Description: Highlights: • Optode respirometry is an effective new method for copepod respiration measurements. • Respiration rates and Q10 values were established for tropical Atlantic key species. • Respiration was influenced by body mass, temperature and species-specific behavior. • Depth of occurrence did not have a significant effect on standardized respiration. • The oxygen minimum zone did not yet fundamentally impact copepod ecophysiology Abstract Zooplankton respiration plays an important role in the carbon cycling of pelagic ecosystems. The rate of oxygen consumption in zooplankton is affected by the physical environment, vertical distribution range and species-specific behavior. Especially in tropical oceans, oxygen minimum zones (OMZs) may influence zooplankton metabolic processes and vertical distribution and thus structure zooplankton communities. Here we present respiration rates of tropical Atlantic copepods in relation to environmental factors, especially O2 concentration, and species-specific characteristics. Copepods were sampled during two research stays on the Cape Verde Island São Vicente in March/April and May/June 2010. Minimum O2 concentrations of 51 μmol kg− 1 (pO2 of 4.25 kPa) at 400 m depth were recorded within the OMZ. Respiration rates of epi- and mesopelagic calanoid copepods were measured by optode respirometry at three different ambient temperatures (13, 18, and 23 °C) to establish the effect of temperature on metabolic rates. Mass-specific oxygen consumption ranged from 27 μmol O2 gDM− 1 h− 1 in copepodids C5 of Lophothrix sp. at 13 °C to 774 μmol O2 gDM− 1 h− 1 in Pleuromamma xiphias copepodids C5 at 18 °C and was mainly controlled by body mass and temperature. Mass-specific respiration rates were highest in surface-dwelling organisms and decreased with increasing depth. To allow for a comparison of shallow and deep-living copepods, respiration rates were standardized to a common temperature of 18 °C and a mean body dry mass of 0.5 mg, applying a Q10 of 2.0 and a body mass exponent of − 0.56. Temperature- and body mass-corrected respiration rates did not decrease with increasing depth indicating that neither depth of occurrence, nor current hypoxic conditions within the OMZ had a fundamental, persistent effect on zooplankton respiration.

Description: Highlights: • Environmental conditions cause specific zooplankton life strategies. • No ontogenetic or diel vertical migration in the life cycle of Calanus chilensis. • Spatial expansion of Calanus chilensis secondary production far offshore. • Compacted surface biomass of Calanus chilensis allows easy foraging by anchovy. Abstract: Calanid copepods of the genera Calanus and Calanoides are key components of zooplankton communities in upwelling systems. Here, we compare the life-history traits of Calanus chilensis from the Humboldt Current Systems (HCS) off northern Peru and its counterpart Calanoides natalis from the northern Benguela Current System (BCS) off Namibia. A comprehensive data set of the distribution and abundance patterns of these species along extensive horizontal and vertical scales is presented. C. chilensis from the HCS was almost exclusively restricted to the surface layer (50–0 m) above the oxygen minimum zone (OMZ), whereas C. natalis from the BCS inhabited the entire water column down to 800 m performing ontogenetic vertical migration (OVM) through the OMZ. Resting stages of C. natalis at depth accumulated high amounts of lipid (30–60% of dry mass, DM), whereas C. chilensis did not rely on lipid reserves. These findings confirm that the life cycle of C. chilensis does not include OVM with diapause at depth. Surprisingly, the regional distribution of C. chilensis secondary production extended much further offshore (〉200 km from the coast) than is typical of other coastal upwelling systems. Deviating environmental conditions forced the two key calanid species to develop specific, but different life strategies for HCS and BCS. Compacted biomass concentrations of C. chilensis in the surface layer from the shelf (≤3 g DM m−2) to offshore waters (≤1.5 g DM m−2) facilitate easy and efficient foraging by predators such as juvenile Peruvian anchovies. In contrast, a large fraction of the C. natalis biomass occurs within the OMZ and is thus out of reach for hypoxia-sensitive predators. Calanoid copepods (e.g. C. chilensis) play a crucial role as important prey for growth and recruitment of small pelagic fish. Thus, the compacted biomass and high productivity of C. chilensis at the surface derived from its adaptive life-history traits (no OVM) may explain the superior trophic transfer efficiency and hence enormous fisheries yield of the HCS compared to the BCS.

Description: Hyperiid amphipods are predatory pelagic crustaceans that are particularly prevalent in high-latitude oceans. Many species are likely to have co-evolved with soft-bodied zooplankton groups such as salps and medusae, using them as substrate, for food, shelter or reproduction. Compared to other pelagic groups, such as fish, euphausiids and soft-bodied zooplankton, hyperiid amphipods are poorly studied especially in terms of their distribution and ecology. Hyperiids of the genus Themisto, comprising seven distinct species, are key players in temperate and cold-water pelagic ecosystems where they reach enormous levels of biomass. In these areas, they are important components of marine food webs, and they are major prey for many commercially important fish and squid stocks. In northern parts of the Southern Ocean, Themisto are so prevalent that they are considered to take on the role that Antarctic krill play further south. Nevertheless, although they are around the same size as krill, and may also occur in swarms, their feeding behaviour and mode of reproduction are completely different, hence their respective impacts on ecosystem structure differ. Themisto are major predators of meso- and macrozooplankton in several major oceanic regions covering shelves to open ocean from the polar regions to the subtropics. Based on a combination of published and unpublished occurrence data, we plot out the distributions of the seven species of Themisto. Further, we consider the different predators that rely on Themisto for a large fraction of their diet, demonstrating their major importance for higher trophic levels such as fish, seabirds and mammals. For instance, T. gaudichaudii in the Southern Ocean comprises a major part of the diets of around 80 different species of squid, fish, seabirds and marine mammals, while T. libellula in the Bering Sea and Greenland waters is a main prey item for commercially exploited fish species. We also consider the ongoing and predicted range expansions of Themisto species in light of environmental changes. In northern high latitudes, sub-Arctic Themisto species are replacing truly Arctic, ice-bound, species. In the Southern Ocean, a range expansion of T. gaudichaudii is expected as water masses warm, impacting higher trophic levels and biogeochemical cycles. We identify the many knowlegde gaps that must be filled in order to evaluate, monitor and predict the ecological shifts that will result from the changing patterns of distribution and abundance of this important pelagic group.