Polar Front Region Research Articles

Different temperature adaptation in Arctic and Atlantic heterotrophic bacteria in the Barents Sea Polar Front region

In the northern Barents Sea, at and around the Polar Front, carbon cycle variables were investigated during 2weeks in late summer of 2007. Arctic Water primary production in the experimental period averaged 50mmol C m−2day−1, as estimated from satellite sensed chlorophyll. In Atlantic waters, which appeared to just have passed the culmination of a late summer bloom, primary production was 125mmol C m−2day−1. Total organic carbon (TOC) averaged 82.4μM C in the mixed layer, and the values showed a gradient with highest values to the southeast and lowest to the northwest. The distribution of TOC was not related to the distribution of Atlantic and Arctic waters, although the highest values were found in Atlantic Water. Integrated bacterial production in the mixed layer, as estimated from thymidine incorporation rates, averaged 6.3% of primary production. In Atlantic Water, over the depth of the mixed layer, bacterial production rate averaged 0.40mmol C m−3day−1, which was 6.6 times the average in Arctic Water and 2.3 times the average in the front regions. Below 30m depth, bacterial production rates were generally higher in the Arctic Water than in the Atlantic Water. Moreover, when production rates of bacteria were compared according to temperature, the rates in Arctic Water were systematically higher than the rates in Atlantic Water. This difference implies that the heterotrophic bacteria from the Arctic have adapted towards higher growth efficiency than the bacteria in Atlantic Water.

Propagation of planetary-scale zonal mean wind anomalies and polar oscillations

Global atmospheric variables can be physically decomposed into four components: (1) the zonal time averaged climate symmetric component, (2) the time averaged climate asymmetric, (3) the zonal-mean transient symmetric anomaly, and (4) the transient asymmetric anomaly. This study analyzes the relationships between the intra-seasonal and inter-annual variability of planetary scale decomposed zonal and meridional winds in the tropopause, and oscillations such as those from the El Nino-Southern Oscillation (ENSO), the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO). The tropical inter-annual zonal mean wind anomalies in the tropopause are linked with the ENSO cycle and can propagate into the subtropics, mid-latitudes, and polar front regions via abnormal meridional vertical cells. Similarly, tropical intra-seasonal (40–60-d) zonal wind anomalies can reach the subtropics and mid-latitudes. The polar intra-seasonal zonal wind anomalies in the tropopause can propagate toward high-latitude areas. Thus, the AO and the AAO are the result of the interaction and propagation of these planetary scale zonal wind anomalies.

Spatial Variation in Turbulent Heat Fluxes in Drake Passage

High-resolution underway shipboard atmospheric and oceanic observations collected in Drake Passage from 2000 to 2009 are used to examine the spatial scales of turbulent heat fluxes and flux-related state variables. The magnitude of the seasonal cycle of sea surface temperature (SST) south of the Polar Front is found to be twice that north of the front, but the seasonal cycles of the turbulent heat fluxes show no differences on either side of the Polar Front. Frequency spectra of the turbulent heat fluxes and related variables are red, with no identifiable spectral peaks. SST and air temperature are coherent over a range of frequencies corresponding to periods between ~10 h and 2 days, with SST leading air temperature. The spatial decorrelation length scales of the sensible and latent heat fluxes calculated from two-day transects are 65 ± 6 km and 80 ± 6 km, respectively. The scale of the sensible heat flux is consistent with the decorrelation scale for air–sea temperature differences (70 ± 6 km) rather than either SST (153 ± 2 km) or air temperature (138 ± 4 km) alone. These scales are dominated by the Polar Front. When the Polar Front region is excluded, the decorrelation scales are 10–20 km, consistent with the first baroclinic Rossby radius. These eddy scales are often unrepresented in the available gridded heat flux products. The Drake Passage ship measurements are compared with four recently available gridded turbulent heat flux products: the European Centre for Medium-Range Weather Forecasts high-resolution operational product in support of the Year of Coordinated Observing Modeling and Forcasting Tropical Convection (ECMWF-YOTC), ECMWF interim reanalysis (ERA-Interim), the Drake Passage reanalysis downscaling (DPRD10) regional product, and the objectively analyzed air–sea fluxes (OAFlux). The decorrelation length scales of the air–sea temperature difference, wind speed, and turbulent heat fluxes from these four products are significantly larger than those determined from shipboard measurements.

Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)

This paper presents iron (Fe) profiles in the upper 1000 m from nine short-term (transect) stations and three long-term (process) stations occupied in the Australian sector of the Southern Ocean during the SAZ-Sense expedition in austral summer (January–February) 2007. Strong vertical and horizontal gradients in Fe concentrations were observed between the 18 sampled profiles (i.e. 0.09–0.63 nmol/l dissolved Fe (dFe)). Average dFe concentrations in surface gggwaters in the northern Sub-Antarctic Zone (SAZ-N) West (station P1) were 0.27±0.04 nmol/l. This is lower in the SAZ-N East region (station P3 and around) where average dFe values in the mixed layer were 0.48±0.10 nmol/l. The Polar Front (PF) station (P2) exhibited the lowest average surface Fe values (i.e., 0.22±0.02 nmol/l). Iron concentrations in deep waters down to 1000 m were more uniform (0.25–0.37 nmol/l dFe), which is in accordance with values reported elsewhere in remote waters of the Southern Ocean, but lower than those observed in the North Atlantic and North Pacific basins. A strong decoupling was observed between dFe and nutrient cycles at all stations. Particulate Fe levels were generally very low for all SAZ stations (<0.08–1.38 nmol/l), with higher values observed at stations collected near Tasmania and in the SAZ-N East region. The intrusion of subtropical waters, enriched with Fe from sediments or dust further north, is thought to mediate Fe input to the SAZ-N and STZ areas, while input from below would be the main source of Fe in the PF region. We applied the tracer Fe ⁎ (Fe ⁎= [dFe]− R Fe:P×[PO 4 3−], where R Fe:P is the algal uptake ratio) to estimate the degree to which the water masses were Fe limited. In this study, Fe ⁎ tended to be negative and decreased with increasing depths and latitude. Positive Fe ⁎ values, indicating Fe sufficiency, were observed in the (near-)surface waters collected in the SAZ-N East and near continental sources, where primary production was higher and ultimately limited by the lack of macro-nutrients, not Fe. Micro-organisms residing in the SAZ-N West and PF on the other hand experienced negative Fe ⁎, indicating a strong co-limitation by low silicic acid concentration and Fe supply (and light in the case of PF).

Spatial variability of the primary production and chlorophyll a concentration in the drake passage in the austral spring

The spatial distribution of the primary production (PP) and the chlorophyll a concentration (Chl) were investigated during two research cruises in the Drake Passage area in October–November of 2007 and 2008. The algorithm evaluating the integral PP (PPint) for the water column in this area was developed based on the data on the surface chlorophyll (Chls) and the incident solar irradiance obtained in 2004–2008 in the Atlantic Sector of the Southern Ocean. The results obtained both by the experimental and model approaches suggested that the Polar Front (PF) region of the Drake Passage was characterized by low values of both the PPint (<100 mg C/m2 per day) and Chls (0.08–0.20 mg/m3) in October–November. Low values of the Chls and relatively high phaeophytine a concentrations indicated the winter succession state of the phytoplankton community in the Antarctic Ocean and the southern Polar Frontal Zone (PFZ). The seasonal warming of the surface water layers and the developing pycnocline resulted in a phytoplankton bloom and a Chls concentration of more than 1 mg/m3 in mid-November in this area and the Subantarctic waters.

