Description: A pronounced seasonal variability in primary productivity is the crucial factor in determining the life strategies of many polar zooplankton species, whereas oligotrophic tropical oceans experience little change with a low productivity. Lipid accumulation represents an important energetic adaptation in pelagic organisms to cope with the pronounced seasonal productivity in polar oceans. We compare important global euphausiid species from the Arctic via the tropics to the Antarctic, focussing on the genera Euphausia and Thysanoessa. While the Antarctic krill Euphausia superba is the species with the highest biomass of all metazoans on earth, krill species are clearly less relevant in other oceans. In polar krill species total lipid accumulation is usually very pronounced and may be utilised for metabolic maintenance during overwintering or for reproductive processes in spring. Most polar krill species store large amounts of the unusual depot lipid phosphatidylcholine (lecithin), a polar lipid with highly unsaturated fatty acids. However, they exhibit strong differences in their neutral lipid compounds, which may either consist of wax esters or triacylglycerols, but also of both types of lipid classes. Their major end-products of the fatty acid and fatty alcohol biosynthesis are quite different and usually species-specific. In contrast, tropical euphausiids do not rely on lipids as energy reserves and exhibit the usual lipid and fatty acid compositions of biomembranes with high amounts of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). In polar euphausiids, the major end-products of the fatty acid and fatty alcohol biosynthesis are quite different and usually species-specific. The ecophysiological implications of these deviating lipid characteristics may determine biogeographical zonation patterns and affect the vulnerability of polar krill species to global warming.

Description: Total lipids, lipid class and fatty acid compositions were studied to elucidate their role in the life cycle strategies of the Antarctic silverfish Pleuragramma antarcticum, one of the few truly pelagic fish species in the Antarctic realm. Various size groups (larvae to juveniles, 10-105 mm length) of this notothenioid fish have been collected in the southern Weddell Sea during different seasons (mainly spring: October/November and summer: January/February). Total lipid data of P. antarcticum revealed little variation between the seasons. Lipid levels were largely determined by the developmental stage and size, with lower lipid levels (〈20% of dry mass DM) found in the larvae (10-16 mm) and intermediate concentrations in the younger juveniles, which increased strongly to maximum amounts of lipid (〉40%DM) in the older juveniles (〉55-105 mm). Lipid class composition was clearly related to total lipid levels, with neutral lipids (triacylglycerols) comprising 〈20% of total lipid (%TL) in the younger lipid-poor specimens and 〉65%TL in the older lipid-rich fishes. Triacylglycerols and phospholipids showed an inverse relationship. Hence, the relative importance of these polar lipids, typically components of biomembranes, decreased with increasing lipid and triacylglycerol levels. Principal fatty acids were 14:0, 16:0, 18:1(n-9), 18:1(n-7), 20:5(n-3) and 22:6(n-3). Elevated portions of the long-chain mono¬unsaturated fatty acids, 20:1 and 22:1, in the lipid-rich specimens suggest the ingestion of two dominant calanoid copepod species, the wax ester-rich Calanoides acutus and the triacylglycerol-rich Calanus propinquus. They are the only species known to biosynthesize larger amounts of these monounsaturated compounds in high-Antarctic waters, which may therefore serve as dietary marker fatty acids for higher trophic levels. Apparently, wax ester moieties (fatty acids and alcohols) ingested with prey, e.g. copepods, are converted to triacylglycerols via fatty acids or metabolized by P. antarcticum. We discuss the function of these high-energy and low-density lipid compounds as energy reserve and/or buoyancy aid in this swimbladderless fish species.

Description: The aim of this study was to determine the macro-parasitic infestation level of oysters from the southern German Bight focussing on copepods of the genus Mytilicola. Crassostrea gigas, Ostrea edulis and Mytilus edulis were collected at five locations: three nearshore sites in the eastern Wadden Sea and two offshore cultivation sites in the German Bight. To reveal seasonal variations one sampling site was investigated in winter and summer. At the nearshore sites, Mytilicola orientalis was regularly detected in C. gigas. Prevalences ranged between 32.3% and 45.1%, intensity between 3.0 ± 0.6 and 8.2 ± 1.5. Infestation rates of C. gigas within the southern German Bight decreased from west to east: Apparently, M. orientalis has started its range extension along the German coast with gradual retardation eastwards but generally followed the invasion route of its main host, the Pacific oyster. Interestingly, we detected not only M. intestinalis but also M. orientalis as an intestinal parasite in M. edulis, which has sofar not previously been described as host within this region. We conclude that M. orientalis is flexible in its host choice. Furthermore, in the eastern Wadden Sea infestation rates of oysters and mussels by copepods are similar. These results deviate from the patterns observed for the northern Wadden Sea in terms of infestation level and host specificity. No macro-parasites were found in oysters and mussels from the offshore sites. This absence can be considered as potentially beneficial for aquaculture activities in the open ocean in terms of stamina and physiological performance.

Description: Antarctic and Arctic zooplankton species have developed very similar life strategies and energetic adaptations to the harsh environment via sophisticated modes of lipid accumulation. A very efficient biosynthesis, storage and utilization of lipids enable especially herbivorous species to buffer the pronounced seasonality of food supply in the polar oceans. Lipid levels usually peak at the end of the productive season in autumn and reach minimum levels in spring. In many species lipid deposits are not primarily used for maintenance during winter but are conserved to fuel reproductive processes at the end of the dark season. The dependence on seasonal primary production is also reflected by the respective lipid compositions. Detailed lipid analyses of dominant Antarctic and Arctic copepods revealed that the herbivorous Calanus and Calanoides species have developed the most complex lipid biochemical pathways. They biosynthesise large amounts of wax esters with long-chain monounsaturated fatty acids and alcohols (20:1, 22:1) as major components. In contrast, the Antarctic Calanus propinquus and C. simillimus synthesise primarily triacylglycerols consisting mainly of long-chain monounsaturated fatty acids with 22 and even 24 carbon atoms (2 major isomers), which is very unusual among plankton species. In contrast, the lipids of omnivorous and carnivorous taxa such as Metridia or Euchaeta are deficient in such long-chain fatty acids and alcohols, although their lipid reserves mainly consist of wax esters. Our investigations underscore that lipids are a key factor in high latitude ecosystems, especially for the lower trophic levels. The extremely lipid-rich herbivorous species ensure an efficient lipid-based energy transfer and represent high-calory food for fish and warm-blooded animals like birds and mammals.

