Anders Jonsson

Climate warming is increasing thermal stratification depth, strength, and duration in lakes, leading to more frequent hypolimnetic oxygen depletion. Most research has focused on eutrophic temperate lakes, which differ significantly from boreal lakes that dominate Earth’s landscape. However, assessing the impact of hypoxia, confounded by browning, warming, and altered stratification, on biogeochemistry and ecological processes in boreal lakes is particularly challenging. Here, we test how oxygenating a hypoxic hypolimnion affects water chemistry, bacterial and primary production (BP and PP), and detritus degradation in a shallow humic boreal lake divided into two basins in an experimental four-year Before-After Control-Impact (BACI) design. After two control years, we oxygenated the hypolimnion of one basin during two stratified periods without disturbing the seasonal development of the thermocline. Hypolimnetic oxygen concentrations moderately impacted lake biogeochemistry. Reoxygenation altered nitrification pathways (increased NO ₃ ⁻ ) of the hypolimnion, and slightly decreased epilimnion and lake BP (− 6.1% of annual average) and green tea degradation (− 6.0%), whereas Rooibos degradation slightly increased (7.3%). Other water chemistry parameters remained within natural variation. We compared our BACI approach, which separates natural variation, to the simpler Before vs After approach, which does not. We find that studies not accounting for seasonal and among-year variability may overestimate the effects of oxygenation on hypolimnion biogeochemistry, as much of the observed impact is due to natural climate variation. Climate warming and altered stratification patterns are therefore likely to impact boreal lake algal and bacterial production and degradation more than hypolimnion hypoxia during the stratified period.

Nitrogen (N) deposition and dissolved organic carbon (DOC) levels in northern lakes are shifting due to climate change and atmospheric deposition declines, altering the availability of light and nutrients in these ecosystems. Yet their impacts on the biomass, stoichiometry, and the structure of planktonic food chains remain uncertain. We therefore investigated zooplankton-to-seston biomass ratios (Z:S in C, N, and P) across 34 Swedish lakes with varying N deposition, DOC concentration, and fish predation control. Mean Z:S values were 2.9% for C, 7.5% for N, and 7.7% for P, with substantial regional variation. Z:S ratios were higher in lakes with lower atmospheric N deposition, improved seston quality, and greater calanoid copepod dominance in zooplankton. The strong link between zooplankton stoichiometry and community composition underscores the role of calanoids in regulating nutrient dynamics in northern lakes. Fish predation reduced zooplankton biomass but did not significantly alter Z:S ratios or zooplankton community composition. Meanwhile, increasing DOC dampened the higher Z:S in low N deposition lakes by reducing calanoid dominance and promoting more uniform zooplankton assemblages. Our findings suggest that lake browning counteracts the expected increase in Z:S ratios associated with recovery from atmospheric N deposition, potentially altering nutrient transfer in lake food webs.

Widespread increases in lake browning, which affects primary production, have been observed in northern lakes. While lake browning is attributed to increases in terrestrially derived total organic carbon (TOC) and total iron (Fe), Fe does not consistently correlate with increasing TOC over time. This temporal mismatch between TOC and Fe indicates that we still do not fully understand the causes of lake browning, especially in the context of gradually changing climatic conditions. In this study, we utilized Fennoscandian (Swedish and Finnish) 30-year (1990-2020) time series data for 102 lakes to describe possible reasons for the temporal decoupling between TOC and Fe. Using wavelet coherence analysis, we found evidence of interannual response of median TOC and Fe concentrations to changes in precipitation patterns. Although TOC and Fe were increasing in most lakes following an increase in precipitation (indicating they were temporally coupled), 41% of the lakes—typically larger with shorter water residence times—exhibited a declining trend in Fe. This decline was associated with short-term (2 to 4 years) increases in precipitation, leading to temporal decoupling. Furthermore, we found no evidence that rising air temperatures or declining sulfur (S) deposition influence the coupling or decoupling patterns between TOC and Fe. Our findings suggest that natural variability of lake types and their responses to changing climatic conditions (here primarily changes in precipitation) are the main factors contributing to temporal decoupling of TOC and Fe in some Fennoscandian lakes.

Zooplankton community composition of northern lakes is changing due to the interactive effects of climate change and recovery from acidification, yet limited data are available to assess these changes combined. Here, we built a database using archives of temperature, water chemistry and zooplankton data from 60 Scandinavian lakes that represent broad spatial and temporal gradients in key parameters: temperature, calcium (Ca), total phosphorus (TP), total organic carbon (TOC), and pH. Using machine learning techniques, we found that Ca was the most important determinant of the relative abundance of all zooplankton groups studied, while pH was second, and TOC third in importance. Further, we found that Ca is declining in almost all lakes, and we detected a critical Ca threshold in lake water of 1.3 mg L⁻¹, below which the relative abundance of zooplankton shifts toward dominance of Holopedium gibberum and small cladocerans at the expense of Daphnia and copepods. Our findings suggest that low Ca concentrations may shape zooplankton communities, and that current trajectories of Ca decline could promote widespread changes in pelagic food webs as zooplankton are important trophic links from phytoplankton to fish and different zooplankton species play different roles in this context.

