Monitoring marine plankton ecosystems. I: Description of an ecosystem approach based on plankton indicators

A procedure is presented that allows monitoring of the dynamic regime of pelagic ecosystems in the North Atlantic Ocean. It uses a diversity index (mean number of species per continuous plankton recorder [CPR] sample) and species assemblage indicators calculated from all calanoid copepods (108 species) identified by the CPR survey since 1958. In any region of the North Atlantic covered by the CPR survey, the procedure calculates the values of these ecosystem indicators using a constant number of samples, selected randomly, and checks any bias associated with the spatial and temporal heterogeneity of the sampling. The procedure is described and illustrated in this study using CPR data in the central North Sea. The usefulness of this tool is demonstrated by a study of the sensitivity of the indicators to the number of CPR samples employed in the estimation of monthly or annual means. Firstly, it is shown that on an annual scale the procedure improves the quality of estimations. However, when long-term changes of a plankton indicator are examined for a particular month, it is crucial to use an appropriate number of CPR samples to detect a significant link between the biological and hydro-climatic environment. Secondly, the importance of using species assemblage indicators is demonstrated. They allowed the detection and the ecological interpretation of both episodic events and the regime shift in the central North Sea. It is shown that the regime shift in the North Sea had a strong impact on plankton community structure and was marked by an increase in diversity related to a progressive increase in warm-water species and a decrease in cold-water species. This study also has important implications for researchers using the CPR data. It demonstrates the robustness of the CPR data, but on the other hand also shows that the spatial and temporal heterogeneity should be assessed with care before interpreting the long-term changes of a CPR-derived plankton indicator.