Material supply to the abyssal seafloor in the Northeast Atlantic

Progress in Oceanography 50 (2001) 27–63 www.elsevier.com/locate/pocean

Material supply to the abyssal seafloor in the Northeast Atlantic R.S. Lampitt

a,*

a

, B.J. Bett a, K. Kiriakoulakis b, E.E. Popova a, O. Ragueneau c, A. Vangriesheim d, G.A. Wolff b

Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK c UMR CNRS 6539, IUEM, Technopole Brest-Iroise, 29280 Plouzane´, France d IFREMER DRO/EP, BP 70, 29280 Plouzane´, France

b

Abstract Downward particle flux was measured using sediment traps at various depths over the Porcupine Abyssal Plain (water depth 苲4850 m) for prolonged periods from 1989 to 1999. A strong seasonal pattern of flux was evident reaching a maximum in mid-summer. The composition of the material changed with depth, reflecting the processes of remineralisation and dissolution as the material sank through the water column. However, there was surprisingly little seasonal variation in its composition to reflect changes in the biology of the euphotic zone. Currents at the site have a strong tidal component with speeds almost always less than 15 cm/sec. In the deeper part of the water column they tend to be northerly in direction, when averaged over periods of several months. A model of upper ocean biogeochemistry forced by meteorology was run for the decade in order to provide an estimate of flux at 3000 m depth. Agreement with measured organic carbon flux is good, both in terms of the timings of the annual peaks and in the integrated annual flux. Interannual variations in the integrated flux are of similar magnitude for both the model output and sediment trap measurements, but there is no significant relationship between these two sets of estimates. No long-term trend in flux is evident, either from the model, or from the measurements. During two spring/summer periods, the marine snow concentration in the water column was assessed by time-lapse photography and showed a strong peak at the start of the downward pulse of material at 3000 m. This emphasises the importance of large particles during periods of maximum flux and at the start of flux peaks. Time lapse photographs of the seabed show a seasonal cycle of coverage of phytodetrital material, in agreement with the model output both in terms of timing and magnitude of coverage prior to 1996. However, after a change in the structure of the benthic community in 1996 no phytodetritus was evident on the seabed. The model output shows only a single peak in flux each year, whereas the measured data usually indicated a double peak. It is concluded that the observed double peak may be a reflection of lowered sediment trap efficiency when flux is very high and is dominated by large marine snow particles. Resuspension into the trap 100 m above the seabed, when compared to the primary flux at 3000 m depth (1800 mab) was lower during periods of high primary flux probably because of a reduction in the height of resuspension when the material is fresh. At 2 mab, the picture is more complex with resuspension being enhanced during the periods

* Corresponding author. Fax: +44-(0)23 80596247. E-mail address: [email protected] (R.S. Lampitt). 0079-6611/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 7 9 - 6 6 1 1 ( 0 1 ) 0 0 0 4 7 - 7

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of higher flux in 1997, which is consistent with this hypothesis. However there was rather little relationship to flux at 3000 m in 1998. At 3000 m depth, the Flux Stability Index (FSI), which provides a measure of the constancy of the seasonal cycle of flux, exhibited an inverse relationship with flux, such that the highest flux of organic carbon was recorded during the year with the greatest seasonal variation.  2001 Elsevier Science Ltd. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Materials and methods . . . . . . . . . . . . . . 2.1. Hydrography . . . . . . . . . . . . . . . . . . 2.2. Meteorology, upper mixed layer dynamics, 2.3. Downward particle flux . . . . . . . . . . . 2.4. Marine snow concentration . . . . . . . . . 2.5. Benthic phytodetrital coverage . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . primary production and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Appendix A

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. General characteristics and hydrography . . . . . . . . . . . . . . . . 3.2. Meteorology, upper mixed layer dynamics and primary production 3.3. Downward particle flux . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Swimmer contamination . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. General pattern of flux and composition . . . . . . . . . . . . . . 3.3.3. Reliability of the flux measurement . . . . . . . . . . . . . . . . . 3.3.4. Near bottom collections and resuspension . . . . . . . . . . . . . 3.3.5. Global comparison of primary flux . . . . . . . . . . . . . . . . . . 3.3.6. Relationship between primary flux and primary production . . . 3.3.7. Seasonal and interannual variability . . . . . . . . . . . . . . . . . 3.4. Flux related observations . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. Marine snow concentration . . . . . . . . . . . . . . . . . . . . . . 3.4.2. Benthic phytodetrital load . . . . . . . . . . . . . . . . . . . . . . . 3.5. Measured and modelled primary flux . . . . . . . . . . . . . . . . . . 4.

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1. Introduction A major challenge facing biogeochemists is to derive reliable estimates of material loss from the upper mixed layer. Once lost from this part of the water column, contact with the atmosphere is removed and hence its influence on global climate is postponed for decades or even centuries. An understanding of the downward flux of particulate matter in the mid-water environment is of crucial importance in studies of

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the upper ocean, for which this flux represents a loss. However, from the perspective of the benthos, which is the focus of most papers in this volume, this downward particle flux can be considered as the primary supply term. The level of flux, its seasonal variability and the composition of the settling material are all influenced by the biogeochemical processes in the water column and also the hydrographic conditions that may advect material to, or away from, the site of interest. In contrast to studies in shelf and coastal seas, studying the open ocean offers an advantage of the separation between the upper ocean where biogenic particles are produced and consumed, and the deep-sea floor where they are only consumed. The two environments are effectively separated, so that measurements of downward flux in the lower part of the water column provide reliable indications of the supply to the seafloor (Lampitt & Antia, 1997). The techniques by which such fluxes may be measured have been the subject of much controversy, and the measurements are often substantially influenced by the method chosen. They include models based on radionuclide budgets (Buesseler, Bacon, Cochran, & Livingston, 1992), biogeochemical models (Fasham & Evans, 1995), the accumulation rates of radioisotopes, refractory compounds and faunal markers in deepsea sediments (McManus, Anderson, Broecker, Fleisher, & Higgins, 1998; Marchant, Hebbeln, & Wefer, 1999), and the oxygen consumption of the sediment biological community (Jahnke, 1996). The only direct method of measurement is by the use of particle interceptor traps, which collect material as it settles through the water column. Lampitt and Antia (1997) collated and summarised all of the available open ocean flux data, which satisfied rigorous quality criteria, to derive global estimates of organic carbon flux based on the relationship between total primary production and flux at 2000 m depth. They also examined global trends in deepwater flux and regional variations in the degree of seasonality in the fluxes, using an objective index. The data set they used was small, only 76 years of trapping data were then available for the entire globe. Despite evidence for there being significant interannual variations in particle fluxes (e.g. Newton, Lampitt, Jickells, King, & Boutle, 1994), long term data spanning periods greater than 3 years were available for only three sites. Since then several further long-term data sets have been published (Baldwin, Glatts, & Smith, 1999; Wong et al., 1999; Takahashi, Fujitani, Yanada, & Maita, 2000). The objectives of our present study were: 1. To measure directly the downward particle flux at an abyssal site, 2. To interpret the observations in terms of the factors controlling particle production in the surface waters and, 3. To assess the effect of this flux on the biogeochemical processes of the underlying sediment. This paper therefore ‘looks’ in two directions; both up to the surface and down to the seabed. The study site, in the region 48°50’N 16°30’W (Fig. 1) on the Porcupine Abyssal Plain (PAP), has been the main focus of an on-going study that started in 1992. It is about 350 km to the northeast of the site of the 1989 JGOFS North Atlantic Bloom Experiment (NABE), from which data are also included. There is considerable background information on the upper ocean processes at the NABE site (Ducklow & Harris, 1993) and with respect to these, the NABE and PAP sites may be considered as similar. However, from a benthic perspective the PAP site is significantly different, because the seabed is flatter. The absence of topographic undulations makes the site less spatially variable. During 1997–99 the benthos provided a strong focus for the EU funded BENGAL research project (Billett & Rice, 2001). There are occasional benthic storms occurring in the general region during which bottom currents have been reported to reach speeds of up to 27 cm/sec (Klein & Mittelstaedt, 1992), but no such storms have been recorded either at PAP (see below) or at the EDYLOC site 270 km to the southeast (Vangriesheim, 1988). At EDYLOC (47°N, 14°30’W), there are strong semi-diurnal and inertial oscillations and mesoscale variability at periods of 125 to 200 days. At PAP any influence of the slope and continental shelf is thought to be very slight, with negligible advection of particulate material (Weaver, Wynn, Kenyon, & Evans, 2000).

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Fig. 1. Location of the study site on the Porcupine Abyssal Plain (PAP) and of the JGOFS site at which the North Atlantic Bloom Experiment was carried out in 1989.

