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

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.

The Validity of Optical Properties as Tracers of Terrigenous Dissolved Organic Carbon during Extensive Remineralization in Coastal Waters

“…The different rates of change are to be expected because coastal water samples contain overall less CDOM and FDOM, and consist of a mixture of marine and terrestrial DOM, while the river samples still consisted of tDOM even at the end of the incubations. The results of bio-and photoincubation for water from the peatland-draining river suggest that Southeast Asian peatlandderived tDOM behaves similar to other highly humified photo-labile but bio-refractory tDOM during remineralization (Chen & Jaffé, 2014;Dempsey et al, 2020). The fact that our environmental data do not demonstrate such clear relationships between optical properties and tDOC loss therefore suggests that natural tDOC remineralization in this region proceeds through manuscript submitted to Global Biogeochemical Cycles interactive degradation processes that do not leave clear signatures in the optical properties.…”

Cyclical browning in a subtropical lake inferred from diatom records

“…Diatom assemblage response to monthly variability in transparency and the hydrologic drivers and limnological consequences of transparency variability were evaluated using the 14-year phytoplankton record (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019). Secchi depth was used to indicate the consequences of browning on water transparency, since prior studies showed a strong positive correlation between the frequent measures of Secchi depth, and less frequent measures of water color and total organic carbon (TOC, 98% of which is DOC; Gaiser et al, 2009a;Dempsey et al, 2020). The relationship between hydrologic drivers and limnological correlates of Secchi depth variability were evaluated using the monthly means of all meteorological and limnological variables.…”

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

“…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).…”

“…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.…”