Processes responsible for the formation and maintenance of sub-surface chlorophyll maxima in the Gulf of Finland

Vertical cross-sections of temperature, salinity and Chl a fluorescence distributions in the Gulf of Finland were mapped on 11, 19–20 and 25 July 2006. The sub-surface Chl a maximum layers with thickness varying between 1.5 and 9 m and intensity up to 7.6 μg l−1 were observed in the lower part of the seasonal thermocline within the depth range of 14.5–35 m. Nutrient (PO43−, NO2− + NO3−) analyses of water samples collected from the thermocline revealed the coincidence of the location of Chl a maxima and nutriclines. We suggest that the observed Chl a maxima were formed by dinoflagellate Heterocapsa triquetra capable for vertical migration and nutrient uptake in dark. The upward flux of nutrients caused by estuarine circulation and vertical turbulent mixing created favourable conditions for the formation and maintenance of sub-surface Chl a maxima. We explain the observed horizontal patchiness of sub-surface Chl a maxima by meso-scale processes – by the accumulation of phytoplankton along the depressed isopycnals at the base of anti-cyclonic circulation cells and by the horizontal convergence of waters in the downwelling area.