2. Materials and methods 2.1. Hydrography Water currents near the seabed were measured using Aanderaa current meters with a sampling interval of 60 minutes. These were deployed at a height of 3 mab (attached to the ‘Bathysnap’ system; see below) and within 50 m of each of the traps on the sediment trap moorings (approx 1000, 3000 and 4700 m depth). Additional measurements made in the very near bottom layer in 1996–97 are reported by Vangriesheim, Springer, and Crassous (2001). 2.2. Meteorology, upper mixed layer dynamics, primary production and modelled particle loss The model structure consists of 3 homogeneous layers. The seasonal cycle of the lower boundary of the Upper Mixed Layer (UML) was obtained using the parameterisation of the UML physics based on a model of Kraus–Turner type (e.g. Ryabchenko, Gorchakov, & Fasham, 1998). The model requires the wind speed, air temperature and surface salinity to be specified. Air temperature and wind speed were taken from NCEP re-analysis provided by NOAA-CIRES, Climate Diagnostics Center, Boulder, Colorado, USA from their web site (http://www.cdc.noaa.gov/). Values of surface salinity were obtained by linear interpolation of

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mean monthly values from Levitus, Burgett, and Boyer (1994). Lower boundaries of the second and third layers were fixed at 125 and 250 m. The ecosystem model is based on the 7-compartment model of Fasham and Evans (1995) but with an additional state-variable of fast-sinking detritus, which is assigned a sinking rate of 50 m/day (Fig. 2). Data on the precise provenance of such fast sinking material are almost impossible to extract from the published literature. Based on microscopic analyses of collected marine snow particles (Alldredge & Gotschalk, 1990; Lampitt, Wishner, Turley, & Angel, 1993; Lampitt, pers. obs.) and an intuitive understanding of the oceanic system, we have assumed that the fast sinking material consists of 50% of phytoplankton, 10% of dead zooplankton, and 40% faecal pellets. To match these requirements, we prescribe the sources of the fast-sinking detritus to be 0.7% of zooplankton mortality, 10% of phytoplankton mortality, 2% of zooplankton grazing on phytoplankton, detritus and bacteria. All other model parameters were taken from Fasham and Evans (1995), except zooplankton maximum ingestion rate (g) was assumed to be 1.1 instead of 1.0/d, and zooplankton ingestion half-saturation constant (k3) was set at 0.9 instead of 1.0 mmol/m3. These slight changes to the Fasham and Evans’ (Fasham & Evans, 1995) grazing parameters were made to give better agreement with primary production measured during JGOFS NABE in 1989. The amount of nitrate entrained during the winter convection, when the upper mixed layer (UML) depth is ⬎250 m, depends on the vertical profile below the lower model boundary. This profile was derived from the measurements made during JGOFS and although there were substantial variations even at a depth of 1000 m (range 16–21 mmol/m3), we assume the profile to be linear with the nitrate concentration, N(z) (mmol/m3) described by: N(z)⫽az⫹b where z is depth (m), a=0.015 mmol/m3, b=5 mmol/m3. Below the UML and beyond the reach of the model, particle sinking rate was taken as 120 m/d for comparison with observations of fluxes at 3000 m depth. This rate was adopted on the basis of the time lag observed between pulses of surface production and arrival of material on the seafloor (Lampitt, 1985). However, we accept that at any particular time variations in the characteristics of the particle pool are likely to result in very wide variations in sinking rates between particles, as well as variations in the mean sinking rate throughout a season. Remineralisation during sinking was based on the Martin curve (Martin,

Fig. 2.

Structure of the biogeochemical model.

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Knauer, Karl, & Broenkow, 1987). The effect of these assumptions is that the flux at 3000 m depth is 10% of that at 250 m (the model output depth) and lags by 22.9 days. Primary production was derived using the UML model, but was also calculated using satellite-based colour sensor data for 1997–1999, inclusive. The surface imprint of the 3000 m ‘statistical funnel’ is that area of ocean containing the source of the particles collected in the 3000 m trap. The size and location of this area of sea surface depends on the particle sinking rates and the hydrography of the region (Siegel & Deuser, 1997). Faster sinking particles will originate from closer to the mooring position than those sinking more slowly. Since we know nothing about the spectrum of sinking speeds of the particles collected in the traps, nor do we have an adequate description of the hydrography in the region, only a very crude estimate of the statistical funnel is possible. Based on the assumption of a sinking rate of 120 m/d and a residual current speed of 9 cm/s, the potential surface imprint has a diameter of 400 km. We have selected a circle centred on the trap location to calculate the source primary production from the satellite data. A very similar area was used for the JGOFS site to the southwest of our mooring (Waniek, Koeve, & Prien, 2000). The primary production algorithm was derived from a compilation of 14C 24-hour incubations integrated to the euphotic depth obtained between 1982 and 1995 in the outer Celtic Sea and the ocean to the west, which can be considered representative of the PAP region (Joint & Groom, 2000). The algorithm derived was: Log10PP⫽0.567∗log10CK⫹2.830 where PP is primary production (mg C/m2/d) and CK is the estimated chlorophyll concentration (mg/m3) This is almost identical to an empirical model derived by Falkowski et al. (1998) giving confidence in its broader applicability. The algorithm has been applied to the daily equal angle (nominally 9 km resolution) standard mapped ADEOS and SeaWiFS images and the average of all valid pixels within the 400 km circle was used as an estimate of primary production at the trap site. 2.3. Downward particle flux Particle flux in the midwater zone has been described at the JGOFS site (Honjo & Manganini, 1993; Newton et al., 1994; Boyd & Newton, 1995; Lampitt, Newton, Jickells, Thomson, & King, 2000) and data are also available for the benthic environment (Thomson et al., 1993; Elderfield & Thomas, 1995). The site is at the same depth as, or slightly shallower than the lysocline (Biscaye, Kolla, & Turekian, 1976; Lisitzin, 1996). A succession of sediment trap moorings were deployed at 48°N 19°30’W in 1989 and 1990 and subsequently at about 49°N 16°30’W (Fig. 1; Table 1). Each mooring carried time series sediment traps (Parflux 7G-13) (Honjo & Doherty, 1988) of mouth area 0.5 m2 at nominal depths of 1000, 3000 and 4700 m (the latter trap being 90–100 m above the seabed). The JGOFS protocol was adopted for sample handling (SCOR 1990; Newton et al., 1994). Swimmers were picked out under a dissecting microscope. They were identified to taxonomic group, but only their total abundances are reported here. Fluxes of organic and inorganic carbon, nitrogen and opaline silica were determined as previously described by Newton et al. (1994) for 1989/99 material, and as described by Kiriakoulakis et al. (2001) and Ragueneau et al. (2001) for the 1992–1999 material. Another trap was deployed on the Module Autonome Pluridisciplinaire (MAP) lander (Technicap PPS4/3, 0.05 m2, 12 bottles) (Vangriesheim & Khripounoff, 1990) with the mouth of the trap 2 mab. The material collected in the trap on the lander was analysed as follows: 1. Total carbon and nitrogen contents were measured in duplicate with a Carlo-Erba NA 1500 autoanalyzer. 2. Organic carbon content was measured with a Leco WR12 elemental analyser after removing carbonates with a 2 N HCl solution (Weliky, Suess, Ungerer, Mueller, & Fischer, 1983; Vangriesheim et al., 2001).

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Table 1 Sediment trap moorings at the PAP site Mooring ID Deployment station Location

I III V IX X XI XII XV XVIII XIX XX XXIIIa XXV XXVI MAP 1 MAP 2

485 11880#11 52908 53316#1 M30/1 12812#2 12930#35 13077#62 13200#96 13368#55 13627#25 55002#2 12913#2 13368#9

Sampling dates

Water depth (m)

North

West

Start

Finish

47°58.37⬘ 47°58.74⬘ 47°45.20⬘ 48°55.10⬘ 48°59.45⬘ 48°58.20⬘ 49°16.47⬘ 48°59.98⬘ 49°4.47⬘ 49°0.42⬘ 48°59.41⬘ 48°59.83⬘ 48°59.51⬘ 48°58.43⬘ 48°56.06⬘ 48°56.52⬘

19°33.05⬘ 19°31.97⬘ 19°28.80⬘ 16°18.20⬘ 16°22.68⬘ 16°29.00⬘ 16°28.20⬘ 16°21.11⬘ 16°17.96⬘ 16°18.14⬘ 16°12.97⬘ 16°13.73⬘ 16°26.24⬘ 16°25.82⬘ 16°31.93⬘ 16°25.74⬘

18/04/89 23/07/89 22/04/90 12/04/92 07/09/92 29/04/94 12/09/94 12/10/95 15/09/96 23/03/97 27/07/97 20/03/98 04/10/98 08/10/99 18/08/96 22/03/98

16/07/89 15/04/90 16/09/90 30/08/92 LOST 07/09/94 LOST 19/05/96 19/03/97 20/07/97 22/03/98 13/09/98 07/09/99 01/09/00 07/09/97 13/09/98

4559 4570 4572 4810 4810 4840 4840 4812 4807 4874 4869 4869 4860 4830 4840 4848

The organic carbon content of particles was uncorrected for the formalin treatment. Inorganic carbon content was calculated as being the difference between the total and the organic carbon contents. In calculating carbonate fluxes, it was assumed that all inorganic carbon was represented by CaCO3, and a carbonate to inorganic carbon ratio of 8.33 was used. 2.4. Marine snow concentration During deployments in 1990 and 1992, an underwater 35 mm still camera was attached to the sediment trap mooring and photographed a volume of water defined by an orthogonal collimated beam from a strobe light (Lampitt, Hillier, & Challenor, 1993). The total volume of water illuminated and visible to the camera was 40l, but only a portion of this volume has been used for the analyses. The frame interval was 1023 minutes in 1990 (19 April–19 September) and 256 minutes in 1992 (6 April–16 July). The system was attached to the mooring at a depth of 270 m during 1990, and at 1170 m during 1992. Analyses of the images were carried out using a Kontron Vidas system. The negative of each frame was imported using a high definition CCD camera. The maximum and minimum dimension of each particle was measured, and used to calculate a particle’s volume and the equivalent spherical diameter (ESD). Each particle was attributed to one of 6 logarithmic size classes on the basis of its ESD. The abundance and volume concentration within each of these classes was determined. 2.5. Benthic phytodetrital coverage The time-lapse camera system ‘Bathysnap’ (Lampitt & Burnham, 1983) was deployed in the region of the sediment trap mooring on a number of occasions. About 2 m2 of seabed was photographed repeatedly at frame intervals of between 2.4 and 8 hours. From each frame, a record was made of the percentage of predefined quadrats that had more than 50% of their surfaces covered by phytodetritus (Lampitt, 1985). This provides a semi-quantitative assessment of the amounts of phytodetritus lying on the sediment surface at any one time.

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3. Results and discussion 3.1. General characteristics and hydrography The PAP site is at the centre of an extensive, monotonously-flat area of abyssal plain (Billett & Rice, 2001). This is in contrast to the NABE site, where sediment traps were deployed during 1989 and 1990 (Fig. 1), and where the rough topography is likely to enhance spatial variability in the benthic environment. Such variability can result in problems when interpreting the data, and was the reason for the change in location for this long-term study. The currents within 150m of the seabed at PAP (Fig. 3) were almost always ⬍15 cm/s and never exceeded 20 cm/s (see also Vangriesheim et al., 2001). No evidence of benthic storms was reported for the PAP site, which contrasts to the NOAMP area (45–49°N, 17–23°W) immediately to the south. In the NOAMP region the bottom topography is more rugged, with seabed hills rising

Fig. 3. Temporal variation in current speed at either 3 or approximately 150 m above the seabed (mab). Heights are indicated in each panel. Time axes are in decimal years with a 50 day scale bar appropriate for all data sets.

