Local effects of wind speed and direction on the phytoplankton of the Southern Bight

Abstract

The causes of the exceptionally early blooms of Ceratium spp. in the Southern Bight of the North Sea during the early 1970s are re-assessed. This localised event is found to coincide with a change in the meridional circulation over the North Sea in the late winter and early spring, with abnormally strong northerlies during the 1960s giving way to strengthened southerlies during the early 1970s. Such a change in wind direction can be expected (both from observation and simulation) to bring about a less widespread distribution of turbid coastal water in the Southern Bight and the marked reduction in windfetch may also have served to decrease watercolumn turbidity through its effect on the incidence of long-period swell. These processes may be reinforced by associated changes in the quantity of turbid runoff discharged by the Rhine. The advanced blooms of Ceratium spp. in the early 1970s are attributed both to these effects and to an increased frequency of light winds (decreased wind mixing) in the early months of the year.