Distribution of Roseobacter RCA and SAR11 lineages and distinct bacterial communities from the subtropics to the Southern Ocean

We assessed the composition of the bacterioplankton in the Atlantic sector of the Southern Ocean in austral fall and winter and in New Zealand coastal waters in summer. The various water masses between the subtropics/Agulhas-Benguela boundary region and the Antarctic coastal current exhibited distinct bacterioplankton communities with the highest richness in the polar frontal region, as shown by denaturing gradient gel electrophoresis of 16S rRNA gene fragments. The SAR11 clade and the Roseobacter clade-affiliated (RCA) cluster were quantified by real-time quantitative PCR. SAR11 was detected in all samples analysed from subtropical waters to the coastal current and to depths of > 1000 m. In fall and winter, this clade constituted < 3% to 48% and 4-28% of total bacterial 16S rRNA genes respectively, with highest fractions in subtropical to polar frontal regions. The RCA cluster was only present in New Zealand coastal surface waters not exceeding 17 degrees C, in the Agulhas-Benguela boundary region (visited only during the winter cruise), in subantarctic waters and in the Southern Ocean. In fall, this cluster constituted up to 36% of total bacterial 16S rRNA genes with highest fractions in the Antarctic coastal current and outnumbered the SAR11 clade at most stations in the polar frontal region and further south. In winter, the RCA cluster constituted lower proportions than the SAR11 clade and did not exceed 8% of total bacterial 16S rRNA genes. In fall, the RCA cluster exhibited significant positive correlations with latitude and ammonium concentrations and negative correlations with concentrations of nitrate, phosphate, and for near-surface samples also with chlorophyll a, biomass production of heterotrophic prokaryotes and glucose turnover rates. The findings show that the various water masses between the subtropics and the Antarctic coastal current harbour distinct bacterioplankton communities. They further indicate that the RCA cluster, despite the narrow sequence similarity of > 98% of its 16S rRNA gene, is an abundant component of the heterotrophic bacterioplankton in the Southern Ocean, in particular in its coldest regions.

Benthic organic carbon flux and oxygen penetration reflect different plankton provinces in the Southern Ocean

For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At 11 sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this data set as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O 2 profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mg C m −2 d −1. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (∼3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from microsensor data suggests that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.

Phytoplankton production characteristics in the Eastern Atlantic and Atlantic sector of the Southern Ocean in October–November 2004

Production parameters of surface phytoplankton were measured along three transects: La Manche-Cape Town (I); Cape Town-54°S (II); 0°-49°W (along 54°S) (III). The Canary upwelling waters were most productive along transect I, where the surface chlorophyll a (Chl 0) and the surface primary production (PP 0) were as high as 4.3 mg/m3 and 173 mg C/m3 per day, respectively. Mosaic patterns in the distribution of these parameters were recorded in the northeastern regions of the South Subtropical Anticyclonic Gyre (Chl 0 = 0.03–0.35 mg/m3; PP 0 = 1.6–12.6 mg C/m3 per day). Along transect II, the average twofold southward increase in Chl 0 (from 0.2 to 0.4 mg/m3) and the concurrent decline of the phytoplankton assimilation activity ( AN 0) resulted in deviations from typical latitudinal changes inPP 0. At most sites, PP 0 values varied between 6 and 15 mg C/m3 per day. Negligible changes in Chl 0 (0.36–0.85 mg/m3), PP 0 (8–19 mg C/m3 per day), and AN 0 (0.7–1.6 mg C/mg chl a per hour) were registered for the oceanic waters along transect III. Along all the transects, PP 0 depended on Chl 0 to a greater extent than AN 0. The values of the latter parameter were largely determined by the water temperature and showed a slight correlation with the insolation. Along transect II, the integrated primary production (PP int) and the layer-integrated chlorophyll a in the upper 200 m (Chl 0–200) generally varied from 180 to 360 mg C/m2 per day and from 30 to 70 mg/m2, respectively. In the Polar Front region, an increase in Chl 0–200, PP int, Chl 0, and PP 0 up to respective values of 190 mg/m2, 520 mg C/m2 per day, 1.2 mg/m3, and 32 mg C/m3 per day was observed. A comparison of the water column (0–100 m) stability with the vertical distribution of the primary production and chlorophyll content along transect II implies that the thick (>100 m) upper mixed layer (UML) formed in response to the strong water cooling and wind forcing was largely responsible for the limited primary production in the Subantarctic and Antarctic regions. The large UML thickness resulted in an intense removal of plant cells from the photosynthetic layer and light starvation of a significant (up to 60%) part of the phytoplankton community.

