°C) observed during the cruise. The salinity and density profiles indicate that mixing of the upper 100-m layer was occurring at these stations as shown by the downward bend of the isohalines and isopycnais, though the isotherms are not so conspicuous. The near-shelf stations (14 to 16) indicate that the low salinity overshadows the temperature in bringing the isopycnals downwards (Fig. 2B), It seems that salinity variations during the cruise had more effect on the density of the upper layer than temperature variations (Fig. 2C). Thus, stability is maintained even at the stations with temperature inversions near the surface, such as Stas 4, 5, 6, 7, and 17, except at Sta. 2 where a high salinity core ( > 34.2 × 10 -3) at about 20 m made the water between 20 and 40 m slightly unstable. This could have been a transient phenomenon due to sinking of an isolated body of water of higher salinity. Such sinking might have occurred near the shelf break and the isolated water might have been carried to the site.

Primary production in the Weddeil Sea

1021

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The relative light levels at the northern and central stations (2 to 10, 17, 18) indicate that the water columns were evidently clear, with 1 ~ light level reaching between 38 and 76 m (Fig. 2D). The depth of the euphotic zone at the southern stations was much shallower than at the northern stations, ranging between 16 and 23 m. 41

1022

SAYED Z. EL-SAYED and SATORU TA(;UCrtl

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Fig. 3. Spatial distribution of nitrate (A), phosphate (B), silicate (C), and dissolved organic carbon (D) along the cruise track. Nutrients The distributions of nitrate-N, phosphate-P, and silica-Si in a vertical transect in the upper 150 m of the water column exhibit a marked boundary between Stas 10 and 11 (Figs 3A, B, C). At the southern stations (11 to 16) the three nutrients were lower than at the other stations.

Primary productionin the WeddellSea

1023

Dissolved organic carbon (DOC) Dissolved organic carbon concentrations were lower (< 1.0 g m -a) at the southern stations than at the northern stations, where values > 2.0 g m -a were common (Fig. 3D). The standing crop of dissolved organic carbon in the water column (to the depth of the euphoric zone) ranged from 14 g m- 2 at Sta. 13 to 96.8 g m - 2 at Sta. 6. Waters to the north of Sta. l0 were characterized by higher amounts of DOC (> 35 g m-2), while south of Sta. 11 values lower than 25 g m-2 were recorded.

Chlorophyll a and phaeopigments The concentration of chlorophyll a and phaeopigments, in marked contrast to the inorganic nutrients, were much higher at the southern stations than at the northern ones. The northern stations were generally poor in chlorophyll a (i.e., < 0.1 mg m-3), with more or less uniform distribution within the euphotic zone (Fig. 4A). The increase in the concentration of chlorophyll a at the southern stations was quite evident; most of the changes occurred between Stas 10 and 11. Waters north of Sta. 10 were characterized by lower amounts of chlorophyll a ( < 1 0 m g m -2) than waters south of Sta. 11 (>20 mg m-2). The distribution of phaeopigments was similar to that of chlorophyll a, with low concentrations at the northern stations (< 10 mg m-2) and higher values (> 20 mg m-2) at all the southern stations (Fig. 4B).

Particulate organic carbon and nitrogen There was a marked break in the distribution of both particulate organic carbon (POC) and nitrogen (PON) between Stas 10 and 11 (Figs 4C, D). At the southern stations (11 to 16) POC ranged from 19 to 136mg m -3 and from 8.1 to 66mg m -a at the northerncentral stations. PON had a similar pattern of distribution to that of POC, ranging from 2.8 to 22 mg m -3 at the southern stations and from 0.91 to 9.8 mg m -a at the northern stations. Average concentration of POC and PON were higher at the southern stations, 36mgCm-3and6mgNm-3