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1000 m above the level of the abyssal plain. There Klein and Mittelstaedt (1992) recorded storms lasting between 3 and 27 days during which near-seabed currents velocities reached a maximum of 27 cm/s. Topographically induced gyres are a common feature at the NOAMP site making its hydrography far more complex than at the PAP site. At the PAP site, the currents in the lower part of the water column tend to flow northward (see also Vangriesheim et al., 2001). Only in the upper water column was this general trend less predictable and occasionally, as in 1998, the flow reverses and flows towards the south. As expected, there is a strong tidal component to the water movements. The water column appeared to behave as an entity from the seabed at least to 3000 m and sometimes to as shallow as 1000 m depth, any changes in direction occurring synchronously throughout much of the water column. Some of these fluctuations may have resulted from the passing of deep-penetrating eddies, which interact with the generally northerly flow of Antarctic Bottom Water and Lower Deep Water (McCartney, 1992). A detailed analysis of the hydrography of the PAP region is beyond the scope of this paper, but the relevant conclusions are: 1. The low current speeds will have ensured that the sediment traps are likely to have performed with high efficiency. The efficiency of sediment traps declines shapely at current speeds ⬎15 cm/sec (Baker, Milburn, & Tennant, 1988) and when currents exceed this level, particle flux data should be discarded. Despite this critical technical limitation on sediment trap sampling, only a few exceptional papers have included data on in situ current speeds (e.g. Khripounoff, Vangriesheim, & Crassous, 1998). 2. Assuming the particles were sinking 120 m/day, material collected by the traps will have been derived from an area of the euphotic zone with a diameter ⱕ400 km. This area has been used in the analysis of primary production using satellite images (Fig. 4).

3.2. Meteorology, upper mixed layer dynamics and primary production According to the Krauss–Turner model, in winter ocean mixing is largely controlled by changes in air– sea heat flux, whereas in summer, wind-driven mixing is the dominant process. In the North Atlantic, the depth of the winter mixed layer (WML) ranges from 25 m in equatorial regions to ⬎600 m in the northeast (Marshall, Nurser, & Williams, 1993). The meridional gradient of the WML is very steep in the PAP region, so that only minor interannual latitudinal shifts in the processes responsible for winter mixing will produce large variations in depth of mixing. The model produces large interannual variations in the WML depth and in the pattern of mixing after the formation of the seasonal pycnocline (Fig. 5). Observations of WML depths have been calculated from various sources using CTD profiles and are based either on the depths of attainment of a fixed density difference from the surface or on the depth of a particular density gradient. These support the model output, although the critical estimates of the mixed layer depth during the first week of March are unavailable. An interesting observation from the mixed layer model is that in spite of the large interannual variations in mixing pattern, the seasonal pycnocline starts to develop every year within a surprisingly narrow time window between 15 and 30 March. The interannual variation in mixing results in variations in the nutrient supply and hence to significant changes in the total primary production calculated using the upper ocean model (Fig. 5). It should be noted, however, that both the seasonal cycle of primary production and the annual integrated value are very sensitive to the profile of nitrate adopted in the model. Even at a depth of 1000m large variations occur in nutrient concentrations over spatial scales of 10s of kilometres (S. Holley, pers. comm.), so the linear profile derived from JGOFS measurements may be subject to significant uncertainty. The satellite colour sensors have been used to provide independent estimates of surface pigment concentrations within the 400 km circle surrounding the PAP site, and there is good agreement during the summer

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Fig. 4. Monthly average chlorophyll concentration as determined from SeaWIFS images for 1998 for the months of March to September. The white ring has a diameter of 400 km and represents the expected source area for material collected in the 3000 m sediment trap whose position is indicated by the white spot. The location of the NABE site is shown by the white cross. (Courtesy of S. Groom, NERC remote sensing data analysis service.)

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Fig. 5. Upper curve: Mixed Layer Depth (MLD) as calculated from the Krauss–Turner model (solid line) and measured (red triangles). Lower curve: Total net primary production (PP) derived from the upper ocean biogeochemical model (solid line) with 14C measurements (green triangles) made during the JGOFS North Atlantic Bloom Experiment (NABE) in 1989 (Lochte, Ducklow, Fasham, & Stienen, 1993), during UK Biogeochemical Ocean Flux Study in 1990 (Bury, Boyd, Preston, Savidge, & Owens, 2001) and on Discovery cruise 227 in 1997 (E. Ferna´ ndez, pers. comm.).

between the model’s output and the average pigment concentration within the area (Fig. 6a). In the spring, however, the modelled concentrations are substantially higher, the satellite data show that the bloom occurs in mid to late May, some two months later than the time of the bloom predicted by the model! Primary production calculated from the satellite images is very much dependent on the algorithms used (Fig. 6b) and we are presenting data derived from the 400km circle (125,000 km2) around the trap site (Groom & Joint, pers. comm.). Such values are almost always lower than the model output, sometimes by as much as 60%. For comparison, we also present data on average monthly integrated values derived by Falkowski and Behrenfeld (pers. comm.) averaged over 42 pixels around the trap site (10,000 km2). Finally, we present the mean monthly values derived from the CZCS sensor on the Nimbus satellite for the years 1978–1986 using both the derivations of both Antoine (pers. comm.) and Longhurst, Sathyendranath, Platt, and Caverhill (1995). The approach adopted by Behrenfeld and Falkowski is to use a global dataset derived from a wide variety of water types and provinces and to incorporate many different data sources despite their inherent methodological differences. In constrast the approach of Groom and Joint is only to use data from areas very close to the PAP site, which have used consistent methodology. Data from a number of direct measurements at the site using the 14C technique are also shown in Fig. 5. From these it can be seen that although the model derives similar levels of production, it tends to predict the spring increase occurring earlier than that observed. As with all studies on primary production, there is a wide variation in the estimates, which generates much controversy about which are the ‘true’ values. Even the direct measurements by 14C uptake experiments are controversial, and there is considerable uncertainty as to whether the measurements are closer to the net or the gross primary production. The model output provides an estimate of net primary production and so if anything, should be somewhat lower than estimates that have been normalised to the 14C uptake rate. The satellite images give a strong indication that productivity remains low during April and May, although the model indicates enhancement from early April. This may indicate deficiencies in the model, but the downward particle flux consistently starts to increase in mid-April, suggesting that the spring bloom has started in mid-March, if the sinking rate of 120 m/d is valid. A point to note from Fig. 4 is that in spite of the large number of data points, which contribute to each pixel of the monthly composite, there is still a substantial spatial variability.

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Fig. 6. (a) Chlorophyll concentration in the upper mixed layer as derived from the satellites by Groom and Joint (pers. comm.) ADIOS (1997) and SeaWIFS (1998/1999) and from the biogeochemical model. (b) Total primary production derived from the model and from various treatments of satellite data. 14C measured values are also shown (E. Ferna´ ndez, pers. comm.).

The conclusion from these comparisons is that during the summer months the model output is within the range of independent estimates of total primary production for the site, both measured directly and satellite-derived. The increase in model chlorophyll in March that is not reflected in the satellite data is a surprising aspect of the data sets and demonstrates that the two approaches have not converged at this important time of the year. Nevertheless the broad agreement in terms of primary production gives us some confidence in the model output for this and hence, in its description of particle flux. It is important to remember that total primary production is not the driving force controlling downward particle flux. It

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39

is new production, supported by upwelling nitrate that is likely to govern the rates of particle export from the upper mixed layer when considered over an annual cycle. 3.3. Downward particle flux 3.3.1. Swimmer contamination The contamination of sediment trap samples by swimming zooplankton is a controversial issue, which undoubtedly has a large effect on estimates of particle flux, particularly within the upper 1000 m of the water column depending on how the swimmer problem is dealt with (Michaels, Silver, Gowing, & Knauer, 1990). To date the extent of this contamination is seldom reported and the relative credibility of data cannot, therefore, be assessed. At the PAP site, the level of contamination varies greatly at any one depth and there may be enhanced contamination during periods of high flux. At 1000 m water depth, the contamination was, as expected, greatest (Fig. 7), but it was still much less than in traps set in the upper few hundred metres of the water column (Lampitt, 1989). 3.3.2. General pattern of flux and composition There is a strong seasonal pattern to the fluctuations in downward particle flux at all levels in the water column, with rates increasing at all depths after the end of March. In the lower part of the water column, rates decrease to winter levels after mid to late September, whereas at 1000 m depth, winter values resume

Fig. 7. Numerical flux of swimmers into traps and dry weight flux after removal of swimmers. Data are presented at the midpoint of the sampling intervals.

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after the end of June. There is clearly considerable inter-annual variation, both in the timing of the major flux peaks and in their magnitude (see below). Flux of the major components of the settling material at all four depths is shown in Figs. 8–12 and in Appendix A. Although there are some significant gaps in the records, which were sometimes the result of instrument failure, the seasonal cycle of flux is clearly evident for all components and at all depths. When data are available for most of a year, only slight interpolations have been needed to calculate the integrated annual flux, and the results of these are given for the various components of the material (Table 2). The values are within the range expected for this latitude and biogeographical region as compiled by Lampitt and Antia (1997), but an interesting point is that the organic carbon and nitrogen fluxes were at their highest levels in 1989 during the time of the JGOFS North Atlantic Bloom Experiment. This suggests that this may have been an unusual year for the region. It is widely believed that the composition of settling material reflects the biology and biogeochemistry of the euphotic zone and the rates of remineralisation and dissolution of the material as it sinks through the water column (Jickells, Newton, King, Lampitt, & Boutle, 1996). Inter-annual changes in upper ocean community structure may have an effect on the mass flux (Boyd & Newton, 1995), or may simply affect the composition of sedimenting material, while the mass flux remains constant (Deuser, Jickells, King, & Commeau, 1995). One would thus expect to see seasonal changes in the concentrations of the various broad categories of biogenic component (organic carbon, inorganic carbon, nitrogen, opaline silica) and in the ratios of these components. The average composition of the collected material as expressed by these gross characteristics at the four depths (Table 3) shows no major changes with depth. However the more detailed analyses of Kiriakoulakis et al. (2001) indicate there were important depth-related changes and

Fig. 8.

