Abstract High‐latitude systems are warming faster than the global average, altering the rates of silicon (Si) mobilization from terrestrial to aquatic systems. Dissolved Si (DSi) concentration and its ratio with other nutrients exert strong controls on algae blooms in fresh and marine receiving waters, especially in high‐latitude regions where diatoms often dominate riverine and coastal autotroph communities. Here we present an examination of decadal scale changes in river DSi concentrations, loads, and nutrient ratios in 70 high‐latitude rivers (>58°) across North America, Europe, Asia, and Antarctica. We examined monthly and annual changes in DSi, dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP) concentrations and yields using sequential breakpoint analysis (i.e., SiZer) that allows for variable trend identification within a single time series. Except for the Antarctic streams, we found a predominant trend of increasing DSi relative to DIN and DIP for both annual concentrations and yields, driven primarily by declines in DIN and DIP rather than increases in DSi. Contrary to expectations, we found little evidence that changes in discharge or shifts in seasonality (i.e., month‐specific trends) drove these altered interannual nutrient dynamics. However, interannual changes in DSi concentrations were associated with hydroclimatic variability (precipitation, evapotranspiration, snow cover, air temperature) and DIP availability. Taken together, our analysis identifies large‐scale shifts in exports of Si, N, and P in high‐latitude rivers, with implications for algae productivity and composition in the highly productive waters of high‐latitude ecosystems.

Riparian and coastal habitats characterize the land-water interface and contain a disproportionate share of the earth’s biodiversity and vital ecosystem services. While terrestrial land use change and habitat loss – the primary cause of biodiversity decline – has been mapped globally, it has not been disaggregated along the land-water interface. The same is true for land abandonment and habitat recovery. We used a temporally harmonized Corine Land Cover product for 39 European countries to quantify loss and gains of (semi-)natural land from urban and cropland expansion and abandonment within riparian and coastal zones. Between 2000 and 2018, Europe lost 16,337 km2 of natural habitat to land use expansion but was partially offset by 8,910 km2 of (semi-)natural habitat recovery associated with land abandonment, resulting in a net loss of 7,427 km2. Habitat loss strongly concentrated near water: 25% occurred within 1 km and 96% within 10 km of the land–water interface. Coastal and riparian zones experienced disproportionately high losses relative to their extent, with net habitat loss intensities 2.6-fold and 1.4-fold greater, respectively, than inland areas. Urban expansion was the dominant driver of habitat loss, causing nearly five times more net loss than cropland expansion despite occupying a much smaller share of land area, and urban-driven losses increased sharply toward the water’s edge. These findings indicate that effective and representative conservation at Europe’s land–water interface requires spatially targeted strategies that prioritise limiting residential urban expansion in coastal zones, managing dryland agricultural expansion in riparian areas, and focusing mitigation efforts near the water’s edge. Given the concentration of human presence and activity at the land–water interface, conservation and restoration efforts in coastal and riparian zones are likely to maximise the delivery of ecosystem services, including climate change mitigation, recreation, and human health benefits.

Arctic fjords support important, productive ecosystems whose nutrient budgets are highly sensitive to climate-driven deglaciation. Here we present over three decades of data describing the nitrogen biogeochemistry of streamflow in the Kongsfjord and Adventfjord regions (West Spitsbergen, Svalbard) to show that microbial nitrate production by nitrification can substantially increase nitrate export to these fjords by runoff. Stable isotopes show that the nitrification of rock or “geogenic” nitrogen is particularly important, especially where shale-rich sedimentary rocks are present. In these cases, the release of nitrified nitrate into runoff may increase total riverine nitrate export by up to ~25 times, providing a potentially significant resource for utilization by downstream ecosystems. Since the influence of nitrification becomes more pronounced as ground thaw progresses, it could sustain primary production as the Arctic growing season lengthens, especially in nitrate-limited fjords. The widespread occurrence of shale bedrock across the Arctic also reveals a likely influence of geogenic nitrogen within the entire region. Microbial nitrate production can increase Arctic riverine nitrate export by up to ~25 times, especially where shale-rich rocks are found, according to data from Svalbard. Primary Handling Editors: Annie Bourbonnais and Alice Drinkwater.

