Description: Zooplankton grazing on bacterio- and phytoplankton was studied in the Gulf of Aqaba and the Northern Red Sea during Meteor Cruise Me 44-2 in February-March 1999. Protozoan grazing on bacterioplankton and autotrophic ultraplankton was studied by the Landry dilution method. Microzooplankton grazing on phytoplankton 〉6 µm was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual zooplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae 〈6 µm were high (bacteria: 0.7 to 1.1 d-1, ultraphytoplankton: 0.7 to 1.3 d-1), while grazing rates on Synechococcus spp. were surprisingly low and undetectable in some experiments. Mesozooplankton grazing was weak, cumulative grazing rates being ca. 2 orders of magnitude smaller than the grazing rates by protozoans. Among mesozooplankton, appendicularians specialised on smaller food items and calanoid copepods on larger ones.

Description: 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.

Description: AbstractThe main reproductive period of herbivorous Antarctic copepods is coupled to the phytoplankton bloom in spring, while omnivorous or carnivorous species apparently reproduce year round. However, our knowledge of the reproduction during autumn and winter is limited. Therefore this study during a cruise with RV `PolarsternŽ aimed to measure reproductive activity in two dominant copepod species Metridia gerlachei and Ctenocalanus citer in late autumn (April/May). For this purpose, gonad development stages of both species were determined from species taken with Bongo net in the eastern Bellingshausen Sea (Antarctic Peninsula region), and related to the ambient feeding conditions represented as chlorophyll a concentration. In addition, experiments were conducted with M. gerlachei to estimate in-situ egg production rates and to study the influence of feeding and starvation on gonad development. Gonad maturity of M. gerlachei differed considerably between stations and decreased over time. The correlation of chlorophyll a and the proportion of mature females, though significant, was weak and highly variable. In experiments, the gonads of feeding females were in better condition than those of their starving counterparts, showing that reproductive activity in M. gerlachei is related to ambient feeding conditions. The in-situ egg production rates measured during 24h incubations were low, ranging from 0 to 1.9 eggs female-1 d-1. Calculated egg production rates based on preserved samples, however, were considerably higher with a maximum of 9.1 eggs female-1 d-1 suggesting that the incubation method might not be applicable in M. gerlachei. Mature females of C. citer were found at each station (20 to 60%). The gonad stage composition was rather similar at all stations showing no clear temporal and spatial trend. In this species, the proportion of mature females was not related to the chlorophyll a concentration, indicating either that the ambient phytoplankton stock was sufficient to fuel maturation or that other food sources were used. Our study shows that both species are producing during the austral autumn, indicating that their reproduction is partially decoupled from the spring phytoplankton bloom.Key words: austral autumn, copepods, reproduction, Metridia gerlachei, Ctenocalanus citer

Description: The aim of the expedition MET 44/2 (Feb. - Mar. 1999) with RV „Meteor“ was to study the biological and physical relationships in the Gulf of Aqaba and the northern Red Sea. The investigated samples of zoo- and phytoplankton were taken in that program. The selected stations form a transect from the head of the Gulf of Aqaba to the northern Red Sea. Day and night catches were taken. Position VI in the southern Gulf of Aqaba was sampled three times over two weeks to investigate the changes between the winter and the spring situation. The zooplankton samples were taken with a multiclosing net (9 nets) with a meshsize of 150 µm over the whole water column. The phytoplankton samples came from the Rosette from different dephts of the first 100 m. Profiles of temperature and salinity show slight differences between the situation in the Gulf of Aqaba and the northern Red Sea. The profiles for temperature indicate a small decrease of temperatur between 300 and 500 m. The nutrient concentrations imply strong differences between the investigated stations. The copepods were the most frequent group of the whole zooplankton community at all investigated stations with 76 - 79 % in the northern Red Sea and 84 - 88 % in the Gulf of Aqaba. Appendicularians, mollusks, ostracods and chaetognaths contribute more than 1% to the community. The vertical distribution shows at all stations a similar pattern. The upper 200 - 300 m hold the highest amount of the whole zooplankton community. Ostracods can be in high amounts in deeper regions too. Station 157 is an exception concerning the vertical distribution. There is an even distribution in the upper 400 m. Differences between day and night stations can be clearly seen. The highest zooplankton concentration are distributes in deeper regions at day stations than at night. The dominant group of the copepods were the calanoids on all stations with 62 - 79 %. Oithonids and poecilostomatids take part with 10 - 20 % of the Copepod community. The highest concentration of copepods is in the upper 300 m. The meaning of the calanoids is decreasing with depth while the poecilostomatids are increasing. Pleuromamma indica, a calanoid species, was taken to investigate distributional differences between the sex and the copepodit stages. The youngest copepodit stages (CI - CII) did not occur in the samples. Copepodit III stages only occurred in the northern Red Sea and the southern Gulf of Aqaba. The females domineer in the Gulf of Aqaba, while the northern Red Sea is domineeres by the males. Day and night differences were observed for the older Copepodit stages (CIV - CV) and the adults. The investigation of Position VI shows an increase of the standing stock and a higher concentration if the zooplankton to the upper 100 m. The chl a concentration does not increase, but the maximum is changing from 10 down to 50 m in the watercolumn. The community analysis shows that the two stations in the southern Gulf of Aqaba are similar. Including the different depth horizonts, the analysis shows that the cluster are build vertically. The community change is at the depth of the above mentioned temperature change. The phyto- and microplankton distribution of the upper 100 m shows different pictures. While at the stations of the southern Gulf (165, 167) the abundances are very high, we found low abundances at the other stations. The vertical distribution is different as well. The differences can be seen in the chl a concentrations. The dominant group at all stations are the coccolithophorids, followed by diatoms and dinoflagellates.