Scholarly article on topic 'Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean'

Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Friederike Ebersbach, Philipp Assmy, Patrick Martin, Isabelle Schulz, Sina Wolzenburg, et al.

Abstract The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilisation experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January–March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, ≤5mgGXeq.m−2 d−1, and a few phytodetrital aggregates were found in the sediment traps. Only a few per cent of the POC flux was found in the traps consisting of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.

Academic research paper on topic "Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean"

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Particle flux characterization and sedimentation patterns of protistan plankton during the iron fertilization experiment LOHAFEX in the Southern Ocean

Friederike Ebersbach, Philipp Assmy, Patrick Martin, Isabelle Schulz, Sina Wolzenburg, Eva-Maria Nothig

Äff É-.ÎA DEEP-SEA RESEARCH

"ST,'.™"™" PARTI

Océanographie Research Papers

www.elsevier.com/locate/dsri

PII: S0967-0637(14)00057-0

DOI: http://dx.doi.org/10.1016/j.dsr.2014.04.007

Reference: DSRI2330

To appear in: Deep-Sea Research I

Received date: 18 May 2013 Revised date: 10 April 2014 Accepted date: 15 April 2014

Cite this article as: Friederike Ebersbach, Philipp Assmy, Patrick Martin, Isabelle Schulz, Sina Wolzenburg, Eva-Maria Nothig, Particle flux characterization and sedimentation patterns of protistan plankton during the iron fertilization experiment LOHAFEX in the Southern Ocean, Deep-Sea Research I, http://dx.doi.org/10.1016/j.dsr.2014.04.007

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Particle flux characterization and sedimentation patterns of protistan plankton during the iron fertilization experiment LOHAFEX in the Southern Ocean

Friederike Ebersbacha'b'*, Philipp Assmyab1, Patrick Martin0'2, Isabelle Schulzab, Sina Wolzenburga, Eva-Maria Nothig b

a Center for Marine Environmental Sciences, University of Bremen

b Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany

c National Oceanography Centre, Southampton, SO14 3ZH, UK

1 Present address: Norwegian Polar Institute, Fram Centre, 9296 Troms0, Norway

2 Present address: Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

* Corresponding author. Tel. +49(0)42169316181, friederike.ebersbach@gmx.net Abstract

The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilization experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January - March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, <5 mg GX eq. m-2 d-1, and few phytodetrital aggregates were found in the sediment traps. Only a few percent of the POC flux found in the traps consisted of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward

POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.

Research Highlights

- Faecal pellets and reprocessed faecal material dominated POC flux

- Small cells dominated the bloom and were exported via mesozooplankton grazing

- TEP fluxes were low, but likely enabled grazing on, and thus export of, small cells

Keywords

Nano- and picoplankton, export flux, iron fertilization, protists, sediment trap, Southern Ocean

Abbreviations

prot C: protist carbon POC: particulate organic carbon TEP: transparent exopolymer particles 1. Introduction

The downward flux of particulate organic carbon (POC) out of the surface ocean, known as the biological pump (Volk and Hoffert, 1985; De La Rocha, 2007), exerts an important control on atmospheric CO2 concentrations (Parekh et al., 2006; Kwon et al., 2009). Although most of the sinking POC is remineralized in the upper few hundred meters, some sinks below the permanent thermocline and is removed from contact with the atmosphere for climatically-relevant time-scales (Lutz et al., 2002; Buesseler and Boyd, 2009; Kwon et al., 2009). The magnitude of the POC flux is strongly dependent on the nature of the particles that are exported from the surface, which is governed by the structure of the phytoplankton and grazer community in the surface and the mesopelagic (100-1000 m) (Boyd and Trull 2007; Buesseler and Boyd, 2009).

However, even basic questions remain debated, such as which types of particles, and which phytoplankton taxa, contribute most to the flux out of the euphotic zone, and which have a greater tendency to pass through the mesopelagic zone without being remineralized. It is now appreciated that phytodetrital aggregates, not just zooplankton faecal pellets, are a key component of the particle flux (Turner, 2002). Export of phytodetrital aggregates is probably mediated by transparent exopolymer particles (TEP), which promote aggregation. However, few studies have measured downward TEP fluxes (Passow et al., 2001; Martin et al., 2011; Assmy et al., 2013). Moreover, an outstanding question concerns the degree to which the very

small, ubiquitous nano- and picoplankton contribute to downward flux. The long-held view that only large-celled phytoplankton, such as diatoms, contribute directly to particle flux (Michaels and Silver, 1988) has recently been challenged based on results from modelling and from biochemical and carbon isotopic analyses (Richardson and Jackson, 2007; Close et al., 2013). Our overall still poor understanding of the biological pump limits our ability to predict responses to environmental changes (Taucher and Oschlies, 2011).

The Southern Ocean consists largely of High-Nitrate-Low-Chlorophyll (HNLC) areas, where primary production is limited by iron (Fe) availability (Martin, 1990; Boyd et al., 2007). This has raised the question of whether enhancing Fe supply to the Southern Ocean would stimulate the biological pump and sequester anthropogenic CO2 (Lampitt et al., 2008a; Smetacek and Naqvi, 2008).

Five artificial Fe fertilization experiments have been carried out in the Southern Ocean: SOIREE (Boyd et al., 2000), SOFeX-N and -S (Coale et al., 2004), EisenEx (Assmy et al., 2007), EIFEX (Smetacek et al., 2012), and SAGE (Harvey et al., 2011). A further two studies examined naturally Fe fertilized waters downstream of Southern Ocean islands, CROZEX (Pollard et al., 2009) and KEOPS (Blain et al., 2007). Although carbon export at 200 m was enhanced during SOFeX, EIFEX, CROZEX and KEOPS (Coale et al., 2004; Salter et al., 2007; Blain et al., 2007; Smetacek at el., 2012), enhancement of deep export (>1000 m) was only observed during EIFEX and CROZEX (Pollard et al., 2009; Smetacek at el., 2012), although particle stocks during KEOPS were enhanced down to 400 m in fertilized relative to unfertilized waters (Jouandet et al., 2011). However, many artificial Fe fertilization experiments were too short to follow the demise of the Fe-induced blooms (Smetacek and Naqvi, 2008). The LOHAFEX experiment (loha means iron in Hindi) was hence designed to follow the build-up and demise of the bloom, and to fertilize an area large enough to minimise the effects of dilution with unfertilized waters (Smetacek and Naqvi, 2008).

