Shallow lakes tend to exist in one of two ecological extremes, either a clear-water macrophyte dominated state, or a turbid-water phytoplankton dominated state. These states have been described theoretically through the concept of alternative stable states and stability landscapes. Our study focuses on a suite of shallow lakes from the Prairie Pothole Region of Minnesota, USA; a subset of the study lakes are currently in the clear state, and the others are either turbid or have been observed to be switching back and forth from turbid to clear states in recent years. We collected sediment cores from these lakes, lead-210 dated them, and looked at multiple downcore proxies of ecological condition including diatom community assemblage, biogenic silica, and total phosphorus. The goal of the study was to determine the historical state of these lakes and their stability prior to major landscape alterations.
Results/Conclusions
In this suite of shallow lakes from the Prairie Pothole Region we find that the diatom communities do not respond to short-term (1-10 years), recent changes between clear and turbid states. Instead, diatom communities in lakes that are currently turbid, or have been flipping back and forth from clear to turbid in recent years, show only one significant shift in the diatom community assemblage over the past 150-250 years; and lakes that have been clear over our period of observation show stability over the longer term. This suggests that short-lived shifts into the clear water state are governed by the long-term state of the lake following a regime shift, and that the ecology of the lake does not fully return to it’s pre-disturbance state during these temporary clear periods. This fits into ecological theory in that the state of these lakes seems to be inherently unstable once some threshold is crossed. These results also have important implications for lake management, where clear-water states in some of these shallow ecosystems have become only temporary, managed features.