Sunlight‐induced carbon dioxide emissions from inland waters

Abstract

The emissions of carbon dioxide (CO 2 ) from inland waters are substantial on a global scale. Yet the fundamental question remains open which proportion of these CO 2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55°N to 69°N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m À2 yr À1 , which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr À1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO 2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which corresponds to the absorbed photons. Therefore, the slope of the regression line represents the wavelength-and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr À1 under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO 2 emissions contribute up to about one tenth to the global CO 2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.

Transformations in DOC along a source to sea continuum; impacts of photo-degradation, biological processes and mixing

“…Furthermore, we demonstrated that when DOC has a higher proportion of chromophoric structures (as indicated by a high SUVA254) it is more susceptible to being completely mineralised to CO2. This supports findings from previous studies (Dehaan 1993;Koehler et al 2014) and highlights how DOC leaching from peatlands is particularly susceptible to photo-degradation. The 14-17% reduction in DOC concentration of the unfiltered peatland stream solution after exposure to a dose of solar radiation equivalent to one sunny mid-summer day demonstrates that a significant proportion of DOC may be photo-degraded during transport down the river system, despite the relatively short residence time.…”

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

“…Thus, the relative role that UV light processing played for the DOC quality transformations was much larger in clear lakes than in highly colored lakes. In absolute terms, however, some studies suggest that the absolute light-induced color loss is similar for brown and clear lakes given equal incoming surface irradiation, even if this loss in color is distributed in different ways over the water column (Granéli et al, 1996;Koehler et al, 2014). In our study the mean absolute a 420 loss in the four clearest epilimnetic sites was 1.0 m −1 year −1 (Fig.…”

“…A fundamen-tally important water-column process that generates carbon dioxide (CO 2 ) is the microbial degradation of terrestrially derived DOC (Lapierre et al, 2013;Fasching et al, 2014). Significant amounts of DOC can also be mineralized by ultraviolet (UV) sunlight in lakes (Koehler et al, 2014) and running waters (Cory et al, 2014). However, while much research attention has been drawn to CO 2 production from these different processes, surprisingly little is known about the relative role played by biological and photochemical processes for DOC quality transformations and, in particular, for the removal of color.…”

Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic ecosystems

“…Recent studies indicate that sunlight can play an important role in dissolved organic matter (DOM) degradation in thermokarst-impacted lakes and ponds (Alaska, Cory et al, 2013Cory et al, , 2014Canadian High Arctic, Laurion and Mladenov, 2013; sub-Arctic Sweden, Koehler et al, 2014). To understand why sunlight can be an important control on DOM degradation in these systems, we review controls on DOM photodegradation and the specific characteristics of thermokarst lakes that can maximize opportunities for photodegradation.…”

“…Clouds generally decrease surface UV, but the effect of clouds can be offset by ozone levels, making it difficult to predict surface UV based only on latitude and date across the Arctic (Vavrus et al, 2010;Bernhard et al, 2013). Although less UV generally reaches surface waters in the Arctic compared to lower latitudes due to lower solar zenith angles, thermokarst lakes and ponds can often have high concentrations of light-absorbing DOM that is susceptible to photo-degradation, thus potentially counter-balancing the lower UV (Alaska, Cory et al, 2014; sub-Arctic Sweden, Koehler et al, 2014).…”