This manuscript focuses on the contribution of intact cells of phytoplankton and protozooplankton to the downward carbon flux during LOHAFEX, and reports the TEP distribution in the water column and downward TEP fluxes.

2. Material and Methods

2.1 Study area

LOHAFEX was conducted during austral summer in the Antarctic Polar Frontal Zone in the Atlantic Sector of the Southern Ocean (48°S 15°W). A ~300 km2 patch in the centre of a cyclonic eddy was fertilized with 2 t of iron (as 10 t of FeSO4 * 7 H2O) on January 27 (d0)

(Fig. 1), and marked with surface-tethered buoys equipped with GPS. Another 2 t of iron were applied 18 days later. Stations inside the patch (IN-stations) were distinguished from control stations outside the patch (OUT-stations) based on the photosynthetic quantum efficiency (Fv/Fm ratio) of phytoplankton, and concentrations of chlorophyll, pCO2, and the inert tracer SF6. The fertilized patch was studied for 39 days (27 January to 6 March 2009), making LOHAFEX the longest iron fertilization experiment to date.

2.2 Sediment traps

Funnel-shaped neutrally buoyant PELAGRA sediment traps (Lampitt et al., 2008b) were deployed at 200 and 450 m depth (Table 1). Multiple deployments of 1-6 days each were made throughout the experiment to collect as close to a contiguous record of particle flux as possible. Each trap has four separate collection funnels, each leading to a 500 mL Nalgene collection cup. Collection cups were programmed to open 18-24 h after deployment (by sliding under the funnel), and to close again before ascending to the surface.

Trap collection cups were filled prior to deployment with 2% borate-buffered formaldehyde in filtered (0.2 jjm) seawater with NaCl added to 0.5%. Recovered samples were split on board using a rotary splitter, and splits for plankton counts were stored at 4°C.

Polyacrylamide gels were deployed in some trap cups to preserve sinking particles intact. Gels were prepared prior to the cruise, and photographed on board after trap recovery following Ebersbach and Trull (2008). Particles identified in gels were classed as intact faecal pellets, reprocessed faecal material and phytodetritus (S 1). Faecal pellets (fp in S 1) were distinguished by having a clearly-defined shape, owing to the membrane surrounding the pellet. Most pellets were cylindrical, but some were oval. Loose, very fluffy aggregates with no defined shape were classed as phytodetritus (pd in S 1). Particles that were more compact and with a better-defined shape than phytodetritus, but more loose than faecal pellets, were classed as reprocessed faecal material (fm in S 1). We attribute such reprocessed material to coprorhexy and coprochaly of faecal pellets by copepods (Lampitt et al., 1990; Noji et al., 1991), which was apparently extensive during LOHAFEX (Martin et al., 2013).

2.3 Microscopic analyses and data processing

The sinking material collected with the PELAGRA traps was examined using inverted light and epifluorescence microscopy (Axiovert 135, 200; Zeiss, Oberkochen, Germany) following the method of Throndsen (1995). Subsamples of 10 or 50 mL, depending on the density of the material, were settled in sedimentation chambers (Hydrobios; Kiel, Germany)

for 48 h. Protists were identified and a minimum of 500 cells counted at either 100, 200, or 400 x, depending on cell size. Depending on their abundance, cells were counted in transects, quarter, half, or whole chambers, and identified to species level where possible.

The biovolume of each taxon was estimated according to Hillebrand et al. (1999) by measuring 10-20 individuals per taxon at 400 x magnification. Biovolumes were converted into carbon content after Menden-Deuer and Lessard (2000). Using the taxon-specific carbon content per cell, and the abundance of each taxon in the sediment trap samples, we calculated the carbon flux contributed by each taxon (in mg C m 2 d-1). This is referred to below as protist carbon (prot C) flux.

Prot C comprised both phyto- and protozooplankton. Amongst the phytoplankton, we distinguished diatoms, flagellates, silicoflagellates, coccoid cells, and dinoflagellates. All cells with a flagellum were classed as flagellates. We did not distinguish between auto- and heterotrophic flagellates but between size classes (2.5-5, 5-10, and 10-20 jm) for the purpose of prot C calculation. Coccoid cells were defined as circular autotrophic cells up to 2 jm in size that lacked flagella. Protozooplankton was divided into dinoflagellates, ciliates, foraminifera, radiolaria, and heliozoa. Dinoflagellates were not differentiated into auto- and heterotrophic but into thecate and athecate taxa. Where possible, we distinguished between different species of diatoms, flagellates, and dinoflagellates; all taxa identified are listed in Supplementary Table 1.

For diatoms and tintinnid ciliates (tintinnids), we also enumerated intact empty and broken frustules and intact empty and damaged loricae according to Assmy et al. (2007, 2013). To enumerate broken frustules, only fragments consisting of >50% of the frustule were counted to prevent double-counting broken frustules. Damaged loricae were either crushed or missing part of the lorica. Empty diatom frustules can result from natural cell death, viral infection, parasite infestation, and protozoan or metazoan grazing, while broken frustules are mainly due to copepod grazing (Assmy et al., 2007; Assmy et al., 2013). Empty tintinnid loricae can result either from disruption of the delicate ciliate cell during fixation, or from the same mortality factors as for empty diatom frustules except metazoan grazing. Due to the stiff, leathery consistency of tintinnid loricae also it is mainly copepod grazing that results in damaged specimens (Assmy et al. 2013). Although intact empty and broken diatom frustules contain very little or no carbon, we calculated their equivalent carbon content as for full cells of each species to yield a ratio of full to empty and broken frustules (F:EB). This ratio illustrates the role of mortality factors versus sinking of intact cells for the total diatom flux. Only the carbon content of full diatom cells was used to calculate total prot C flux and the

relative contribution of diatoms to C flux. The lorica carbon content on the other hand was estimated to account on average for 10% of the total carbon content of tintinnids. This estimate is considerably lower than the lorica carbon content given by Gilron and Lynn (1989) because the loricae of the dominant species, in particular Acanthostomella norvegica, were very thin (1-2 jm) and already partly digested.

