Spatio‐temporal variation in climate and weather, allochthonous carbon loads, and autochthonous factors such as lake metabolism (photosynthesis and respiration) interact to regulate atmospheric CO2 exchange of lakes. Understanding this interplay in diverse basin types at different timescales is required to adequately place lakes into the global carbon cycle, and predict CO2 flux through space and time. We analyzed 18 years of data from seven moderately hard lakes in an agricultural prairie landscape in central Canada. We applied generalized additive models and sensitivity analyses to evaluate the roles of metabolic and climatic drivers in regulating CO2 flux at the intra‐annual scale. In all basins, at mean conditions with respect to other predictors, metabolic controls resulted in uptake of atmospheric CO2 when surface waters exhibited moderate primary production, but released CO2 only when primary production was very low (5‐13μg, L−1) or when dissolved nitrogen was elevated (>2000μg, L−1), implying that respiratory controls offset photosynthetic CO2 uptake under these conditions. Climatically, dry conditions increased the likelihood of ingassing, likely due to evaporative concentration of base cations and/or reduced allochthonous carbon loads. While more research is required to establish the relative importance of climate and metabolism at other time scales (diel, autumn/winter), we conclude that these hard fresh waters characteristic of continental interiors are mainly affected by metabolic drivers of pCO2 at daily‐monthly timescales, are climatically controlled at interannual intervals, and are more likely to in‐gas CO2 for a given level of algal abundance, than are softwater, boreal ecosystems. Publication cover image Accepted Articles Accepted, unedited articles published online and citable. The final edited and typeset version of record will appear in the future. Figures References Related Information Metrics Details This article is protected by copyright. All rights reserved. Keywords hardwater lake CO2 carbon dioxide algae eutrophication Publication History Accepted manuscript online: 19 May 2018 Manuscript accepted: 11 May 2018 Manuscript revised: 10 May 2018 Manuscript received: 20 March 2018