Primary production Primary production values less than 0.5 mg C m-3 day-1 characterized all the northern stations (Fig. 4E). The least productive was Sta. 6 (0.039 g C m -2 day-l), but the production of the southern stations, particularly those off the Filchner Ice Shelf, was higher. Station 14 had the highest rate of production observed during the entire cruise (0.68 g C m - 2 day- 1). At almost all the stations where primary production experiments were carried out, we noted that production extended well below the euphotic zone, taken here as the depth of 1~o surface light level. The contribution of primary production below the euphotic zone was nearly one-fourth of the total production in the water column (24.8_+ 36.9~) with the northern--central stations contributing 33.0_+47.5~ compared to 14.2~ for the southern stations. The photosynthetic fixation below the euphoric zone during the present cruise was much higher than reported from the Ross Sea (Eltanin cruise 51 ; see FAy, 1973), or from the Indian sector of the Antarctic (Eltanin cruise 46; EL-SAvEDand Jix'rs, 1973). During the two latter cruises, carbon fixation below the photic zone did not exceed 5 to 10% of that in the entire water column. The percentage contribution of the nannoplankton to total chlorophyll a averaged

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Fig. 4. Spatial distribution of chlorophyll a (A), phaeopigments (B), particulate organic carbon (C), nitrogen (D), and primary production (E) along the cruise track.

Primary production in the We,ddell Sea

1025

74.1 + 34.2~o for the entire water column. The contribution of the nannoplankton to the total standing crop of phytoplankton averaged 63.3 + 36.6~ for the southern stations and 80.2+35.5~ for the northern--central stations. The value for the southern stations was close to the 60.1~ for the Ross Sea (Eitanin cruise 51) reported by FAY (1973). Nannoplankton contributed 48.7+ 17.6~o of the primary production at the southern stations. The average contribution for all the stations during the cruise was 74.2~o, which is higher than that for the Ross Sea (54~) obtained by FAY (1973). DISCUSSION

Chlorophyll a concentrations and primary production data serve to emphasize regional differences in the Weddell Sea. The present observations corroborate those by EL-SAVED and MANDELLI (1965), who noted pronounced regional variations in the productivity parameters between the northern, eastern, and southern regions of the Weddell Sea. Perhaps the most conspicuous feature of the present cruise was the markedly lower productivity of the northern and central regions than of the more productive shelf waters at the head of the Weddell Sea. Chlorophyll a, phaeopigments concentrations, and primary production levels were all markedly different in the northern-central and the southern stations, permitting us to divide the Weddell Sea into two main provinces, a relatively poor northern-central province and a much more productive southern province. To the former, one can add the impoverished eastern waters of the Weddell Sea (see EL-SAYEDand MANDELLI, 1965). Although the waters off the Filchner Ice Shelf were the most productive observed during the present cruise, they were much less productive than the waters off the Ronne Ice Shelf (in the southwestern Weddell Sea), where a spectacular bloom of the diatom Thalassiosira tumida (Janisch) Hasle was observed in February 1968 (EL-SAVED,1971b; HASLE, HEIMDAL and FRYXELL,1971). Station 17 near Stas 9 and l0 (in the central province) and Sta. 18 near Stas 2 and 3 (in the northern province) were occupied one and two weeks later, respectively, to ascertain any changes in the top 12 parameters listed in Table 2. The ice cover at both Stas 17 and 18 was nil (Table 1). Of the 12 parameters, five showed significant (P < 0.05) variations after one week and nine after two weeks. Even though the concentration of chlorophyll a did not increase, primary production doubled within a week at Sta. 17. Thus, the spatial variation in the biological parameters in the northern-central province is more likely to be larger than the temporal variation taking place within a period of one week. Several investigators have drawn attention to the importance of the stability of the water column in controlling production (SVERDRUP, 1953; PINGREE, 1978). HART (1934), HASLE (1956), and EL-SAYEDand MANDELLI(1965) attributed the low productivity of the Southern Ocean to the comparatively low stability of surface waters, which prevents the organisms from remaining in the optimum light zone long enough for extensive production. The density (at) data at various light levels during the present study indicate two different water masses. Southern stations (11 to 16) were characterized by lower a, values at lower light levels, i.e., lower than 27.30 at the 1% light level, 27.50 at the 0.19/o light level, and 27.70 at the 0.01% light level. The location of the six southern stations agreed with the Shelf Domain defined by CARMACK(1974) during austral summer. Figure 5A shows the mean a, at different light levels for the two provinces. In the southern province, water was well mixed within the euphotic zone and stratified below the photic zone. Water in the

