An experiment in phytoplankton ecology and applied fishery management: effects of artificial aeration on troublesome algal blooms in a small eutrophic loch

Abstract. A sharp rise in the concentration of soluble reactive phosphorus antl a marked decrease in the level of nitrate were associated with intermittent thermal stratification and localized de-oxygenation in Coldingham Loch (Berwickshire, Scotland) in summer. These conditions apparently favoured plunktonic blue-green ‘algae’ (Cyanobacteria) such asAphani-zomenon, which grew to bloom proportions and threatened the trout angling fishery for which the loch is managed. Various measures for ameliorating the situation are discussed and two bubble-gun units were installed in the deepest part of the loch (ca 12m), to distribute compressed air and mix the column. The installation was operated continuously during the periods May-September of 1984 and 1985. Temperature and chlorophyll measurements indicated good vertical and horizontal mixing. In spite of virtually no effect on rates of release of phosphorus from the sediments, mixing prevented surface bloom formations of Aphanizomenon and reduced its biomass; maxima of 17 and 52m filament 1^?1 in 1984 and 1985 respectively contrast markedly with peaks of 130 and 630m 1^?1 recorded in 1983. Total phytoplankton abundance was also lower. In 1984, the main difference occurred at the ‘critical’ late-summer period with chlorophyll concentrations of ca 20 μg 1^?1 being only one-half of those measured at the same time in 1983, and Secchi disc transparencies of ca 3·0 m compared to values of ≤ l·5min 1983. An even greater response was recorded in 1985; throughout May to September, chlorophyll levels rarely exceeded 13μg 1^?1 while water transparency usually exceeded 3·0m.