Interactions between light and wave exposure differentially affect epilithic algal biomass and productivity in two large lakes of different trophy

Light and nutrient availability determine the depth distribution of littoral attached algae, but erosional forces, such as wind waves, structure shallow, nearshore habitats. We measured the biomass and productivity of algae on cobbles (epilithon) in the wave-influenced erosional zones of two large lakes with contrasting trophic status, eutrophic Lake Erie and oligotrophic Lake Huron. We used fetch and wind speed to create an index of wave exposure for 10 sample sites and used regression models to determine the variation in algal biomass and productivity explained by physicochemical parameters. Lake Erie epilithon had greater biomass (average = 59–249 mg chlorophyll a m−2) and higher productivity (76–92 mg C m−2 hr−1) than Lake Huron epilithon (1–147 mg m−2 and 14–57 mg C m−2 hr−1, respectively). In Lake Erie, light and wave exposure had significant and opposite effects on algal biomass and productivity. Maximum algal biomass occurred between 1.5 and 3 m in Lake Erie, where the effects of waves begin to diminish and light was still relatively abundant. Conversely, Lake Huron biomass was uniformly low along the depth gradient. Although depth or wave exposure did not explain variation in biomass, Lake Huron epilithic productivity was best explained by the interaction between depth and wave exposure. Rocky nearshore areas in large lakes are dynamic habitats for attached algae, leading to high spatial variation in biomass and productivity. Understanding how physicochemical dynamics influence benthic algal biomass and productivity are necessary to predict their distributions in a changing world.