Submerged macrophytes are key components in many freshwater and marine ecosystems, contributing to ecosystem functions and services. In temperate shallow lakes, spring epiphyton shading can be decisive for submerged macrophyte development, potentially leading to macrophyte collapse and a shift to undesired, turbid conditions. Global change can alter epiphyton phenology, however, the consequences for submerged macrophytes and their stabilizing effects on clear-water conditions remain to be elucidated. Based onInformed by field data, we propose a general epiphyton shading phenology for submerged macrophytes in temperate shallow lake ecosystems. We express the temporal dynamics of epiphyton shading in terms of onset and relative increase (slope) of epiphyton development as well as epiphyton grazing impacts (onset, duration) using a Boltzmann function. This function is added to the ecosystem model PCLake+ as a customizable, macrophyte-specific shading factor. We then assess how changes in the epiphyton phenology and the presence of grazing on epiphyton affects submerged macrophyte biomass in a generic temperate shallow model lake under control and warm winter scenarios. The model results from the model provide a proof-of-concept that epiphyton shading can provoke macrophyte loss and shifts between alternative equilibria. Threshold values for critical shifts depend on epiphyton shading phenology. Earlier onset and longer duration of grazing can maintain macrophytes in nutrient or climate conditions under which they would otherwise collapse. Our results show the pivotal importance of epiphyton phenology in determining lake ecosystem-wide responses stressing the need for better incorporation of epiphyton into both models and monitoring.

Here we present the data resulting from transient runs of the adapted PCLake+ model (25 years) which were generated by varying parameters of the Boltzmann shading curve (by shifting the midpoint), by introducing grazing periods with a) different onsets, and b) different duration, and by running two temperature scenarios (control and warmer winters). The resulting time series were aggregated for variables submerged vegetation and pelagic phytoplankton chlorophyll-a to a) cumulative biomass developed in the macrophyte growth period, and b) years till collapse of macrophytes in the modelled lake. These data were used for plotting the figures published in the paper: "Periphyton phenology determines stability of submerged vegetation dominance in temperate shallow lakes"