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RESEARCH PRODUCT

Can lake restoration by fish removal improve the status of profundal macroinvertebrate assemblages?

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Summary Removal of fish to restore lake pelagic food webs and water quality has been shown to increase the abundance of benthic invertebrates in shallow areas due to decreased predation pressure, while responses of profundal macroinvertebrate assemblages are less well documented. We used multivariate analyses and a Before-After-Control-Impact design to assess the impacts of fish removal (in total 101 000 kg of percid and cyprinid fish) on profundal macroinvertebrate species composition and abundance and ecological status in two basins of a eutrophic boreal lake. In the deeper main basin, macroinvertebrate community composition and classification of ecological status were not affected by fish removal, although changes in chironomid abundance and relative abundances of predominant oligochaete taxa were observed. By contrast, in the shallower basin, community response to fish removal was more pronounced and characterised by a marked decrease in abundances of pollution-tolerant chironomid and oligochaete taxa, resulting in change in community composition and improvement in the ecological status. Fish removal resulted in no change in trophic state of Lake Jyvasjarvi; hence, the mechanism(s) responsible for the observed changes in profundal assemblages are unclear. A decrease in turbidity and an increase in Secchi depth suggest that concentrations of suspended solids may have decreased, resulting in lower planktic microbial production, indirectly affecting macroinvertebrate community structure. Our results indicate that a 3-year fish reduction may have been too short to achieve complete recovery of the benthic communities, suggesting that prolonged food-web management together with a greater reduction in nutrient loading is needed.