Abstract
AbstractMany boreal lakes are experiencing an increase in concentrations of terrestrially derived dissolved organic matter (DOM)—a process commonly labeled “browning.” Browning affects microbial and photochemical mineralization of DOM, and causes increased light attenuation and hence reduced photosynthesis. Consequently, browning regulates lake heterotrophy and net CO2‐efflux to the atmosphere. Climate and environmental change makes ecological forecasting and global carbon cycle modeling increasingly important. A proper understanding of the magnitude and relative contribution from CO2‐generating processes for lakes ranging in dissolve organic carbon (DOC) concentrations is therefore crucial for constraining models and forecasts. Here, we aim to study the relative contribution of photomineralization to total CO2 production in 70 Scandinavian lakes along an ecosystem gradient of DOC concentration. We combined spectral data from the lakes with regression estimates between optical parameters and wavelength specific photochemical reactivity to estimate rates of photochemical DOC mineralization. Further, we estimated total in‐lake CO2‐production and efflux from lake chemical and physical data. Photochemical mineralization corresponded on average to 9% ± 1% of the total CO2‐evasion, with the highest contribution in clear lakes. The calculated relative contribution of photochemical mineralization to total in‐lake CO2‐production was about 3% ± 0.2% in all lakes. Although lakes differed substantially in color, depth‐integrated photomineralization estimates were similar in all lakes, regardless of DOC concentrations. DOC concentrations were positively related to CO2‐efflux and total in‐lake CO2‐production but negatively related to primary production. We conclude that enhanced rates of photochemical mineralization will be a minor contributor to increased heterotrophy under increased browning.
Highlights
We estimated dissolved inorganic carbon (DIC) photoproduction in boreal lakes using modeled spectra of irradiance and apparent quantum yield (AQY), and spectra of attenuation coefficients and absorption extrapolated from the measured photosynthetically active radiation (PAR) to the UV region from 70 lakes in Norway and Sweden
We found that DIC photoproduction contributed on average 9% Æ 1% to the CO2 emission from the lakes
Regarding that this percentage decreases with increased dissolve organic carbon (DOC) concentrations and that water temperatures as well as DOC and nutrient concentrations in boreal lakes are increasing (Larsen et al 2011b; O’Reilly et al 2015), we expect that the relative contribution of sunlight for CO2 production in boreal lakes may decline in the future
Summary
Humic substances are a major source of energy to heterotrophs in aquatic ecosystems with high terrestrial influence and the subsequent increase in heterotrophic CO2 production may indirectly stimulate autotrophs. Photomineralization of DOC to dissolved inorganic carbon (DIC) might be a significant part of the DIC production and carbon cycling in humic lakes, adding to the high respiratory activity of heterotrophic prokaryotes and low autotrophic CO2-fixation. In order to simulate photochemical mineralization, knowledge of the reactivity across the whole spectrum of photochemically active wavelengths is needed. This photochemical reactivity or apparent quantum yield (AQY) of DIC photoproduction is defined as moles photochemically produced DIC per mole photons absorbed by the DOC pool (Miller et al 2002). A significant share of the AQY variability between lakes can be explained by simple optical parameters (Koehler et al 2016), allowing for estimates of photochemical DIC production when system-specific AQY spectra are not available
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