PHYTOPLANKTON VARIABILITY ON THE FAROE SHELF

Mixing by the strong tidal currents on the Faroe Shelf causes the shal lower part of the shelf water to be vertically and horizontally almost homogeneous. This water mass is separated from the surrounding, seaso nally stratified, waters by a tidal front, The system is maintained by a persistent anticyclonic circulation system of currents driven by ti dal rectification. Since 1990, nutrient concentrations and phytoplankt on biomass and species composition in the shelf water have been studie d in a systematic monitoring programme. The observations demonstrate l arge variations. In some years, nitrate becomes almost totally deplete d in summer and the diatoms that dominate during the spring bloom are replaced by flagellates (e.g. Phaeocystis pouchetii). In other years, nitrate concentrations remain fairly high and diatoms continue to domi nate the phytoplankton through summer. The inter-annual differences ar e more likely caused by variations in primary production rather than b y variations in nutrient inflow. Furthermore, variations in primary pr oduction are more likely caused by grazing, due to the large differenc es in zooplankton abundance, rather than by variations in light condit ions or stratification as affected by winds and tidal currents. (C) 19 98 International Council for the Exploration of the Sea.