Solar ultraviolet radiation (UVR; 290-400 nm) is primarily attenuated by dissolved organic carbon (DOC) in boreal lakes. DOC is a mixture of organic molecules originating from in-lake metabolic processes (clear; autochthonous), or degradation of organic litter in a lake’s catchment (brown; allochthonous). I have used mesocosm and whole-lake experiments at the Experimental Lakes Area (ELA) to study mechanisms of change in properties of DOC, and the effects of these changes on underwater UVR exposure.

UVR was the primary factor in DOC bleaching. Hydrogen peroxide (H2O2; 700 nM), photochemically produced via photo-Fenton reactions when DOC absorbs UVR, also bleached DOC independently of UVR. Unlike UVR, H2O2 can bleach DOC throughout the mixed layer. DOC in H2O2-treated and UV-shielded enclosures became more autochthonous-like, and DOC in UV-exposed enclosures and surrounding Lake 239 remained primarily allochthonous, despite bleaching. Some important species of phytoplankton were negatively sensitive to UVR. However, chlorophyll and algal biomass were highest in UV-shielded, H2O2-treated enclosures, indicating that UVR and H2O2 had independent and opposite effects on phytoplankton. Increased algal nutrient availability may have occurred as a result of altered nutrient chemistry or differing trophic sensitivities to H2O2.

Whole-lake acidification also caused a switch in DOC quality from allochthonous to autochthonous-like in Lake 302S. This acid-induced change was due to increased chemical oxidation or precipitation or UV-absorbent aromatic portions of allochthonous DOC molecules, leaving relatively UV-transparent aliphatic chains behind. Acid-induced reduction of DOC concentration and colour increased UVR exposure within Lake 302S by up to ~3000% during peak acidification. Three – to 10-fold increases in photoprotective algal pigments were observed with acidification. A survey of 8 lakes suggests that increased UVR exposure plays an important role in structuring shallow benthic algal communities, especially after acidification, including changing assemblages, production of photoprotective pigments, and respiration. In contrast, invertebrate communities appear to reflect the characteristics of the algal mats they inhabit, depending more on food quality and perhaps availability of shelter from UVR.

These physical, chemical, and biological interactions suggest that changes in the nature of DOC and increased UV-transmittance, that follow acidification, ozone depletion, and drought, may have important roles in biogeochemical processes within lakes.