Harmful phytoplankton events caused by variability in the Irish Coastal Current along the west of Ireland

Abstract

Frequent sampling in summer along the western and northwestern coasts of Ireland showed the rapid onshore development of blooms of potentially harmful phytoplankton species. In both 1998 and 1999, concentrations of Gyrodinium cf. aureolum rose by four orders of magnitude to over one million cells per litre in Donegal Bay(northwestern Ireland) in less than 10days. The rapid development of these populations was linked to advection resulting from unfavourable wind-forcing of the Irish Coastal Current (ICG) which runs northwards along the western Irish coast. Current measurements showed that after a particular sequence of changes in wind direction phytoplankton populations could be rapidly advected from areas of slack circulation on the shelf via the ICC into aquaculturally sensitive coastal zones such as Donegal Bay. The model presented is similar to one already demonstrated for the occurrence of toxic events in the bays of southwestern Ireland. Other historical harmful events along the west and northwest coasts relating to substantial losses in both finfish and shellfish culture could also be explained using the model. These include the G. aureolum bloom of 1992, the Prorocentrum balticum bloom in 1997.