Evidence of alternative states in freshwater lakes: A spatially-explicit model of submerged and floating plants

Freshwater systems provide iconic examples of ecological tipping points. In some lakes and ponds, changes to nutrient levels can drive sudden shifts between primary producer communities dominated by either submerged or floating plants. Several models, ranging in complexity, have been developed to understand the interaction between these primary producer groups. Previous field studies suggest that spatial (e.g., water body size) and temporal (e.g., seasonality) processes are important for the dynamics of this system in nature, but these processes cannot be included in most models without a significant increase in model complexity. Therefore, I developed a spatially- and temporally-explicit model of this system with moderate model complexity that extends a previous model, in which alternative states are known to occur. I found that under low (approximately ≤2mg total nitrogenL−1) or high (approximately ≥6mg total nitrogenL−1) nutrient levels, either submerged or floating plants dominated, respectively. At intermediate nutrient levels, simulations resulted in different final plant states, depending on the initial cover of floating and submerged plants, providing evidence for alternative states. Under most conditions, stable intermediate states were uncommon. Water body size had a large effect on the dynamics of the system, as observed in the field, but only if wind strengths increased with water body size and there was a prevailing wind direction. Surprisingly, species composition and trait diversity did not appear to have major effects on the final plant states. This model allows the integration of processes on multiple scales of biological organization, from species traits and composition, to climate and seasonality or ecosystem-level properties, and it complements the growing realization that spatial context has significant impacts on the dynamics of alternative states in nature.