Abstract
Abstract. Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12 %–25 % of the total carbon flux from soils and bedrock. This CO2 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 CO2 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 CO2. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance – SUVA320; spectral slope ratio – Sr), 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, ∼ 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.
Highlights
Lakes are closely linked to their surrounding terrestrial ecosystems
For the photodegradation-only treatments, exposure to sunlight resulted in a significant production of dissolved inorganic carbon (DIC) and increases in slope ratio (Sr) and significant decreases in dissolved oxygen (DO), dissolved organic carbon (DOC), and specific ultraviolet absorbance at 320 nm (SUVA320) relative to the biodegradation treatments
We showed that photodegradation can be more important than biodegradation in processing watershed inputs of terrestrial DOC on short timescales in the surface waters of a lake
Summary
Lakes are closely linked to their surrounding terrestrial ecosystems. As the lowest point in the landscape, they receive a significant influx of terrestrially derived dissolved organic carbon (DOC) and nutrients (Williamson et al, 2009; Wilkinson et al, 2013). Climate and land use changes are altering the link between lakes and their surrounding landscapes by strengthening the flow of material during extreme rain events and large wildfires or weakening it during extended periods of drought (Strock et al, 2016; Williamson et al, 2016). In northeastern North American and western European lakes, there has been as much as a doubling of DOC concentrations due to recovery from anthropogenic acidification and climate change (Monteith et al, 2007; Williamson et al, 2015; de Wit et al, 2016).
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