Upwelling and its Influence on Nutrient Concentration in the Coastal Area of the Hanko Peninsula, Entrance of the Gulf of Finland

Upwelling was found to be a rather common phenomenon in the coastal area of the Hanko Peninsula. Intensive hydrographic and current measurements were made during 10 upwelling episodes in 1987-88. A clear relationship was noted between wind and current direction, and wind direction and change in salinity at 35 m depth during moderate or strong winds. Upwelling was observed as a sudden increase in salinity (ΔS 1) at 35 m depth and as a slight increase in salinity in the surface layer. When thermal stratification prevailed, the surface layer temperature dropped by as much as 10 °C, and nutrient concentrations increased; that of PO4-P from 1-2 μg PO4-Pl-1 to 4-8 μg PO4-Pl-1 and that of NH4-N from 0•5-2 μg NH4-Nl-1 up to 20 μg NH4-Nl-1. During upwelling, a strong offshore flow was observed in the surface layer and a compensated onshore flow in a lower layer. In the study area the coastline followed the east-west axis and the upwelling occurred when the wind was from the directions between 190°-280°, and the wind impulse was at least 5000 kg m-1 s-1 (50 h of moderate wind). The minimum wind impulse required for upwelling seemed to be inversely proportional to the power of 5-10 of buoyancy. This means that during thermal stratification the necessary wind impulse is fairly constant and that the more homogeneous the water column is the more wind energy is needed (up to 14 000kg m-1 s-1).