Impact of intensive cage fish farming on the phytoplankton and periphyton of a Scottish freshwater loch

摘要

Nutrients, phytoplankton and periphyton were monitored in a 71 ha shallow, unstratified lake used for intensive cage culture of rainbow trout. Inorganic nitrogen, ortho-phosphate and suspended solids were significantly higher near the cages and the bottom and, although declining during summer, nutrients did not reach levels which limit phytoplankton growth.Microcystis aeruginosadominated the phytoplankton, with surface chlorophyllareaching 189 µg l−1in August, but with no subsequent bloom collapse or deoxygenation. A sub-dominant community of ‘vernal’ diatoms andPediastrumspp. persisted. Periphyton was dominated byMelosira italica-subarctica. Algal species and water quality showed the lake to be highly eutrophic. Chlorophyll values predicted from a phosphorus-dependent eutrophication model agreed with observations but light limitation by self-shading and suspended farm wastes, aided by wind-induced turbulence, is believed to control algal growth rates and biomass. Implications for environmental management of intensive freshwater cage farms are dis