2.4 TEP analyses

TEP were measured both in the sediment trap samples and in the water column. Water samples of 250 mL were collected from the upper 500 m with Niskin bottles attached to a CTD rosette, and were processed in duplicate within a few hours of collection following Passow and Alldredge (1995). Samples were filtered onto 0.4 jm polycarbonate filters, stained with Alcian blue and stored at -20° C in sealed polycarbonate tubes. TEP in sediment trap samples were stained according to Passow et al. (2001).

Stained filters were then dissolved in 80% H2SO4, and the absorbance at 787 nm measured on a Pharma Spec UV-1700 spectrophotometer. The Alcian blue solution was calibrated against Gum Xanthan and TEP expressed as Gum Xanthan equivalents (jg GX eq. L-1).

Water column TEP inventories were integrated from zero to 100, 200, and 500 m. TEP fluxes, in mg GX eq. m-2 d-1, were calculated from the TEP content of trap material.

3. Results

3.1 Summary of physical and biological developments following fertilization

The fertilized patch rotated for four and a half weeks inside the closed eddy core and was then filamented towards the end of the experiment due to entrainment of the LOHAFEX eddy by a neighbouring anti-cyclone. The fertilized patch could be tracked for 39 days post-fertilization, making LOHAFEX the longest iron fertilization experiment to date. Trap trajectories mirrored the circulation in the upper 200 m, determined by shipboard ADCP surveys and revealed by the drift of the buoys, indicating a vertically coherent circulation down to at least 450 m (Martin et al., 2013). Most probably, the traps thus collected material sinking down from immediately above, allowing a reliable distinction between IN- and OUTpatch traps.

Upon Fe addition, Fv/Fm increased from initially 0.33 to 0.4-0.5 throughout the experiment, while nitrate decreased from 20 to 17.5 jmol L-1, and Si(OH)4 remained low at 0.6-1.6 jmol L-1. Chl a standing stocks doubled after fertilization to ~80 mg m- , but

remained ~40 mg m-2 outside the patch (Schulz et al., in prep.). 14C primary production peaked at 130 mmol C m-2 d-1 inside the patch, but remained <80 mmol C m-2 d-1 outside (M. Gauns, personal communication). Net community production (NCP) rose after fertilization, averaging 21 mmol m-2 d-1 ±20% inside the patch, but was 0 mmol m-2 d-1 outside (Martin et al., 2013).

Non-diatom phytoplankton, mostly unidentified flagellates and coccoid cells, made up the bulk of protist plankton, while diatoms, dinoflagellates, and Phaeocystis antarctica accounted for <5%, 6%, and 10% of protist plankton inside the patch, respectively (Fig. 2). The community composition was very similar outside of the patch, and did not change much over time (Schulz et al., in prep.). Although bacterial production, estimated by thymidine and leucine uptake, roughly doubled inside the patch the composition and abundance of the bacterial and archaeal community stayed remarkably constant throughout the experiment (Thiele et al., 2012). Stocks of protozooplankton and copepods <1 mm (including all naupliar and copepodite stages as well as adults of Oithona spp.) also stayed relatively stable inside and outside the patch (IN: 253±66 mg C m-2, OUT: 262±98 mg C m-2; integrated over the upper 80 m) (Schulz et al., in prep.). While stocks of copepods >1 mm, dominated by Calanus

simillimus, were in the same order of magnitude IN and OUT but quite variable

(~1700±1000 mg C m ; integrated over the upper 200 m) (Mazzocchi et al. in prep.). Grazing rates of C. simillimus were especially high, with faecal pellet production rates implying grazing of >30% on average of NCP (range 0.7-240%) (Gonzalez et al., in prep.).

Downward POC fluxes were low throughout the experiment and did not differ between fertilized and unfertilized waters (Martin et al., 2013): PELAGRA traps caught mostly <12 mg POC m-2 d-1 at both 200 m and 450 m, while export at 100 m as diagnosed from 234Th profiles was 75 mg C m-2 d-1.

3.2 Flux characteristics from polyacrylamide gels

Numerous gels were deployed, but owing to technical problems with the traps only six were recovered successfully: three at the start and one at the very end of the experiment inside the patch (450 m), and two outside of the patch at the end of the experiment (200 m) (Table 1). Moreover, unlike in studies using cylindrical sediment traps (Ebersbach and Trull, 2008; Ebersbach et al., 2011), the particles collected here may have been altered while travelling down the collection funnel. Most particles were deposited in a circle (S 2), indicating that they moved down the funnel instead of sinking undisturbed into the gel. The results must hence be interpreted with caution. Nevertheless, the gels indicated that particle

flux consisted predominantly of faecal pellets and reprocessed faecal material, while fragile phyto-detrital aggregates were rare (Fig. 3). No major changes in the contribution by different particle classes were evident after fertilization, nor differences between IN and OUT patch (Fig 3).

3.3 Protist carbon (prot C) fluxes

Since there were only three 200 m traps, yielding samples very similar in composition

to those from 450 m (Tables 2), we focus here on results from the 450 m traps.

Total prot C fluxes were very low, always <0.9 mg m d" , and accounted for <10% of total POC flux (Figs. 2, 4; Table 2). Prot C fluxes were composed of diatoms, unidentified flagellates, autotrophic coccoid cells, Phaeocystis antarctica, and thecate and athecate dinoflagellates. Tintinnids and foraminifera were present only as empty or damaged/broken individuals, and the majority of diatom frustules were broken. All other taxa were present at very low abundances.

In contrast to the prot C standing stock in the surface, which was dominated by flagellates and coccoid cells, the prot C in the sediment traps was dominated by thecate dinoflagellates and flagellates, with the other taxa contributing a similar proportion as to the surface stock (Fig. 2). However, these proportions varied strongly over time: while thecate dinoflagellates contributed almost all prot C early on, their relative contribution declined in favour of flagellates (Fig. 5).

