Is there light after depth? Distribution of periphyton chlorophyll and productivity in lake littoral zones

Periphyton and phytoplankton contribute to the base of lake food webs, and both groups of microalgae are influenced by resources and physical forcing. Spatial variation in light availability interacts with the physical dynamics of the water column to create predictable depth gradients in resources and disturbance that may differentially affect periphyton vs phytoplankton. We characterized the depth distribution of chlorophyll and productivity of periphyton on sediments (epipelon) and phytoplankton in the euphotic zones of 13 oligomesotrophic lakes that span a large size gradient (0.017–32,600 km2). Epipelic chlorophyll usually increased with depth in the epilimnion. Light was the primary driver of the consistent within-lake patterns in periphyton productivity across this lake-size gradient. In 5 lakes, epipelic periphyton exhibited a unimodal distribution of productivity with depth in the photic zone, but no evidence of photoinhibition was found for periphyton. Rather, patterns in sediment N and P and observed changes in biofilm structure were consistent with determination of epipelic biomass by disturbance at depths ≤1 m in the smaller lakes and by light limitation at depths >1 m. Further quantification of the effects of disturbance on epipelon is needed. Nonetheless, our data demonstrate that the perceived high spatial variability in periphyton biomass and productivity is not an impediment to development of robust models of whole-lake primary production that include both phytoplankton and periphyton.