The unstable seasonality of Calanus finmarchicus in the fair isle current

Seasonal variation in the abundance of marine plankton taxa is commonly exp ressed in the form of 'mean annual cycles', in which the mean abundance ove r several years for a particluar season is plotted against the time of year . Typically, such mean annual cycles are used to compare patterns between d ifferent spatial locations. We wish to argue, however, that this can be ver y misleading since the shape of the underlying seasonal cycle itself may ch ange with time as well as location. Thus, it is possible that the mean annu al cycle may never actually occur in the environment in any location, nor n eed it characterize the underlying cyclicity in any one year. Moreover, ign oring changes in the shape of seasonal cycles over time can obscure importa nt demographic shifts in plankton populations. It is therefore essential to test statistically for any changes in the shape of annual cycles before as suming their stability through time. Here we report on continuous plankton recorder (CPR) data of the calanoid copepod Calanus finmarchicus in which t here is a demonstrable change over time in the pattern of seasonal fluctuat ions in abundance, and we discuss briefly the demographic implications of t his change.