Annual variability in the seasonal cycles of chlorophyll, nutrients and zooplankton on the North-West European continental shelf

Seasonal cycles of salinity, nutrients, chlorophyll and zooplankton at 8 locations on the West-European shelf were analysed with respect to their timing and magnitude in the period 1980-1984. A late spring bloom with low chlorophyll values (2-4 mg.m-³) is observed in the Irish Sea, off the Scottish east coast and the Channel entrance. An early spring bloom and relatively high chlorophyll values (7-12 mg.m-³) are found in the southern Bight and the Skagerrak, whereas a late spring bloom with high chlorophyll concentrations (24 mg.m-³) is found along the Dutch and Belgian coast. In contrast to the other regions, the peak of the phytoplankton cycle in the German Bight (Helgoland) occurs in the summer period instead of the spring period. The peak in the yearly abundance of copepods shifts from May-June in the south to July-August in the north of the shelf. In the Irish Sea and the Channel entrance two seasonal copepods peaks are observed. The January nitrate values in Irish Sea, the southern Bight and the Skagerrak are about 20 % higher than those in the Atlantic input signal in the Channel entrance and east off Scotland. The January DIP values in these regions are comparable to those of the input signal, but the Irish Sea forms an exception here the level is increased by 10-20%. If compared with the Atlantic input signal the January values for nitrate and DIP at the Dutch and Belgium coast are about 10 and 4, and in the German Bight (Helgoland) 4 and 3 times higher, respectively. At most sites changes in the seasonal cycles of chorophyll coincide with changes in nutrient concentrations, wheras the maximum level of the seasonal signal is related to the nutrient levels