Concomitantly, we observed marked increases in the equivalent C flux of empty and broken diatom frustules (Fig. 6), even though total biogenic silicon (BSi) flux if anything decreased slightly during this period (Table 2). Moreover, POC fluxes of empty and damaged tintinnid loricae also increased (Supplementary Table 3), indicating the importance of grazing during LOHAFEX. Likewise, the vast majority of foraminiferan tests were empty (Supplementary Table 4), and some of these showed signs of grazing, such as broken spines and/or tests (Fig. 7).

3.4 TEP concentrations in the water column and TEPflux

TEP concentrations were quite low and decreased strongly with depth, but remained steady over time and were very similar inside and outside of the patch (Fig. 7; Table 3). Depth-integrated TEP over the upper 100 m was <10 g GX eq. m-2 (Table 3). TEP and Chl a in the surface layer were not correlated (R2 = 0.15, p = 0.28).

TEP fluxes as caught in the sediment traps were very low throughout the experiment (always <5 mg GX eq. m-2 d-1), and showed no marked temporal trends or IN versus OUT patch differences (Table 2).

4. Discussion

4.1 Sediment trap collections

We cannot prove beyond doubt that the traps collected particles from only inside or only outside of the fertilized patch. However, a ship-board ADCP showed that the surface circulation was homogenous down to at least 200 m, and the trajectories of all traps closely reflected the surface flow (Martin et al., 2013). Given such an apparently consistent circulation from the surface down to the depth of the traps, it is most likely that they did collect particles derived from a relatively small area of surface water not far from the trap location. It is therefore unlikely that trap collections were strongly biased by particles originating from far away.

4.2 Composition of the flux

While intact protists comprised the bulk of surface POC, as found generally in the open ocean (Martiny et al., 2013), direct sinking of intact cells (prot C) contributed only a very minor fraction of the downward POC flux below 200 m (Fig. 2; Table 2). This is in sharp contrast to mass sinking events after diatom blooms, in which the majority of downward POC flux may be contributed by intact cells or resting stages of a subset of diatom species (Assmy et al., 2013; Rynearson et al., 2013). The highest prot C contribution to POC flux during LOHAFEX was found at the start (Table 2), reflecting pre-fertilization particle export (234Th was already depleted at Day 0 (Martin et al., 2013)).

The taxonomic composition of the prot C flux differed from that found in the surface, notably in the high proportion of thecate dinoflagellates and the presence of empty theca (Fig. 2, 5; Table 2). Intact diatoms did not contribute notably more to prot C fluxes than to the standing stock, and although intact flagellates contributed less to prot C flux than to the standing stock, they nevertheless contributed nearly a third of prot C flux (Fig.2; Table 2). Even intact coccoid cells <2 jm were found in the trap (Table 2), clearly showing that small coccoid cells and flagellates can contribute as intact cells to the biological pump, without needing to pass through the microbial loop. This adds direct observational evidence to previous studies based on modelling (Richardson and Jackson, 2007) and on the biochemical and isotopic composition of suspended particles in the mesopelagic (Close et al., 2013).

However, the low contribution of prot C to total downward POC flux (Fig. 2; Table 2), and the scarcity of phytodetritus in the gels (Fig. 3), indicate that direct aggregation and sinking of any protist classes was not a significant pathway for POC export.

Instead, the predominance of faecal pellets and disintegrating (reprocessed) faecal material (Fig. 3) points to mesozooplankton grazing as the primary vector for downward flux in the low-flux LOHAFEX system. The importance of mesozooplankton grazing for export of small-celled phytoplankton was highlighted by Wilson and Steinberg (2010), who reported the wide-spread presence of cyanobacterial aggregates in copepod guts. Likewise, Waite et al. (2000) reported intact picoplankton cells embedded within organic aggregates including faecal material, though only comprising <0.15% of downward POC flux. Thus, the recognisable prot C in our sediment traps probably represents primarily the minor fraction that survived gut passage intact, and was then liberated from disintegrating faecal pellets in the traps. Flagellates and coccoid cells are far less distinct than diatom frustules, tintinnid loricae or dinoflagellate thecae (Table 2), and they would have been missed if embedded in larger particles. Their contribution might hence have been under-estimated, although our conclusion that intact protist cells contributed only a small proportion of POC flux is unaffected.

Increased particle disintegration could explain the apparent increase over time in empty and broken diatom frustules, empty and damaged tintinnid loricae, and in flagellate contribution to prot C - while no similar increase was found in total BSi flux (Table 2). It therefore seems likely that later traps contained a greater proportion of more strongly disintegrating faecal material, indicating more intense reprocessing of faecal pellets later in the experiment. The decrease of faecal pellets with depth, and the concomitant increase in unrecognisable detritus particles observed in PA gels points strongly to intense reprocessing of faecal pellets (Martin et al., 2013).

That the majority of diatom frustules and foraminifera were empty or broken (Fig.6, Supplementary Table 4), and all tintinnid loricae empty or damaged (Supplementary Table 3), clearly points to mesozooplankton grazing as the primary route for particle flux. While empty loricae might be caused by the detachment of the ciliate cell during sample fixation, no such detached ciliates were found in the trap, suggesting that the cells were grazed at the surface. Damaged loricae, in contrast, can only be explained by deformation of the tough, leathery lorica by crustacean grazing, as is the case for broken diatom frustules and foraminiferan tests. However, even though these protistan taxa with hard body parts leave recognisable remains even after crustacean grazing, their empty, broken and damaged hard parts still did not vastly dominate the sediment trap samples (Fig. 7). A large fraction of the total POC flux

was therefore most likely derived from small cells rendered unrecognisable by mesozooplankton grazing, supporting the idea that nano- and picoplankton can contribute a significant proportion of the low downward particle fluxes out of "retention" systems (Wassmann, 1998) with high grazing pressure, such as LOHAFEX.

The very low proportion of full diatom cells compared to empty and broken frustules in the trap samples (Fig. 6) underscores the role of certain diatoms as "silica sinkers" in the Southern Ocean. This is particularly so for Fragilariopsis kerguelensis, the most abundant diatom species in the traps, and the one with the lowest F:EB ratio of the three most abundant diatom species (Supplementary Table 2). During the iron fertilization experiment EIFEX, F. kerguelensis was also primarily responsible for silica export in form of empty frustules, rather than carbon export, which was enhanced due to mass sinking of other diatom species (Assmy et al., 2013).

