Physiological characteristics of plankton organisms and their role in ecosystem functioning. Results from GENUS I and II

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, Physiolog