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Optical and hydrographic consequences of freshwater runoff during spring phytoplankton growth in a Scottish fjord

McKee, David and Cunningham, Alex and Jones, K. (2002) Optical and hydrographic consequences of freshwater runoff during spring phytoplankton growth in a Scottish fjord. Journal of Plankton Research, 24 (11). pp. 1163-1171. ISSN 0142-7873

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Abstract

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.