1026

SAVFD Z, EL-SAYFD and SATORU TAGU(HI

Table 2. Mean and 950% confidence limits of photosr,ntheticall)' actice radiation (PAR~, ice cover, depth ~[ euphotic zone, integrated amounts of nitrate, phosphate, and silica in the euphotic zone, mean concentration of dissolved organic carbon (DOC), particulate organic carbon (POC), and nitrogen (PON) in the euphotic zone, integrated amounts of chlorophyll a and phaeopigments in the euphoric zone, primary production, day-zooplankton, night~ zooplankton, and day night average zooplankton abundance Jor the northern central province and the southern province. Probability based on "t'-test is shown. Null hypothesis is that values are the same in the two province~

Parameters P A R (cal cm 2 d a y l j Ice cover (oktas)

Depth of euphotic zone (m) Nitrate (mg-at. N m - 21 Phosphate (rag-at. P m - z ) Silica (mg-at. Si m - z) D O C (g m -3) P O C (mg m -3) P O N (mg m -3) Chlorophyll a (mg m - 2) Phaeopigrnents (mg m - 2 ) Primary production (g C m - 2 day - 1 ~ Day zooplankton (mg wet m 31 Night zooplankton (mg wet m - 3 ) All zooplankton (mg wet m 3 )

N o r t h e r n central province ............................. M e a n _+95"o CI 55.4_+ 17.3 2.1 _+0.8 56.6 + 6.47 1300 +- 235 101 + 20.6 2930 .-+459 1.14__+0.22 26.5 ± 7.12 3.44_+ 1.02 4.36 +_+1.75 4.44-+-+ 1.16 0.104 +- 0.092 6.47 + 4.5 84.9 + 33.1 30.2 ± 30.1

Southern province M e a n _-+95~!0 CI 53.8 i+_17.5 3.3 _+4.2 20.3 -+ 3.21 281 + 52.6 31.2 ± 6.81 741 -+ 131 0.834-+0.148 52.3.+-+ 19.5 8.72_+3.30 31.6 + 9.51 29.5 -+9.11 0.410 +- 0.23 56.9 -+ 68.4 118 ± 101 99.8 ± 37.3

P (}.8765 0.2632 0.0000 0.0000 0.0001 0.0000 0.0291 (/.0012 0.0002 0.000(I 0.0000 0.0023 0.0000 (I.6485 0.0226

northern-central province, on the other hand, was stratified within the photic zone but unstable below it. Based on the vertical distribution of a, values, all measurements were grouped into the two provinces (Fig. 5A) and their means were calculated (Figs 5B to H). An average standard error for the mean was 101~. Due to the stable density gradient below the euphotic zone together with sufficient light and nutrient supply in the euphotic zone, high concentrations of chlorophyll a, with small vertical variation, were observed in the southern province. A well-mixed euphotic zone provides higher production than a stagnant euphotic zone (MARRA, 1978). Below the euphotic zone the concentration of chlorophyll a decreased sharply (Fig. 5E) due to little light penetration for photosynthesis (Fig. 2D). Throughout the present study, primary production significantly (r = 0.88; P < 0.001) depended on chlorophyll a concentration, as given by the following equation : production (g C m - 2 day - 1 ) = (12 + 51 chlorophyll a + 46. This suggests that phytoplankton in the northern-central province might grow at least as fast as in the southern province. The faster-growing phytoplankton at the northern-central province, however, could not remain in the euphotic zone due to the unstable water structure below the euphotic zone (Fig. 5A). This suggestion is further supported by the fact that the phytoplankton, which we assume sank below the euphotic zone in the northern-central province, still had high photosynthetic capacity as evidenced in their contributing more than 30~o of the primary production of the water column, compared to 157/o in the southern province (Fig. 5F). The high production contributed to higher concentrations of particulate organic carbon and nitrogen in the southern province (Figs 5G,H).

