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Microbial communities play a fundamental role in lake nutrient cycling, yet their composition and functional diversity in response to environmental gradients remain poorly understood. Specifically, little is known about how the supply of dissolved macronutrients, including inorganic and bioavailable organic fractions, shape microbial community structure, and functional diversity in lakes that are strongly subsidized by terrestrial inputs. Boreal lakes, with varying concentrations of total and bioavailable dissolved organic carbon (DOC), nitrogen (N) and phosphorus (P), provide an ideal setting to investigate these dynamics. Here, we hypothesize that microbial pathways related to N and P acquisition, as inferred from marker-gene data, are more represented under relative deficiency of available N and P resources, respectively. To test this, we analysed the rRNA-inferred microbial community composition and metabolic functional diversity across 34 south-Swedish lake outlets in relation to bioavailable nutrient supply. Results show that DOC and P were key drivers of microbial community structure, with bulk DOC concentrations being most relevant for bacteria (16S rRNA), while bioavailable fractions of DOC and P were relatively more influential for eukaryotic communities (18S rRNA). Predicted N- and P-related metabolic pathways correlated with nutrient ratio imbalances, supporting our hypothesis that microbial communities adjust their metabolic strategies in response to relative nutrient demand. These findings demonstrate that accounting for nutrient ratios and bioavailability, in addition to bulk concentrations, helps provide an improved mechanistical understanding of microbial functional potentials in lakes.

Dissolved organic matter (DOM) is a major source of macronutrients to freshwaters, yet it has variable and poorly understood bioavailability. Because intrinsic variation in bioavailability is caused by chemical structures of organic nutrients, DOM composition data should improve predictions of bioavailable resource pool sizes. We hypothesized that bioavailable organic carbon (C) and nitrogen (N) fractions are made up of freshly produced humic- and protein-like DOM, respectively, whereas bioavailable phosphorus (P) is linked to microbially-derived DOM with potential organophosphate content and/or to humic-like structures associated with DOM-Fe-phosphate complexes. These hypotheses were tested from surface waters collected at eight, unproductive and organic matter-rich stream and lake sites, from which we performed C, N and P microbial bioassays with flow cytometry in combination with analyses of DOM composition using fluorescence excitation-emission matrix (EEM) analysis. Bioavailable C followed the predicted patterns, with strong links to fluorescent features indicating recently produced DOM. Surprisingly, bioavailable N was relatively poorly related to DOM composition, including protein-like fluorescence, and was instead driven mainly by the amount of inorganic N. The bioavailable P showed patterns in support of the hypothesized link to microbially-derived organic components, whereas its relationships to free or complex-bound forms of inorganic phosphate were inconclusive. Thus, the strength of the hypothesized patterns varied. Nonetheless, in addition to the variability in bioavailable nutrient concentrations explained by standard bulk nutrient variables, we show that DOM composition variables made significant and unique contributions to explaining the variance in bioavailable C (19%), N (13%) and P (18%). Therefore, improved regression models for bioavailable nutrient concentrations could be achieved by including DOM composition among the explanatory variables. Overall, DOM composition analysis is a promising tool to improve prediction and develop our understanding of bioavailable macronutrients in organic matter-rich freshwaters.

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.

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 NO3−) 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.

Coastal eutrophication results from increased riverine loads of inorganic nutrients, including phosphorus (P), which may co-occur with increased dissolved organic carbon (DOC) loading. These DOC molecules are often pigmented, causing water darkening, but they also contain dissolved organic P (DOP), which could exacerbate eutrophication. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive effects of increased DOC, higher inorganic nutrient concentrations, and water darkening in coastal ecosystems. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of labile DOC (glucose), inorganic nutrients and pigmented compounds that cause darkening. Whereas the evidence for labile DOC (glucose) effects on BDOP was weak, inorganic nutrient enrichment caused increases in BDOP concentrations in clear-water mesocosms. By contrast, in experimentally darkened waters, the addition of inorganic P did not contribute to BDOP but mainly persisted in its inorganic form. Our results suggest that water management efforts aimed at preventing or reversing coastal darkening could increase the removal of excess inorganic P from the water due to light-enhanced algal uptake. However, the total dissolved bioavailable P pool may not decrease but rather shift from dominance by inorganic to organic forms. Therefore, mitigating both coastal darkening and eutrophication in these ecosystems is essential for reducing total bioavailable P to a level that supports their ecological balance and functionality.

