Description: Respiration and ammonium excretion rates at different oxygen partial pressure were measured for calanoid copepods and euphausiids from the Eastern Tropical South Pacific and the Eastern Tropical North Atlantic. All specimens used for experiments were caught in the upper 400 m of the water column and only animals appearing unharmed and fit were used for experiments. Specimens were sorted, identified and transferred into aquaria with filtered, well-oxygenated seawater immediately after the catch and maintained for 1 to 13 hours prior to physiological experiments at the respective experimental temperature. Maintenance and physiological experiments were conducted in darkness in temperature-controlled incubators at 11, 13 or 23 degree C (±1). Before and during experiments, animals were not fed. Respiration and ammonium excretion rate measurements (both in µmol h-1 gDW-1) at varying oxygen concentrations were conducted in 12 to 60 mL gas-tight glass bottles. These were equipped with oxygen microsensors (ø 3 mm, PreSens Precision Sensing GmbH, Regensburg, Germany) attached to the inner wall of the bottles to monitor oxygen concentrations non-invasively. Read-out of oxygen concentrations was conducted using multi-channel fiber optic oxygen transmitters (Oxy-4 and Oxy-10 mini, PreSens Precision Sensing GmbH, Regensburg, Germany) that were connected via optical fibers to the outside of the bottles directly above the oxygen microsensor spots. Measurements were started at pre-adjusted oxygen and carbon dioxide levels. For this, seawater stocks with adjusted pO2 and pCO2 were prepared by equilibrating 3 to 4 L of filtered (0.2 µm filter Whatman GFF filter) and UV - sterilized (Aqua Cristal UV C 5 Watt, JBL GmbH & Co. KG, Neuhofen, Germany) water with premixed gases (certified gas mixtures from Air Liquide) for 4 hours at the respective experimental temperature. pCO2 levels were chosen to mimic the environmental pCO2 in the ETSP OMZ or the ETNA OMZ. Experimental runs were conducted with 11 to 15 trial incubations (1 or 2 animals per incubation bottle and three different treatment levels) and three animal-free control incubations (one per experimental treatment). During each run, experimental treatments comprised 100% air saturation as well as one reduced air saturation level with and without CO2. Oxygen concentrations in the incubation bottles were recorded every 5 min using the fiber-optic microsensor system and data recording for respiration rate determination was started immediately after all animals were transferred. Respiration rates were calculated from the slope of oxygen decrease over selected time intervals. Chosen time intervals were 20 to 105 min long. No respiration rate was calculated for the first 20 to 60 min after animal transfer to avoid the impact of enhanced activity of the animal or changes in the bottle water temperature during initial handling on the respiration rates and oxygen readings. Respiration rates were obtained over a maximum of 16 hours incubation time and slopes were linear at normoxia to mild hypoxia. Respiration rates in animal-free control bottles were used to correct for microbial activity. These rates were 〈 2% of animal respiration rates at normoxia. Samples for the measurement of ammonium concentrations were taken after 2 to 10 hours incubation time. Ammonium concentration was determined fluorimetrically (Holmes et al., 1999). Ammonium excretion was calculated as the concentration difference between incubation and animal-free control bottles. Some specimens died during the respiration and excretion rate measurements, as indicated by a cessation of respiration. No excretion rate measurements were conducted in this case, but the oxygen level at which the animal died was noted.

Description: Near-bottom zooplankton communities have rarely been studied despite numerous reports of high zooplankton concentrations, probably due to methodological constraints. In Kongsfjorden, Svalbard, the near-bottom layer was studied for the first time by combining daytime deployments of a remotely operated vehicle (ROV), the optical zooplankton sensor moored on-sight key species investigation (MOKI), and Tucker trawl sampling. ROV data from the fjord entrance and the inner fjord showed high near-bottom abundances of euphausiids with a mean concentration of 17.3 ± 3.5 n/100 m**3. With the MOKI system, we observed varying numbers of euphausiids, amphipods, chaetognaths, and copepods on the seafloor at six stations. Light-induced zooplankton swarms reached densities in the order of 90,000 (euphausiids), 120,000 (amphipods), and 470,000 ind/m**3 (chaetognaths), whereas older copepodids of Calanus hyperboreus and C. glacialis did not respond to light. They were abundant at the seafloor and 5 m above and showed maximum abundance of 65,000 ind/m**3. Tucker trawl data provided an overview of the seasonal vertical distribution of euphausiids. The most abundant species Thysanoessa inermis reached near-bottom concentrations of 270 ind/m**3. Regional distribution was neither related to depth nor to location in the fjord. The taxa observed were all part of the pelagic community. Our observations suggest the presence of near-bottom macrozooplankton also in other regions and challenge the current view of bentho-pelagic coupling. Neglecting this community may cause severe underestimates of the stock of elagic zooplankton, especially predatory species, which link secondary production with higher trophic levels.

