Macrophyte-dominated and turbid states of shallow lakes: Evidence from Alberta Lakes

Ecosystems may exist in more than one state. This provocative ecological theory was initially developed and addressed largely through modeling studies, but over the last 2 decades a number of diverse ecosystems have been shown to display alternate states. Here I present data from 30 lakes located in a relatively limited geographic region (Alberta, Canada) to seek evidence for alternate states in shallow prairie lakes. The lakes that I surveyed fell into the following three clusters based on their turbidity and maximum depth: (a) deep, relatively clear dimictic lakes, (b) shallow clear polymictic lakes, and (c) shallow turbid polymictic lakes. The shallow lakes had similar proportions of lake bottom supporting the growth of rooted submerged macrophytes, yet the macrophyte biomass was approximately five times higher in shallow clear lakes than in shallow turbid lakes. An analysis of covariance (ANCOVA) revealed that there was a significantly higher concentration of chlorophyll per unit of dissolved phosphorus in shallow lakes classified as turbid compared to those classified as clear. A separate ANCOVA also showed that the higher turbidity in the turbid lakes was not due simply to higher chlorophylla(a) concentrations; rather, it appears that increased sediment resuspension is the source of the higher turbidity seen in these lakes. Food web effects appear to be small. Although these shallow Alberta lakes contain no benthic fishes, as do many European lakes that have been observed over time, the general patterns are consistent with the theory that alternate states can exist in shallow lakes.