Particle fluxes and δ15N variation of sinking particles in the central Bransfield Strait (Antarctica)

Sediment-trap deployment in the central Bransfield Strait during 1999/2000 shows distinct seasonal variation of particle fluxes, in which more than 99% of total annual flux occurred during a limited time. A more positive relationship between biogenic silica and total organic carbon fluxes in the intermediate-water sediment trap demonstrates that the surface-water production (mainly diatoms) played an important role in the supply of biogenic particles during the short period. The low δ15N values of particulate organic particles are attributed to high nitrate concentration in the Antarctic surface waters. The gradual increasing δ15N values in the bottom-water trap from the productive to unproductive season are likely associated with a switch in source from diatom aggregates to some remains of zooplankton, supplement to the horizontal contribution of lithogenic particles presumably from nearby shallow environment. Thus, the δ15N enrichment of sinking particles in the bottom-water trap during December to early February seems to be due to the laterally transported materials, assuming that the δ15N values decrease with depth in the Antarctic Polar Frontal region.

Reproductive response of the copepod Rhincalanus gigas to an iron-induced phytoplankton bloom in the Southern Ocean

The reproductive response of Rhincalanus gigas to the build up of a phytoplankton bloom in the Southern Ocean was studied during the European iron fertilization experiment (EIFEX). Egg production experiments were conducted over a period of approximately 5 weeks during development of a diatom dominated bloom. R. gigas showed a clear response to increasing chlorophyll a concentrations and the total egg production of the R. gigas population was highest just after the peak of the bloom at day 29 after fertilization. The average peak production was 50 eggs female−1 day−1. The percentage of egg producing females increased from about 0 to 90% during the course of the experiment. Accordingly, the maturation of the gonads reflected the positive response towards enhanced chlorophyll a concentrations. The fast reproductive response indicate that R. gigas was food limited during the period of this study in the Antarctic Polar Front region (APF).

Deep chlorophyll a maxima (DCMs) in pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron concentrations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 297:71-81 (2005) - doi:10.3354/meps297071 Deep chlorophyll a maxima (DCMs) in pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron concentrations O. Holm-Hansen*, M. Kahru, C. D. Hewes Scripps Institution of Oceanography, University of California-San Diego, La Jolla, California 92093-0202, USA *Email: oholmhansen@ucsd.edu ABSTRACT: A deep chlorophyll a maximum (DCM) at depths between 60 and 90 m in waters south of the Antarctic Polar Front (APF) occurs only in pelagic waters where the chlorophyll a concentrations in the upper mixed layer (UML) are very low (generally <0.2 mg m–3). Dissolved Fe concentrations in these waters with DCMs are also very low (generally <0.2 nM) and are probably a limiting factor for phytoplankton growth and biomass. DCMs occur in the upper portion of the temperature minimum layer (TML), which is the winter residue of the Antarctic Surface Water (AASW). The higher phytoplankton biomass at these depths is thought to result from higher Fe concentrations in the winter remnant of the AASW as compared to that found in the overlying UML. A survey of the literature indicates that DCMs are located predominately over the deep ocean basins where enrichment of surface waters with Fe from either coastal sediments or from upwelling processes would be minimal. DCMs are not found in coastal waters or in pelagic regions where complex bottom topography causes upwelling of deep water with sufficiently high Fe concentrations to enhance surface chlorophyll a concentrations. Such enrichment of surface waters overlying or downstream of topographical seamounts or ridges that rise to within a few thousand meters of the surface usually results in elevated phytoplankton biomass in the UML and no DCM due to decreased solar irradiance in the TML. The effect of such enrichment of Fe in surface pelagic waters that results from upwelling processes is most pronounced in the Scotia Sea, in the Polar Frontal region downstream of South Georgia, over the Southwest Indian Ridge, over the Kerguelen Plateau, and over the Pacific Antarctic and Southeast Indian Ridges. KEY WORDS: Chlorophyll a · Deep chlorophyll a maxima · Southern Ocean · Iron · Bathymetry Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 297. Online publication date: August 01, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Zooplankton dynamics in the eastern Atlantic sector of the Southern Ocean during the austral summer 1997/1998—Part 1: Community structure

