Phytoplankton ecology of the larger Scottish lochs

Summary

Lake phytoplankton biomass and species composition of 15 of Scotland's larger lochs are viewed here as major responses to the interplay between eutrophication pressures from the catchment, and to a suite of physical, chemical and biotic attributes of the waters. Pressures varied considerably: from Morar to Lubnaig in terms of catchment-to-loch area ratio; from Awe, Katrine, Morar and Ness to Leven on the basis of losses of phosphorus from the catchments; and from St Mary's down to Shiel on the basis of altitude. Values for factors determining the impact of the pressures also range over an order-of-magnitude: loch mean depth from the shallowest (Leven and Menteith) to the deepest (Ness and Morar); predicted annual mean total phosphorus levels (highest in Leven and lowest in Morar, Shiel and Katrine); theoretical hydraulic flushing rate (lowest in Katrine and highest in Veyatie). Rankings based on these features predicted tolerably well the relative standings of the lochs in terms of overall phytoplankton biomass — with mean annual chlorophyll concentrations from c. 0.5 μg l^-1 in Morar to 100 μg 1 ^-1 in Leven — although the algal crops in Leven depend very much on Daphnia population densities. The relative likelihood of eutrophic or oligotrophic species predominating was also reasonably well anticipated. A comparison of these rankings with those based on the amounts of chlorophyll observed per unit loading of the major limiting nutrient (P) showed that Leven and Menteith are the most efficient at converting the nutrient resources into biomass (respectively, up to 100, and c. 60 mg chlorophyll_a m^-3 g^-1 P m ^-2 y^-1), with the clear waters Katrine and Morar, and the humic-stained Tay belying their otherwise oligotrophic or dystrophic character. These contrast with values of 0.6–2.5 mg chlorophyll_a m ^-3 g^-1 P m^-2 y^-1 obtained for Ness. Reference to algae recorded at the turn of the century show that Leven has been markedly enriched. However, shifts are also indicated for Earn and Lubnaig which are susceptible to enhanced inputs of bio-available P from fish-farms, and for Katrine.