Mixing by the strong tidal currents on the Faroe Shelf causes the shallower part of the shelf water to be vertically and horizontally almost homogeneous. This water mass is separated from the surrounding, seasonally stratified, waters by a tidal front. The system is maintained by a persistent anticyclonic circulation system of currents driven by tidal rectification. Since 1990, nutrient concentrations and phytoplankton biomass and species composition in the shelf water have been studied in a systematic monitoring programme. The observations demonstrate large variations. In some years, nitrate becomes almost totally depleted 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 dominate the phytoplankton through summer. The inter-annual differences are more likely caused by variations in primary production rather than by variations in nutrient inflow. Furthermore, variations in primary production are more likely caused by grazing, due to the large differences in zooplankton abundance, rather than by variations in light conditions or stratification as affected by winds and tidal currents.