Abstract Zooplankton abundance, biomass and community structure in the top 300 m layer were studied along the 6°E meridian between 49 and 65°S during a Scandinavian/South African Joint Global Ocean Flux Study (December 1997–January 1998) onboard the S.A. Agulhas. Along the transect, total abundance and biomass of zooplankton ranged from 8 to 384 ind m−3 and 1.4 to 64.1 mg DW m−3, respectively. The general pattern in zooplankton distribution observed was an increase in density between Spring Ice Edge and the Antarctic Polar Front (APF) regions. Copepods numerically dominated zooplankton along the entire transect accounting for 68–97% of the total abundance and 24–90% of the total zooplankton standing stock. Other important contributors to total biomass, particularly within the Winter Ice Edge (WIE) and APF regions, were cnidarians and ctenophores (up to 50%), pteropods (up to 35%), tunicates (up to 30%), chaetognaths (up to 25%) and euphausiids (up to 23%). Numerical analyses identified three major zooplankton groupings, coinciding with the three regions of investigation. A dramatic change in the zooplankton composition occurred at the APF region as several copepod and euphausiid species were found on either side of the front only. A second change in the species composition was observed at the WIE, emphasizing the ecological importance of the frontal zone associated with the winter ice edge.

Zooplankton dynamics in the eastern Atlantic sector of the Southern Ocean during the austral summer 1997/1998—Part 2: Grazing impact

Zooplankton grazing impact using the gut fluorescence technique was investigated in the top 200 m water layer along the 6°E meridian between 49°50′ and 60°25′S at six Biostations during a Scandinavian/South African Joint Global Ocean Flux Study (December 1997–January 1998) on board the SA Agulhas. Copepods were found to be the most conspicuous grazers along the entire transect, generally accounting for ≈40% of total zooplankton grazing. Pelagic pteropods, Limacina helicina and Clio sulcata, were the second most important grazers within the Seasonal Ice Edge (SIE) region, while the tunicate Salpa thompsoni and euphausiids, mainly Euphausia superba and E. frigida, were the second and third most important consumers of the phytoplankton production within the Winter Ice Edge (WIE) and Antarctic Polar Front (APF) regions. The overall ingestion rates of the zooplankton community ranged from 24 to 277 mg C m−2 day−1. The lowest and highest ingestion rates were recorded within the SIE and APF regions. The zooplankton grazing impact was low within the SIE region accounting for <10% of daily primary production. The highest phytoplankton consumption rate (56% of daily primary production) was observed in the WIE region, while the APF region was characterized by modest (23–33%) grazing impact of zooplankton.

Coupling of epipelagic and mesopelagic heterotrophic picoplankton production to phytoplankton biomass in the Antarctic polar frontal region

We assessed relationships between phytoplankton standing stock, measured as chlorophyll a (Chl a), primary production (PP), and heterotrophic picoplankton production (HPP), in the epipelagic zone (0–100 m) as well as in the mesopelagic zone (100–1,000 m) in the polar frontal zone of the Atlantic sector of the Southern Ocean in austral summer (late December to January) and fall (March to early May). Integrated epipelagic HPP was positively correlated to integrated PP in summer (data for fall are not available) but not to integrated Chl a. However, integrated mesopelagic HPP was positively correlated to Chl a in summer as well as fall. The mesopelagic fraction of HPP as a percentage of total HPP was also positively correlated to Chl a, whereas the epipelagic fraction of HPP was negatively correlated to it. These results indicate that with increasing phytoplankton standing stock, constituted mainly of highly silicified diatoms, the focus of its consumption by heterotrophic picoplankton shifts from epipelagic to mesopelagic waters. With a growth efficiency of 30%, our HPP data indicate that in both the epipelagic and mesopelagic zone heterotrophic picoplankton consume 20% of PP. Mesopelagic heterotrophic picoplankton consumed around 80% of the sinking flux, measured from depletion of 234Th, which is a lower fraction than that reported from the central and subarctic Pacific. Our analysis indicates that it is important to include mesopelagic HPP in comprehensive assessments of the microbial consumption of PP, phytoplankton biomass, and particulate organic matter in cold oceanic systems with high rates of export production.

