The role of solar radiation in structuring the shallow benthic communities of boreal forest lakes

We have attempted to quantify the roles of physical, chemical, and biological interactions structuring the attached algal and invertebrate communities in eight boreal lakes at the Experimental Lakes Area, northwestern Ontario, varying in dissolved organic carbon (DOC) concentrations (3-9 mg L-1). Attached benthic communities on rocky, south-facing shores were sampled at 0.1, 0.3, 0.7, and 1.5 m depths. Multivariate redundancy analysis explained 58% of the variance in algal communities and 75% of the variance in invertebrate communities. Algal composition was determined most by photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) fluxes, whereas the composition of associated invertebrates was driven by a combination of exposure to solar radiation and food quality and quantity. Communities on rock surfaces exposed to high solar fluxes in these lakes had very high concentrations of a photoprotective scytonemin-like pigment (SLP-A), and were dominated by filamentous green algae. Conversely, deeper, more shielded communities were dominated by diatoms and had low concentrations of SLP-A. This suggests that previous paleolimnological evidence of SLP-A increases might reflect changes in littoral attached algal communities rather than planktonic ones. Decreases in DOC concentrations as a result of climate warming will result in increased penetration of solar radiation in boreal lakes. As a result, in the event of decreased precipitation, we predict that the dominance of UVR-resistant epilithic algae in littoral zones will occur in deeper waters than before, with coincident reductions in densities of many invertebrate taxa in algal biofilms increasingly exposed to high solar fluxes.