Bodo von Bodungen

Primary production and sedimentation during spring in the Antarctic Peninsula region

Phytoplankton biomass and composition, primary productivity (in situ simulated and in vitro incubations) and sedimentation rates (measured with free-drifting sediment traps suspended at 100 m depth) were recorded in the Bransfield Strait area of the Antarctic Peninsula during November to December 1980. Three distinct and persistent zones were encountered: low biomass comprising flagellates and diatoms in the Drake Passage and Scotia Sea (zone I): high to moderate biomass of Phaeocystis and diatoms in the northern and central Bransfield Strait (zone II); and moderate biomass (Thalassiosira spp. in the process of forming resting spores) in the vertically homogeneous water on the northern Antarctic Peninsula shelf (zone III). Nutrient concentrations were high throughout; zooplankton grazing relative to phytoplankton biomass and production was heavy in zone I but negligible in the other 2 zones. Rates of primary production in zones I, II, and III averaged 230, 1660 and 830 mg C m−2 d−1, respectively. Assimilation numbers were low throughout (< 1 mg Chl a)−1 h−1) and growth physiology of the zonal phytoplankton assemblages was basically similar. Sedimentation rates recorded by 2 traps in zone II were low (97 and 138 mg C m−2 d−1) and higher (546 mg C m−2 d−1) in a third trap which collected mostly euphausiid faeces. Sedimentation was heaviest in zone III (450 to 1400 mg C m−2 d−1) where collections of the 3 traps deployed were dominated by intact diatom frustules (Thalassiosira spp.). Spore formation and heavy sedimentation of diatoms thus also occurs at the end of Antarctic blooms in spite of high ambient nutrients. As approximately two-thirds of the diatoms in traps were resting spores, we suggest that sinking of cells represents a seeding strategy which ensures regional persistence of neritic assemblages. Species-specific differences in seeding strategies may well be important in determining spatial and temporal patterns of Antarctic phytoplankton abundance. This aspect of phytoplankton biology is likely to have far-reaching implications, not previously considered, for the structure of Antarctic food webs.

Particle flux, and composition of sedimenting matter, in the Greenland Sea

Vertical flux of particulate material was recorded with moored sediment traps during 1988/1989 in the Greenland Sea at 72 degrees N, 10 degrees W. This region exhibits pronounced seasonal variability in ice cover. Annual fluxes at 500 m water depth were 22.79, 8.55, 2.39, 3.81 and 0.51 g m(-2) for total flux (dry weight), carbonate, particulate biogenic silicate, particulate organic carbon and nitrogen, respectively. Fluxes increased in April, maximum rates of all compounds occurred in May-June, and consistently high total flux rates of around 100 mg m(-2)d(-1) prevailed during the summer. The increasing flux of biogenic particles measured in April is indicative of an early onset of algal growth in spring. Small pennate diatoms dominated in the trap collections during April, and were still numerous during the high flux period when Thalassiosira species were the most abundant diatoms. During May-June, up to 22% of the Thalassiosira cells collected were viable-looking cells. The faecal pellet flux increased after the May-June event. Therefore we conclude that the diatoms settled as phytodetritus, most likely in rapidly sinking aggregates. From seasonal nutrient profiles it is concluded that diatoms contribute 25% to new production during spring and 50% on an annual basis. More than 50% of newly produced silicate particles are dissolved above the 500 m horizon. High new production during spring does not lead to a pronounced sedimentation pulse of organic matter during spring but elavated vertical export is observed during the entire growth period

The Plankton Tower. IV. Interactions Between Water Column and Sediment in Enclosure Experiments in Kiel Bight

Neritic ecosystems in the boreal zone generally maintain more plankton biomass over a longer period of the year than off-shore systems in the same latitude. Productivity is higher particularly during the summer stratification, between the spring and autumn phytoplankton blooms brought about by nutrients from sources other than pelagic remineralization. Plankton biomass levels maintained by recycling within a pelagic system tend to decrease with time if limiting nutrients bound in sedimenting particles are not replenished. In neritic environments, surface waters can receive nutrients from the land, but depending on water depth and local weather and geomorphology, replenishment can also come from nutrient-rich subthermocline water and sediments. In deeper bodies of water with a steep coastline, such as fjords, the sediment contribution will be less important (Takahashi et al. 1977) than in shallow water systems with more of their sediment surface within the euphotic zone (von Bodungen et al. 1975, Rowe et al. 1975).

Modelling the pelagic nitrogen cycle and vertical particle flux in the Norwegian sea

A 1D Eulerian ecosystem model (BIological Ocean Model) for the Norwegian Sea was developed to investigate the dynamics of pelagic ecosystems. The BIOM combines six biochemical compartments and simulates the annual nitrogen cycle with specific focus on production, modification and sedimentation of particles in the water column. The external forcing and physical framework is based on a simulated annual cycle of global radiation and an annual mixed-layer cycle derived from field data. The vertical resolution of the model is given by an exponential grid with 200 depth layers, allowing specific parameterization of various sinking velocities, breakdown of particles and the remineralization processes. The aim of the numerical experiments is the simulation of ecosystem dynamics considering the specific biogeochemical properties of the Norwegian Sea, for example the life cycle of the dominant copepod Calanus finmarchicus. The results of the simulations were validated with field data. Model results are in good agreement with field data for the lower trophic levels of the food web. With increasing complexity of the organisms the differences increase between simulated processes and field data. Results of the numerical simulations suggest that BIOM is well adapted to investigate a physically controlled ecosystem. The simulation of grazing controlled pelagic ecosystems, like the Norwegian Sea, requires adaptations of parameterization to the specific ecosystem features. By using seasonally adaptation of the most sensible processes like utilization of light by phytoplankton and grazing by zooplankton results were greatly improved.

Biologie und Ökologie

Der entscheidende Faktor fur die Pflanzen- und Tierwelt der Ostsee ist der Salzgehalt. In dem sehr jungen Brackwassermeer konnte sich kaum eine eigenstandige Flora und Fauna entwickeln. Die meisten Organismen sind uber die Nordsee eingewandert. Im nordlichsten Teil gibt es noch einige eiszeitliche Relikte, die hier von ihrem Hauptverbreitungsgebiet in der Arktis isoliert wurden. Mit abnehmendem Salzgehalt geht die Zahl der marinen Arten immer weiter zuruck. Das wird besonders deutlich in der zentralen Ostsee, wo die Salinitat nur noch zwischen 5 und 12%o liegt. In diesem Bereich konnen sich auch nur wenige Suswasserorganismen behaupten. Fur sie ist der Salzgehalt wiederum zu hoch. Bei den meisten von ihnen stellen 3%o die Grenze dar. So finden sich Suswasserarten vor allem in den Kustenbereichen mit starken Zuflussen, aber auch in der nordlichen Bottenwiek und im ostlichsten Teil des Golfs von Finnland. Die marinen und auch die limnischen Arten stosen in der Ostsee immer irgendwo an ihre Verbreitungsgrenzen, so das sie hier empfindlicher gegenuber naturlichen und anthropogenen Stresfaktoren sind als in den Kerngebieten ihrer Verbreitung.