Ecology and physiology of calanoid copepods in relation to the oxygen minimum zone in the eastern tropical Atlantic

Abstract

Calanoid copepods often contribute up to 95% of zooplankton communities and are key components in the energy transfer from primary producers to higher trophic level consumers such as fish, seabirds and whales. Through feeding, respiration and excretion, copepods considerably affect the cycling of organic and inorganic matter and thus essentially influence marine carbon fluxes. Increasing hypoxic conditions in tropical oceans influence the distribution, abundance and ecophysiology of pelagic organisms, but the effects of hypoxia on marine organisms are not yet fully understood and have become a field of increasing scientific interest. Since the oxygen minimum zone (OMZ) in the eastern tropical Atlantic is not yet as pronounced as in other tropical oceans, studies on zooplankton in relation to hypoxic conditions are still limited. This study aims to analyse ecophysiological characteristics of calanoid copepods from the eastern tropical Atlantic to identify different life strategies. Special focus is given to the current impact of the OMZ on copepod distribution and metabolic processes as well as to possible ecophysiological adaptations to hypoxia. Different methodological approaches (assessment of abundance and vertical distribution by Multinet catches, measurement of respiration rates) and biochemical analyses (enzyme activities (ETS&LDH), total lipid, fatty acids and stable isotopes (13C&15N)) were applied to assess specific ecophysiological characteristics. Five different life strategies of abundant copepod species could be identified. The active converters are distributed in warm, well oxygenated epipelagic waters and are characterised by high activity levels and high metabolic rates as well as by low lipid content and low δ15N ratios indicating continuous feeding and a herbivorous to omnivorous feeding behaviour. Mesopelagic copepods were classified as opportunistic predators with a predominantly carnivorous feeding mode indicated by high concentrations of the fatty acid 18:1(n-9), high δ15N ratios and high carnivory ratio. Low metabolic rates but high lipid and wax ester contents represent valuable traits to cope with the food scarcity of mesopelagic habitats. Diurnal vertically migrating (DVM) copepods such as Pleuromamma spp. were categorised as adaptive migrants . This life strategy is primarily characterised by a bimodal distribution pattern, low Q10 values and variable metabolic rates. Trophic biomarkers point towards an omnivorous diet and increased LDH activity may support DVM and a temporal stay within the OMZ. Ontogenetically vertically migrating (OVM) species (i.e. Calanoides carinatus) are described as dormant survivors and have a bimodal distribution pattern. In C. carinatus this life strategy is characterised by diapausing copepodids C5 in meso- to bathypelagic depths that have low metabolic rates and high wax ester levels as lipid reserve. High concentrations of phytoplankton marker fatty acids, low δ15N ratio and a low carnivory index point towards herbivorous feeding during active periods at the surface. Eucalanid copepods were identified as thrifty floaters , representing a distinctive life strategy among calanoid copepods. Their broad vertical distribution and high abundance especially within the OMZ as well as their low Q10 values point towards a high temperature and hypoxia tolerance. Sluggish movement, high lipid and wax ester contents as well as low metabolic rates may indicate dormancy, which possibly facilitates survival within OMZs. Lowered metabolic rates and potential dormancy in eucalanid copepods as well as DVM and increased LHD activities in Pleuromamma spp. were identified as possible ecophysiological and behavioural adaptations to hypoxic conditions. In conclusion, this study identified five different life strategies of abundant calanoid copepods from epi- to bathypelagic depths in the eastern tropical Atlantic. It illuminates the current impact of hypoxic conditions and discusses specific ecophysiological and behavioural adaptations to the OMZ. The results imply that certain copepod groups can better cope with hypoxia than others and may thus be able to survive intensifying OMZs in the future.