Dry weight flux as determined by sediment traps at the four depths.

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Fig. 9.

41

Organic carbon flux as determined by sediment traps at the four depths.

some possible seasonal variations show up when data from particular times of the year are grouped together. The gross composition reported here (Table 3) may lead to the conclusion that apart from the lithogenic fraction, losses of most components occur as the material sinks through the water column. However, we have doubts about the reliability of the 1000 m data, which we discuss blow that prevent us from drawing such conclusions. In terms of elemental ratios (Table 3), the POC:PIC ratio is of considerable importance as it reflects the ability of the sinking particles to sequester CO2 from the atmosphere. If the ratio is ⬎0.7, the biological pump will favour the uptake of CO2 from the atmosphere, whereas if this ratio is lower the tendency will be to release CO2 from the sea. As has been reported elsewhere (Honjo & Manganini, 1993; Lampitt & Antia, 1997) that the POC:PIC ratio tends to decrease with depth because POC is preferentially remineralised, and this trend was also evident in our data. In the global data set compiled by Lampitt and Antia (1997), there is no apparent decrease in this ratio below a depth of 2000 m, and for these deeper annual trap data (n=34, range 0.35–2.67) the average ratio was 0.79. Since that compilation was completed, further multi-year data sets from the Pacific have also found similar or somewhat higher values; 0.65 in eastern subarctic (Wong et al., 1999), 1.08 in the central subarctic (Takahashi et al., 2000) and even higher in the eastern Pacific off California (Baldwin et al., 1999). The C:N ratios show surprisingly little variation, increasing slightly but significantly between 1000 and 3000 m. There was, however, no significant difference in the ratio between the 3000 m and 100 mab traps (Kiriakoulakis et al., 2001). These ratios are comparable to those reported for the NABE site (Honjo & Manganini, 1993), and indicate a decreasing order of reactivity from TPN, POC, BSi to PIC. As will be discussed below, the data from 3000m are thought to provide the best estimate of primary downward flux in the region and in Fig. 13, the seasonal patterns of material composition at this depth

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Fig. 10.

Inorganic carbon flux as determined by sediment traps at the four depths.

are presented. No consistent seasonal trends in the composition of the material is evident apart from lowered % PIC and enhanced %BSi in summer. There is considerable variation between traps at any particular time of year, but the variability within each trap deployment is much less. This suggests that the variability in the whole data set is not the result of analytical error but reflects inter-annual differences. This absence of any seasonal pattern contrasts with other studies. At station M in the Northeast Pacific there were pronounced maxima in winter POC (苲9%) and nitrogen (苲1.2%) and corresponding minima (苲5% and 苲0.5%, respectively) in mid-summer (Baldwin et al., 1999). In our data set inter-annual differences are most clearly seen in the data for the autumns of 1989, 1996 and 1999, during which the % POC rose from a normal level of ca. 6% to ⬎14%. As can be seen from Figs. 9 and 10, organic carbon flux was not especially high during these periods and the compositional change seems to reflect a decline in PIC flux resulting in a marked increase in the POC:PIC ratios (Fig. 13d). A widely held view is that the early summer deposition of material or indeed even the majority of downward flux is mediated by diatoms. One would therefore expect to see increases in the early summer in %BSi (Fig. 13g) and in the ratios BSi:PIC and BSi:POC (Fig. 13h and f). This trend was only very weakly evident in the data and was certainly not statistically significant. From this one might conclude that diatoms are clearly important mediators of flux in some environments (e.g. Central subarctic Pacific; Takahashi et al., 2000), but this may not be the case everywhere.

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Fig. 11.

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Total nitrogen flux as determined by sediment traps at the four depths.

3.3.3. Reliability of the flux measurement An important point shown in Table 2 is that the flux was not decreasing consistently with increasing depth. These data are thus not compatible with a concept of steady downward settlement of material from a homogeneous region, during which dissolution and remineralisation take place. Walsh, Dymond, and Collier (1988) discussed this possibility when, as is often the case, they observed the flux at 1000 m depth was less than that deeper in the water column. They discussed several hypotheses to explain this pattern, including particle transport during diel migration of zooplankton and deep water microbial production, but no conclusive explanation has yet been advanced. During the middle and late summer, the flux at 1000 m is usually much less than that at 3000 m and 4700 m. Winter flux levels at 2 mab are always substantially lower than that at either 100 mab or 3000 m depth. Suggestions are that the large aggregates are not collected efficiently by the traps (Beaulieu & Smith, 1998; Sherrell, Field, & Gao, 1998) or that at high levels of flux, the traps may become blocked (Takahashi et al., 2000) encouraging caution in the interpretation of certain sediment trap data (see below). When the data are compared to model estimates of downward flux, the conclusion that we draw is that under certain circumstances the flux is underestimated. Although one cannot make an objective judgement as to whether the measured flux at one depth provides a better estimate of actual downward flux than at another, the flux at 1000 m depth demonstrated far more interannual variability than at the other depths. For example, the dry mass flux during May 1996 was about 5% of the May 1998 value (Fig. 8), and very much less than elsewhere in the water column. The 1000 m data set shows characteristics of being subject to random errors, with large and sudden changes in flux

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Fig. 12.

Opaline silica flux as determined by sediment traps at the four depths.

unrelated to meteorological changes and not mirrored elsewhere in the water column. We suggest that these may be associated with fluctuations in the characteristics of the settling material, or in the nature of the zooplankton swimmers entering sample cups (Woodstock & Lampitt, unpublished data) rather than demonstrating a real fluctuations in downward flux. The annual integrated flux at 1000 m was consistently lower than that at either 3000 m, 100 mab or, when available, 2 mab (Table 2); this situation is only credible if the 1000 m traps were consistently receiving material from a region of lower particle supply. Our understanding of the hydrography of the region is simplistic, but based on this and our assumptions about sinking rates of particles, material should have reached 1000 m in about 8 days after travelling horizontally about 60 km. If the current directions were uniform throughout the water column (not the case in 1998), then material reaching 3000 m would have originated a further 苲135 km upstream, and would have taken 苲17 days longer to reach the trap. In this case there would have been consistently lower levels of export over horizontal distances of about 130 km upstream of the mooring site. While such a scenario remains possible, we would expect to see a comparable spatial pattern in surface chlorophyll concentrations. The satellite images (Fig. 4) give no indication of such a trend; the monthly averages demonstrated substantial variability within the 400 km circle of the statistical funnel, but no consistent trends. Furthermore, the efficiency of sediment traps at 1000 m calculated from budgets of 230Th may be as low as 10%, whereas deeper traps have far higher efficiencies of up to 95% (Scholten et al., 2001). These observations agree with comparisons between sediment trap fluxes and benthic accumulation rate data (Jickells et al., 1996; Lampitt et al., 2000), where deep traps provide flux estimates that are similar to accumulation rates over thousands of years.

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Table 2 Annual integrated flux (g/m2/y). Figures in parentheses were not measured directly but were calculated from the dry weight flux and the composition of all the material collected at that depth (Table 3). Flux at 2 mab is dominated by flux during 1997 during which there were no autumn data. Autumn values were therefore taken from 1996. Also shown in this table is the Flux Stability index (FSI) at 3000 m depth calculated from the raw data and also calculated for 2000 m depth using the relationship between FSI and depth described in Lampitt and Antia (1997)

Dry weight 1989 1990 1994 1997 1998 1999 Mean % Variance Organic carbon 1989 1990 1994 1997 1998 1999 Mean % Variance Total nitrogen 1989 1990 1994 1997 1998 1999 Mean % Variance Inorganic carbon 1989 1990 1994 1997 1998 1999 Mean % Variance Biogenic silica 1998 1990 1994 1997 1998 1999 Mean % Variance

1000 m

3000 m

4700 m

12.6 13.1 9.0 11.5 19.5

22.2 13.5 15.1 21.3 26.1 13.5 18.6 28.4

26.4 22.7 14.4 24.7 27.7 17.2 22.2 23.9

2.36 1.03 (1.00) 1.03 1.30 0.81 1.21 49.0

2.27 (0.73) 0.96 1.13 0.82 1.16 48.7

0.393 0.176 (0.155) 0.136 0.156 0.076 0.18 59.8

0.419 0.228 (0.123) 0.138 0.134 0.098 0.19 63.4

1.08 0.86 (0.94) 1.55 1.82 0.50 1.12 42.8

1.54 1.63 (1.04) 1.85 2.09 1.22 1.56 24.9

1.87 1.52 (2.00) 3.77 (3.45) (1.79) 2.40 39.8

1.74 1.66 (1.39) 2.41 (2.68) (1.66) 1.92 26.1

0.78 (1.00) (1.09) (0.75) 0.95

0.141 (0.138) (0.094) 0.12

0.78 (0.81) (0.55) 0.75

1.67 (1.92) (1.32) 1.64

2 mab

27.5

27.5

1.92

1.92

0.225

0.22

1.98

1.98

(continued on next page)

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Table 2 (continued)

FSI (DW) (days) 1989 1990 1994 1997 1998 1999

1000 m

3000 m

@ 3000m 59.0 86.7 82.0 82.5 87.0 93.1

@ 2000m 48.7 85.2 72.1 72.6 77.1 83.2

4700 m

2 mab

Table 3 Time weighted composition of material collected during the entire series 1000 m

