Gravelines : première étude de surveillance du site. Description statistique et interprétation écologique des données

The objective of this report is to characterize the evolution of the pelagie coastal ecosystem (studied by sampling hydrobiological parameters, phytoplankton and zooplankton) off the Gravelines' nuclear power plant (french coast of the North Sea). The sampling program (contractually defined by the Centre National pour l'Exploitation des Océans and Electricité de France) is composed of several successive phases. The results obtained for the first three of them (september 1974 - december 1975 ; august 1976 - july 1977 ; january 1978- june 1979) are presented in this report, with special emphasis on the third one realized just before the power plant's activity started (march 1980). It is shown that, from 1974 to 1979, some subsets of the ecosystem (such as the zooplanktonic community) exhibit a stable dynamic structure, with however large quantitative interannual fluctuations.On the opposite, a much less accurate picture of the seasonal pattern of phytoplanktonic species succession is obtained. It is proposed to define a baseline for the system from the information obtained between 1974 and 1979, during which period a mean stationarity may be assumed. However, the temporal extent of our data basis is insufficient for ascribing interannual differences to some trend ; moreover, the implications of "empty boxes" in data basis are discussed, especially particulate matter and microzooplankton, which have been shown by many workers to be of crucial importance in the understanding of relationships between primary and secondary producers. From a technical point of view, the study uses widely inertia analysis methods (Principal components and correspondence analyses), conceived as a synthetic descriptive tool. The problem of preparation of tables to be processed (coding, transformation of data) is emphasized.