Optical and hydrographic consequences of freshwater run-off during spring phytoplankton growth in a Scottish fjord

A combination of in situ measurements and radiative transfer modelling were used to study optical conditions in the inner basin of Loch Etive, a Scottish fjord, in March and April 2000. The basin was strongly stratified with three layers separated by marked pycnoclines. The surface layer averaged 5 m in depth and was heavily stained with coloured dissolved organic matter (CDOM) which reduced the euphotic depth to between 7 and 10 m. Approximately 20% of the photosynthetically available radiation (PAR) in the water column was absorbed by phytoplankton, 44% by CDOM and 36% by sea water. Detectable concentrations of the major inorganic nutrients (nitrate, phosphate and silicate) occurred at all depths, but significant phytoplankton populations (averaging 6 mg chlorophyll a m(-3)) were found only in the reduced-salinity surface layer. The freshwater input therefore acted both as a source of buoyancy which promoted phytoplankton growth near the surface and as an attenuator of visible light which inhibited growth deeper in the water column.