4.3. The role of TEP for export processes

TEP concentrations were relatively low (Table 3; Fig. 8), although TEP concentrations even within the Southern Ocean have been reported to range from 10-2000 jg GX eq. L-1 (Passow et al., 1995; Hong et al., 1997; Corzo et al., 2005; Ortega-Retuerta et al., 2009). TEP

fluxes in the sediment traps were also low (Table 2) compared to fluxes of

~100 mg GX eq. m d" reported during collapse of a North Atlantic diatom bloom (Martin et al., 2013) and a time-series off California (Passow et al., 2001). However, 0-100 m TEP stocks during LOHAFEX were in a similar range as the 2-8 g GX eq. m-2 reported during EIFEX (Assmy et al., 2013), at the end of which a mass sinking event of diatom aggregates was observed (Smetacek et al., 2012). Absolute TEP values should be compared with caution, since subtle differences in standard preparation and sampling protocols between studies could introduce systematic biases. However, as TEP comprise a potentially broad group of polysaccharides, TEP measured in different locations or at different times might differ physically and chemically such as to affect aggregation and downward flux differently. Neither downward flux nor TEP stocks varied much throughout LOHAFEX (Table 2, 3); in contrast, towards the end of EIFEX the TEP stock more than doubled. During EIFEX, TEP certainly facilitated aggregation but the main binding agent seemed to have been autolysed cytoplasm released after cell death, as has been reported for the giant diatom Coscinodiscus wailesii (Armbrecht et al. 2014).

Neither POC and TEP fluxes, nor prot C and TEP fluxes were correlated (R2 = 0.01, p = 0.80 and R2 = 0.01, p = 0.82, respectively), again suggesting that direct export of TEP-rich

aggregates was low, although the lack of large changes in POC flux may be responsible for the lack of a correlation.

TEP flux during LOHAFEX may have been largely due to mesozooplankton grazing on detritus aggregates in the surface. Mesozooplankton can ingest TEP (Passow and Alldregde, 1999; Ling and Alldregde, 2003) and aggregates of cells otherwise too small to feed on (Wilson and Steinberg, 2010), and while no large-scale TEP-mediated aggregation and mass sinking event was observed during LOHAFEX, TEP may have been critical to making the pico- and nanoplankton that dominated the bloom more accessible to crustacean grazers, thus indirectly mediating downward POC flux.

5. Conclusions

LOHAFEX resulted in a moderate bloom dominated by pico- and nanoplankton, especially small flagellates, which acted as a retention system with high grazing pressure by copepods. The low downward particle fluxes we observed were dominated by faecal material, which appeared to be reprocessed by copepods engaging in coprorhexy and coprochaly. Although sinking particles contained hard parts of large-celled taxa, mostly showing signs of grazing damage, these groups contributed a small proportion of downward POC flux. Instead, a large fraction of the POC flux was most likely contributed by pico- and nanoplankton, and indeed many individuals of these small-celled taxa were still recognisable in sinking particle samples recovered at 450 m depth. Although TEP concentrations were low and of similar magnitude as observed during EIFEX, TEP-mediated aggregation and mass sinking of intact phytoplankton cells was not a significant pathway for export during LOHAFEX. Instead, TEP possibly enabled mesozooplankton grazing on small phytoplankton, promoting their export via faecal pellets.

A cknowledgements

We thank LOHAFEX co-chief scientists Victor Smetacek and Wajih Naqvi, and the captain and crew of RVPolarstern. Kevin Saw prepared and deployed the PELAGRA traps. We are grateful to Uta Passow, Dieter Wolf-Gladrow and Ulrich Bathmann for thoughtful discussions, and to three anonymous reviewers for their valuable comments on the manuscript. F. E., I. S. and P. A. were funded through DFG-Research Center / Cluster of Excellence „The Ocean in the Earth System" and supported by GLOMAR - Bremen

International Graduate School for Marine Sciences. P. A. was additionally supported by the

Centre for Ice, Climate and Ecosystems (ICE) at the Norwegian Polar Institute.

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Figures

Fig. 1. Composite MODIS image of the LOHAFEX study area showing Chla surface concentrations in the period 12 to 14 February. The iron induced bloom is encircled (its elongated form is just an "illusion" because the "circular" patch had moved over the three days the MODIS image was assembled). Note the size of the much larger natural phytoplankton blooms in the northeast of the (artificial) LOHAFEX bloom for comparison. (Source: http://modis.gsfc.nasa.gov/)

Fig. 2. Contribution of total protist carbon (prot C) to total POC (a and c), and composition of prot C (b and d) inside the patch - showing the large differences between surface standing stock (upper graphs) and flux (lower graphs). Surface data are averaged over all IN stations (integrated over the upper 80 m), and flux data show averages of the 450 m deep IN deployments of the sediment traps. Within the composition of prot C, thecate and athecate dinoflagellates, unidentified flagellates, Phaeocystis antarctica, coccoid cells and diatoms are distinguished.

Fig. 3. Detailed images of the gels as they were obtained during LOHAFEX: a) Gel 1-3 (IN, parallel samples, d0-d2), b) Gel 4/5 (OUT, parallel samples, d26), c) Gel 6 (IN, d34). See Table 1 for deployment details. The majority of the collected particles appear to be of faecal origin; in the beginning the material seems to be more compact (a)), while at the end more intact (and degraded) faecal pellets and reprocessed material was found (b), c)). Note that the proto-zooplankton almost disappeared in Gel 4-6 from the second half of the experiment. (Scale bar: 1 cm)

Fig. 4. Total protist carbon (prot C) flux determined from PEL AGRA trap samples: Development of IN and OUT fluxes at a) 200 and b) 450 m. IN and OUT stations are labelled in the graphs.

Fig. 5. Relative protist carbon (prot C) flux of the C flux carrying groups: thecate and athecate

dinoflagellates, unidentified flagellates, Phaeocystis antarctica, coccoid cells and diatoms

over the course of the experiment at a) 200 and b) 450 m. IN and OUT stations are labelled in

the graphs; see Table 1 for deployment details of the sediment traps.

Fig. 6. Diatom POC flux (full frustules) and equivalent C flux (empty and broken frustules) at a) 200 m, and b) 450 m over the course of the experiment. IN and OUT stations are labelled in the graphs; see Table 1 for deployment details of the sediment traps. On the 2nd y-axis the ratio of full to empty and broken frustules (F:EB ratio) is given for each trap sample.

