Coarse-Scale Horizontal Distribution of Phytoplankton in a Semi-Enclosed Coastal Area

Abstract. The horizontal distribution of phytoplankton in relation to changes in the physico-chemical conditions of a eutrophic semi-enclosed embayment is described. Phytoplanktonic populations derived from a station interval of 6 miles (coarse scale variations) were compared by quantitative (standing stock) and qualitative (species composition, diversity, Diatom/Flagellate ratio) criteria. The results showed that quantitative and qualitative parameters differed in their sensitivity and were affected differently by environmental perturbations. Along a horizontal transect with defined variations in nutrient concentrations, changes in standing stock levels were not significant, but differences in species composition existed and caused coefficients of variation between stations for species diversity up to 50% and of the Diatom/Flagellate ratio up to 120%. Furthermore, distributional differences among certain flagellate species (Prorocentrum micans, Prorocentrum balticum) and diatom species (Nitzschia closterium, Chaetoceros curvisetus) were recorded which showed that the flagellate species were distributed more uniformly than the diatoms along a transect with progressive decrease in nutrient concentrations.