Description: Variability in upwelling events may lead to periods of constrained food availability in the northern Benguela upwelling system (NBUS), thereby affecting the physiological state and metabolic activity of euphausiids. Most attention has so far been paid to seasonal effects but little is known about regional variability.Metabolic activity (expressed by respiration and excretion rates) and physiological state (expressed by reproductive effort and moult activity) in Euphausia hanseni were examined at different stations during austral summer (minimum upwelling) and austral winter (maximum upwelling). Overall, regional differences in physiological state, influencing metabolic activity, were greater than seasonal ones, indicating favourable conditions for growth and reproduction year-round. Higher respiration rateswere found for females in more advanced stages of sexual development.Moult stage did not affect oxygen consumption rates, however. The physiological state of E. hanseni at the time of capture may serve as ameaningful indicator of the associated hydrographic conditions in the NBUS,to be further used in eco-system analysis on seasonal or long-term time scales. A latitudinal comparison of species highlights the extraordinary physiological plasticity of euphausiids.

Description: Modern Oxygen Minimum Zones (OMZs) are extensive and prevail in most of the world oceans. Anthropogenic and climate induced changes will cause OMZs to expand, while coastal hypoxia is assumed to increase in extent and severity. Particularly in Eastern Boundary Upwelling Ecosystems (EBUEs), such as the Humboldt and the Benguela upwelling system, severe OMZs prevail. Animals living in these areas have to physiologically and/or behaviourally adapt to the low oxygen levels or will be excluded from these areas or at least their vertical distribution ranges will be limited. It is assumed that some areas may experience a shift from an abundant and diverse regime to one that is lean and dominated by vertical migrators. Further, temperature is known to impact the hypoxia tolerance of animals negatively. Euphausiids are known as pronounced diel vertical migrators, thus facing different levels of oxygen and temperature within 12 hours and are an important trophic link between primary producers and higher trophic levels throughout the world oceans. The critical oxygen pressure (Pcrit) and the regulation index (RI), as a method to differentiate between oxygen conformity and regulation, were used to assess hypoxia tolerances of different species from different ecosystems. Furthermore, diel vertical migration behaviour was monitored in some species. We will present and compare hypoxia tolerances of different dominant euphausiids species, show how oxygen availability may affect their vertical migration behaviour and assess how future climate scenarios (warming waters and decreasing oxygen content) may alter horizontal and/or vertical distribution of these species.

Description: Das interdisziplinäre GENUS-Projekt (Geochemistry and Ecology of the Namibian Upwelling System) ist ein vom BMBF gefördertes Verbundprojekt mit dem Ziel den Zusammenhang zwischen Klimavariabilität, biogeochemischen Stoffkreisläufen und der Ökosystemstruktur des nördlichen Benguelagebietes besser zu verstehen. Ein Schwerpunkt des GENUS-Projektes liegt dabei auf der Quantifizierung des aktiven Kohlenstoffflusses durch die vorherrschenden Zooplanktonarten. Das Benguelaauftriebsgebiet vor der Küste Namibias gehört zu den produktivsten Meeresgebieten weltweit und hat somit einen starken ökonomischen Einfluss auf die Anrainerstaaten und versorgt diese mit wichtigen marinen Ressourcen (Fische, Krebstiere u.a.). Im nördlichen Benguelagebiet wird das Zooplankton ganzjährig von Copepoden, Dekapoden und verschiedenen Krillarten dominiert. Viele dieser Planktonorganismen unternehmen ausgeprägte Vertikalwanderungen und transportieren dadurch Kohlenstoff, den sie während der Nacht in den oberen Wasserschichten als Nahrung aufgenommen haben, aktiv in tiefere Wasserschichten. In der Tiefe wird die Nahrung katabolisiert und der Kohlenstoff als anorganischer Kohlenstoff über die Atmung wieder ausgeschieden. Dabei ist sowohl die Horizontalverteilung des Planktons, als auch deren Wanderungsverhalten von verschiedenen biotischen und abiotischen Parametern beeinflusst. Einerseits, wahrscheinlich bedingt durch hydrographische und topographische Unterschiede, ist die Planktonbiomasse im nördlichen Bereich des Untersuchungsgebietes deutlich höher als im südlichen Bereich. Andererseits passen einzelne Arten ihr Vertikalwanderungsverhalten, d.h. die Wanderungsamplitude, an die vorherrschenden hydrographischen Bedingungen, vor allem Temperatur und Sauerstoffverfügbarkeit, an. Dies führt zu einer hohen räumlichen Variabilität in den aktiven Kohlenstoffflüssen. Zusätzlich führen Unterschiede in den Habitatpräferenzen (Schelf/Schelfhang/Offener Ozean) dazu, dass der aktive Kohlenstofftransport auf dem Schelf maßgeblich von Copepoden und einer Krillart bestimmt wird, wohingegen am Schelfhang und im offenen Ozean Krill und Dekapoden diesen Prozess dominieren. Vergleiche der aktiven mit den passiven Kohlenstoffflüssen (mittels Sedimentfallen) zeigen, dass das Zooplankton über dem Schelf in etwa 1-8% des organischen Kohlenstoffflusses ausmacht, wohingegen sie 7-73% zu den Flüssen im Hangbereich und offenen Ozean beitragen. Analysen des Einflusses der Temperatur und Sauerstoffverfügbarkeit auf das Verhalten und die Physiologie der Tiere legen dabei nahe, dass klimatische Veränderungen in den Meeresregionen auch den Beitrag des Planktons zu den Kohlenstoffflüssen regional, aber auch im globalen Maßstab, signifikant verändern werden.

