The unstable seasonality of Calanus Finmarchicus in the Fair Isle Current

Seasonal variation in the abundance of marine plankton taxa is commonly expressed in the form of ‘mean annual cycles', in which the mean abundance over several years for a particluar season is plotted against the time of year. Typically, such mean annual cycles are used to compare patterns between different spatial locations. We wish to argue, however, that this can be very misleading since the shape of the underlying seasonal cycle itself may change with time as well as location. Thus, it is possible that the mean annual cycle may never actually occur in the environment in any location, nor need it characterize the underlying cyclicity in any one year. Moreover, ignoring changes in the shape of seasonal cycles over time can obscure important demographic shifts in plankton populations. It is therefore essential to test statistically for any changes in the shape of annual cycles before assuming their stability through time. Here we report on continuous plankton recorder (CPR) data of the calanoid copepod Calanus finmarchicus in which there is a demonstrable change over time in the pattern of seasonal fluctuations in abundance, and we discuss briefly the demographic implications of this change.