Waterfowl, macrophytes, and the clear water state of shallow lakes
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The importance of lake ecosystems for waterfowl remains a topic of debate. In order to assess how temporal variations in lake features, specifically shifts between alternative stable states, may interact with the waterfowl fauna, we performed a long-term (22 years) study of the shallow Lake Krankesjön, southern Sweden. Lower total numbers of waterfowl occurred during periods with low macrophyte cover and turbid water, than when submersed macrophytes flourished and the water was clear. Some specific functional groups of waterfowl, such as herbivores, invertebrate, and fish feeders, showed a positive relation to clear water and high macrophyte cover. Hence, our data suggest that some migratory waterfowl may select lakes based on water quality, thereby adjusting their large-scale migratory routes. On the other hand, omnivorous waterfowl exhibited their highest abundances during turbid conditions. Furthermore, waterfowl not primarily relying on food from the lake showed no response to fluctuations in turbidity or macrophyte cover, but followed regional trends in population dynamics. In our study lake, L. Krankesjön, we estimated that waterfowl remove less than 3% of the macrophyte biomass during a stable clear-water state with lush macrophyte beds. However, during transition periods between alternative stable states, when macrophyte biomass is lower and the plants already stressed, the consumption rate of waterfowl may have a stronger effect on lake ecosystem functioning.
KeywordsWaterfowl Bird Alternative stable state Lake Macrophyte Herbivory Grazing
This study was partly funded by grants from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), the Swedish Research Council (VR), and the Linné Grant on animal migration (“CAnMove”; VR). The Swedish Bird Survey is supported by grants from the Monitoring Programme of the Swedish Environmental Protection Board. We are most grateful to the hundreds of volunteers who performed the census of the birds within The Swedish Bird Survey. We also thank Irmgard Blindow and Hans Källander for providing valuable input to this study, and Romi Burks for thorough editing of the manuscript.
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