Observations of a thin near surface layer in an estuarine environment: An exceptional bloom of the dinoflagellate Akashiwo sanguinea in the Lee estuary (Lough Mahon), Co. Cork, in September 2010

Observations on the occurrence of an exceptional phytoplankton bloom in the Lee estuary, Co. Cork, in September 2010 are reported. A thin layer of phytoplankton dominated by the dinoflagellate Akashiwo sanguinea (Dinophyceae), with a cell concentration of 16,900 cells ml−1 and chlorophyll a concentration of 680 μg l−1, was observed at a depth of 1 m. The layer extended over a horizontal distance of 3 km and was located in the estuarine pycnocline. Levels of dissolved oxygen supersaturation were elevated at 190 (%) saturation (15.9 mg O2 l−1 ) and the highest biochemical oxygen demand associated with the bloom was greater than 25 mg O2 l−1. Using biovolume data to calculate carbon content, and a water exchange rate of 0.06 d−1 and dilution factor of 0.1, the potential oxygen demand following bloom collapse was estimated to be 7.22 mg O2 l−1. The potential impact of this thin layer on the water quality of the estuary is discussed.