3000 m

100 mab

2 mab

% Dry weight %POC %PIC %N %Si %Lithogenic

8.34 6.18 1.05 14.68 15.79

6.62 6.20 1.02 13.22 25.81

5.11 7.22 0.85 9.65 27.99

6.08 4.14 0.76

Molar ratios POC:PIN C:N Si:PIC Si:Corg

1.19 8.32 1.54 1.23

1.05 7.64 0.86 0.81

0.70 6.99 0.59 0.75

0.98 10.43

The deep flux of POC in the PAP region may be compared with the metabolic demands of the underlying benthic community as expressed by their oxygen consumption. Using benthic chambers Witbaard, Duinveld, Van der Weele, Berghuis, and Reyss (2000) observed the sediment community oxygen consumption rate to be 0.45 mmol/m2/d at the PAP site on four occasions during 1997–1998. Although this was somewhat higher than the annual mean deposition rate of POC of 0.27 mmol/m2/d, it was within the range of the observed rates, 0.17–0.53 mmol/m2/d. In order to calculate remineralisation rates of sinking particles Walsh et al. (1988) normalised their measured fluxes to aluminium, which is considered to be a refractory tracer. Some important but unconstrained assumptions are required for this normalisation: 1. the particles containing the tracer behave in the same way as those containing organic carbon and, 2. the effective particle sinking rate was 100m/d. We do not feel sufficiently confident of these assumptions to repeat these calculations. We consider the fluxes measured at PAP by the traps at 1000m are unreliable because of their erratic pattern, their smaller magnitude compared to the fluxes at the other depths, and from likely trapping inefficiency at this depth. This conclusion does not, however, cast doubt on the compositional data discussed above. At the present we can assume that the efficiency of collection by sediment traps is independent of particle composition.

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Fig. 13. Seasonal variation in composition of material collected in sediment traps at 3000 m depth. Left hand column of graphs are percentages of the dry weight of material collected while the right hand column are molar ratios.

3.3.4. Near bottom collections and resuspension Material collected in the sediment traps at 100 mab and 2 mab gives a measure of ‘apparent flux’ as opposed to the primary flux measured higher in the water column (Richardson & Hollister, 1987; Lampitt et al., 2000). Traps set within 1000 m of the seabed collect material that is thought to be a mixture of particles of three different sources: 1. Particles that have settled directly from the overlying water column (primary flux). 2. Mineral particles which have been resuspended from the sediment, either locally or from some distance away. 3. Freshly deposited particles which have been resuspended from the local seabed (i.e. the rebound flux: see Walsh, Fischer, Murray, & Dymond, 1988; Lampitt et al., 2000). The flux measured just above the seafloor is enhanced by both rebound and by adsorption of mineral particles. However it must also be influenced by the further remineralisation and dissolution the primary

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flux has been exposed to during the additional time it spends in the water column sinking from 3000 m. In accordance with this we found that the organic carbon and nitrogen fluxes near the bottom were similar to those at 3000 m (Table 2), whereas the more refractory components such as calcite (and hence dry weight) showed a marked increase. A nearby trap at 10 mab showed an even greater enhancement of dry weight flux with values typically 10–100% higher than those at 100 mab (Witbaard et al., 2000). We have examined the samples from 1989 and 1990 to determine the nature of this resuspended material and the processes involved (Lampitt et al., 2000). The ratio of the flux at 100 mab to that at 3000 m (i.e. 1400 mab) was termed the resuspension factor (RF), with higher values indicating a higher level of resuspension. It was found that during periods of low primary flux, RF values were higher. As expected these low flux periods occurred during winter, but coincidentally during the winter of 1990, the local near-bottom current speeds were also higher. There were, therefore, two possible causes for the inverse relationship between RF and primary flux. One is that higher current speeds resulted in greater resuspension, and the other is that at higher primary flux levels, recently deposited material forms a blanket over the seabed that prevents resuspension to 100 mab. The larger data set we report on here, supports the previous observation of an inverse relationship between RF and primary flux (Fig. 14). In this case, however, there was no relationship between current speed and RF (Fig. 3), and it must be concluded that the relationship with current speed found for 1989/90 was fortuitous. Lampitt (1985) observed that recently deposited aggregates were resuspended more easily than the underlying sediment, but that this material very rapidly re-settled back on to the seabed as soon as the current speeds decreased. We suggest that the reduced RF observed during the periods of high flux reflects a process whereby recently deposited material prevents smaller sized particles reaching the 100 mab trap; for example by their incorporation into the larger aggregates, which may be confined to the BBL. If this hypothesis is correct we would expect to see very different results emerging from the data relating to the 2 mab trap. The RF at 2 mab shows a less clear picture, largely because of the very low fluxes observed during the summer flux peak of 1998 and during the winter of 1996/97. Such decreases in flux of about 50% in the lower part of the water column are not consistent with our understanding of rates of remineralisation and dissolution, and suggest that the processes very close to the seabed may be quite different from those above. Kiriakoulakis et al. (2001) noted that one sediment trap sample from 2 mab had a very different organic composition in terms of its lipid content compared to the samples from

Fig. 14. Relationship between dry mass flux at 3000 m depth and the degree of resuspension expressed as the Resuspension Factor (RF). RF is defined as the ratio of flux at 100 mab (or 2 mab) to that at 3000 m depth.

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elsewhere in the water column. They speculate on the role that the benthopelagic fauna may play in introducing material into the traps very near the seabed. The implication from Fig. 14 is that scavenging of particulate and dissolved substances by resuspended material is reduced during periods of high flux, although such scavenging will increase when the primary flux increases. 3.3.5. Global comparison of primary flux From the observations discussed in Section 3.3, it must be concluded that the only reliable measure of primary downward flux in this region was that measured at 3000 m depth. This is therefore the data set for comparison with other published values at similar depths and with the model output we describe here. Comparative data from elsewhere have been compiled by Lampitt and Antia (1997). Their focus was on studies of organic carbon flux and they excluded all data if: 1. They were obtained within the top 1000 m or lower 500 m of the water column, 2. There was evidence of advection or, 3. The data failed to cover an annual cycle. Since then further data sets have been published from the North Pacific (Wong et al., 1999; Baldwin et al., 1999; Takahashi et al., 2000) and Indian Ocean (Honjo, Dymond, Prell, & Ittekkot, 1999). From the perspective of deep-water fluxes, the PAP site appears to be typical of mid-latitude provinces (sensu Longhurst, 1995) that are characterised by having a nutrient-limited spring primary production peak. Fig. 15 shows the deep water flux in each of the five open-ocean clusters of provinces, ranked according to the degree of flux stability as determined by Lampitt and Antia (1997). It can be seen that apart from fluxes of opaline silica, the fluxes at PAP are comparable to similar sites elsewhere in the world. In the case of silica, the range of flux in type 2 provinces is larger and the flux at PAP is somewhat lower than at other sites. As previously discussed, the absolute levels of flux fail, somewhat surprisingly, to follow the trend of flux stability. 3.3.6. Relationship between primary flux and primary production The relationship between annual primary production as determined by Longhurst et al. (1995) and the organic carbon flux normalised to 2000 m shows that at lower levels of primary production (⬍250 g/m2/y), there is a near-linear relationship between primary production and the fluxes (Lampitt & Antia, 1997). However, at higher levels of primary production, there is no further enhancement of organic carbon flux, which seemed to reach an upper limit of about 4 g/m2/y. These new data from PAP fit into this scheme well, with PP levels taken as 165 g/m2/y (Fig. 16). However, new data from the Arabian Sea (Honjo et al., 1999) are not consistent with there being an upper limit to organic carbon fluxes, since they observed values in excess of 7 g/m2/y at their most productive location. It should be emphasised that these regressions rely on primary production estimates based on the CZCS satellite data, and on assumed relationships between primary production and sea surface colour, a topic that is still developing rapidly. Large uncertainties remain, as is evident from the wide range of estimates of primary production at the PAP site (Fig. 6). 3.3.7. Seasonal and interannual variability Seasonal variations in flux have already been extensively discussed (e.g. Berger & Wefer, 1990; Honjo, Dymond, Collier, & Manganini, 1995; Lampitt & Antia, 1997). The Flux Stability Index (FSI) is a index of such variations and is defined as the number of days required for half of the annual flux to be collected in any one year (Lampitt & Antia, 1997), so that high values indicate a stable (less variable) pattern of flux. Lampitt and Antia (1997) demonstrated that the clusters of provinces with common temporal patterns of upper ocean biology described by Longhurst (1995), had characteristic levels of FSI. Analysis of our

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Fig. 15. Global trends in deep water flux arranged according to the plankton climate categories proposed by Longhurst (1995). The data pertain to all deep ocean locations where the criteria proposed by Lampitt and Antia (1997) are satisfied. The categories are arranged by increasing Flux Stability Index as previously determined.

extended data set (Table 2) demonstrates that at PAP the FSI has varied little over time, and continues to be within the range of values for the mid-latitude types of province (Lampitt & Antia, 1997); the range for dry weight FSI at 2000 m being 35–85 days. The global analysis also indicated that regions with higher FSI have lower fluxes (Lampitt & Antia, 1997), although the relationship was very weak. The present data for PAP, suggests that at a single site the inverse relationship persists (r2=0.85; Fig. 17). However, it is obvious from Fig. 17 that the regression depends heavily on the low FSI and high annual flux found in 1989, and poses the question whether this was an exceptional year for the region. Further work is required to examine this, but it would indeed be unfortunate if the extensive observations carried out during NABE only gave insights into what may turn out to have been an exceptional year. At present, the reasons for such variations in FSI or annual particle flux are not apparent. 3.4. Flux related observations The model output at 3000 m depth provides a framework against which to compare the measured flux data (Fig. 18) and various other aspects relevant to particle flux, namely the concentration of marine snow particles and the quantity of phytodetrital material lying on the seabed.

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Fig. 16. Relationship between organic carbon flux normalised to a depth of 2000 m using the ‘Martin curve’ and total primary production as determined by Longhurst et al. (1995) from CZCS satellite data. Data from the polar regions are distinguished from the remainder as they appear not to conform to the general pattern. The line is the hyperbolic tangent curve derived by Lampitt and Antia (1997) for data published prior to 1996 which did not include several of the high flux values included in this graph.

