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Lakes may function as either sinks or sources of CO2. Their response to climate change is uncertain, as we lack continuous data of lake CO2 efflux and its drivers. This is especially true in the littoral zone of lakes, which can be very dynamic from the continuous injection and remobilization of terrestrial nutrients. This study used high-frequency measurements of CO2 exchange during the ice-free season by prototype low-power floating Forced Diffusion (FD) autochambers. We quantified the net surface flux of CO2 across a transect of the littoral zone of a small deep oligotrophic lake in eastern Nova Scotia, Canada, and examined potential drivers. The littoral zone was a net source for CO2, on average emitting 0.171 ± 0.023 µmol CO2 m-2 s-1, but we did observe significant temporal variation across diel and seasonal periods, as well as with distance from shore. While no pelagic environmental driver appeared to explain this variability in CO2 exchange, our study suggests that factors which vary on a fine spatial scale within the littoral zone may effectively regulate CO2 exchange. If environmental drivers of pelagic CO2 exchange are unrelated to CO2 exchange in the littoral zone, this may have large implications for current mechanistic understandings of lake carbon dynamics, and for upscalings of fluxes. This work shows the spatial and temporal variability of littoral CO2 efflux, as well as the utility of low-power FD automated chambers for observing lake-atmosphere net CO2 exchange.

The present study examined potential effects of seasonal variations in photoreactive dissolved organic matter (DOM) on methylmercury (MeHg) photodemethylation rates in freshwaters. A series of controlled experiments were carried out using natural and photochemically preconditioned DOM in water collected from one lake in June, August, and October. Natural DOM concentrations doubled between June and August (DOC = 10.2–21.2 mg C L−1) and then remained stable into October (DOC = 19.4 mg C L−1). Correspondingly, MeHg concentrations peaked in August (0.42 ng L−1) along with absorbances at 350 nm (A350) and 254 nm (SUVA254). Up to 70% of the MeHg was photodemethylated in the short 48-hour irradiation experiments with June having significantly higher rates than the other sampling months (p 0.10). However, MeHg photodemethylation efficiencies (quantified in mole MeHg lost/mole photon absorbed) were higher in treatments with less photoreactive DOM. Congruently, MeHg photodemethylation efficiencies also decreased over summer by up to 10× across treatments in association with increased photoreactive DOM, and were negatively correlated with DOM concentration. These results suggest that an important driver of MeHg photodemethylation is the interplay between MeHg and DOM with greater potential for photodemethylation in freshwaters with more photobleached DOM and lower DOM content. This article is protected by copyright. All rights reserved

The carbon isotopic composition (delta(13)C) of recently assimilated plant carbon is known to depend on water-stress, caused either by low soil moisture or by low atmospheric humidity. Air humidity has also been shown to correlate with the delta(13)C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration. However, there are other reasons why the delta(13)CO(2) of soil efflux may change with moisture conditions, which have not received as much attention. Using a combination of greenhouse experiments and modeling, we examined whether moisture can cause changes in fractionation associated with (1) non-steady-state soil CO(2) transport, and (2) heterotrophic soil-respired delta(13)CO(2). In a first experiment, we examined the effects of soil moisture on total respired delta(13)CO(2) by growing Douglas fir seedlings under high and low soil moisture conditions. The measured delta(13)C of soil respiration was 4.7 per thousand more enriched in the low-moisture treatment; however, subsequent investigation with an isotopologue-based gas diffusion model suggested that this result was probably influenced by gas transport effects. A second experiment examined the heterotrophic component of soil respiration by incubating plant-free soils, and showed no change in microbial-respired delta(13)CO(2) across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired delta(13)CO(2), but they indicate that the expected impacts of photosynthetic discrimination may be similar in direction and magnitude to those from gas transport-related fractionation. Gas transport-related fractionation may operate as an alternative or an additional factor to photosynthetic discrimination to explain moisture-related variation in soil-respired delta(13)CO(2).