PHYTOPLANKTON BIOMASS, LIGHT ATTENUATION AND MIXING IN THE SHANNON ESTUARY, IRELAND

Distributions of suspended matter, light attenuation and chlorophyll a were measured in the Shannon Estuary over the period 1988-1990. Light attenuation was found to be highly correlated with levels of suspended matter, and the availability of underwater irradiance was found to be the dominant influence on phytoplankton biomass and chlorophyll levels over most of the estuary. Though levels of suspended matter, and hence light attenuation, were highest in the upper estuary, depths of mixing were relatively shallow resulting in less light limitation of phytoplankton growth than elsewhere. Turbidity maxima in the upper estuary were associated with chlorophyll maxima, the magnitude of which appeared to be related to river discharge.