Photomineralization in a boreal hydroelectric reservoir: a comparison with natural aquatic ecosystems

In order to evaluate the role of photochemistry in the carbon dioxide (CO₂) generation from a 10-year-old boreal reservoir, the photomineralization of dissolved organic matter (DOM) was assessed and compared to a boreal river as well as to boreal and temperate lakes during July and August, 2003. Sterile water samples were irradiated by sunlight over the whole photoperiod and subsequently analyzed for CO₂. Mean energy-normalized apparent photochemical yield of CO₂ (an index of DOM photoreactivity normalized for the energy absorbed by samples) was significantly higher in the reservoir (27.7 ± 13.0 mg CO₂·m⁻³·kJ⁻¹) and the boreal river (35.8 ± 2.3 mg CO₂·m⁻³·kJ⁻¹) than in the boreal lakes (15.5 ± 5.1 mg CO₂·m⁻³·kJ⁻¹). The DOM photoreactivity of the temperate lakes (20.9 ± 8.1 mg CO₂·m⁻³·kJ⁻¹) was not statistically different from any type of boreal water bodies. There was no significant difference in either the integrated photoproduction of CO₂ (273–433 mg CO₂·m⁻²·d⁻¹) or the potential photochemical contribution to CO₂ diffusive fluxes (56–92%) among these water bodies. DOM photoreactivity was significantly affected by the cumulative hydrological residence time (CHRT) when considering the whole data set. However, when considering only the boreal water bodies, iron (Fe) and manganese (Mn) also intervened. The fact that DOM photoreactivity was related to CHRT as well as to Fe and Mn concentrations, which are respectively permanent and long-lasting features of the reservoir, suggests that the photoproduction of CO₂ is not likely to decrease over time. This process may therefore play a substantial role in the long-term CO₂ emissions from boreal reservoirs during the summer, its potential contribution to CO₂ diffusive fluxes being estimated at 56 ± 29 %.