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Carnivorous zooplankton - their role in the arctic pelagic ecosystem

Final Activity Report Summary - ZOO ROLE (Carnivorous zooplankton - their role in the arctic pelagic ecosystem)

The Marie Curie intra-European project ZOO ROLE focused on how carnivorous zooplankton function in the Arctic pelagic ecosystem. The plankton community contains the highest biomass of aquatic ecosystems. The study of plankton, and the variables that influence their dynamics, therefore, has a clear importance for the understanding of aquatic ecosystems.

Plankton are often small organisms that by definition are passive drifters of the sea. Phytoplankton, together with the macro algae, are the basis for most marine food webs. By photosynthesis they convert solar energy, carbon dioxide, water and nutrients into organic compounds. Herbivorous zooplankton constitute the next trophic level, and they are in turn eaten by other carnivorous zooplankton or fish. Carnivorous zooplankton have been found to have a substantial impact on prey communities and selective predation by zooplankton may influence the trophic structure of the zooplankton community. Important and common predatory groups, such as the chaetognaths and carnivorous copepods are particularly understudied in the Arctic. A thorough understanding of the dynamics of polar food webs is a key to enable accurate predictions of effects on marine food webs by global warming and to manage fisheries in a sustainable way.

The main objective was to study the role of carnivorous zooplankton in the Arctic and in particular to determine if they exert a predation control on the large dominating calanoid copepods Calanus spp. Copepods are the numerically dominant zooplankton in the sea. Copepods are of primary importance in marine ecosystems because they form a direct link between phytoplankton and many carnivorous zooplankton, fish and whales.

Two groups of carnivores were analysed: chaetognaths and carnivorous copepods. Chaetognaths, commonly called arrow worms, are common in all oceans. They are elongated and transparent and their length varies with species from 3 mm to over 100 mm. All chaetognaths are carnivorous and they are important zooplankton predators. The carnivorous copepod Pareuchaeta norvegica is a common constituent of the zooplankton in many oceanic systems and its length is 5-7 mm. P. norvegica prey upon small copepods and fish larvae.

Chaetognaths were analysed for gut content and feeding by carnivorous copepods was assessed by measuring egestion of faecal pellets. Simultaneously, predator and prey distribution was determined in the field and used to estimate predation impact. The work was conducted during three cruises in West Greenland.

I made cutting edge contributions to the knowledge of carnivorous zooplankton in Arctic waters. Results revealed that small copepods were the main prey for the two species of chaetognaths (Eukrohnia hamata and Sagitta elegans). Comparing prey abundance and gut content for E. hamata shows that some specific copepods were selectively preyed. The number of prey in the guts of the chaetognaths was among the highest recorded and they fed 0.5 to 1 prey per day. Chaetognaths are often considered to be dominant predators, but the impact by the copepod Pareuchaeta norvegica exceeded chaetognath predation 10 fold. The gut content of P. norvegica ranged from 2 to 7 prey per individual. The prey production rate was low at all times and the predation impact of P. norvegica was in some areas 20% of the copepod population production. Such strong predation is likely to have effects on population dynamics of copepods and illustrate the importance of this species in the pelagic ecosystem. The investigations in Greenland have and will result in several publications.