Description: Polar environments like the high Arctic Kongsfjord are characterized by pronounced seasonality leading to strong variations in primary production. Food sources are particularly scarce during winter. Herbivorous krill, such as the arcto-boreal Thysanoessa inermis are key components in the ecosystem of Kongsfjord and strongly rely on phytoplankton as a food source. Therefore, during polar night such species must be adapted to survive long periods without significant nutritional input. We investigated physiological mechanisms and the allocation of energy resources to try to explain how T. inermis manages to survive the Arctic winter. Adult specimens caught in late summer were kept under starvation conditions for 28 days. Changes in metabolic rates (respiration and excretion) and biochemical composition (protein, lipid and fatty acid analyses) were monitored. In contrast to the Antarctic krill, Euphausia superba, and the subtropical E. hanseni, the arcto-boreal species did not reduce metabolism but utilized lipid reserves for survival. Assessed from total lipid stores and energy demand, the potential survival period was estimated at 63 days without food uptake, which is not sufficient to survive the entire winter. Results were compared to specimens that overwintered in-situ and discussed in relation to other euphausiids. In conclusion, T. inermis is well adapted to survive the Arctic winter provided that alternative food sources are available, but has a different strategy to cope with starvation than krill species from other latitudes.

Description: There is growing interest in using recirculating aquaculture systems (RAS) to raise noble crayfish Astacus astacus a valuable and once plentiful food species in Europe, now a highly endangered species. The growth and survival of A. astacus was compared in growth trials in RAS and open-pond systems (OPS) over a period of 2 months. Energy and lipid content of available diets and crayfish tissue were also determined. Growth of A. astacus during summer was significantly (p 〈 0.01, one sample t test) higher in OPS (SGR 1.23) than in RAS even at the highest feeding ration provided at 5 % bw/d−1 (RAS HI SGR 0.78 ± 0.06). OPS crayfish also had significantly (p 〈 0.01 OPS vs. all RAS treatments; Pairwise Wilcoxon) higher lipid content (8.51 %) than RAS crayfish (RAS HI 5.73 %, RAS MED 6.93 %, RAS LOW 5.92 %). Survival rates in RAS were, however, 100 % compared with previous observations in OPS of approx. 70 %. While results showed OPS growth exceeds than that in RAS in the short term, RAS survival rates and annualized growth performance may outweigh this disadvantage, particularly if optimal artificial diets for RAS holding are provided. Feed and crayfish analysis indicated that culturing A. astacus in RAS require a diet protein content exceeding 30 % and lipid content of 〈13 %, indicating that the carp diet supplied was not optimal. RAS culture allows this valuable species to be cultured in controlled, disease-free enclosed systems—resulting in high-value food products as well as high-quality seedlings for restocking purpose.

Description: The high-Arctic Kongsfjord is influenced mainly by cold Arctic water but also by warmer Atlantic water masses. In recent years, the proportion of the Atlantic inflow from the south has increased. Concurrently, one temperate-boreal (Meganyctiphanes norvegica) and one subtropical-temperate (Nematoscelis megalops) krill species are now regularly found in the Kongsfjord, in addition to the previously prevailing arcto-boreal species Thysanoessa inermis and T. raschii. In view of the recent changes in these species’ biogeographic distributions, we compared their physiological tolerances. Using non-invasive optical oxygen sensors, respiration measurements served to characterize metabolic responses to temperature variations. Thysanoessa spp. appear more cold-stenotherm than the other 2 krill species: the upper level of respiratory capacity is reached at 12°C and they are less tolerant to decreasing oxygen concentrations. This finding is consistent with their arcto-boreal distribution. In contrast, M. norvegica and N. megalops showed a higher tolerance to temperature changes, a robust nutritional condition and sexual maturity. Such physiological plasticity may explain the recent northward expansion of their geographic range.