A critical comparison of two long-term zooplankton time series from the central-west North Sea

Long-term changes in relative and absolute zooplankton abundance and species composition mere compared between the Dove Marine Laboratory and Continuous Plankton Recorder (CPR) time series in the central-western North Sea. The two time series employ different sampling mechanisms, with the Dove series being obtained by net sampling at a fixed station off the Northumberland coast, while the CPR series utilizes a network of fixed towed routes. It was found that there was a significant correlation (r = 0.64, P < 0.001) between the relative year-to-year fluctuations of the two series and a significant agreement in the pattern of year-to-year variations in species composition of the dominant taxa (P = 0.01) over the majority of the time period. However, examination of absolute zooplankton abundances found large differences between the two time series. Differences in mesh size and in the sample processing methodologies were not wholly responsible for these. Model-derived catching efficiencies for the two sampling devices suggested that differences in absolute abundances may have been mainly due in same tara to a greater degree of active avoidance of the CPR sampling device by zooplankton, although it is likely that passive avoidance ar a result of hydrodynamic factors has a role.