This is of the same order of magnitude as the CO 2 emissions from land use change, or the carbon transport from continents to the ocean (Ciais et al., 2013), making CO 2 emissions from lakes important in the global carbon cycle. Lakes are concentrated in boreal regions, which contain roughly 30% of global lakes (Downing et al., 2006;Verpoorter et al., 2014), and together with the arctic region contribute 17% of global lake CO 2 emissions (Aufdenkampe et al., 2011). Potential climate change effects in boreal lakes, including increased runoff (Larsen et al., 2011;Weyhenmeyer et al., 2015) and increased carbon mineralization rates (Bergström Abstract Lakes are generally supersaturated in carbon dioxide (CO 2 ) and emitters of CO 2 to the atmosphere. However, estimates of CO 2 flux ( CO 2 E F ) from lakes are seldom based on direct flux measurements and usually do not account for nighttime emissions, yielding risk of biased assessments.Here, we present direct CO 2 E F measurements from automated floating chambers collected every 2-3 hr and spanning 115 24 hr periods in three boreal lakes during summer stratification and before and after autumn mixing in the most eutrophic lake of these. We observed 40%-67% higher mean CO 2 E F in daytime during periods of surface water CO 2 supersaturation in all lakes. Day-night differences in wind speed were correlated with the day-night CO 2 E F differences in the two larger lakes, but in the smallest and most wind-sheltered lake peaks of CO 2 E F coincided with low-winds at night. During stratification in the eutrophic lake, CO 2 was near equilibrium and diel variability of CO 2 E F insignificant, but after autumn mixing CO 2 E F was high with distinct diel variability making this lake a net CO 2 source on an annual basis.We found that extrapolating daytime measurements to 24 hr periods overestimated CO 2 E F by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated CO 2 E F by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO 2 emission estimates.Plain Language Summary Considerable carbon cycling occurs within lakes, and carbon inputs from the catchment can be processed internally, stored in sediment and biomass or transported downstream. Additionally, carbon is exchanged with the atmosphere, resulting in lake uptake or atmospheric emission of carbon dioxide. Carbon dioxide exchanges from lakes have globally significant implications, but may be highly variable in time in ways that are not yet accounted for in emission estimates. Here, we estimated carbon dioxide exchange during multiple days and nights in three lakes with different nutrient levels during summer and autumn. For the most nutrient rich lake we also covered the period of water column mixing in autumn, which constitutes a critical time for carbon exchange. When carbon dioxide emission exceeded uptake, we found 40%-67% higher average exchange rates during daytime than nighttime. In contrast, the most nutrient...

Lakes evade significant amounts of carbon dioxide (CO2) to the atmosphere; yet the magnitude and origin of the evasion are still poorly constrained. We quantified annual CO2 evasion and its origin (in‐lake net ecosystem production vs. lateral inputs from terrestrial ecosystems) in 14 high‐latitude lakes through high‐frequency estimates of open water CO2 flux and ecosystem metabolism and inorganic carbon mass‐balance before and after ice breakup. Annual CO2 evasion ranged from 1 to 25 g C m−2 yr−1 of which an average of 57% was evaded over a short period at ice‐breakup. Annual internal CO2 production ranged from −6 to 21 g C m−2 yr−1, of which at least half was produced over winter. The contribution of internal versus external source contribution to annual CO2 evasion varied between lakes, ranging from fully internal to fully external with most lakes having over 75% of the evasion sustained through a single source. Overall, the study stresses the large variability in magnitude and control of CO2 evasion and suggests that environmental change impacts on CO2 evasion from high‐latitude lakes are not uniform.

Benthic gross primary production (GPP) is often the most important part of aquatic food webs in northern lakes, which are gradually warming and receiving increased terrestrial colored dissolved organic carbon loadings due to global change. Yet, measurements of benthic GPP are fairly uncommon, and methods and unit dimensions of benthic GPP are unstandardized and rarely compared. In this study, we measured benthic GPP in 27 headwater lakes from three regions in northern Sweden and analyzed potential constraining drivers of benthic GPP z rates at discrete depths and estimates of benthic GPP averages across the whole lake, as well as across the littoral zone. We also compared in situ measurements of benthic GPP averages across the whole lake with modeled values using the "autotrophic structuring model." We found that benthic GPP z rates were best explained by, and positively related to, available light (i.e., a function of depth and water color) and temperature. Benthic GPP averages across the whole lake, on the contrary, were best explained by the relative size of the littoral area, which is a measure that combines lake bathymetry and water color. The comparison between in situ measured and modeled estimates of benthic GPP averages across the whole lake revealed that (1) the autotrophic structuring model underestimates GPP at low values and overestimates GPP at high values compared with measured data, and that (2) measured values were related to temperature, which is not included as a variable in the autotrophic structuring model. Considering future predicted changes impacting northern latitude lakes, our results suggest that increased lake water temperatures can to some extent mitigate the negative impacts of reduced light availability from lake browning on benthic GPP z rates. The combined impact of these changes on benthic GPP averages across the whole lake will depend on, and be moderated by, lake bathymetry determining the relative size of the littoral area.

Metabolism is one of the most fundamental ecosystem processes, but the drivers of variation in metabolic rates among lakes dominated by benthic primary producers remain poorly constrained. Here, we report the magnitudes and potential drivers of whole-lake metabolism across 43 Swedish arctic-alpine lakes, based on the free-water diel oxygen technique with sondes deployed during the open-water season near the surface and bottom of the lakes. Gross primary production (GPP) and ecosystem respiration (R) were strongly coupled and ranged from 0.06 to 0.45 mg and 0.05 to 0.43 mg L À1 d À1 among lakes. On average, GPP and R decreased eightfold from relatively shallow to deep lakes (mean depth 0.5-10.9 m) and twofold from concave to convex lakes (mean depth: maximum depth 0.2-0.5). We attribute this to light limitation and shape-specific sensitivity of benthic GPP to disturbance by lake ice. Net ecosystem production (GPP-R) ranged from À0.09 to 0.14 mg L À1 d À1 and switched, on average, from positive to negative towards deeper lakes and lakes richer in dissolved organic carbon (DOC; 0.5-7.4 mg DOC L À1 ). Uncertainties in metabolism estimates were high (around one and three times mean R and GPP), especially in deep lakes with low insulation and diurnally variable wind speed. Our results confirm the role of DOC in stimulating net heterotrophy and highlight novel effects of lake shape on productivity in benthic-dominated lake ecosystems and its response to changes in lake ice cover.