Biogeosciences, 17, 6327–6340, 2020 https://doi.org/10.5194/bg-17-6327-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. The relative importance of photodegradation and biodegradation of terrestrially derived dissolved organic carbon across four lakes of differing trophic status Christopher M. Dempsey1, Jennifer A. Brentrup2,a, Sarah Magyan1, Lesley B. Knoll3, Hilary M. Swain4, Evelyn E. Gaiser5, Donald P. Morris6, Michael T. Ganger1, and Craig E. Williamson2 1Biology Department, Gannon University, Erie, PA, USA 2Global Change Limnology Laboratory, Department of Biology, Miami University, Oxford, OH, USA 3Itasca Biological Station and Laboratories, College of Biological Sciences, University of Minnesota, Lake Itasca, MN, USA 4Archbold Biological Field Station, Venus, FL, USA 5Department of Biological Sciences and Institute of Environment, Florida International University, Miami, FL, USA 6Earth and Environmental Sciences Department, Lehigh University, Bethlehem, PA, USA acurrently at: Biology and Environmental Studies, St. Olaf College, Northfield, MN, USA Correspondence: Christopher M. Dempsey ([email protected]) Received: 6 May 2020 – Discussion started: 2 June 2020 Revised: 4 October 2020 – Accepted: 22 October 2020 – Published: 15 December 2020 Abstract. Outgassing of carbon dioxide (CO2) from fresh- mum of 1 % was photomineralized. In addition to document- water ecosystems comprises 12 %–25 % of the total carbon ing the importance of photodegradation in lakes, these results flux from soils and bedrock. This CO2 is largely derived from also highlight how lakes in the future may respond to changes both biodegradation and photodegradation of terrestrial dis- in DOC inputs. solved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the signif- icance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly under- 1 Introduction stood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate Lakes are closely linked to their surrounding terrestrial lakes of differing trophic status to simulate the effects of in- ecosystems. As the lowest point in the landscape, they re- creases in terrestrial DOC from storm events. We assessed ceive a significant influx of terrestrially derived dissolved or- the relative importance of biodegradation and photodegra- ganic carbon (DOC) and nutrients (Williamson et al., 2009; dation in oxidizing DOC to CO2. We measured changes in Wilkinson et al., 2013). Climate and land use changes are DOC concentration, colored dissolved organic carbon (spe- altering the link between lakes and their surrounding land- cific ultraviolet absorbance – SUVA320; spectral slope ra- scapes by strengthening the flow of material during extreme tio – Sr), dissolved oxygen, and dissolved inorganic carbon rain events and large wildfires or weakening it during ex- (DIC) in short-term experiments from May–August 2016. In tended periods of drought (Strock et al., 2016; Williamson all lakes, photodegradation led to larger changes in DOC and et al., 2016). Long-term changes in DOC concentrations are DIC concentrations and optical characteristics than biodegra- variable and appear to be regionally controlled. In northeast- dation. A descriptive discriminant analysis showed that, in ern North American and western European lakes, there has brown-water lakes, photodegradation led to the largest de- been as much as a doubling of DOC concentrations due to re- clines in DOC concentration. In these brown-water systems, covery from anthropogenic acidification and climate change ∼ 30 % of the DOC was processed by sunlight, and a mini- (Monteith et al., 2007; Williamson et al., 2015; de Wit et al., 2016). However, DOC concentrations in Greenland lakes Published by Copernicus Publications on behalf of the European Geosciences Union. 6328 C. M. Dempsey et al.: The relative importance of photodegradation and biodegradation (Saros et al., 2015) and the Mississippi River (Duan et al., Since light attenuation varies so strongly among lakes 2017) have been decreasing. A long-term study of the Florida of differing trophic status, testing the relative importance Everglades showed that some study sites were decreasing in of DOC processing via photodegradation or biodegradation DOC concentration, but the majority of sites were not chang- with mechanistic experiments is needed. Previous research ing (Julian et al., 2017). As DOC inputs into aquatic ecosys- on DOC degradation has primarily occurred in high DOC tems have increased, stabilized, or decreased, long-term stud- lakes, but in clear-water lakes, 1 % of surface UVA and pho- ies have focused on understanding the mechanisms behind tosynthetically active radiation (PAR), which are the