Spatial, seasonal and long-term fluctuations of plankton in relation to hydroclimatic features in the English Channel, Celtic Sea and Bay of Biscay

Plankton collected by the Continuous Plankton Recorder (CPR) survey were in vestigated for the English Channel, Celtic Sea and Bay of Biscay from 1979 to 1995. The main goal was to study the relationship between climate and pl ankton and to understand the factors influencing it. In order to take into account the spatial and temporal structure of biological data, a three-mode principal component analysis (PCA) was developed. It not only identified 5 zones characterised by their similar biological composition and by the sea sonal and inter-annual evolution of the plankton, it also made species asso ciations based on their location and year-to-year change. The studied speci es have stronger year-to-year fluctuations in abundance over the English Ch annel and Celtic Sea than the species offshore in the Bay of Biscay. The ch anges in abundance of plankton in the English Channel are negatively relate d to inter-annual changes of climatic conditions from December to March (No rth Atlantic Oscillation [NAO] index and air temperature). Thus, the negati ve relationship shown by Fromentin & Planque (1996; Mar Ecol Frog Ser 134:1 11-118) between year-to-year changes of Calanus finmarchicus abundance in t he northern North Atlantic and North Sea and NAO was also found for the mos t abundant copepods in the Channel. However, the hypothesis proposed to exp lain the plankton/NAO relationship is different for this region and a new h ypothesis is proposed. In the Celtic Sea, a relationship between the plankt omic assemblage and the air temperature was detected, but it is weaker than for the English Channel. No relationship was found for the Bay of Biscay. Thus, the local physical environment and the biological composition of thes e zones appear to modify the relationship between winter climatic condition s and the year-to-year fluctuations of the studied planktonic species. This shows, therefore, that the relationship between climate and plankton is di fficult to generalise.