Photochemical mineralisation in a boreal brown water lake: considerable temporal variability and minor contribution to carbon dioxide production

Abstract. Sunlight induces photochemical mineralisation of chromophoric dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) in inland waters, resulting in carbon dioxide (CO₂) emissions to the atmosphere. Photochemical rate modelling is used to determine sunlight-induced CO₂ emissions on large spatial and temporal scales. A sensitive model parameter is the wavelength-specific photochemical CDOM reactivity, the apparent quantum yield (AQY). However, the temporal variability of AQY spectra within inland waters remains poorly constrained. Here, we studied a boreal brown water lake in Sweden. We measured AQY spectra for photochemical DIC production monthly between June and November 2014 and parameterised a photochemical rate model. The total AQY between 280 and 600 nm increased about 3-fold during the open-water period, likely due to a high rainfall event with consecutive mixing in autumn that increased availability of highly photoreactive CDOM. However, the variability in AQY spectra over time was much smaller than previously reported variability in AQY spectra between lakes. Yet, using either the AQY spectrum from the least or from the most photoreactive water sample resulted in a 5-fold difference in simulated annual DIC photoproduction (2012–2014), with 2.0 ± 0.1 and 10.3 ± 0.7 g C m⁻² yr⁻¹, respectively. This corresponded to 1 and 8 % of the mean CO₂ emissions measured from this lake. We conclude that (1) it may be recommendable to conduct repeated AQY measurements throughout the season for more accurate simulation of annual photochemical DIC production in lakes and (2), in agreement with previous studies, direct CDOM photomineralisation makes only a minor contribution to mean CO₂ emissions from Swedish brown water lakes.