primary the change, but less research has addressed the fate of DOC wavelengths active in photodegradation (Osburn et al., 2001), once it enters a lake. can reach significant depths. In some oligotrophic lakes, By attenuating light in the water column and also provid- UVA may reach up to 7 m for UVA and 14 m for PAR. In ing a source of energy, DOC serves an important role in lakes some of the clearest lakes in the world, such as Lake Tahoe, by regulating the balance between photosynthesis and respi- PAR can reach depths > 45 m (Rose et al., 2009a, b). Geo- ration (Williamson et al., 1999) and, thus, the flux of CO2 to graphic location and time of year influence the amount of the atmosphere (Cole et al., 1994). Previous studies indicated solar radiation lakes receive. In the subtropics, PAR and UV that most lakes are net heterotrophic, where the breakdown of light have high intensity across the spectrum all year round, organic carbon exceeds production (Kling et al., 1991; Cole whereas in temperate regions those wavelengths are strongest et al., 1994). Estimates suggest that lakes respire about half during the summer months. of the annual 2 Gt flux of carbon to the oceans each year as Watershed land use and lake trophic status have also been CO2 (Cole et al., 1994; Tranvik et al., 2009; Tranvik, 2014). shown to influence DOC composition and reactivity (Lu et The traditional paradigm has been that the dominant mecha- al., 2013; Hosen et al., 2014; Larson et al., 2014; Evans et nism causing the release of excess CO2 from lakes is the bac- al., 2017). DOC from forested systems was more reactive and terial respiration of DOC (biodegradation), with photomin- had different CDOM properties when compared to disturbed eralization (conversion of DOC to CO2) accounting for only environments (Lu et al., 2013; Williams et al., 2016; Evans et 10 % of bacterial rates (Granéli et al., 1996; del Giorgio et al., 2017). Studies examining how terrestrial DOC inputs are al., 1997; Jonsson et al., 2001). However, research on over processed in lakes are needed, especially with the increasing 200 Arctic lakes, rivers, and streams revealed that sunlight frequency of extreme rain events (Rahmstorf and Coumou, dominated the processing of DOC, and photomineralization 2011; Westra et al., 2014; Fischer and Knutti, 2015). Future rates were, on average, 5× greater than dark bacterial res- climate change projections suggest that, for northern ecosys- piration rates (Cory et al., 2014). In addition, the source of tems, a 10 % increase in precipitation could lead to a 30 % inland water CO2 remains uncertain, due in large part to a increase in the mobilization of soil organic matter (de Wit lack of measurements (Raymond et al., 2013; Lapierre et al., et al., 2016). Extreme rain events deliver fresh DOC not ex- 2013; Weyhenmeyer et al., 2015), and predicting DOC reac- posed to prior sunlight into lakes, which can lead to signifi- tivity has been challenging (Evans et al., 2017). Quantifying cant reductions in light availability and increases in thermal the dominant degradation pathways for terrestrial DOC from stability and lake heterotrophy (Jennings et al., 2012; Klug et a range of lakes will improve estimates of carbon fluxes, par- al., 2012; de Eyto et al., 2016; Zwart et al., 2016). As DOC ticularly for mineralization rates that currently have a high concentrations change globally, understanding the processes degree of uncertainty (Hanson et al., 2014). that determine the fate of DOC will help predict the systems Many past studies have focused on testing the effects of most likely to release more CO2. photodegradation and biodegradation on DOC quantity in- Here our aim was to (1) determine the relative importance dividually, but they have not simultaneously evaluated how of photodegradation and biodegradation for altering terres- these two processes alter the colored dissolved organic car- trial DOC quantity and CDOM from lakes of varying trophic bon (CDOM; Granéli et al., 1996; Koehler et al., 2014; Va- status; (2) quantify the percentage of the initial DOC pool chon et al., 2016a). CDOM is the fraction of dissolved or- that was photomineralized, partially photodegraded, biode- ganic matter that is capable of absorbing light. The effects of graded or remained unprocessed; and (3) compare the effects sunlight on DOC are not isolated to only increasing mineral- of photodegradation on DOC quantity and CDOM across ization rates. Photodegradation can also decrease the color four lakes to understand differences in how terrestrial DOC and molecular weight of DOC, which can increase light from the watersheds of different lake types responds to pho- availability