Stability of alternative stable states in shallow lakes

Background/Question/Methods

Shallow lakes exhibit two alternative stable states of turbid with high phytoplankton abundance or clear with high abundance of submerged macrophytes. Alternative states are well documented, but little is known about the factors influencing the probability a lake will be turbid or clear and about stability of the contrasting states. We studied probability of alternative states and lake stability by sampling 123 shallow lakes across six ecoregions of Minnesota (USA) in July for three consecutive years (2009-2011). We measured total phosphorus levels (TP) and estimated abundance of phytoplankton (as chlorophyll a, hereafter chla), submerged macrophytes, and fish. K-means cluster analysis was used to classify lakes in each year as turbid or clear and to estimate probability of clear versus turbid. We also estimated lake stability by calculating coefficients of variation for macrophyte mass and chla across the three years. We then used an information-theoretic approach to identify the most parsimonious models for predicting probability of state and stability (coefficients of variation) using both chla and macrophytes for turbid and clear lakes. Candidate model variables were lake size and depth, ecoregion (forested versus prairie), TP, and summed abundance of planktivorous and benthivorous fish (fish).

Results/Conclusions

Across the three years, 75% of lakes were stable-clear, 8% were stable-turbid, and 17% shifted states. Probability of being clear was best predicted using a piece-wise regression model based on TP (r² = 0.49). Lakes had a 95% chance of being clear up to 90 ug/L TP, with probability declining to 20% at 520 ug/L TP. Stability of macrophyte mass in turbid lakes was inversely related to TP and fish biomass (r²= 0.60), and in clear lakes stability was highest in prairie lakes with low TP and fish biomass (r²= 0.17).  Chla stability in clear lakes was highest in low TP forested lakes (r²= 0.22), while chla stability in turbid lakes was highest in low TP lakes (r²= 0.75). Our results indicate TP levels are more closely related to likelihood of a clear state than fish biomass, but lakes persist in a clear state over a wide range of TP. Moreover, lake stability is also related to TP and secondarily to fish biomass. Interestingly, stability of both clear and turbid lakes was inversely related to TP. However, models explained more variance in stability of turbid lakes compared to clear lakes, leaving other factors influencing stability of clear states unknown.