Long-term changes in zooplankton and the climate of the North Atlantic

Long-term variations in zooplankton abundance in the north-east Atlantic, the North Sea, and in freshwater UK lakes are investigated by means of the Continuous Plankton Recorder survey and the Windermere and Esthwaite lakes data. Inter-annual variability of plankton abundance in these data sets shows strong correlation with two modes of climatic variability in the North Atlantic: the latitudinal shifts of the north wall of the Gulf Stream and the North Atlantic Oscillation. Detailed analyses reveal that the connection between environmental forcing and plankton response depends on various mechanisms, i.e., timing and intensity of the spring phytoplankton bloom resulting from changes in stratification levels, changes in temperature, and, in the case of the copepod Calanus finmarchicus, advection of the population into the North Sea at the end of the winter season. Future attempts to predict changes in marine ecosystems on the basis of climate scenarios will require focusing major effort on biological–physical modelling and large-scale plankton population ecology. The maintenance of long-term monitoring programmes is also essential to determine whether the climate–plankton connections observed during several decades will persist in the future or will be overruled by other mechanisms and principally human-induced perturbations.