Fig. 7. Images showing the flux composition as examined with the microscope (from bulk sediment trap samples; a, b, c, f: 450 m; e, d: 200 m): a) dinoflagellate Gyrodinium sp. and reprocessed faecal material and (loose) phyto-detritus, b) broken radiolaria embedded in reprocessed faecal matter, c) empty tintinnid loricae incorporated in reprocessed faecal material and phyto-detrital aggregates, d) foraminifera with broken spines and dinoflagellate Ceratium pentagonum incorporated in reprocessed faecal material, e) broken frustules of Rhizosolenia sp., thecae of C. pentagonum, and reprocessed faecal matter and phyto-detritus, f) reprocessed faecal material, phyto-detrital aggregates and recognisable individual faecal pellets.

Fig. 8. TEP distribution in the upper 500 m of the water column: profiles a) IN, and b) OUT.

S 1. Particle types found in the PA gels: i) fm: reprocessed faecal material, ii) pd: phyto-detritus, iii) fp: intact faecal pellet. Scale bar: 1 mm.

S 2. Image of entire PA gel (Gel 6; see Table 1 for deployment time), in which particulate flux of a 24 h sampling period, IN, from the second half of the LOHAFEX study is preserved. The uneven distribution of the particulate matter points to a strong impact of the trap design on the collection procedure (see 3.2 for a detailed discussion). The diameter of the gel is 10 cm.

Table 1. PELAGRA trap deployments. While trap #2 was only for gel deployments, trap #9 and #11 delivered gels and bulk samples for biogeochemical analysis.

# Collection days Depth Patch Gels Start position End position

1 d0 - d2 210 IN 48.024°S, 15.811°W 47.872°S, 15.881°W

2 d0 - d2 450 IN 1, 2, 3 48.027°S, 15.802°W 47.885°S, 15.884°W

3 d10 - d15 440 prob. IN 47.732°S, 15.125°W 47.849°S, 15.470°W

4 d13 - d15 200 prob. IN 47.895°S, 15.265°W 47.799°S, 15.519°W

5 d17 - d21 470 OUT 47.503°S, 15.441°W 47.479°S, 14.886°W

6 d22 - d26 440 IN 47.655°S, 15.595°W 47.586°S, 14.561°W

7 d23 - d28 440 IN 47.351°S, 15.417°W 47.894°S, 14.425°W

8 d24 - d29 430 OUT 47.300°S, 15.556°W 48.385°S, 14.626°W

9 d26 - d27 230 OUT 4, 5 47.514°S, 15.451°W 48.373°S, 14.754°W

10 d29 - d33 460 IN 48.086°S, 14.467°W 48.979°S, 15.135°W

11 d34 - d37 440 IN 6 48.796°S, 15.237°W 49.041°S, 15.285°W

666 667

Table 2. Protist C (prot C) flux and surface standing stock (integrated over 80 m) of diatoms, flagellates, coccoid cells and dinoflagellates. Standing stock represents the average over all IN or all OUT stations*.

Diatoms Unidentified Phaeocystis Coccoid cells

Flagellates antarctica

PELAGRA % of % of POC % of % of POC % of % of POC % of POC % of POC

trap POC surf. stock POC surf. stock POC flux surf. stock flux surf. stock

flux flux

# ¡(IN), d0- 0.52 4.00 1.06 50.34 0.00 9.47 0.04 23.45

# 4 (IN), 0.36 4.00 1.22 50.34 0.06 9.47 0.08 23.45

d13-d15

# 9 (OUT), 0.25 3.74 1.85 49.73 0.07 8.78 0.11 16.35

d26-d27

450m (IN)

# 3, d10- 0.20 4.00 0.37 50.34 0.05 9.47 0.01 23.45

# 6, d21- 0.14 4.00 0.53 50.34 0.01 9.47 0.02 23.45

# 7, d23- 0.05 4.00 0.19 50.34 0.01 9.47 0.01 23.45

# 10, d28- 0.14 4.00 0.96 50.34 0.06 9.47 0.06 23.45

# 11, d33- 0.24 4.00 2.15 50.34 0.07 9.47 0.12 23.45

# 5, d17- 0.87 3.74 1.32 49.73 0.27 8.78 0.04 16.35

# 8, d24- 0.43 3.74 0.70 49.73 0.01 8.78 0.05 16.35

* total POC for standing stock is the sum of protist C, bacteria C and copepods (<1 mm) C from Schulz et al. (in prep) ** total POC and BSi fluxes from PELAGRA traps ( Martin et al., 2013)

Dinoflaggellates Totalprot C Total POC

Thecate Athecate Total

% of POC flux % of POC surf. stock % of POC flux % of POC surf. stock % of POC flux % of POC surf. stock % of POC flux % of POC surf. stock flux** mg C m -2 d-1

12.06 1.90 0.72 3.36 12.77 5.26 14.40 92.51 5.56

1.75 1.90 0.13 3.36 1.88 5.26 3.53 92.51 4.78

0.74 1.18 0.17 3.16 0.91 4.34 3.12 82.94 28.39

5.66 1.90 0.22 3.36 5.87 5.26 6.45 92.51 8.44

1.36 1.90 0.06 3.36 1.41 5.26 2.11 92.51 9.18

0.14 1.90 0.02 3.36 0.16 5.26 0.41 92.51 22.96

0.46 1.90 0.10 3.36 0.55 5.26 1.72 92.51 12.82

2.52 1.90 0.16 3.36 2.67 5.26 5.18 92.51 12.85

3.85 1.18 1.14 3.16 4.99 4.34 7.22 82.94 2.83

1.39 1.18 0.10 3.16 1.48 4.34 2.66 82.94 10.97

BSi TEP

flux** mg Si m-2 d-1 POCBSi mol:mol flux mg GX eq m-2 dt1

2.38 0.56 5.47 19.84 0.00 5.14

4.41 4.50 8.44 3.00 2.31

1.68 3.16

4.48 4.78 6.37 10.01 13.04

3.95 8.13

1.36 0.03 0.00 2.24 0.78

1.83 2.61

Table 3. TEP concentration in the water column (in GX eq).