Diet of juvenile cod (age 0?2) in the Barents Sea in relation to food availability and cod growth

Diet investigations were carried out on 0-, 1- and 2-year-old Northeast Arctic cod (Gadus morhua) sampled in the Barents Sea during 1984–2002. Stomach-content analyses showed that the 0 and 1 group cod fed mainly on crustaceans, with krill and amphipods composing up to 70% of their diet. Krill (Thysanoessa spp. and Meganyctiphanes norvegica) and amphipods (Themisto spp.) were mainly found in cod stomachs sampled in the central and close to the Polar Front region in the Barents Sea where these prey organisms are reported to be abundant in summer. A shift in the main diet from crustaceans to fish was observed from age 1 to age 2. The diet of 2-year-old cod mainly comprised capelin (Mallotus villosus) and other fish, and to a lesser degree, krill and amphipods. Shrimp (mainly Pandalus spp.) was also an important prey in both age 1 and 2 cod. A statistically significant positive relationship was obtained between capelin stock size and the amount of capelin in the diet of 2-year-old cod. Results from this study also show that the larger age-2 cod preyed more on capelin in winter and that larger cod (>22 cm) prefer larger capelin (>12 cm). During periods of low capelin abundance, the 2-year-old cod shift their diet more to crustaceans, such as krill and amphipods. A positive significant relationship was also obtained between Total Fullness Index (TFI) and the amount of capelin in the diet and between TFI and the growth of 2-year-old cod, indicating that the growth of age-2 cod is to a large extent dependent on the amount of capelin consumed. Growth of age-1 cod was also positively correlated to TFI.

Macrozooplankton communities and environmental variables in the Barents Sea marginal ice zone in late winter and spring

The horizontal distribution of macrozooplankton (≥3 mm) was studied with respect to changes in environmental conditions along 3 transects across the marginal ice zone (MIZ) in the NW Barents Sea during late winter (March 2000) and spring (May 1999). The water masses, which were identified by their salinity and temperature characteristics, largely determined the macrozooplank- ton composition, but season (late winter vs spring), geographical area (latitude and longitude) and bottom depth were also important. Almost 80% of the total macrozooplankton variability could be explained by changes in these environmental variables. Ice cover and the algal bloom situation had no significant impact on the horizontal macrozooplankton distribution early in the season (March to May). We sampled 3 main water masses: cold and less saline Arctic water (ArW), warm and saline Atlantic water (AtW) and mixtures of these (MIX); 5 distinct macrozooplankton communities were found, 2 in ArW (ArW bank and ArW slope), 2 in MIX (warm, >1°C, and cold, <0°C, MIX) and 1 in AtW. These communities were characterised by differences in species abundance rather than differ- ences in taxonomic composition. Numerically important macrozooplankton were Calanus glacialis, C. hyperboreus, Thysanoessa inermis and Aglantha digitale. These were also important in terms of wet biomass together with Beroe cucumis, Clione limacina and Sagitta elegans. Good indicator species for ArW were C. glacialis, C. limacina, Mertensia ovum and Parathemisto libellulla, and for AtW Thysanoessa spp. (T. inermis, T. longicaudata and T. raschii). Characteristic for MIX, i.e. the Polar Front region, was low macrozooplankton abundance, biomass and species richness compared to AtW and ArW. Seasonal variability (late winter vs spring) accounted for 21% of the total macro- zooplankton variability. Particularly the herbivores showed large seasonal variability in abundance and biomass, most probably due to their seasonal migration patterns.

