Long-term changes in zooplankton biomass concentration and mean size over the Northwest European shelf inferred from Continuous Plankton Recorder data

Data from the Continuous Plankton Recorder (CPR) survey over the past 40 years have shown that the abundance of copepods in many parts of the North Atlantic has declined, indicating geographical shifts in the plankton communities. Because the CPR does not sample all zooplankton species with equal efficiency, these observations may give a biased view of the overall changes. Here, we compensate for CPR undersampling by using previously published species-specific correction factors derived from comparisons of catches made with WP-2 ring-nets and the CPR. Based on such corrected data, the southern North Sea showed the highest concentrations of biomass, in contrast to maps based on uncorrected data, in which the areas of highest biomass were in the northern North Sea. Trend analysis confirmed the previously reported general decrease of total biomass. There has also been a general decrease in the mean size of zooplankton over time in the northern North Sea, but this has not been observed elsewhere. The results indicate the importance of smaller zooplankton species in the ecology of the Northwest European shelf. The changes in community structure may have general implications for energy transfer efficiency to higher trophic levels, and for the sustainability of fisheries resources