Alternate stable states and the shape of the lake trophic distribution

Ecological theory applied to small and shallow lakes suggests that water clarity and primary productivity may be bimodal, reflecting alternate stable states. This hypothesis was tested using remote sensing data. We used estimated Secchi disk transparency derived from reflected light measured by Landsat satellite photosensors. Lake trophic state indices (TSI) were estimated from the transparency estimates. Because alternate stable state theory is typically applied to productive lakes, we predicted that planktonic primary productivity would be especially bimodal in the small and shallow eutrophic lakes of southern Wisconsin. We found that overall, trophic state for over 8000 lakes in Wisconsin was multimodal (at least bimodal). Frequency distributions for lake size categories appeared to be distinctly bimodal. The largest and smallest lakes for all of Wisconsin had significantly bimodal curves, and southern (more productive) lakes had a more distinct multimodal curve than northern lakes. These results support the basic predictions of alternate stable state theory. The significantly trimodal distribution for southern lakes suggests that they may exist in more than just two alternate stable states.