Description: In the framework of the GENUS –“Geochemistry and Ecology of the Namibian Upwelling System” research program, trophic interactions and carbon pathways throughout the food web of the coastal upwelling system are being quantified. In contrast to earlier studies, special focus is been given to lower trophic levels in higher taxonomic resolution. Energy demands of various zooplankton taxa, including copepods, euphausiids, decapods and fish larvae, have been quantified with standardized methodology via optode respirometry. Dietary spectra and trophic levels were analyzed by trophic biomarker approaches based on fatty acid composition and stable isotopes (15N, 13C), respectively. All empirical data are assembled for an Ecopath with Ecosim (EwE) food-web model. The EwE model distinguishes between shelf and offshore communities. The conceptual food‐web model consists of three groups of primary producers, i.e. diatoms, dinoflagellates, and cyanobacteria, as well as many consumers such as Calanoides carinatus as the key herbivorous copepod in the Benguela upwelling system, other copepods, Euphausia hanseni, other zooplankton, and pelagic fishes including sardine, anchovy, and horse mackerel. Empirical data show that zooplankton and particularly copepods encompass a wider range of trophic levels from herbivory to secondary or even tertiary consumers (δ15N from 4 to 12‰), while anchovy had rather low δ15N of about 7‰. Respiration rates and metabolic activities of copepods could be parameterized for the model by an energy budget approach based on ambient temperature, body mass, and activity level. Calanoid copepods consumed 78mg C m‐2 d‐1 in shelf regions and 21mg C m‐2 d‐1 in oceanic regions. Locally, C. carinatus could remove up to 90% of the diatom biomass per day. The community consumption of pelagic decapods ranged from 7 mg C m‐2 d‐1 to 〉20mg C m‐2 d‐1 with highest values in the northernmost part of the study area. Overall, pelagic decapods apparently play a more prominent role in the northern Benguela ecosystem than previously assumed and may exert a substantial predation pressure on calanoid copepods. GENUS results emphasize that the trophic interactions within zooplankton and lower trophic levels are more complex than just linking primary producers with pelagic fish and should be taken into account in the process of developing realistic food‐web models of coastal upwelling systems. Keywords: foodweb, zooplankton, trophic interactions, energy flux

Description: The goal of the GENUS project (Geochemistry and Ecology of the Namibian Upwelling System) is to analyse the interrelationships between climate change, oceanic nutrients, greenhouse gases and the ecosystem structure in the coastal upwelling area off Namibia. The biological/ecological work package focused on the structure of the Northern Benguela Upwelling System (NBUS) and its energy flows under changing environmental conditions. Physiological constraints and adaptations were detected in several taxonomic groups investigated within the project, such as several copepod species, euphausiids and fish larvae. Temperature and oxygen distribution in the water column were identified as main drivers in modulating the distribution and ecology of many species. The extension and position of the Oxygen Minimum Zone (OMZ) seems to have a significant impact on the life cycles, vertical distribution and trophic condition of various species. In copepods we find a variety of adaptation mechanisms. While some species avoid the OMZ, others use it for resting and predator avoidance during daytime. Such vertical migrations contribute significantly to vertical carbon flux. The same holds true for decapods (4.4 mg C m‐2 d‐1). Respiration rates of 16 copepod species were determined to average 54.6 ±32.8 ml O2 d‐1 gDM‐1. Calanoides carinatus diapausing C5 stages reduced respiration at depth by 82% compared to surface activity. Further adaptations were found in euphausiid species: While Euphausia hanseni is capable to use the OMZ as a retreat by reducing its metabolic activity at lower temperatures and unfavourable trophic conditions, Nematoscelis megalops generally maintains a low level metabolism adapted to a constant life in the OMZ, and avoids crossing the thermocline. Special features of early life stages of Trachurus capensis, the fish species actually showing highest commercial landings, were analysed to elucidate their potential advantages in life performance, compared to other small pelagic species such as sardines or anchovies. The species showed short‐term hypoxia tolerance down to 30% oxygen saturation and even survived 10% saturation. Combined with the ability to switch from smaller to larger copepod prey and to surpass vulnerable early stages much faster than competitor species, this could explain the dominance of Trachurus capensis in the NBUS. In addition, competitors and predators of fish larvae such as jellyfish or chaetognaths showed little or no response to low oxygen concentrations gaining advantage over e.g. sardines and anchovies. Isotope analyses of various pelagic species and their food revealed a complex picture of the trophic levels of species including developmental stages and provide the basis for trophic flow models. The results will also serve to calibrate carbon dynamics and nutrient flux models that were developed in another GENUS work package. Data will be compared with nutrient distribution patterns and dynamics that may influence primary production and impact zooplankton distribution and higher trophic levels such as fish, seabirds or mammals. Results clearly show that continuing ocean warming coupled with expansion of the OMZ may alter horizontal and vertical distribution of species and the food web structure of the ecosystem. Keywords: Benguela Current, OMZ, Pelagic Ecosystem, Physiology