Catchment Effects on the Horizontal Distribution of Phytoplankton in Five of Scotland's Largest Freshwater Lochs

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Abstract
(1) Lochs Morar, Ness, Shiel, Awe and Lomond were each visited on three occasions, when horizontal variations in surface temperature, conductivity and chlorophyll a fluorescence were measured along transects running the length of each loch. (2) The dominant species of phytoplankton behaved as passive tracers of the physical flow. Large-scale (> 5 km) patches of phytoplankton were produced by local differences in growth rate and not by wind-induced water movements. (3) In all the lochs, the development of phytoplankton patchiness was directly related to the development of chemical patchiness as measured by conductivity. The potential for chemical patchiness could, in turn, be related to geological and land-use differences between the sub-catchments surrounding each loch. (4) Some theoretical models of patch formation by differential production are discussed, and used to demonstrate the importance of advective rather than diffusive processes in inland waters. (5) The relative importance of catchment and hydraulic effects on the horizontal distribution of phytoplankton in the five lochs is discussed.