Algal blooms in high turbidity, a result of the conflicting consequences of turbulence on nutrient cycling in a shallow water estuary

The fate of inorganic nutrients was studied together with the organic composition and chloropigment content of suspended particles in a shallow and well mixed estuary (Great Ouse, North Sea, England) from March 1990 to January 1991. Monthly sampling was made at high water neap tide yielding comparisons during a one year cycle. During early spring and summer phytoplankton development occurred inside the turbid estuary, chlorophyll a concentrations peaking above 100 μg l −1 , and nutrients displayed a non conservative behaviour related to the biological uptake. During autumn and winter the situation reversed, with low chlorophyll a concentration and more conservative behaviour of nutrients in the estuary. The reasons for the occurrence of phytoplankton blooms in turbid environment and their relation with turbidity and light-penetration are discussed. This study demonstrates that despite a thin euphotic zone and a totally mixed water column there is some convincing evidence for primary production development in shallow water estuary. It is possible to indeed rationalize the phytoplankton behaviour in terms of optical depth mixing approach. Photosynthetic uptake of nutrient inside estuaries may be of considerable important for understanding the fate of anthropogenic nutrients to the offshore waters, in this case the North Sea.