The phytoplankton ecology of the larger Scottish lochs

Summary

Lake phytoplankton biomass and species composition are viewed here as major responses to the interplay between eutrophication pressures from the catchment, and a suite of physical, chemical and biotic attributes (sensitivity factors) of the waters themselves. The relationship is explored for 15 of Scotland's larger lochs. With the exception of Leven and possibly Lubnaig and the Lake of Menteith, these are high quality waters. Pressures varied considerably: from Morar to Lubnaig in terms of catchment-to-loch area ratio; from e.g. 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: i.e. loch mean depth from the shallowest (Leven and Menteith) to the deepest (Ness and Morar); predicted annual mean total phosphorus levels (which agree tolerably well with measured values where these exist), are highest in Leven and lowest in Morar, Shiel and Katrine; theoretical hydraulic flushing rate is lowest in Katrine and highest in Veyatie. Rankings of the waters on the basis of these features predicted tolerably well the relative standings of the lochs in terms of overall phytoplankton biomass — with mean annual chlorophyll concentrations varying from c. 0.5 µg 1^-1 in Morar to 100 µg 1^-1 in Leven— with the latter depending 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 per g P m^-2 y^-1), with the clear waters Katrine and Morar, and the humic-stained Tay belying their otherwise oligotrophic or dystrophic facies. These contrast with values of 0.6–2.5 mg chlorophyll_a m^-3 per g 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 e.g. fish-farms, and for Katrine.