The relative importance of photodegradation and biodegradation of terrestrially derived dissolved organic carbon across four lakes of differing trophic status

Abstract

<jats:p>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, the optical characteristics of the DOC (SUVA320 and 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 ~ 2 % was photo mineralized. 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. </jats:p>

Decreasing Photoreactivity and Concurrent Change in Dissolved Organic Matter Composition with Increasing Inland Water Residence Time

“…Hence we show, for the first time, that photoreactivity can be quantitatively related to the WRT of inland waters. The complete photochemical mineralization may account for a limited fraction of the total loss of DOC in most water bodies (Allesson et al, 2021;Dempsey et al, 2020), estimated to correspond to about 10% of total emission of CO2 from lakes in the region studied here (Koehler et al 2014). We demonstrate that, still, photochemical reactions may be important in shaping the composition of DOM.…”

Attenuation of photosynthetically active radiation and ultraviolet radiation in response to changing dissolved organic carbon in browning lakes: Modeling and parametrization

“…For example, photo‐processing by UV radiation alters both the color and concentration of DOC (Zhang et al 2010; Cory et al 2014), and these changes in DOC would therefore feed back into the estimation of both K d PAR and K d UV320 . In fact, photo‐processing is often more important than biodegradation of carbon (Dempsey et al 2020) and can account for up to 95% of total carbon processing in Arctic systems (Cory et al 2014). In Lake Giles, photo‐processing of DOC resulted in nearly a 50% decrease in DOC absorption coefficients in just 1 week (Dempsey et al 2020), again highlighting the importance of incorporation of DOC absorption or color in addition to DOC concentration for estimating light attenuation.…”

“…In fact, photo‐processing is often more important than biodegradation of carbon (Dempsey et al 2020) and can account for up to 95% of total carbon processing in Arctic systems (Cory et al 2014). In Lake Giles, photo‐processing of DOC resulted in nearly a 50% decrease in DOC absorption coefficients in just 1 week (Dempsey et al 2020), again highlighting the importance of incorporation of DOC absorption or color in addition to DOC concentration for estimating light attenuation. Further, in lakes experiencing diverging surface water vs. deepwater temperature patterns and consequent increases in thermal stability, residence time of water and DOC tends to increase, which is an important control of DOC processing in lakes (Cory et al 2015; Catalán et al 2016).…”