Zooplankton ecology and pelago-benthic coupling

ObjectivesThe aims of the present study are the analyses of zooplankton communities in the Bellingshausen Sea during the transition from the summer to the winter state. To characterize the "autumn state" our research focussed on the following issues: horizontal and vertical distribution, population structure, maturity of gonads, reproduction, gut content, feeding activity (natural phyto-plankton suspension, ice algae), as well as the significance of zooplankton for the particle flux with special emphasis on calanoid copepods and metamorphed larvae of benthic animals.Work at seaThe major gear employed for the distributional studies of mesozooplankton was the multiple opening and closing net equipped with five nets of 55 µm each. Stratified vertical hauls on the shelf and slope stations covered the entire water column between the surface and maximal 700 m, while at oceanic stations, the net was deployed down to 1000m. The depth ranges were defined according to the temperature profiles at the respective station. Sampling was carried out in the high Antarctic Bellingshausen sea on a transect off Adelaide island towards the continental slope of the Antarctic Peninsula. Additional sampling was carried out in the sea-ice zone in the southern Bellingshausen sea. Altogether, 24 hauls were carried out resulting in a rich set of more than 120 samples. The net samples were preserved in 4% buffered formalin, at some stations in 100% ethanol for molecular genetic purposes.In order to study interactions between the pelagic and the benthos, we deployed the multiple opening and closing net, the mulibox corer (MUC) and the box corer (GKG) at four so-called process stations, and ran additionally feeding and defecation experiments. The MUC allows the simultaneous sampling of up to 12 cores, including the above benthic surface water column. Both the upper 5cm sediment obtained with each core and the above water column were deep frozen (-80°C and 30°C respectively); the water sample was sieved through 55µm mesh size before. Where possible, the study of the benthos was complemented by sampling the benthic surface water layer using an epibenthic sledge (EBS) pulling two nets of 80µm and 300µm above the seafloor for 10 minutes. The EBS allows the study of demersally drifting larvae close to settlement.For the experimental, biochemical and histological work, live specimens were caught by means of a Bongo net (100 and 335 µm mesh size) over the entire water column. Feeding and defecation experiments were carried out at the process stations with the dominant zooplankton species (euphausiid larvae and the copepods Metridia gerlachei and Calanoides acutus). All experiments were run at 0°C in a cooled laboratory container in dim light. The food offered was the natural phytoplankton suspension from the rosette samples of the upper 50 m and with ice algae from melted ice cores. The concentrations of chlorophyll a were determined at the beginning and end of the experiments and were measured on board. Additionally, subsamples for microscopic counting were also taken to obtain information on preferential feeding on different size classes. The respective species and size composition will be determined on these preserved samples in the laboratory in Bremerhaven.At a total of 18 stations egg production experiments were conducted with Metridia gerlachi. For each experiment, 36 females were incubated singly in cell wells (volume: 10ml filtered seawater) for 24h at 0°C and checked for eggs every 8 hours. Other species such as Calanoides acutus and Rhincalanus gigas were only rarely found, and thus only a few females were incubated. In addition to the experiments, female Rhincalanus gigas, Calanoides acutus, Paraeuchaeta antarctica, Metridia gerlachei and few Ctenocalanus citer were preserved for histological analysis.Meroplanktonic larvae, mainly echinoderm and polychaete larvae, were deep frozen (-80°C) for further lipid and stomach content analyses in the laboratory.Preliminary ResultsIn general, we found low zooplankton abundances. Plankton communities were dominated at most stations by euphausiid larvae, by the large calanoid copepod Metridia gerlachei and by numerous small cyclopoid and calanoid copepod species. A thorough investigation of the samples will elucidate the seasonal development of the zooplankton community. These data will be discussed with respect to the life strategies of the species and relationships to hydrography and primary production.Up to this moment only some preliminary observations on the occurrence of invertebrate larvae can be presented. On the basis of preliminary observations of subsamples obtained from bongo net samples (mainly 100+300µm mesh size), phoronid and polychaete larvae (mainly nereids and spionids) seemed to be by far most abundant at all stations. This might change with the analyses of the samples from the multinet with smaller mesh size (55 µm). Echinoderms were also found at several stations, represented by asteroid brachiolaria and ophiopluteus in the water column. For this group, it seemed likely that larvae were more abundant at stations situated close to the continental slope off Adelaide Island, but has yet to be proven. Molluscs were represented as veligers in the water column, whereas bivalves have as of yet only been found as settled juveniles in one multicorer sample already analysed. As representative of a rare taxonomic group with a low diversity in the Antarctic, priapulid larvae have been found in several samples on the station transect off Adelaide Island. Generally, it can be concluded that invertebrate reproduction in the Antarctic takes also place in autumn, and that the occurrence of pelagic larval development of benthic invertebrates in Antarctic autumn can no longer be questionable!All copepod species studied were actively swimming. However, first results of the experimental work show low feeding rates, probably indicating the onset of a decreasing metabolic rates towards the winter.Egg production of Metridia gerlachei was extremely low at all stations. Only a maximum of 2 out of the 36 females spawned within the 24 hours at rates between 4 and 35 eggs per female and day. Thus, mean population egg production rate was less than one egg f-1 d-1. This agrees with the overall low state of gonad development in M. gerlachii as revealed of a first check of the gonad morphology. The gonads of Calanoides acutus and Rhincalanus gigas also seemed to be also in a very reduced stage, and consequently egg production was always zero.Feeding experiments with about 200 Metridia gerlachii females and highly concentrated ice algae as food showed that although the females were feeding, the gonads remained immature. From macroscopic observation, no difference was dected between starving and feeding females. This, however, has to be confirmed by histological investigation