Contrasting responses of fish taxonomic and functional diversity to eutrophication in Danish shallow lakes

Shallow lakes may shift from a clear state dominated by submerged macrophytes to a turbid state dominated by phytoplankton under increasing nutrient stress. The implications of such an ecosystem state shift for fish taxonomic and functional diversity remain poorly understood. To address this question, we analyzed fish diversity in 96 Danish shallow lakes that varied greatly in nutrient levels and morphometry. We first fit a multiple linear model and used hierarchical variance partitioning to examine the relative importance of multiple environmental variables for changes in fish diversity metrics. Then we used the results to guide the construction of a piecewise structural equation model (pSEM) to quantify the direct and indirect effects of abiotic and biotic variables on fish diversity. Liner regression models demonstrated that the functional divergence of fish communities increased with chlorophyll a and decreased with reduced submerged macrophyte cover, whereas species richness and other diversity measures showed the opposite response. Chlorophyll a explained more variance in the selected fish diversity metrics than any other predictor. Loss of macrophytes due to eutrophication could lead to habitat homogenization and food resource simplification and, in turn, replacement of species belonging to the functionally diverse benthic and lithophilic species guilds by members of the more species rich but functionally redundant pelagic guild. Our study showed that functional divergence may better detect habitat degradation in lake ecosystems than taxonomic diversity, and it highlights the importance of macrophytes for maintaining the ecosystem functions of shallow lakes.