The potential contribution of fecal pellets from large copepods to the flux of biogenic silica and particulate organic carbon in the Antarctic Polar Front region near 170°W

Abstract During a spring and a summer cruise, we determined the amounts of biogenic silica (bSi) and particulate organic carbon (POC) encapsulated into fecal pellets by the community of large copepods in the Antarctic Polar Front region, in order to estimate the potential contribution of sinking pellets to bSi and POC flux from the surface layer. Grazing by large copepods can be an important mechanism for enhancing vertical flux of POC and bSi because particles in the diatom size range are excellent foods for mesozooplankton, bSi is inert in copepod guts, and copepod fecal pellets (which have a chitinous membrane that retards dissolution of pellet contents) have sinking rates of 100s m d−1. Rates of pellet production were measured in deck incubations with natural seawater as food medium. BSi and POC contents of pellets were measured in the laboratory. These data were applied to measured abundances of large copepods and the resultant rates of bSi and POC pellet production were compared to flux measurements made by other investigators. Comparison between pellet production by the community of large copepods and flux at 100 m determined by thorium isotopes indicated that pellet production was equivalent to 22–63% of POC flux and 42–107% of bSi flux during the spring cruise. The potential contribution of pellets to flux at 100 m in the summer was considerably smaller, between 2% and 7% for POC and between 1% and 5% for bSi (with one exception at the southernmost station where there was a diatom bloom). We conclude that, in the spring especially, fecal pellets can contribute significantly to the bSi and POC sinking from the surface layer within the Antarctic Polar Front region. However, sediment traps deployed at a depth of 1000 m during the spring cruise indicated total fluxes of both bSi and POC were less than pellet production in the upper 100 m, indicating consumption or degradation of sinking material is significant between 100 and 1000 m. Fecal pellet production of POC was 4–10 times greater than the flux of POC at 1000 m and pellet production of bSi was 0.4–5 times the flux of bSi at 1000 m. Although pellets can contribute significantly to total flux, a large fraction of pellet material must be remineralized during sinking.

Seasonal variation in production, retention and export of zooplankton faecal pellets in the marginal ice zone and central Barents Sea

Vertical distribution and sedimentation of faecal pellets (FPs) as well as the production rates of FPs by larger copepods were studied during three cruises to the Barents Sea in March and May 1998, and July 1999. Three to five 24-h stations were selected during each cruise, where at least one main station was located in Arctic water (ArW), one in the polar front region (PF) and one in Atlantic water (AW). A winter scenario was encountered in March with very low concentrations of FPs in the water column, most of the time well below 0.1 mg faecal pellet carbon (FPC) per cubic meter, and with sedimentation rates below 3 mg FPC m −2 day −1 at all depths and stations. Increased concentrations of FPs were observed in May and the maximum biomass of FPs was found in ArW (4.8 mg FPC m −3). This was reflected in high vertical flux of FPs in the ArW, just below the chlorophyll maximum (∼150 mg FPC m −2 day −1). FPC sedimentation explained ∼40% of the total particulate organic carbon (POC) export at 90 m depth at this station. Copepod FP production was moderate to high in May, reflecting favourable feeding conditions. Large spatial variation in the estimated retention potential of FPs was observed, ranging from 96% in AW to ∼40% in the PF region. The July scenario did not differ very much from that observed in May. The lowest suspended concentrations and vertical flux of FPs were again observed in AW, in spite of the high pellet-production rate. FPC explained 34% of the POC export out of the upper layer in ArW, 40% in the PF region, but only 8% in AW. The calculated retention potential of 70% of the produced copepod FPs in AW decreased to 60% and 47% in the PF region and ArW, respectively. Krill FPs comprised a significant fraction of both suspended and sedimented FPC throughout the central Barents Sea. The data show that spatial and temporal variations in the FP “retention filter” are extensive and evidently of importance for the patterns of vertical flux of organic matter and the regulation of pelagic–benthic coupling in the Barents Sea.

