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Current estimates of carbon dioxide (CO2) evasion from Arctic lakes are highly uncertain because few studies integrate seasonal variability, specifically evasion during spring ice-melt. We quantified annual CO2 evasion for 14 clear-water Arctic lakes in Northern Sweden through mass balance (ice-melt period) and high-frequency loggers (open-water period). On average, 80% (SD: ± 18) of annual CO2 evasion occurred within 10 d following ice-melt. The contribution of the ice-melt period to annual CO2 evasion was high compared to earlier studies of Arctic lakes (47% ± 32%). Across all lakes, the proportion of ice-melt : annual CO2 evasion was negatively related to the dissolved organic carbon concentration and positively related to the mean depth of the lakes. The results emphasize the need for measurements of CO2 exchange at ice-melt to accurately quantify CO2 evasion from Arctic lakes.

Aquatic microbial responses to changes in the amount and composition of dissolved organic carbon (DOC) are of fundamental ecological and biogeochemical importance. Parallel factor (PARAFAC) analysis of excitation–emission fluorescence spectra is a common tool to characterize DOC, yet its ability to predict bacterial production (BP), bacterial respiration (BR), and bacterial growth efficiency (BGE) vary widely, potentially because inorganic nutrient limitation decouples microbial processes from their dependence on DOC composition. We used 28-d bioassays with water from 19 lakes, streams, and rivers in northern Sweden to test how much the links between bacterial metabolism and fluorescence PARAFAC components depend on experimental additions of inorganic nutrients. We found a significant interaction effect between nutrient addition and fluorescence on carbon-specific BP, and weak evidence for influence on BGE by the same interaction (p = 0.1), but no corresponding interaction effect on BR. A practical implication of this interaction was that fluorescence components could explain more than twice as much of the variability in carbon-specific BP (R² = 0.90) and BGE (R² = 0.70) after nitrogen and phosphorus addition, compared with control incubations. Our results suggest that an increased supply of labile DOC relative to ambient phosphorus and nitrogen induces gradually larger degrees of nutrient limitation of BP, which in turn decouple BP and BGE from fluorescence signals. Thus, while fluorescence does contain precise information about the degree to which DOC can support microbial processes, this information may be hidden in field studies due to nutrient limitation of bacterial metabolism.

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 GPPz 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 GPPz 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 GPPz 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⁻¹ d⁻¹ 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 −0.09 to 0.14 mg L⁻¹ d⁻¹ 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⁻¹). 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.

Lake ecosystems receive, transmit and process terrestrial carbon and thereby link terrestrial, aquatic and global carbon cycles. Most lakes evade CO2 to the atmosphere, but the annual magnitude of CO2 evasion, as well as sources and mechanisms underpinning CO2 evasion from lakes are still largely unresolved. CO2 evasion from lakes can be sourced from direct external input from the catchment, but CO2 can also be produced in-lake from organic carbon breakdown. Both sources have been shown to be of importance to individual systems, but a landscape perspective is still missing. Globally, most lakes are in northern high latitudes, but due to infrequent seasonal sampling the magnitude of CO2 evasion on an annual scale is largely unknown, as are constraining variables of in-lake metabolism (i.e. production and consumption of CO2). As a consequence of these knowledge gaps, there is little possibility to predict future lake carbon cycling, for instance due to changing dissolved organic carbon (DOC) input or lake temperature resulting from global warming.

In this thesis I aim to resolve these knowledge gaps surrounding the magnitude, cycling and sources of CO2 evasion from high-latitude (mainly arctic) lakes. By combining the estimates of annual CO2 evasion and metabolism, I investigated the magnitude of CO2 evasion, as well as the contribution of the internal carbon processing to CO2 evasion. Inclusion of ice-melt evasion allows to assess the importance, and drivers, of ice-melt CO2 evasion on the annual scale. Furthermore, by pooling lakes from multiple different lake surveys I was able to analyse the lake and landscape variables associated with high-latitude lake metabolism. Finally, through use of an experimental pond facility I manipulated dissolved organic carbon input and temperature to explore the effects of future climate conditions on lake carbon cycling and CO2 evasion.

I found that both external input and internal CO2 production can contribute to CO2 evasion from lakes, but it is often dominated (>75%) by a single source and forest cover increased the amount to which the internal source contributed to annual CO2 evasion. I also found that the concentration of DOC in the lakes was inversely correlated to the proportion of CO2 lost at ice-melt. As a result, the ice-melt season is of significant importance to the annual CO2 evasion from low DOC high-latitude lakes, and omission can underestimate the magnitude of annual CO2-evasion by ~50%. Metabolism in these types of clear-water, low nutrient systems is dominated by benthic (on the sediment) production. Consequently, in-lake metabolism in these high-latitude clear-water lakes is largely constrained by lake depth and basin shape, and the potential for ice-scouring to disturb the benthic system in littoral areas. Convex lakes with predominantly shallow sediments were thus less productive compared to concave lakes where benthic production is less affected by ice-scouring. Finally, increasing DOC inputs (e.g. as a result of changes in climatic conditions) positively related to the amount of CO2 produced within and evaded from the lakes. However, warming was found to decrease in-lake CO2 production and evasion, potentially via increased nutrient limitation of carbon mineralization (i.e. more energy expanded for nutrient uptake in order to break down organic carbon in warmer water), and changes in community structure (e.g. different macrophytes). This thesis thus clearly outlines the annual magnitude (both open water and the specific importance of ice-melt), source contribution (quantified for many lakes rather than single systems) as well as the lake and landscape factors of note to source contribution (i.e. forest cover and DOC input increased internal cycling, especially in shallow and concave systems). Taken together, the results advance understanding the mechanisms behind cycling and evasion of CO2 in earth’s most common lake type.

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.

Lakes are generally supersaturated in carbon dioxide (CO2) and emitters of CO2 to the atmosphere. However, estimates of CO2 flux ((Formula presented.)) 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 (Formula presented.) 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 (Formula presented.) in daytime during periods of surface water CO2 supersaturation in all lakes. Day-night differences in wind speed were correlated with the day-night (Formula presented.) differences in the two larger lakes, but in the smallest and most wind-sheltered lake peaks of (Formula presented.) coincided with low-winds at night. During stratification in the eutrophic lake, CO2 was near equilibrium and diel variability of (Formula presented.) insignificant, but after autumn mixing (Formula presented.) was high with distinct diel variability making this lake a net CO2 source on an annual basis. We found that extrapolating daytime measurements to 24 hr periods overestimated (Formula presented.) by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated (Formula presented.) by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO2 emission estimates.