Macrophyte growth forms shift along the trophic gradient of lakes

A shift in phototrophic organisms occurs along lake trophic gradients characterized by a change from macrophyte- to phytoplankton-dominated states. Before lakes reach a turbid and phytoplankton-dominated state, shifts from meadow-forming Characeae to canopy-forming macrophyte species can occur where Characea are present, especially in oligotrophic to mesotrophic lakes with sand or gravel substrate. However, eutrophication intensity causing this shift has not previously been examined. We analysed data from 132 lakes located in Mecklenburg-Vorpommern (Germany) with a generalized linear model (GLM) and random forest (RF) models complemented with grid approximation to determine if (1) species richness of macrophytes declines along eutrophication gradients, (2) the chlorophyll <i>a</i> (Chl-<i>a</i>) concentrations above which the abundance of Characeae declines, and (3) the Chl-<i>a</i> concentrations above which the abundance of canopy-forming species declines. The number of macrophyte taxa declined gradually following a log-linear trend and with increasing Chl-<i>a</i> concentrations. Based on the RF models, the abundance of Characeae decreased abruptly at 5–13 µg L⁻¹ Chl-<i>a</i>, whereas canopy-forming species showed a monotonous and slight unimodal decreasing response at 35–103 µg L⁻¹ Chl-<i>a</i>. The results support the theory of shifts in growth forms along eutrophication gradients in lakes and provide, for the first time, estimates of Chl-<i>a</i> concentrations required for these shifts. Changes in growth forms are obvious indicators for eutrophication and can serve as an additional incentive to improve lake trophic status.