Summary Climate change and land‐use intensification are increasing productivity in subarctic lakes. Simultaneously, fish and invertebrate species adapted to temperate conditions are expanding their range northwards into subarctic habitats. Community level studies are required to predict long‐term effects of these dual stressors on subarctic freshwater ecosystems. We conducted a space‐for‐time study examining the fish, benthic invertebrate and pelagic zooplankton communities in littoral, profundal and pelagic habitats in 19 subarctic lakes situated on a temperature, land‐use and productivity gradient in northern Europe. Fish density (ranging between 0.5 and 150.5 fish per net series h−1) and biomass (range between 92 and 5,147 g per net series h−1) increased significantly with increasing lake temperature and productivity. This was associated with significantly decreasing body size (26 to 12 cm total length; 174 to 19 g body mass) and a shift in fish community structure from salmonid (Arctic charr Salvelinus alpinus, whitefish Coregonus lavaretus), to percid (ruffe Gymnocephalus cernua, perch Perca fluviatilis) and ultimately cyprinid (roach Rutilus rutilus, bleak Alburnus alburnus) dominance. Changes in fish community composition were most apparent in littoral and pelagic zones. Benthic macroinvertebrate density peaked in mesotrophic lakes, zooplankton density was highest at either end of the gradient, indicating habitat specific differences in predation pressure and top‐down control. Body size of zooplankton and benthic macroinvertebrates was negatively related to temperature and productivity. These results suggest that climate change and intensification of land‐use practices are gradually turning subarctic lakes into warmer, less transparent and more productive systems harbouring abundant, small‐sized and warmer adapted communities.

Climate change and the intensification of land use practices are causing widespread eutrophication of subarctic lakes. The implications of this rapid change for lake ecosystem function remain poorly understood. To assess how freshwater communities respond to such profound changes in their habitat and resource availability, we conducted a space‐for‐time analysis of food‐web structure in 30 lakes situated across a temperature‐productivity gradient equivalent to the predicted future climate of subarctic Europe (temperature +3°C, precipitation +30% and nutrient +45 μg L−1 total phosphorus). Along this gradient, we observed an increase in the assimilation of pelagic‐derived carbon from 25 to 75% throughout primary, secondary and tertiary consumers. This shift was overwhelmingly driven by the consumption of pelagic detritus by benthic primary consumers and was not accompanied by increased pelagic foraging by higher trophic level consumers. Our data also revealed a convergence of the carbon isotope ratios of pelagic and benthic food web endmembers in the warmest, most productive lakes indicating that the incorporation of terrestrial derived carbon into aquatic food webs increases as land use intensifies. These results, reflecting changes along a gradient characteristic of the predicted future environment throughout the subarctic, indicate that climate and land use driven eutrophication and browning are radically altering the function and fuelling of aquatic food webs in this biome.

Abstract. Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM) and as colloidal Fe oxyhydroxides, principally ferrihydrite. However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation processes along a salinity gradient in an estuary of the northern Baltic Sea efficiently transfer Fe and OM from the dissolved phase into particulate material that accumulates in the sediments. Flocculation of Fe and OM is partially decoupled. This is likely due to the presence of discrete colloidal ferrihydrite in the freshwater Fe pool, which responds differently from DOM to estuarine mixing. Further decoupling of Fe from OM occurs during sedimentation. While we observe a clear decline with distance offshore in the proportion of terrestrial material in the sedimentary particulate organic matter (POM) pool, the distribution of flocculated Fe in sediments is modulated by focusing effects. Labile Fe phases are most abundant at a deep site in the inner basin of the estuary, consistent with input from flocculation and subsequent focusing. The majority of the labile Fe pool is present as Fe (II), including both acid-volatile sulfur (AVS)-bound Fe and unsulfidized phases. The ubiquitous presence of unsulfidized Fe (II) throughout the sediment column suggests Fe (II)-OM complexes derived from reduction of flocculated Fe (III)-OM, while other Fe (II) phases are likely derived from the reduction of flocculated ferrihydrite. Depth-integrated rates of Fe (II) accumulation (AVS-Fe + unsulfidized Fe (II) + pyrite) for the period 1970–2015 are greater in the inner basin of the estuary with respect to a site further offshore, confirming higher rates of Fe reduction in near-shore areas. Mössbauer 57Fe spectroscopy shows that refractory Fe is composed largely of superparamagnetic Fe (III), high-spin Fe (II) in silicates, and, at one station, also oxide minerals derived from past industrial activities. Our results highlight that the cycling of Fe in boreal estuarine environments is complex, and that the partial decoupling of Fe from OM during flocculation and sedimentation is key to understanding the role of Fe in sedimentary diagenetic processes in coastal areas.

Large-scale climatic forcing is impacting oceanic biogeochemical cycles and is expected to influence the watercolumn distribution of trace gases, including methane and nitrous oxide. Our ability as a scientific community to evaluate changes in the water-column inventories of methane and nitrous oxide depends largely on our capacity to obtain robust and accurate concentration measurements that can be validated across different laboratory groups. This study represents the first formal international intercomparison of oceanic methane and nitrous oxide measurements whereby participating laboratories received batches of seawater samples from the subtropical Pacific Ocean and the Baltic Sea. Additionally, compressed gas standards from the same calibration scale were distributed to the majority of participating laboratories to improve the analytical accuracy of the gas measurements. The computations used by each laboratory toPublished by Copernicus Publications on behalf of the European Geosciences Union. 5892 S. T. Wilson et al.: An intercomparison of oceanic methane and nitrous oxide measurements derive the dissolved gas concentrations were also evaluated for inconsistencies (e.g., pressure and temperature corrections, solubility constants). The results from the intercomparison and intercalibration provided invaluable insights into methane and nitrous oxide measurements. It was observed that analyses of seawater samples with the lowest concentrations of methane and nitrous oxide had the lowest precisions. In comparison, while the analytical precision for samples with the highest concentrations of trace gases was better, the variability between the different laboratories was higher: 36 % for methane and 27 % for nitrous oxide. In addition, the comparison of different batches of seawater samples with methane and nitrous oxide concentrations that ranged over an order of magnitude revealed the ramifications of different calibration procedures for each trace gas. Finally, this study builds upon the intercomparison results to develop recommendations for improving oceanic methane and nitrous oxide measurements, with the aim of precluding future analytical discrepancies between laboratories.

Estuaries are important conduits between terrestrial and marine aquatic systems and function as hot spots in the aquatic methane cycle. Eutrophication and climate change may accelerate methane emissions from estuaries, causing positive feedbacks with global warming. Boreal regions will warm rapidly in the coming decades, increasing the need to understand methane cycling in these systems. In this 3-year study, we investigated seasonal and spatial variability of methane dynamics in a eutrophied boreal estuary, both in the water column and underlying sediments. The estuary and the connected archipelago were consistently a source of methane to the atmosphere, although the origin of emitted methane varied with distance offshore. In the estuary, the river was the primary source of atmospheric methane. In contrast, in the adjacent archipelago, sedimentary methanogenesis fueled by eutrophication over previous decades was the main source. Methane emissions to the atmosphere from the study area were highly variable and dependent on local hydrodynamics and environmental conditions. Despite evidence of highly active methanogenesis in the studied sediments, the vast majority of the upwards diffusive flux of methane was removed before it could escape to the atmosphere, indicating that oxidative filters are presently still functioning regardless of previous eutrophication and ongoing climate change.