Dominance of charophytes in eutrophic shallow lakes : when should we expect it to be an alternative stable state?

Submerged plant dominance and a turbid state with few submerged plants have been hypothesized to represent alternative stable states in eutrophic shallow lakes. Here, we analyze the conditions for occurrence of alternative stable states in shallow lakes further, using Charisma, a simulation model describing the growth of Chara aspera. The model includes seasonality and spatial structure, aspects which were absent in earlier models predicting alternative equilibria. The parameterization of the model is largely based on experimental results and field observations. Over a range of conditions, the model does indeed predict alternative stable states. The range of conditions over which alternative stable states exist, appeared most sensitive to the assumed reduction of local turbidity by plants and the maximum growth rate of the plants. Aboveground biomass disappears during winter in most lakes in temperate regions. Our analysis indicates that from an evolutionary perspective there is an optimum biomass allocation strategy with respect to investment in overwintering structures. Too little investment reduces chances to regain dominance in the subsequent year, whereas too much investment in dormant overwintering structures such as seeds and tubers reduces photosynthesis. Interestingly, the optimal investment is lower for obtaining maximal summer biomass than for realizing the maximum stability of the vegetated state. The model also suggests that a short clear-water phase enhances the probability of vegetation survival. The optimal timing for a clear-water phase is at the end of May or in June, as is indeed the case in many lakes. In line with earlier theory, shallow lakes with a `flat' depth profile are predicted to have the strongest hysteresis. In lakes with a depth gradient, the response to changes in turbidity is predicted to depend strongly on horizontal mixing of the water between vegetation stands and the open water. Hysteresis disappears predominantly due to a strong horizontal mixing of water. In case of little mixing, on the other hand, local alternative stable states are predicted to occur