Comparison of changes in the annual variability of the seasonal cycles of chlorophyll, nutrients and zooplankton at eight locations on the northwest european continental shelf (1960–1994)

Abstract

Mean seasonal cycles in time series of salinity, nutrients, chlorophyll and Zooplankton at 8 locations on the Northwest-European shelf were analysed in relation to each other and with respect to their timing and magnitude during a period most of the time series overlap (1980–1984). A relatively late spring bloom in April/May with low chlorophyll values (2–4 mg/m3) is observed in the Irish Sea, off the Scottish east coast and the Channel entrance. An early spring bloom in March/April and relatively high chlorophyll values (7–12 mg/m3) are found in the Southern Bight of the North Sea and in the Skagerrak, whereas a late spring bloom in April/May with chlorophyll concentrations of about 23 mg/m3 is found along the Dutch and Belgian coast. The peak of the phytoplankton carbon cycle in the German Bight (Helgoland) occurs during summer. This is caused by the dominance of large dinoflagellates in this 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, compared to the Atlantic input signal represented by the data from the Channel entrance and east of Scotland, about 20% higher. The values of dissolved inorganic phosphate in January are comparable to those of the Atlantic input signal. However, the Irish Sea forms an exception, here the level is increased by 10–20%. The January values for nitrate and dissolved inorganic phosphate at the Dutch and Belgium coast are about 10 and 4, and in the German Bight (Helgoland) 4 and 3 times higher then the Atlantic input signal. At most sites changes in the seasonal cycles of chlorophyll coincide with changes in nutrient concentrations, whereas the maximum level of the seasonal signal of chlorophyll is related to nutrient levels.