Primary productionin the Weddell Sea

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NITRATE I0

i% "~~

LuO-ot N I"1 I 20

. , , I T~

PARTICULATE 30

T 1 J ....

40

J

ORGANIC NITROGEN

5 G

- ~ l - -

~t.J .

.

.

IO .

.

( m g . m "~l) IS I

20 '

'

'

~q

z ~

i%

o

~OC

H

Fig. 5. Vertical distribution of mean values of at (A), silicate (B), phosphate (C), nitrate (D), chlorophyll a (E), primary production (F), particulate organic carbon (G), and particulate organic nitrogen (H) for the northern-central province (O) and the southern province (VI).

Ice coverage and all biological parameters except the Photosynthetically Active Radiation (PAR) and night zooplankton are significantly different between the two provinces (Table 2). The southern province was characterized by a shallower euphotic zone (20.3 + 3.2 m), higher chlorophyll a concentration (31.6 + 9.5 mg m- 2), and higher primary production (0.410+0.23 g C m -2 day-t). Our estimate of primary production is

1028

SAw~ z. EL-SAYEDand SATORUTAGU(H1

Table 3. Mean and 95110 confidence limits jbr ratios o f nitrate : phosphate, silica . phosphate, pigment ratio.s ~i chlorophyll a : chlorophyll a plus phaeopigments, productivity index, ratio ~f particulate organic" carbon : nitroge~z, ratio o f particulate organic' carbon (.see rext) : chlorophyll a, percent plant carbon o f particulate organic carbon, ratio of night-to-day zooplankton, and ratio o f chlorophyll a content o f ice algae : phytoplankton for the northern central and southern provinces. All ratios are calculated with values integrated to 1%~light levels except zooplankton and i~e algae. See Table 2 .for probability

Parameters Nitrate :phosphate ratio Silica : phosphate ratio Pigment ratio Productivity index (mg C mg chl a - l day 1) Carbon : nitrogen ratio Carbon : chlorophyll a ratio Percent plant carbon Division rate (division day - ~) Night : day zooplankton ratio Ice algae : phytoplankton chlorophyll a ratio

