Description: 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.

Description: Small scale distribution patterns of seabirds in the Antarctic Polar Front (APF) were investigated in relation to other biological, physical and chemical features during the ANT-XIII/2 research cruise of RV Polarstern from Dec 1995 to Jan 1996. The APF is characterized by steep gradients in sea surface temperature and salinity. Within the APF, gradient zones were closely associated with elevated levels of primary production, chlorophyll-a concentrations and zooplankton densities. Even broad-billed prions (Pachyptila vittata-group), which dominated the seabird community by 83% in carbon requirements, showed small scale distributional patterns that were positively related to primary production, chloropyll-a and total zooplankton densities. The findings demonstrate a close, direct link between fine scale physical processes in the APF and biological activity through several food web levels up to that of zooplankton-eating seabirds. Broad-billed prions appeared to forage on very small copepods (Oithona spp.) in close association with the front. Fish and squid eating predators showed poor correlations with small scale spatial structures of the APF. However, in a wider band around the APF, most top predators did occur in elevated densities, showing gradual spatio-temporal diffusion of the impact of the APF on higher trophic levels.

Description: Particle flux data from 27 sites in the Atlantic Ocean have been compiled with a view to determining regional variations in the strength and efficiency of the biological pump and quantifying basin-wide fluxes and estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the Export Ratio (ER = POC FLUX/Primary Production) at 125 m of between 0.1 and 0.4 over the range of production from 50 to 400 gCm-2 yr-2. Significant regional differences in changes of the ER with depth are related to the temporal stability of flux. Sites with more pulsed export have higher shallow ERs and show more rapid degradation of particulate organic carbon (POC) flux with depth, resulting in little variation in fluxes below ca. 3000 m. The opposing effects of organic carbon production and calcification on pCO2 of surface seawater are combined to calculate an "effective carbon flux" at the depth ofeuphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone between 65°N and 65°S amounts to 3.14 Gt C yr-1, of which 5.7% is remineralised above the winter mixed layer, and is thus not available to sequestration on climatically relevant time scales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr-1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (opal: carbonate ratio) is seen across the entire North Atlantic, indicating a basin-wide phenomenon that may be related to large-scale changes in climatic forcing.

Description: Within the high nutrient --- low chlorophyll regime of the Antarctic Circumpolar Current (ACC), high phytoplankton concentrations are frequently observed in the vicinity of the Antarctic Polar Front (APF).As is typical for frontal systems, hydrography in this region is characterized by meanders and eddies as well as up- and downwelling cells which redistribute nutrients and influence the depth of the euphotic zone.To study the processes leading to the observed phytoplankton distribution, a coupled ocean plankton model for ecosystem studies in the ACC has been developed.The ocean component is an eddy-resolving version of the s-Coordinate Primitive Equation Model (SPEM).The model has a horizontal resolution of 1/12$^\circ$ and a vertical resolution increased near the surface.The biological model (BIMAP) comprises two biogeochemical cycles - silica and nitrogen - and a prognostic iron compartment to include possible effects of micronutrient limitation.Model results indicate that part of the ecosystem's regionalvariability can be attributed to the effect of vertical and horizontal advection.However, frontal dynamics alone cannot explain the observed enhanced concentrations of phytoplankton biomass near the APF and the minima in the northern and southern ACC.Simulations which neglect possible effects of iron limitation cannot reproduce the observed large scale phytoplankton distribution.Only when iron limitation is taken into account, the model simulates plankton concentrations in close agreement with observations during the SO-JGOFS cruises.While in the northern ACC phytoplankton growth is limited by silicate, primary production is limited by iron limitation south of the APF.Near the APF, mesoscale iron upwelling enhances primary production, leading to increased phyto- and zooplankton biomass.The meridional structure with two plankton maxima is closely linked to the cross-front overturning circulation.This double-cell circulation with two upwelling branches is caused by the northward sloping large scale bottom topography.keywords: Antarctic Polar Front, ocean-plankton model, phytoplankton distribution, iron limitation

Description: The fugacity of CO2 and abundance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995 to January 1996. Large undersaturations of CO2 in the surface water were observed coinciding with high Chla content. In the major hydrographic regions the mean air-sea fluxes were found to range from -3 to +7 mmol m-2 d-1 making these regions act a sink as well as a source for CO2. In the total 40-dayperiod the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m-2d-1, this based on the Wanninkhof (1992) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in theinorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air-sea flux, Ekman pumping and upwelling are included. Net community production varies between -9 mmol m-2d-1 and +7 mmol m-2 d-1, because of to large oscillations in the dominance of autotrophic (CO2 -fixation) versus heterotrophic (CO2 respiration) activity. Here the mixed layer depth is the major controlling factor. When integrated over time the gross influx and efflux of CO2 from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.

Description: An undulating CTD (SeaSoar) was towed from the Subtropical Front at 421S, 121E to the ice edge at 57.51S, 21W.The instrument measured temperature, salinity, pressure, and light in the top 400m of the water column. Fluorescenceand particle counts also were recorded. Fluorescence was calibrated to chlorophyll a, and particle counts wereconverted to biovolume to provide estimates of phyto- and zooplankton, respectively. The size of particles countedcovered the range 0.258.0 mm.Three major fronts, Subtropical, Subantarctic, and Polar were intersected and identified by their physicalcharacteristics. The Polar Front showed surface and subsurface signatures separated by nearly 300 km. Considerablesmall-scale structure was observed between the fronts, in particular a large (150-km diameter), warm-core eddy wasfound in the northern part of the Subantarctic Zone.Both fronts and eddy were associated with significant biological signals. Phyto- and zooplankton correlated well inthe frontal regions and south of the Polar Front. The Polar Frontal Zone was associated with elevated concentrationsof biomass, whereas concentrations were low at the Subantarctic Front and south of 551S. In contrast, the warm-coreeddy in the Subantarctic Zone contained the highest zooplankton biovolume of the section while chlorophyllconcentration was only moderate. We hypothesise that such a distribution results from grazing.The overall distribution of phyto- and zooplankton changed across the Subantarctic Front. The vertical extent ofbiomass deepened from north to south across the front. Zooplankton tended to be concentrated in relatively narrow,deep-extending bands to the north but were more widely spread to the south. The small-scale hydrographic structurecorrelated significantly with the distribution of plankton, suggesting strong physical controls on both smallzooplankton and phytoplankton.