Protozoan and metazoan zooplankton-mediated carbon flows in nutrient-enriched coastal planktonic communities

The objective of the present study was to study the dynamics of protozoan and metazoan zooplankton food webs in coastal NE Atlantic waters exposed to variable nutrient input. Data were derived from a mesocosm experiment (7 units, 40 m³, 12 m deep) receiving variable nutrient input. The food web included 3 autotrophic groups based on size, and 4 functional heterotrophic groups mainly based on trophic position. Inverse modelling was used to construct networks of carbon flows for the planktonic food web. Heterotrophic nanoplankton, microplankton and mesoplankton (HNP, CIL and COP, respectively) were found to be equally important contributors to grazing and carbon release during undisturbed summer situations. The release of dissolved organic carbon by zooplankton was comparable to that of phytoplankton. Autotrophic food was generally more important for zooplankton than heterotrophic (mean 75 %). Assimilation and growth efficiencies (AE and GE, respectively) of zooplankton groups in undisturbed situations were in the range of 33 to 69 % and 10 to 41 %, respectively. Values were inversely related to gross primary production (GPP). Sedimentation rates of carbon were low. High nutrient input rates increased food availability and most CIL and COP carbon flows. HNP did not respond, and neither did its food, that is, bacteria and picoautotrophs. The response in biomass was generally lower than that for the flows. Values of AE and GE of the zooplankton during high nutrient input and food availability varied between 11 and 29 % and 5.7 and 19 %, respectively, and throughout were lower than at low nutrient input. The sedimentation rate of particulate carbon increased strongly, resulting in an enhanced organic input rate in deep water.