Northern central province Mean ± 93-';~C L

Southern province Mean~95',i; CL

P

12.9 ± 1.30 29.4 _+4.42 0.481 ± 0.059

9,51 _+2.78 24.7 ± 8.68 0.518 £ 0.025

0.0105 0.1814 0.2849

27.9 ± 19.4 7.83 ±0.544 416 _+179 10.9 _+9.38 0.877 ± 0.421 14.4 ± 23.3

12.4 ± 3.77 6.01 _+0.194 34.6 ± 3.54 87.1 ± 9.15 0.495 _+0.128 1,35 ± 1.04

0.1078 0.0001 0.0020 0.0000 0.0750 0.0604

4.68 _+6.66

0.380 ± 0.736

0.0060

comparable to 0.47_+0.15 g C m -2 day -1 (EL-SAYED and MANDELLI, 1965) and confirms the high production off Coats Land. The southern province off Coats Land is one of the most productive areas in the Southern Ocean. Nitrate : phosphate and silica : phosphate ratios (Table 3) suggest that phytoplankton in the southern province seem to utilize more nitrate than in the northern-central province and silica might be utilized at similar rates by phytoplankton at both provinces (Table 3). This suggests that nitrate is a more important nutrient than silica for phytoplankton in the Weddell Sea. Higher pigment ratios and lower C : N ratios of particulate matter suggest either conditions physiologically favorable to phytoplankton in the southern province (Table 3) or that phytoplankton comprise a greater fraction of the total particles. Chlorophyll a and particulate organic carbon were significantly related (r = 0.93; P < 0.01) in the southern province, carbon (mg m - 2 ) = (30_+ 10) chlorophyll a (mg m - 2 ) + 131. Plant carbon : chlorophyll a ratio can be assumed to be 30_+ l0 as suggested by STEELE (1962). Although this assumption has a weakness (BANsE, 1977), it is sufficiently accurate for the present purpose. Thus, if one assumes as a first approximation that the plant carbon : chlorophyll a ratio is 30 for both provinces, because of no nutrient limitations and because of the similarity of PAR in the two provinces (Tables 2,3), plant carbon contributed 10.9 +_9.3~ in the northern--central provinces and 87.1 _ 9.1~ in the southern province (Table 3). HOLM-HANSEN, EL-SAYED, FRANCESCHINIand CUHEL (1977) reported surprisingly low specific growth rates of phytoplankton for the Ross Sea. The division rate (it) can be calculated from the following equation : ~t(division day - 1) = _13.322 log C o + AC, t ~o

primary production in the WeddellSea

1029

where Co is the phytoplankton biomass in carbon and AC is the increase of phytoplankton biomass during time t. In the present study Co was calculated using the plant carbon : chlorophyll a ratio of 30 (equation above), AC was estimated by in situ primary production measurements, and t is one day. The division rates are 0.88+0.42 for the northern-central province and 0.49 + 0.13 for the southern province (Table 3), more than 0.33 division day-1 reported by HOLM-HANSENet al. (1977). Their estimate was based on ATP assays. A conventional ATP assay overestimates its concentration by 33 to 135~o (KARL, 1978), resulting in the overestimation of plant carbon and consequently lower apparent division rates. The ATP assays include all organisms in the sample and leave uncertainty in their ecological application (MAYZAUDand TAGUCm, 1978). Antarctic phytoplankton are considered as obligate psychrophiles, i.e., they are adapted to low temperatures and will not grow at higher temperatures. The overall mean division rate of 0.71 found in the present study is comparable to the maximum division rate (0.75) to be expected at - 2 ° C (EPPLEY, 1972). The present Weddell Sea data support the hypothesis that Antarctic phytoplankton are physiologically adapted to exhibit nearmaximal growth rates at low temperatures. Present rates are somewhat higher than those arrived at by HOLM-HANSENet al. (1977) based on their Ross Sea data. Even though phytoplankton in the southern province seem to be healthy, their apparent division rate and productivity index were lower than in the northern-central province (Table 3). This may be due to differences in carbon :chlorophyll a ratios or to self-shading by higher concentrations of chlorophyll a in the cell (EPI'LEV and SLOAN, 1966; TAGUCrU, 1976). Shading due to a high chlorophyll a content in the water column was observed in the Indian sector of the Antarctic and sub-Antarctic (EL-SAYED and Jirrs, 1973), in the Bransfield Strait (BtJRKrIOLDERand MANDELU, 1965), and off the Antarctic Peninsula (HoRrqE, FOGG and EAGLE, 1969). Although the zooplankton program on the Burton Island cruise was a modest one, the samples showed abundant zooplankton populations in the southern province (Table 2). The populations were mainly of the genera Euchaeta, Calanus, Rhincalanus, and the appendicularia Fritillaria. Euphausia superba, by comparison with the northern-central province, was nearly absent in the southern province. Although EL-SAVEDand MANDELU (1965) showed an inverse phytoplankton and zooplankton relationship in their southern Weddell Sea study, the present study showed a significant positive relationship between chlorophyll a concentration and wet weight of zooplankton caught at noon (P < 0.001). This suggests that a high concentration of chlorophyll a can support a large biomass of zooplankton. It is interesting to note the low ratio (P < 0.06) of night-to-day catch of zooplankton in the southern province (Table 3). Because of high concentrations of food and the long daylight period, zooplankton do not seem to migrate vertically in the southern province. Zooplankton at the northern-central province, however, migrated upward at night. A part of the biological program carried out during the Burton Island cruise was the study of chlorophyll a concentrations in ice cores collected at Stas 8, 10, 11, 13, 14, and 15 by one of us (S.T. ; see ACKLEV,BROCKand TAGROCHI,1979). At the northern-central stations (8, 10), the concentration of chlorophyll a in the ice was higher than in the ambient water (Fig. 6). On the other hand, at the southern stations (11, 13, 14, 15), the concentration of chlorophyll a in the ice was insignificant compared to the much richer phytoplankton chlorophyll a (Table 3). The study of the rate of release of ice algae into the water column by melting in summer is