Fig. 17. Relationship between the Flux Stability Index (FSI) at 3000 m based on dry weight flux for each year where there is a complete annual record and POC flux. The FSI is the number of days required for half of the annual flux to be collected and higher values indicate a more stable (less variable) pattern of flux in the particular year.

3.4.1. Marine snow concentration The marine snow particles photographed have been assumed to sink at 120 m/day (as justified above). For the purposes of comparing the marine snow field to the modelled flux at 3000 m depth, temporal offsets of 23 days have been assumed when the camera was deployed at a depth of 270 m in 1990, and 15 days when the camera was deployed at 1170 m in 1992. No allowance has been made for remineralisation, as the purpose of this exercise is simply to demonstrate the temporal trend. The 5-day running mean (Fig. 19a) for the two years when data were available shows a consistent and strong peak just as the predicted fluxes reach their maximum levels, but decreasing very sharply shortly afterwards, well before the modelled flux declined. As previously reported for 1990 (Lampitt et al., 1993), the peak in volume concentration is mainly generated by marine snow particles in the larger size categories. This suggests that deposition of the larger marine snow particles is a far more dominant component of the flux at the start

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Fig. 18.

Organic carbon flux at 3000 m as measured by sediment trap and as determined by the model.

Fig. 19. Organic carbon flux at 3000 m as determined by the model and (a) Concentration of marine snow particles (0.4–9.6 mm diameter) which would be expected at the same depth and time (5 day running mean), (b) Coverage of the seabed by phytodetrital material advanced by 15 days to simulate the time the material would have been at 3000 m depth. Note the zero offset.

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53

of the depositional pulse than later on during the summer months. The sharp decline in the sediment trapmeasured fluxes in 1992 (at both 3000 m and 100 mab) coincided precisely with the large peak in marine snow concentrations and therefore casts grave doubts on the veracity of the measured decline in flux. The main peak in marine snow concentrations observed in 1990 was also coincident with the minimum between two flux maxima in the trap fluxes, again suggesting that the minimum is an artefact. Sherrell et al. (1998) based on a budget of suspended and sinking particles off California, also concluded that during periods of high flux, the level of flux may be underestimated by sediment traps. Takahashi et al. (2000) suspected that under certain circumstances the trap funnel can become plugged with material preventing further collection; this appeared to be the case in our 1992 data. 3.4.2. Benthic phytodetrital load The arrival of the benthic phytodetrital load at 4800 m showed the same time lag of 15 days as would be predicted from the marine snow data and the model output, assuming a settling rate of 120 m/d. In 1995 there was a dramatic change in the structure of the benthic megafaunal community (Billett et al., 2001) and a disappearance of visible phytodetritus on the sediment surface. The formation of a phytodetrital layer, which previously had been such a significant feature of the deep-sea floor during spring and summer at PAP, has not been observed since 1995. During the three previous years when data were obtained, there was a close relationship between the timing and magnitude of the peak in benthic load and the flux predicted by the model. Unfortunately there are few periods for which there are concurrent data from the sediment traps and photographic time series, but the absence of double peaks in the phytodetrital coverage of the seabed suggests that this was not a characteristic feature. In 1994, the Bathysnap results showed only a very slight decline at a time when the measured flux showed its characteristic major decline coincident with a peak in modelled flux. This again gives us confidence in the model while casting further doubt on the efficiency of the sediment traps during periods of very high flux. The two techniques of examining particles (Bathysnap and sediment trap) are obviously not directly comparable in terms of the particles they record. The two techniques probably have different sensitivities to particles with some specific characteristic of size or composition. Furthermore the Bathysnap observations only record the difference between the rate of supply from above and the rate reduction at the sediment interface, by either incorporation into the sediment or ingestion by the biota or through diagenesis. In spite of this lack of direct comparability, we believe that the two approaches cast a complementary light of the veracity of the sediment trap technique. 3.5. Measured and modelled primary flux Although there are some significant gaps in the record of particle flux, when the entire data set is compared with the model output, there is good agreement in both the timing and magnitude of the flux (Fig. 18). This gives us confidence in the model and suggests that when the observations show significant deviations from the model predictions, there may be artefacts of some sort that are deserving closer inspection. The most notable and consistent deviations are the double peaks in measured flux that were apparent in 1989, 1990, 1994, 1997 and 1999, which are not predicted by the model. The trough between these peaks usually coincided with a peak in the flux predicted by the model. For example in 1992, there was a sudden decrease in flux just as the modelled flux attained its maximum (sample cup change on 7 June with almost zero flux in cups between then and the end of the deployment on 10 August). This sudden and persistent decrease was also observed at 4700 m (100 mab), but not at a site 130 km to the south, where a decline was followed in late June by an increase (Pfannkuche, Boetius, Lochte, Lundgreen, & Thiel, 1999). Apart from the situation in 1992, the observed double peaks may either reflect a deficiency in the model or a reduction in the efficiency of the sediment traps during high flux periods. The annual flux of organic carbon predicted by the model agrees remarkably well with the measured

54

R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

values (Fig. 20) suggesting that any artefacts in the flux recorded by the traps are smoothed over longer time periods. Various comparisons, which have been made between sediment trap measured flux and long term accumulation of refractory materials such as aluminium, have often concluded that the efficiency of sediment traps is high in regions of low current speed (Lampitt et al., 2000 and references therein). The simple explanation for this is that during periods of very high flux, which are dominated by the largest sizes of marine snow particles, the aggregates may adhere to the sides of the sediment trap cone, but may then become less sticky, and fall into the collection cups at a later time. Thus some of the flux from one collection period would boost the measured flux in a subsequent period, but the overall quantity collected would still give a reliable estimate of long-term flux. A significant feature of the annual integrated fluxes is that there has been no apparent trend since 1989, either in the measured or modelled fluxes. Thus the changes observed in the benthic community structure (Billett et al., 2001) cannot be attributed to any gross changes in the organic carbon supply to the seabed. These changes are more likely to reflect stochastic or even long-term cyclic variations, or some subtle changes in the quality of the material supply. The ranges of measured and modelled values are very similar, as are the average measured and modelled annual organic carbon fluxes (1.21 and 1.33 g/m2/y, respectively). It is clear from Fig. 20 that the factors controlling the interannual variability are not adequately described by the model. This is not surprising because the model is one dimensional and does not attempt to encapsulate basin-scale physical descriptions as has been done by others (e.g. Dutkiewicz, Follows, Marshall, & Gregg, 2001). Furthermore, the losses between the upper mixed layer and the trap at 3000 m through remineralisation and dissolution have been assumed to be a constant proportion as described by the Martin remineralisation profile. However, biological processes in the mesopelagic zone are variable and it seems likely that this variability will be found to be dependent to some extent on the downward fluxes on seasonal and annual time scales.

4. Conclusion The BENGAL project has provided the opportunity to make an in depth study of the biogeochemical processes of the seafloor thought to be driven primarily by the supply of organic material from the euphotic zone above.

Fig. 20. Annual integrated flux of organic carbon at 3000 m depth as determined by the model and by measurements where there was adequate temporal coverage.

R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

55

앫 We have presented a time series of particle flux data, which shows considerable seasonal and interannual variability over the past decade with pronounced maxima during the summer. 앫 The annual average flux levels are consistent with other observations in this region and in other similar biogeographical provinces, but the causes of the interannual variability are not understood. 앫 Flux measurements at 1000 m depth were much lower than expected and are considered unreliable as a result of reduced trapping efficiency. At greater depths in the water column, there were indications that the traps became less efficient during periods of high flux so that the total reported flux may be slightly underestimated. 앫 Prior to 1996, coverage of the seabed with phytodetrital material exhibited a clear seasonal signal. Since that year no such coverage has been observed, a change, which is likely to be associated with a shift in the composition of the benthic megafauna. The inference is that the rate at which the material is disappearing has increased as a result of the change in the megafauna, so that large deposits no longer accumulate, rather than that the rate of supply from above has decreased. 앫 This inter-annual variability in the total downward flux does not seem to be correlated with the changes in benthic community structure over the past decade. 앫 The absolute levels of flux are predicted well by an upper ocean biogeochemical model, although the model does not describe well the observed inter-annual variability.

Acknowledgements

This work was funded by EC contract MAS-3 950018 under the MAST III programme. We are indebted to Andy Geary, Ian Waddington, Keith Goy, Phil Taylor, Corinne Woodstock and Davina Gair for help with collection and processing of samples and to Roberta Baldwin, Emilio Ferna´ ndez, Paul Falkowski and Mike Behrenfeld for additional unpublished data. Drs Ken Smith and Ian Walsh made some very constructive comments on the submitted manuscript for which we are most grateful. The Master, officers and crew of the RRS Discovery and RRS Challenger are thanked for their help with sample collection.

Appendix A

Downward particle flux is given in Tables 4–6.

