Effects of Eutrophication on Salmonid Communities in Oligotrophic Lakes

Abstract

Oligotrophic lakes respond to progressive eutrophication by a sequence of predictable events. Increased nutrient loads and subsequent increased plant production result in alterations in the abiotic environment, including changes in the color and transparency of the water, increased turbidity, oxygen depletion in the hypolimnion, and increased chemical stratification. The physico-chemical changes precipitate biotic changes among the phytoplankton, littoral algae, zooplankton, and benthos. The salmonid community may respond initially with an increased body growth rate in various taxa and a higher incidence of parasitism, but later inhibition of natural reproduction occurs, and finally, the taxa are replaced by others that can survive in the changed environment.A relation between natural nutrient loading (expressed in terms of a morphoedaphic index) and yield (both quantitative and qualitative) is proposed as an aid to determining the natural successional status of a lake. Knowing the natural baseline of a particular lake the fisheries managers can judge the nature and size of responses due to cultural nutrient loading and then alter the rate of cultural nutrient loading to modify the ecological effects, or they can use biological engineering to capitalize on the present conditions.Among the most important effects of eutrophication is the increased vulnerability of sedentary discrete stocks to changes in other stresses such as fishing.