1030

SA~ED Z~ EL-SAYED and SAFORU I'AGU( t{I

.J

6

Z)

LOG R = - 0 . 4 1 9 5 =-

~5

1.048 LOG C

.

~3

0 n- 0 0

I

2

3

CHLOROPHYLL a (mg rn-3)

Fig. 6.

Relationship between chlorophyll a content of ice algae to phytoplankton at Stas 8, 10, 11, 13, 14, and 15. 3 NORTH'CENTRAL

/"

:0

SOUTH

'E c>

DD G~

c.D Z

[]

u3 0 0

Fig. 7.

I ~

I 2 CHLOROPHYLL

I 3 a

(rng

t 4

m"3)

Relationship between chlorophyll a concentration and dissolved organic carbon.

beyond the scope of the present investigation. It seems probable, however, that the rate of loss of phytoplankton and released ice algae from the euphotic zone is larger than the release rate of ice algae because of the instability of the water structure below the euphotic zone in the northern-central province (Fig. 5A). This speculation is supported by the fact that no significant relationship was found between ice coverage and the standing crop of chlorophyll a in the euphotic zone. Almost all the dissolved organic carbon in the sea originated ultimately from carbon dioxide fixed by phytoplankton (DuURSMA, 1965). In the present study, the dissolved organic carbon correlated significantly with the concentration

Primary production in the Weddeli Sea

1031

of chlorophyll a in the water column in the southern province (P < 0.001 based on correlation coefficient; see Fig. 7). However, we found no relationship between dissolved organic carbon and chlorophyll a in the northern-central province. The contribution of icealgae chlorophyll a was highly significant in the northern-central province (Fig. 6). The ice algae in that province might be important in contributing to the higher concentration of dissolved organic carbon (Fig. 7). A positive significant relationship (r = 0.75 ; P < 0.001 ) between the ice cover and primary production may be established by the physical structure of the water, but not by the contribution of ice algae to the phytoplankton assemblage in the water column. It seems, then, that the physical structure of the water column is far more significant in controlling primary production than is the contribution of ice algae to the ambient phytoplankton. Thus, the present study demonstrates that water column stability, grazing, and proximity to land masses are the most significant factors controlling phytoplankton production in the Weddell Sea. Acknowledgements--The authors express their thanks to the men of the USCGC Burton Island for their assistance in data collection. We are grateful to KURT BUCK, MICHAEL MEWR, and ROaERT WARNER of Texas A & M University for their assistance in the data collection. GuY A. FRANCESCHINI kindly supplied the data on incident radiation. We thank TAKASrtl ICmW for discussing the physical data with us. JED HIROTA of the University of Hawaii kindly supported the particulate carbon and nitrogen analysis at his laboratory. JAMES FINN of the University of Hawaii assisted in the statistical analysis. This research was supported by National Science Foundation Grant DPP76-80738 to Texas A & M University.

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