Patterns of abundance for Calanus and smaller copepods in the North Sea: time series decomposition of two CPR data sets

Abstract

We present a time series analysis of 34 yr of continuous plankton recorder (CPR) data for 2 taxa: 'total small copepods' and Calanus copepodite stages 5 and 6 in the North Sea. Each was resolved into spatial averages over areas which correspond to the hydrographic regimes of the North Sea as defined by ICES. An iterative method which enables reliable determination of the long-term trends, mean annual cycles and confidence bounds for the monthly means is presented. We find that a purely time-dependent (i.e. trend plus seasonality) model can explain in excess of 70% of the variance in the (log-transformed) data. Nonetheless, the correlation structure in the residuals is indicative of the additional influence of past abundances upon subsequent dynamics. The residuals of many of the adjacent regions within the North Sea exhibit significant cross correlations at lag zero. This suggests that stochastic events can influence zooplankton dynamics over very large areas. We found no evidence that the zooplankton dynamics in up-current regions drive the dynamics of their downstream neighbours. In contrast, hydrographically similar regions tend to share a common seasonal dynamic even when they are not adjacent. KEY \AIORDS: CPR data Time series analysis of spatially resolved data Trend . Seasonal cycle . Autocorrelation . Cross-correlation