1

2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11

I I I I I I I I I I I I

III III III III III III III III III III III

Cup

I

Mooring

23/07/1989 13/08/1989 03/09/1989 24/09/1989 15/10/1989 12/11/1989 10/12/1989 07/01/1990 04/02/1990 04/03/1990 25/03/1990

23/04/1989 30/04/1989 07/05/1989 14/05/1989 21/05/1989 28/05/1989 04/06/1989 11/06/1989 18/06/1989 25/06/1989 02/07/1989 09/07/1989

18/04/1989

13/08/1989 03/09/1989 24/09/1989 15/10/1989 12/11/1989 10/12/1989 07/01/1990 04/02/1990 04/03/1990 25/03/1990 15/04/1990

30/04/1989 07/05/1989 14/05/1989 21/05/1989 28/05/1989 04/06/1989 11/06/1989 18/06/1989 25/06/1989 02/07/1989 09/07/1989 16/07/1989

23/04/1989

Open date (dd/mm/yy) Close date

Table 4 Downward particle flux (mg/m2/d)

PSi

16.2 217.7 86.4 38.3 25.4 24.2 12.7 14.5 13.8 7.2 35.8

48.7 65.6 69.3 80.9 111.1 196.6 215.8 184.9 163.8 121.0 93.1 117.8

38.5

4.02 43.72 12.17 5.87 1.80 1.52 0.72 1.07 1.09 0.58 2.80

3.30 4.20 4.40 5.20 6.70 12.00 14.00 11.30 10.50 11.10 9.00 10.40

1.70

0.63 6.28 3.93 2.38 1.69 1.72 1.03 1.06 0.92 0.56 1.87

3.20 4.20 4.50 5.50 6.80 10.80 10.60 9.80 7.40 5.70 4.20 5.20

2.50

PIC

Dry POC weight

TPN

Dry POC weight

PIC

Flux at 3000 m depth

Flux at 1000 m depth

0.428 7.981 2.084 0.746 0.370 0.278 0.116 0.161 0.196 0.090 0.648

0.465 0.513 0.720 0.833 2.084 2.654 2.478 2.063 1.876 1.284 1.791

TPN

1.00 9.43 6.82 3.34 2.03 1.65 0.54 0.71 0.88 0.39 4.36

3.67 4.56 4.18 4.94 9.07 20.63 26.51 22.18 20.02 12.93 9.87 12.60

3.48

PSi

55.7 199.7 102.2 89.5 56.1 70.2 54.4 49.0 68.8 100.5 38.1

60.0 79.0 66.7 97.8 112.4 136.1 178.7 147.8 146.0 101.5 89.8 119.0

50.1

10.40 38.00 12.56 6.47 3.64 2.72 2.52 2.04 2.49 5.81 2.14

3.40 3.40 3.10 4.20 4.60 6.30 7.60 6.70 6.60 5.20 5.30 7.40

3.00

Dry POC weight

4.70 3.76 5.45 5.97 9.49 17.01 14.38 13.82 8.32 7.85 10.53

2.82

PSi

1.883 2.94 6.969 10.44 2.014 8.36 1.302 8.30 0.803 4.75 0.870 4.77 0.556 3.55 0.447 3.03 0.625 3.56 0.816 3.98 0.617 3.02

no datum 0.510 0.640 0.340 0.538 0.843 1.048 1.930 1.656 1.562 0.994 0.943 1.321

TPN

(continued on next page)

2.10 7.54 5.33 5.26 3.55 5.30 3.92 3.75 5.42 8.37 2.88

4.10 6.00 5.00 7.20 8.50 9.40 12.10 9.50 8.90 6.50 5.70 7.00

3.50

PIC

Flux at 100 m above bottom

56 R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

Cup

1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 7 8 9 10 11

1 2 3 4 5 6 7 8 9 10 11 12 13

Mooring

V V V V V V V V V V V XI XI XI XI XI XI XI XI XI XI XI

XI XI XI XI XI XI XI XI XI XI XI XI XI

Table 4 (continued)

30.04.94 10.05.94 20.05.94 30.05.94 09.06.94 19.06.94 29.06.94 09.07.94 19.07.94 29.07.94 08.08.94 18.08.94 28.08.94

22/04/1990 29/04/1990 13/05/1990 27/05/1990 10/06/1990 24/06/1990 08/07/1990 22/07/1990 05/08/1990 19/08/1990 02/09/1990 12/04/1992 26/04/1992 10/05/1992 24/05/1992 07/06/1992 21/06/1992

10.05.94 20.05.94 30.05.94 09.06.94 19.06.94 29.06.94 09.07.94 19.07.94 29.07.94 08.08.94 18.08.94 28.08.94 07.09.94

29/04/1990 13/05/1990 27/05/1990 10/06/1990 24/06/1990 08/07/1990 22/07/1990 05/08/1990 19/08/1990 02/09/1990 16/09/1990 26/04/1992 10/05/1992 24/05/1992 07/06/1992 21/06/1992

Open date (dd/mm/yy) Close date PSi

107.0 65.4 33.0 28.6 35.3 62.7

68.6 130.5 29.6 15.2 41.0 31.9 88.5 31.8 27.8

64.4 117.9 109.0 123.3 89.2 67.1 67.8 62.6 58.6 55.3 67.3 80.6 79.7

76.4 88.5 41.6 25.3 53.7 100.6 83.9 70.6 62.1 49.0 43.7 57.0 57.4 75.0 90.4 0.0 0.0 0.3 0.3 1.0 0.3

2.90 3.65 1.55 1.26 4.08 6.71 5.43 4.27 3.69 3.21 1.65

7.13 7.07 3.74 1.94 3.10 5.36 4.75 3.76 3.45 2.99 3.45

PIC

Dry POC weight

TPN

Dry POC weight

PIC

Flux at 3000 m depth

Flux at 1000 m depth PSi

0.817 5.47 1.193 7.35 0.305 2.73 0.249 2.62 0.835 7.09 1.624 16.01 1.251 12.65 1.219 9.57 1.094 8.81 0.695 6.15 0.703 4.60

TPN

51.2 63.4 94.0 96.6 60.8 53.6 92.3 75.8 49.3 64.2 88.0 90.1 89.3

64.9 84.5 108.3 36.1 75.5 103.2 82.2 100.2 126.5 118.6 104.2 81.5 43.7 81.5 126.4 2.5 2.5 0.4 0.4 3.2 0.4

3.94 4.47 2.06 1.02 2.66 5.50 3.76 4.68 6.12 5.16 4.70

Dry POC weight 0.798 0.893 0.946 0.216 0.650 1.505 0.890 1.315 1.542 1.110 1.130

TPN

6.20 7.02 5.24 1.69 5.51 11.68 7.88 11.51 14.13 11.63 10.78

PSi

(continued on next page)

4.32 6.00 9.73 3.21 5.87 6.99 6.11 6.79 8.29 8.03 6.71

PIC

Flux at 100 m above bottom

R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63 57

Cup

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

Mooring

XV XV XV XV XV XV XV XV XV XV XV XV XV

XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII XVIII

Table 4 (continued)

15/09/1996 22/09/1996 29/09/1996 13/10/1996 10/11/1996 08/12/1996 05/01/1997 02/02/1997 01/03/1997 15/03/1997 22/03/1997 29/03/1997 05/04/1997

12.10.95 22.10.95 11.11.95 01.12.95 31.12.95 30.01.96 19.02.96 10.03.96 30.03.96 09.04.96 19.04.96 29.04.96 09.05.96 22/09/1996 29/09/1996 13/10/1996 10/11/1996 08/12/1996 05/01/1997 02/02/1997 01/03/1997 15/03/1997 22/03/1997 29/03/1997 05/04/1997 12/04/1997

22.10.95 11.11.95 01.12.95 31.12.95 30.01.96 19.02.96 10.03.96 30.03.96 09.04.96 19.04.96 29.04.96 09.05.96 19.05.96

Open date (dd/mm/yy) Close date PSi

1.86 1.02 0.87

29.3 15.9 11.3 8.1 1.8

1.13 7.91 3.12 2.75 1.56 0.84 0.69

0.50 1.08

11.0 17.2

14.6 35.3 48.1 26.2 28.4 16.3 7.4 6.1 3.9

1.05 1.21 1.03 0.56

7.0 13.1 17.8 9.8

2.94 2.68 2.39 1.24 0.60

1.31

0.158 1.017 0.416 0.408 0.243 0.136 0.085

0.254 0.145 0.139

0.070 0.154

0.099 0.165 0.140 0.077

1.64 5.41 3.95 2.13 2.75 1.90 1.12 0.80 0.73

28.6 30.3 35.9 46.6 33.7 17.8 16.2 15.5 25.0

10.1 50.8 35.8 24.3 26.8 11.9 19.6 30.8 31.8 37.0 27.0 63.5 51.5 4.51 4.26 4.52 3.83 2.27 1.26 0.95 0.94 1.39

0.34 3.43 1.73 1.02 1.09 0.56 0.91 1.53 1.55 1.36 0.76 2.32 1.97 1.52 1.85 2.60 3.74 2.67 1.45 1.21 1.26 1.79

3.30 2.14 2.35 1.06 1.63 2.37 2.63 3.43 2.69 5.83 4.74

PIC

Dry POC weight

TPN

Dry POC weight

PIC

Flux at 3000 m depth

Flux at 1000 m depth

0.837

0.447 0.426 0.485 0.459 0.293 0.163 0.122 0.121 0.182

0.042 0.409 0.229 0.132 0.145 0.069 0.111 0.227 0.200 0.177 0.100

TPN

5.49 5.32 4.31 4.18 4.95 2.08 1.70 2.09 3.49

PSi

28.4 61.2 58.8 55.7 42.9 30.5 28.0 43.5 54.5 79.0 52.6 79.7 65.9

1.99 3.34 2.01 1.35 1.08 0.86 1.03 1.53 1.93 2.22 1.27 2.10 2.23

Dry POC weight 2.17 4.35 4.43 4.78 3.63 2.63 2.28 3.51 4.66 6.58 4.50 7.06 5.56

PIC

0.214 0.355 0.286 0.172 0.132 0.110 0.129 0.187 0.232 0.269 0.176 0.298 0.317

TPN

Flux at 100 m above bottom PSi

58 R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

Cup

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12

13

Mooring

XIX XIX XIX XIX XIX XIX XIX XIX XIX XIX XIX XIX XIX

XX XX XX XX XX XX XX XX XX XX XX XX

XX

08/03/1998

27/07/1997 10/08/1997 24/08/1997 07/09/1997 21/09/1997 05/10/1997 19/10/1997 16/11/1997 07/12/1997 28/12/1997 25/01/1998 22/02/1998

23.03.97 30.03.97 13.04.97 20.04.97 27.04.97 04.05.97 11.05.97 18.05.97 25.05.97 01.06.97 22.06.97 06.07.97 13.07.97

22/03/1998

10/08/1997 24/08/1997 07/09/1997 21/09/1997 05/10/1997 19/10/1997 16/11/1997 07/12/1997 28/12/1997 25/01/1998 22/02/1998 08/03/1998

