Epiphyton phenology determines the persistence of submerged macrophytes: Exemplified in temperate shallow lakes

Abstract

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 on 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 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.