Chla* TEP in the water column

Da surface (10 surface (10 100 m 200 m 500 m

y m) m) 100 m ^ 500 m (int.**) (int.)** (int.)**

№ L-1 mg m'3 mg m'3 mg m'3 g m'2 g m'2 g m'2

IN 88.06 ± 66.38 ± 55.30 ±

d0 0.48 0.73 91.49 ± 4.40 61.25 ± 2.72 7.70 14.04 31.38

d4 0.87 3.88* 86.29 ± 5.24 74.93 ± 62.92 ± 8.07 14.18 -

d9 1.20 9.76 90.77 ± 12.17 68.20 ± 3.02 59.42 ± 7.96 14.78 33.44

d12 0.84 4.08 93.31 ± 5.01 73.62 ± 0.39 8.28 15.04 34.02

d18 1.06 1.34 1.85 69.09 ± 66.81 ± 8.66 15.31 -

d24 1.20 95.14 75.13 ± 1.22 67.73 ± 2.97 59.56 ± 8.58 15.35 35.30

d33 0.94 15.08 83.95 ± 1.41 65.58 ± 3.35 61.32 ± 7.17 13.60 31.68

d36 0.82 3.76 5.47 3.40 5.79 14.02 32.68

T 89.38 ± 64.44 ± 62.36 ±

d16 0.71 6.76 2.23 0.51 8.09 14.82 34.27

d22 0.62 86.56 ± 70.81 ± 65.54 ± 8.27 15.18 35.15

7.45 2.63 2.06

74.82 ± 63.21 ±

d35 0.53 0.15 69.28 3.55 7.32 13.98 33.05

* Chla measurement in the water column at 10 m (unpubl. data M. Gauns)

** TEP concentration integrated over the water column

598 Figures

600 Fig. 1. Composite MODIS image of the LOHAFEX study area showing Chla surface

601 concentrations in the period 12 to 14 February. The iron induced bloom is encircled (its

602 elongated form is just an "illusion" because the "circular" patch had moved over the three

603 days the MODIS image was assembled). Note the size of the much larger natural

604 phytoplankton blooms in the northeast of the (artificial) LOHAFEX bloom for comparison.

605 (Source: http://modis.gsfc.nasa.gov/)

607 Fig. 2. Contribution of total protist carbon (prot C) to total POC (a and c), and composition of

608 prot C (b and d) inside the patch - showing the large differences between surface standing

609 stock (upper graphs) and flux (lower graphs). Surface data are averaged over all IN stations

610 (integrated over the upper 80 m), and flux data show averages of the 450 m deep IN

611 deployments of the sediment traps. Within the composition of prot C, thecate and athecate

612 dinoflagellates, unidentified flagellates, Phaeocystis antarctica, coccoid cells and diatoms are

613 distinguished.

615 Fig. 3. Detailed images of the gels as they were obtained during LOHAFEX: a) Gel 1-3 (IN,

616 parallel samples, d0-d2), b) Gel 4/5 (OUT, parallel samples, d26), c) Gel 6 (IN, d34). See

617 Table 1 for deployment details. The majority of the collected particles appear to be of faecal

618 origin; in the beginning the material seems to be more compact (a)), while at the end more

619 intact (and degraded) faecal pellets and reprocessed material was found (b), c)). Note that the

620 proto-zooplankton almost disappeared in Gel 4-6 from the second half of the experiment.

621 (Scale bar: 1 cm)

623 Fig. 4. Total protist carbon (prot C) flux determined from PELAGRA trap samples:

624 Development of IN and OUT fluxes at a) 200 and b) 450 m. IN and OUT stations are labelled

625 in the graphs.

627 Fig. 5. Relative protist carbon (prot C) flux of the C flux carrying groups: thecate and athecate

628 dinoflagellates, unidentified flagellates, Phaeocystis antarctica, coccoid cells and diatoms

629 over the course of the experiment at a) 200 and b) 450 m. IN and OUT stations are labelled in

630 the graphs; see Table 1 for deployment details of the sediment traps.

632 Fig. 6. Diatom POC flux (full frustules) and equivalent C flux (empty and broken frustules) at

633 a) 200 m, and b) 450 m over the course of the experiment. IN and OUT stations are labelled in

634 the graphs; see Table 1 for deployment details of the sediment traps. On the 2nd y-axis the

635 ratio of full to empty and broken frustules (F:EB ratio) is given for each trap sample.

637 Fig. 7. Images showing the flux composition as examined with the microscope (from bulk

638 sediment trap samples; a, b, c, f: 450 m; e, d: 200 m): a) dinoflagellate Gyrodinium sp. and

639 reprocessed faecal material and (loose) phyto-detritus, b) broken radiolaria embedded in

640 reprocessed faecal matter, c) empty tintinnid loricae incorporated in reprocessed faecal

641 material and phyto-detrital aggregates, d) foraminifera with broken spines and dinoflagellate

642 Ceratium pentagonum incorporated in reprocessed faecal material, e) broken frustules of

643 Rhizosolenia sp., thecae of C. pentagonum, and reprocessed faecal matter and phyto-detritus,

644 f) reprocessed faecal material, phyto-detrital aggregates and recognisable individual faecal

645 pellets.

647 Fig. 8. TEP distribution in the upper 500 m of the water column: profiles a) IN, and b) OUT.

650 S 1. Particle types found in the PA gels: i) fm: reprocessed faecal material, ii) pd: phyto-

651 detritus, iii) fp: intact faecal pellet. Scale bar: 1 mm.

653 S 2. Image of entire PA gel (Gel 6; see Table 1 for deployment time), in which particulate

654 flux of a 24 h sampling period, IN, from the second half of the LOHAFEX study is preserved.

655 The uneven distribution of the particulate matter points to a strong impact of the trap design

656 on the collection procedure (see 3.2 for a detailed discussion). The diameter of the gel is

657 10 cm.

Table 1. PEL AGRA trap deployments. While trap #2 was only for gel deployments, trap #9 and #11 delivered gels and bulk samples for biogeochemical analysis.