30.03.97 13.04.97 20.04.97 27.04.97 04.05.97 11.05.97 18.05.97 25.05.97 01.06.97 22.06.97 06.07.97 13.07.97 20.07.97

Open date (dd/mm/yy) Close date

Table 5 Downward particle flux (mg/m2/d) (moorings XIX–XXV)

PSi

59.5 43.1 23.3 35.7 30.2 29.7 20.1 15.5 4.8 3.4 6.2 6.6

17.0 103.7 278.6 134.6 126.5 66.3 27.4 9.7 67.8 54.2

5.03 3.89 2.23 3.00 2.43 2.49 2.07 1.21 0.24 0.26 0.43 0.42

2.21 7.19 21.26 7.93 7.87 4.05 1.28 0.36 5.46 5.54

4.37 2.51 1.54 2.70 2.56 2.14 1.38 1.39

0.81 7.03 19.97 10.76 9.93 5.78 2.47 0.93 4.53 2.29

4.27 14.83 21.86 17.62 14.69 7.17 2.59 0.79 16.83 12.50

0.689 10.32 0.530 6.22 0.312 3.04 0.405 3.95 0.337 2.93 0.386 2.49 0.327 1.34 0.154 1.14 0.036 0.28 0.047 0.15 0.066 0.64 0.066 0.96

0.345 0.909 2.984 1.080 1.080 0.540 0.173 0.053 0.820 0.837

103.0 81.4 60.7 35.1 25.3 26.1 31.2 35.0 30.4 29.7 25.6 28.7 no sample

15.5 41.1 86.1 92.5 106.4 86.9 74.2 77.1 82.3 145.7 192.2 92.5 106.7 4.52 3.71 2.90 1.83 1.12 1.15 1.69 1.68 1.48 1.26 1.18 1.32

0.96 2.08 4.47 4.95 5.24 4.29 3.11 3.22 4.22 7.38 8.56 3.96 4.69 6.31 5.20 4.22 2.82 2.05 2.15 2.44 2.60 2.33 2.37 1.89 2.02

1.06 3.04 7.29 7.35 8.46 6.94 6.41 6.67 6.51 9.33 13.71 7.08 8.25

PIC

Dry POC weight

TPN

Dry POC weight PIC

Flux at 3000 m depth

Flux at 1000 m depth

3.72 6.27 11.50 10.98 13.48 13.25 13.60 11.67 11.15 34.16 41.57 17.44 20.31

PSi

0.628 23.27 0.490 16.61 0.397 10.84 0.224 4.68 0.164 3.57 0.163 3.41 0.234 3.83 0.245 3.82 0.201 3.72 0.165 3.13 0.153 2.99 0.199 4.33

0.160 0.299 0.629 0.680 0.749 0.669 0.478 0.493 0.635 1.106 1.303

TPN

118.1 118.3 99.7 61.5 57.5 47.7 63.3 58.4 53.6 55.8 39.5 42.3

14.2 25.5 79.1 90.9 75.7 85.5 87.9 81.1 72.1 108.5 159.1 121.7 143.2 5.25 4.41 3.45 2.09 2.06 1.58 2.13 1.91 1.92 1.81 1.24 1.30

0.63 1.16 3.80 3.99 4.20 3.47 2.75 3.14 3.17 4.42 6.12 3.98 5.10

Dry POC weight PSi

0.868 25.70 0.670 21.57 0.532 16.93 0.306 9.69 0.289 5.08 0.206 6.93 0.270 7.93 0.260 7.27 0.292 6.36 0.248 5.86 0.159 4.00 0.161 4.72

0.102 1.93 0.175 2.27 0.543 5.59 0.586 6.11 0.651 4.65 0.474 0.415 0.450 0.441 0.651 4.00 0.904 7.28 0.593 13.90 0.780 13.04

TPN

(continued on next page)

7.50 8.12 6.92 4.62 4.24 3.57 4.78 4.41 4.05 4.38 3.03 3.19

1.16 1.90 6.25 7.73 5.75 7.20 8.04 6.90 5.72 8.95 13.51 9.67 11.09

PIC

Flux at 100 m above bottom

R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63 59

Cup

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

Mooring

XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa XXIIIa

XXV XXV XXV XXV XXV XXV XXV XXV XXV XXV XXV XXV XXV

Table 5 (continued)

11/10/1998 06/12/1998 31/01/1999 28/03/1999 25/04/1999 23/05/1999 06/06/1999 20/06/1999 04/07/1999 18/07/1999 01/08/1999 15/08/1999 29/08/1999

22/03/1998 05/04/1998 19/04/1998 03/05/1998 17/05/1998 31/05/1998 07/06/1998 21/06/1998 05/07/1998 19/07/1998 02/08/1998 16/08/1998 30/08/1998 06/12/1998 31/01/1999 28/03/1999 25/04/1999 23/05/1999 06/06/1999 20/06/1999 04/07/1999 18/07/1999 01/08/1999 15/08/1999 29/08/1999 26/09/1999

05/04/1998 19/04/1998 03/05/1998 17/05/1998 31/05/1998 07/06/1998 21/06/1998 05/07/1998 19/07/1998 02/08/1998 16/08/1998 30/08/1998 13/09/1998

Open date (dd/mm/yy) Close date

PSi

26.0 8.4 11.0 23.8 33.9 7.2 107.9 11.9 34.7 39.6 21.8 61.3

40.5 51.7 110.5 137.1 109.3 93.5 203.2 7.0 2.5 1.5 3.1 27.3 0.6 6.24

7.23 8.25 15.20 18.07 17.93 21.82 47.55 1.29

47.6 23.2 23.5 47.9 63.5 97.4 84.1 36.4 54.7 88.7 27.4 29.5 25.4

30.1 42.4 46.4 84.1 105.7 60.6 112.4 128.2 177.9 178.3 183.0 128.7 86.2 1.85 0.86 1.01 1.84 2.17 5.03 4.03 2.77 4.87 9.33 2.87 4.15 3.81

1.47 1.74 1.98 3.25 4.18 2.66 5.76 7.95 9.68 12.20 11.66 5.14 4.16 2.28 1.02 1.06 1.97 2.86 2.88 2.46 0.88 1.42 1.83 0.61 0.40 1.56

1.87 2.66 3.52 6.57 8.39 4.74 8.05 8.13 11.99 13.39 12.55 10.94 7.07

PIC

Dry POC weight

TPN

Dry POC weight PIC

Flux at 3000 m depth

Flux at 1000 m depth

0.176 0.081 0.101 0.168 0.203 0.526 0.429 0.270 0.519 0.816 0.230 0.298 0.297

0.198 0.229 0.274 0.395 0.486 0.382 0.742 0.949 1.210 1.498 1.226 0.669 0.526

TPN

6.16 7.71 7.40 12.63 13.62 10.42 18.69 25.57 31.80 22.73 20.81 15.10

PSi

55.2 25.8 29.1 40.0 64.4 87.5 94.5 90.5 94.8 118.2 113.5 69.4

22.7 29.2 48.5 53.9 105.5 52.3 95.0 95.9 133.9 194.6 164.7 174.3 131.0

1.73 0.79 0.76 1.16 1.59 4.62 4.91 3.94 4.55 6.46 10.58 4.58

1.11 0.99 1.58 2.12 3.27 2.06 3.58 5.70 6.21 8.17 12.90 6.75 4.51

Dry POC weight

4.61 2.01 2.30 3.15 5.30 6.09 5.74 6.50 7.11 8.77 8.15 5.14

7.32 6.36 9.33 15.51 12.20 14.52 8.09

3.77 4.01 9.08

PIC

0.210 0.098 0.090 0.140 0.213 0.552 0.614 0.488 0.588 0.816 1.237 0.534

0.173 0.140 0.204 0.280 0.390 0.324 0.428 0.681 0.723 0.895 1.318 0.784 0.550

TPN

Flux at 100 m above bottom

3.70 4.56 6.72 6.03 11.22 6.38 12.59 14.34 18.88 26.92 18.51 19.66 13.98

PSi

60 R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

R.S. Lampitt et al. / Progress in Oceanography 50 (2001) 27–63

61

Table 6 Downward particle flux (mg/m2/d) 2 m above bottom Flux at 2 m above bottom

MAP

MAP

Dry weight

POC

1 2 3 4 5 6 7 8 9 10 11 12

18/08/1996 15/09/1996 13/10/1996 08/12/1996 02/02/1997 16/03/1997 13/04/1997 27/04/1997 11/05/1997 01/06/1997 22/06/1997 13/07/1997

15/09/1996 13/10/1996 08/12/1996 02/02/1997 16/03/1997 13/04/1997 27/04/1997 11/05/1997 01/06/1997 22/06/1997 13/07/1997 07/09/1997

132.9 57.1 22.1 7.9 12.4 22.1 5.7 75.7 91.4 332.4 272.4 131.4

5.74 3.75 1.44 0.44 0.70 1.46 0.47 5.09 4.83 17.82 21.25 11.41

1 2 3 4 5 6 7 8 9 10 11 12

22/03/1998 19/04/1998 03/05/1998 17/05/1998 31/05/1998 07/06/1998 21/06/1998 05/07/1998 19/07/1998 02/08/1998 16/08/1998 30/08/1998

19/04/1998 03/05/1998 17/05/1998 31/05/1998 07/06/1998 21/06/1998 05/07/1998 19/07/1998 02/08/1998 16/08/1998 30/08/1998 13/09/1998

36.9 172.6 200.4 201.3 286.3 124.7 90.1 25.1 83.0 53.3 55.9 111.9

1.77 10.53 14.73 9.66 12.45 5.67 4.15 1.18 2.99 2.24 1.90 4.59

PIC

TPN

9.03 3.83 1.39 0.65 1.02 1.62

0.866 0.549 0.221 0.082 0.129 0.203

4.31 5.75 24.50 19.67 9.23

0.599 0.619 2.170 2.165 1.134

PSi

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