pOutgassing of carbon dioxide (COspan classCombining double low lineinline-formula2/span) from freshwater ecosystems comprises 12 %-25 % of the total carbon flux from soils and bedrock. This COspan classCombining double low lineinline-formula2/span is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of COspan classCombining double low lineinline-formula2/span outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to COspan classCombining double low lineinline-formula2/span. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance - SUVAspan classCombining double low lineinline-formula320/span; spectral slope ratio - span classCombining double low lineinline-formulaiS/ir/span), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May-August 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that, in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, span classCombining double low lineinline-formulag1/4/span 30 % of the DOC was processed by sunlight, and a minimum of 1 % was photomineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs./p.
Bibliographical noteFunding Information:
Acknowledgements. We would like to thank Kevin Main, for assistance in collecting Lake Annie water samples, and Erin Overholt, for assistance in the laboratory and logistical support. Additional support was provided by the Robert Estabrook Moeller Research Fellow Award (for CMD and JAB) and the Isabel and Arthur Wa-tres Student Research Award (SM). The authors would like to thank the four anonymous reviewers for their assistance in improving the paper.
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