Global change impacts important environmental drivers for pelagic gross primary production (GPP) in northern lakes, such as temperature, light, nutrient, and inorganic carbon availability. Separate and/or synergistic impacts of these environmental drivers on pelagic GPP remain largely unresolved. Here, we assess key drivers of pelagic GPP by combining detailed depth profiles of summer pelagic GPP with environmental and climatic data across 45 small and shallow lakes across northern Sweden (20 boreal, 6 subarctic, and 19 arctic lakes). We found that across lakes summer pelagic GPP was strongest associated with lake water temperatures, lake carbon dioxide (CO₂) concentrations impacted by lake water pH, and further moderated by dissolved organic carbon (DOC) concentrations influencing light and nutrient conditions. We further used this dataset to assess the extent of additional DOC‐induced warming of epilimnia (here named internal warming), which was especially pronounced in shallow lakes (decreasing 0.96°C for every decreasing m in average lake depth) and increased with higher concentrations of DOC. Additionally, the total pools and relative proportion of dissolved inorganic carbon and DOC, further influenced pelagic GPP with drivers differing slightly among the boreal, subarctic and Arctic biomes. Our study provides novel insights in that global change affects pelagic GPP in northern lakes not only by modifying the organic carbon cycle and light and nutrient conditions, but also through modifications of inorganic carbon supply and temperature. Considering the large‐scale impacts and similarities of global warming, browning and recovery from acidification of lakes at higher latitudes throughout the northern hemisphere, these changes are likely to operate on a global scale.

Accounting for temporal changes in carbon dioxide (CO ₂ ) emissions from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of eddy covariance flux measurements of CO ₂ from 13 lakes and reservoirs in the Northern Hemisphere (NH) and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO ₂ flux at all sites. Accounting for diel variation, only 11% of site-months were net daily sinks of CO ₂ . Annual CO ₂ emissions had an average of 25% (range 3-58%) interannual variation. Nighttime emissions regularly exceeded daytime emissions. Sources of CO ₂ flux variability were delineated through mutual information analysis. Sample analysis of CO ₂ fluxes indicate importance of continuous sampling. Constraining short- and long-term variability is necessary to improve detection of temporal changes of CO ₂ fluxes in response to natural and anthropogenic drivers.

Global environmental changes are causing widespread nutrient depletion, declines in the ratio of dissolved inorganic nitrogen (N) to total phosphorus (DIN:TP), and increases in both water temperature and terrestrial colored dissolved organic carbon (DOC) concentration (browning) in high‐latitude northern lakes. Declining lake DIN:TP, warming, and browning alter the nutrient limitation regime and biomass of phytoplankton, but how these stressors together affect the nutritional quality in terms of polyunsaturated fatty acid (PUFA) contents of the pelagic food web components remains unknown. We assessed the fatty acid compositions of seston and zooplankton in 33 lakes across south‐to‐north and boreal‐to‐subarctic gradients in Sweden. Data showed higher lake DIN:TP in the south than in the north, and that boreal lakes were warmer and browner than subarctic lakes. Lake DIN:TP strongly affected the PUFA contents—especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—in seston, calanoids, and copepods (as a group), but not in cladocerans. The EPA+DHA contents increased by 123% in seston, 197% in calanoids, and 230% in copepods across a lake molar DIN:TP gradient from 0.17 to 14.53, indicating lower seston and copepod nutritional quality in the more N‐limited lakes (those with lower DIN:TP). Water temperature affected EPA+DHA contents of zooplankton, especially cladocerans, but not seston. Cladoceran EPA+DHA contents were reduced by ca. 6% for every 1°C increase in surface water. Also, the EPA, DHA, or EPA+DHA contents of Bosmina, cyclopoids, and copepods increased in lakes with higher DOC concentrations or aromaticity. Our findings indicate that zooplankton food quality for higher consumers will decrease with warming alone (for cladocerans) or in combination with declining lake DIN:TP (for copepods), but impacts of these stressors are moderated by lake browning. Global environmental changes that drive northern lakes toward more N‐limited, warmer, and browner conditions will reduce PUFA availability and nutritional quality of the pelagic food web components.

Accounting for temporal changes in carbon dioxide (CO ₂ ) emissions from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of eddy covariance flux measurements of CO ₂ from 13 lakes and reservoirs in the Northern Hemisphere (NH) and quantify the magnitude and dynamics at multiple temporal scales. We found pronounced diel and sub-monthly oscillatory variations in CO ₂ flux at all sites. Diel variation converted sites to daily net sinks of CO ₂ in only 11% of site-months. Upscaled annual emissions had an average of 25% (range 3-58%) interannual variation. Given temporal variation remains under-represented in inventories of CO ₂ emissions from lakes and reservoirs, revisions in CO ₂ flux are needed using a better representation of sub-daily to interannual variability. Constraining short- and long-term variability is necessary to improve detection of temporal changes of CO ₂ fluxes in response to natural and anthropogenic drivers.

CO₂evasion from freshwater lakes is an important component of the carbon cycle. However, the relative contribution from different lake sizes may vary, since several parameters underlying CO₂flux are size dependent. Here we estimated the annual lake CO₂evasion from a catchment in northern Sweden encompassing about 30,000 differently sized lakes. We show that areal CO₂fluxes decreased rapidly with lake size, but this was counteracted by the greater overall coverage of larger lakes. As a result, total efflux increased with lake size and the single largest lake in the catchment dominated the CO₂evasion (53% of all CO₂evaded). By contrast, the contribution from the smallest ponds (about 27,000) was minor (2flux rates and areal contribution of various sized lakes in assessments of CO₂evasion at the landscape scale.