The occurrence of faecal material in relation to different pelagic systems in the Southern Ocean and its importance for vertical flux

Abstract:During the SO-JGOFS-Polarstern-cruise in Oct/Nov 1992, faecal pellet abundance and distribution were determined in order to assess the impact of zooplankton grazing and defecation within the following three typical Antarctic plankton regimes in the Atlantic sector: the Marginal Ice Zone (MIZ), the southern Antarctic Circumpolar Current (ACC) and the Polar Frontal region (PFr). In contrast to the more southern regions, the PFr was characterised by the occurrence of relatively dense phyto-plankton blooms and high copepod concentrations. Faecal pellets were relatively abundant in the MIZ reaching up to > 105 µg faecal pellet carbon (FPC) m-3, whereas the values in the more northern regions were about one to two orders of magnitude lower: about 6 µg FPC m-3 in the southern ACC and less than 1 µm FPC m-3 in the PFr. Thus, the region with the highest phyto- and zooplankton concentrations showed by far the lowest faecal pellet standing stock concentrations. These results were compared to other regions in the Southern Ocean and to other regions in the world oceans and possible reasons for this situation and the potential ecological impact are discussed. Our investigations show, that not only the biomass of phytoplankton and zooplankton, but also mainly the structures of the plankton communities are decisive for sedimentation potentials of carbon and silica via faecal pellets in the different regions of the ocean.

Organic carbon, biogenic silica and diatom fluxes in the marginal winter sea-ice zone and in the Polar Front Region: interannual variations and differences in composition

Abstract Particle fluxes and composition were examined over 5 years at two mooring sites in the Polar Front Region (site PF: 50°09.S, 5°50.E) and in the marginal winter sea-ice zone (site BO: 54°30.S, 3°20.W) in the eastern Atlantic Sector of the Southern Ocean. Seasonality, interannual variability and the magnitude of total mass fluxes were higher at site BO compared to PF. Five-year averages and standard deviations (1σ) of total mass fluxes were 19.6±18.5 and 24.8±29.9 g m−2 at PF and BO, respectively. Peak fluxes at site BO occurred in January 1995, but the highest peak was measured in February 1991 (almost 1300 mg m−2 d−1) followed by post-bloom sedimentation in March through May. This would imply a time shift of several months between the onset of sea-ice retreat in October and major sedimentation events recorded in January/February with the upper BO traps. At site PF, highest fluxes of about 500 mg m−2 d−1 were found between December and March. Blooms at site BO, influenced by sea ice as indicated by diatom species composition, seem to occur more sporadically (e.g., in 1991 and 1995). Annual diatom fluxes were 11.8×106 and 20×106 valves m−2 during the deployments PF3 (1990) and BO1 (1991), respectively. At PF3, Fragilariopsis kerguelensis (37%) and Thalassionema nitzschioides fo1 (26.5%) dominated diatom flux, while F. kerguelensis (29%) and sea-ice-related algae (40%) were the main contributors to total diatom flux at site BO. During deployment BO1, the bloom collected in February was characterized by a very high molar Si:C of 8.8 that decreased almost continuously during the post-bloom phase, reaching a value of 1 in May. This change, however, was not documented in diatom species composition. We obtained a significant linear increase of biogenic opal with organic carbon fluxes at site PF and a highly significant but exponential relationship at site BO. Higher annual total mass fluxes were recorded at site BO, primarily due to elevated opal and lithogenic fluxes, corresponding to a higher silicate availability in the southern Antarctic Circumpolar Current. In contrast, higher mean organic carbon fluxes were obtained at site PF in accordance with elevated primary production and biomass. We obtained a three-fold higher molar Si:C ratio (5-year mean) for sinking particles collected with the upper BO traps (Si:C=4.0) compared to the PF (Si:C=1.3), consistent with the general pattern of Si and Fe availability. In particular at site BO, the Si:C ratios were usually high, even when accounting for organic carbon decay and biogenic silica (BSi) dissolution in the upper water column. At this study site, the Si:C ratios increased with lithogenic fluxes.