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 30-year (1990–2020) time series data for 102 lakes to describe possible reasons for the temporal decoupling between TOC and Fe. Using Bayesian mixed-effects models and wavelet coherence analysis, we found evidence for differential responses of TOC and Fe concentrations to changes in precipitation, temperature, and sulfur deposition. While TOC appeared more sensitive to the effects of precipitation, temperature and sulfur deposition in individual lakes, Fe concentrations were impacted by complex interactions among these environmental variables. Although TOC and Fe increased in most lakes in response to increased temperature and precipitation, 41% of the lakes—typically with larger catchment-to-lake area ratios and shorter water residence times—exhibited a declining trend in Fe. This analysis encompasses lakes of both significant and non-significant changes over time. This decline in Fe was associated with short-timescale (2–4 years) increases in precipitation, leading to a temporal decoupling between Fe and TOC. Our findings suggest that Fe concentrations do not increase uniformly with rising temperatures and increased precipitation, especially in regions where sulfur deposition has declined due to atmospheric recovery policies.

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.

Dietary uptake is key for transferring potentially toxic contaminants, such as mercury (Hg) and essential dietary nutrients, such as polyunsaturated fatty acids (PUFA), to consumers at higher trophic levels of aquatic food webs. We evaluated the role of diet sources for Hg bioaccumulation and PUFA retention in fish across lake food webs in seven Swedish lakes and two Chinese reservoirs. Fish total Hg (THg) and methyl-Hg (MeHg) differed greatly between the two countries: the Chinese fish contained less than 300 ng g⁻¹ dry weight (d.w.) THg with less than 50% as MeHg, versus the Swedish fishes which contained approximately 2000 ng g⁻¹ d.w. THg and nearly 100% as MeHg. Fatty acids enrichment of linoleic acids (LIN) were more prevalent in the Chinese fishes regardless of size (p < 0.05). Here we examined food web length, fish growth rates, and fatty acids patterns in relation to the quality of fish as a food source for both Hg and FA. Contrary to the expectation that biodilution of Hg throughout the food chain would explain these differences, a more complex picture emerged with high levels of Hg at the base of the food web in the Chinese reservoirs, a decoupling of fatty acid and Hg bioaccumulation, and a major role for both fish stocking and fish feed. It is hoped that this work will provide a nuanced picture of fish quality as a food source in different ecosystems.

Global change may introduce fundamental alterations in phytoplankton biomass and community structure that can alter the productivity of northern lakes. In this study, we utilized Swedish and Finnish monitoring data from lakes that are spatially (135 lakes) and temporally (1995-2019, 110 lakes) extensive to assess how phytoplankton biomass (PB) of dominant phytoplankton groups related to changes in water temperature, pH and key nutrients [total phosphorus (TP), total nitrogen (TN), total organic carbon (TOC), iron (Fe)] along spatial (Fennoscandia) and temporal (25 years) gradients. Using a machine learning approach, we found that TP was the most important determinant of total PB and biomass of a specific species of Raphidophyceae - Gonyostomum semen - and Cyanobacteria (both typically with adverse impacts on food-webs and water quality) in spatial analyses, while Fe and pH were second in importance for G. semen and TN and pH were second and third in importance for Cyanobacteria. However, in temporal analyses, decreasing Fe and increasing pH and TOC were associated with a decrease in G. semen and an increase in Cyanobacteria. In addition, in many lakes increasing TOC seemed to have generated browning to an extent that significantly reduced PB. The identified discrepancy between the spatial and temporal results suggests that substitutions of data for space-for-time may not be adequate to characterize long-term effects of global change on phytoplankton. Further, we found that total PB exhibited contrasting temporal trends (increasing in northern- and decreasing in southern Fennoscandia), with the decline in total PB being more pronounced than the increase. Among phytoplankton, G. semen biomass showed the strongest decline, while cyanobacterial biomass showed the strongest increase over 25 years. Our findings suggest that progressing browning and changes in Fe and pH promote significant temporal changes in PB and shifts in phytoplankton community structures in northern lakes.

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.