# Collection days Depth Patch Gels Start position

End position

1 dO- d2 210 IN 48.024°S, 15.811°W 47.872°S, 15.881°W

2 dO- d2 450 IN 1,2,3 48.027°S, 15.802°W 47.885°S, 15.884°W

3 dio - dl5 440 prob.IN 47.732°S, 15.125°W 47.849°S, 15.470°W

4 dl3 - dl5 200 prob.IN 47.895°S, 15.265°W 47.799°S, 15.519°W

5 dl7 - d21 470 OUT 47.503°S, 15.441°W 47.479°S, 14.886°W

6 d22 - d26 440 IN 47.655°S, 15.595°W 47.586°S, 14.561°W

7 d23 - d28 440 IN 47.351°S, 15.417°W 47.894°S, 14.425°W

8 d24 - d29 430 OUT 47.300°S, 15.556°W 48.385°S, 14.626°W

9 d26 - d27 230 OUT 4,5 47.514°S, 15.451°W 48.373°S, 14.754°W

10 d29 - d33 460 IN 48.086°S, 14.467°W 48.979°S, 15.135°W

11 d34 - d37 440 IN 6 48.796°S, 15.237°W 49.041°S, 15.285°W

Table 2. Protist C (prot C) flux and surface standing stock (integrated over 80 m) of diatoms, flagellates, coccoid cells and dinoflagellates. Standing stock represents the average over all IN or all OUT stations*.

PELAGRA trap

Diatoms

% of POC flux % ofPOC surf, stock

Unidentified Flagellates

% of POC flux % ofPOC surf stock

Phaeocystis antarctica

% of POC flux % ofPOC surf stock

Coccoid cells

% of POC flux % ofPOC surf stock

# 1 (IN), d0-d2 #4 (IN), dl3-dl5 #9 (OUT), d26-d27

450m (IN)

#3, dl0-dl5

# 6, d21-d26 #7, d23-d28 #10, d28-d33 #11, d33-d37

450 m (OUT)

#5, dl 7-d21 #8, d24-d29

0.52 0.36 0.25

0.20 0.14 0.05 0.14 0.24

0.87 0.43

4.00 4.00 3.74

4.00 4.00 4.00 4.00 4.00

3.74 3.74

1.06 1.22 1.85

0.37 0.53 0.19 0.96 2.15

1.32 0.70

50.34 50.34 49.73

50.34 50.34 50.34 50.34 50.34

49.73 49.73

0.00 0.06 0.07

0.05 0.01 0.01 0.06 0.07

0.27 0.01

9.47 9.47 8.78

9.47 9.47 9.47 9.47 9.47

.78 .78

0.04 0.08 0.11

0.01 0.02 0.01 0.06 0.12

0.04 0.05

23. 23. 16.

23. 23. 23. 23. 23.

16.35 16.35

* total POC for standing stock is the sum of protist C, bacteria C and copepods (<1 mm) C from Schulz et al. (in prep)

** total POC and BSi fluxes from PELAGRA traps ( Martin et al.,

Dinoflaggellates

Thecate Athecate

% of POC flux %of POC surf stock % ofPOC flux % ofPOC surf stock

12.06 1.90 0.72 3.36

1.75 1.90 0.13 3.36

0.74 1.18 0.17 3.16

5.66 1.90 0.22 3.36

1.36 1.90 0.06 3.36

0.14 1.90 0.02 3.36

0.46 1.90 0.10 3.36

2.52 1.90 0.16 3.36

3.85 1.18 1.14 3.16

1.39 1.18 0.10 3.16

Total prot C Total POC

Total % ofPOC flux % ofPOC surf stock % ofPOC ßux % ofPOC surf stock flux** mg C m '2 d1

12.77 5.26 14.40 92.51 5.56

1.88 5.26 3.53 92.51 4.78

0.91 4.34 3.12 82.94 28.39

5.87 5.26 6.45 92.51 8.44

S.41 5.26 2.11 92.51 9.18

0.16 5.26 0.41 92.51 22.96

0.55 5.26 1.72 92.51 12.82

2.67 5.26 5.18 92.51 12.85

4.99 4.34 7.22 82.94 2.83

1.48 mJVM 2.66 82.94 10.97

BSi ТЕР

flux** mg Si m'2 d1 POC.BSi mol:mol flux mg GXeq т\Л

2.38 0.56 2.89 5.47 19.84 22.97 0.00 5.14 4.26

4.41 4.50 8.44 3.00 2.31 4.48 4.78 6.37 10.01 13.04 1.36 0.03 0.00 2.24 0.78

1.68 3.16 3.95 8.13 1.83 2.61

Table 3. TEP concentration in the water column (in GX eq).

TEP in the water column

Day surface (10 m) ML-1 surface (10 m) mg m'3 100 m mg m'3 500 m mg m3 100m (int.**) gm2 200m (int.)** gm2 500m (int.)** gm2

dO 0.48 88.06 ±0.73 66.38 ±4.40 55.30 ±2.72 7.70 14.04 31.38

d4 0.87 91.49 ± 3.88* 61.25 ±5.24 - 8.07 14.18 -

d9 1.20 86.29 ± 9.76 74.93 ± 12.17 rjjfel ±3.02 7.96 14.78 33.44

dl2 0.84 90.77 ±4.08 68.20 ±5.01 59.42 ±0.39 8.28 15.04 34.02

dl8 1.06 93.31 ± 1.34 73.62 ± 1.85 8.66 15.31 -

d24 1.20 95.14 69.09 ± 1.22 66.81 ±2.97 8.58 15.35 35.30

d33 0.94 75.13 ± 15.08 67.73 ± 1.41 59.56 ±3.35 7.17 13.60 31.68

d36 0.82 83.95 ±3.76 65.58 ±5.47 61.32 ±3.40 5.79 14.02 32.68

dl6 0.71 89.38 ±6.76 64.44 ± 2.23 62.36 ±0.51 8.09 14.82 34.27

d22 0.62 86.56 ±7.45 70.81 ±2.63 65.54 ±2.06 15.18 35.15

d35 0.53 74.82 ±0.15 69.28 63.21 ±3.55 7.32 13.98 33.05

* Chla measurement in the water column at 10 m (unpubl. data M. Gauns)

** TEP concentration integrated over the water column

Figure 2

Total POC composition

Prot C composition

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