PI curves, photosynthesis (P) versus irradiance, are shown for the bottom layer sample (triangles) and the layer of the second highest sample (circles) of fast ice at St.3 in the Gulf of Finland. The corresponding fits were obtained using MATLAB and used to calculate the photosynthetic efficiency α (shown), maximum photosynthesis, P_max (shown) and quantum yield of photosynthesis Φ (not shown).