Hydrologic, metabolic and chemical regulation of water-column metabolism and atmospheric CO2 exchange in a large continental reservoir during spring and summer

Large continental reservoirs are important sinks of terrestrially-derived carbon (C), yet exhibit high inter-annual variability in net CO2 exchange with the atmosphere due to incompletely understood interactions between supply of solutes (C, nutrients, pH) and lake metabolism. Here we quantified the interaction between hydrology, aquatic metabolism and atmospheric CO2 exchange in the lacustrine Qu'Appelle arm of Lake Diefenbaker, Canada, by comparing total phytoplankton abundance (as Chlorophyll a), net primary productivity (NPP), ratios of microbial productivity (P) to respiration (R), and chemically-enhanced CO2 flux over four springs and summers (2010-2013) with contrasting hydrologic characteristics. Overall, dissolved oxygen (DO) concentrations were correlated inversely (R2adj = 0.63, p < 0.0001) with δ18O values of DO indicating a strong metabolic coupling in this polymictic basin, whereas correlations between concentration and C isotope ratios of dissolved inorganic carbon (DIC) were not significant (R2adj < 0.05). In most years, P:R modeled from DO characteristics was > 1, the basin was slightly autotrophic and CO2 was captured from the atmosphere. In contrast, the Qu'Appelle arm was a substantial source of CO2 during 2012 when river discharge was low, pH was < 8.6, cyanobacteria were common, and P:R was < 1. Multiple least-squares regressions and principal component analysis inferred that hydrologic variability influenced changes in water-column metabolism, whereas independent variation in pH was the main control of atmospheric CO2 exchange (R2adj CO2-pH = 0.84, p < 0.0001). Together, these findings suggest that lacustrine basins of alkaline reservoirs may lack strong coupling of water-column metabolism and CO2 dynamics on seasonal and inter-annual scales.