The Arctic is undergoing rapid environmental transformation, with intensified glacial and permafrost melt fundamentally altering freshwater discharge regimes and biogeochemical fluxes to coastal fjord systems. Here, we investigate how seasonal meltwater dynamics shape protistan plankton communities along a terrestrial – marine gradient in Isfjorden (Svalbard) during the exceptionally warm year of 2018. Sampling across three distinct melt season stages – pre-freshet (May), spring freshet (June), and late summer runoff (August) – revealed pronounced temporal and spatial shifts in community structure, strongly linked to evolving environmental gradients. In May, cold, clear, unstratified waters and marine nutrient inputs supported a typical late spring bloom, led by Phaeocystis pouchetii, which significantly contributed to the particulate organic carbon pool, and was followed by diatoms of the genera Chaetoceros and Thalassiosira. The June freshet triggered sharp stratification and nutrient enrichment from glacial and terrestrial sources, driving an unprecedented proliferation of small flagellates, notably Chrysochromulina and two morphologically distinct, yet unidentified taxa. However, by August, escalating turbidity from intensified meltwater inputs and sediment resuspension severely constrained photic conditions, suppressing protistan biomass despite sustained nutrient availability. Across the season, community dynamics were governed by complex interactions between nutrient supply, light limitation, and physical forcings such as stratification and advection. Our findings suggest that ongoing Arctic warming may increasingly favor opportunistic, small flagellates over traditional diatom-dominated blooms, with major implications for carbon cycling and food web dynamics in Arctic fjords.

Riverine silicon (Si) plays a vital role in governing primary production, water quality, and carbon cycling. Climate and land cover change have altered how dissolved Si (DSi) is processed on land, transported to rivers, and cycled through aquatic ecosystems. The Global Aggregation of Stream Silica (GlASS) database was constructed to assess changes in river Si concentrations and fluxes, their relationship to other nutrients (nitrogen (N) and phosphorus (P)), and to evaluate mechanisms driving the availability of Si. GlASS includes concentrations of DSi, dissolved inorganic N (NO3, NOx, and NH4), and dissolved inorganic P (as soluble reactive P or PO4-P) at daily to quarterly time steps from 1963 to 2024; daily discharge; and watershed characteristics for 421 rivers spanning eight climate zones. Original data sources are cited, data quality assurance workflows are public, and input files to a common load model are provided. GlASS offers critical data to address questions about patterns, controls, and trajectories of global river Si biogeochemistry and stoichiometry.

We analyzed stable carbon and nitrogen isotope values (δ13C and δ15N, respectively) for pan-Arctic coastal primary producers and consumers to detect large-scale regional trends both temporally and spatially. To facilitate comparison, we grouped coastal habitats into fjords, lagoons, shelves, and straits as four "coastscapes". We gathered over 12,000 rows of data collected over 24 years (between 1999 and 2022) from 34 different field campaigns across the coastal Arctic (63 to 81°N and 177°W to 33°E). Our goal was to examine the isotopic patterns in pelagic and sediment particulate organic matter (pPOM and sPOM, respectively) and four consumer groups (deposit feeders, opportunists/scavengers, predators, and suspension feeders) among the four coastscapes. We found that despite the enormous spatial range of data, both pPOM and sPOM became 2.1‰ and 2.2‰ more 13C-depleted per decade, respectively, with parallel decreases in the δ13C values in consumers. The significant decrease is likely attributed to the increased contributions of 13C-depleted terrestrial organic matter across the Arctic coasts from freshwater inputs and coastal erosion in concert with diminishing sea ice that supports sympagic microalgae. Across all Arctic coastscapes, consumer groups exhibited overlapping isotopic composition, notably with wide δ13C ranges that indicated assimilation of multiple organic matter sources, including terrestrial organic matter, organic matter derived from marine phytoplankton and sea ice algae, macroalgae, and potentially benthic microalgae or degraded organic matter. This consistent pattern across coastscapes provides evidence of the trophic plasticity possessed by Arctic consumers, how coastal food webs respond to climate warming, and the signature of terrestrialization imprinted on the pan-Arctic coastal isoscape.

Climate warming is especially pronounced in winter and at high latitudes. Warming winters are leading to the loss of lake ice and changing snow cover on lakes. Historically, lake scientists have paid less attention to the ice cover period, leading to data and theory gaps about the role of winter conditions in lake ecosystem function and the consequences of changing winters. Here we use simple models to show that the latitudinal interaction between ice cover duration and light flux seasonality has profound and underappreciated implications for lakes. Our models focus on light and temperature, two key drivers of ecosystem processes. We show that the relative amount of light arriving in lakes during ice cover increases non-linearly with latitude and that the light climate of high latitude lakes is much more sensitive to changing winter conditions than that of lower latitude lakes. We also demonstrate that the synchronicity between high light and warm temperatures may decrease with latitude, with implications for primary and secondary production. Our results suggest that ice loss may lead to greater relative change to productivity and biotic interactions in higher latitude lakes and also offer several testable predictions for understanding the consequences of climate-induced changes across latitudinal gradients.

Streams and lakes are nested within, and intimately connected with, the surrounding terrestrial landscape. However, these aquatic components of the catchment are commonly neglected and often not included in studies and management plans that focus on terrestrial ecosystems. Developing an improved understanding of biotic and abiotic connectivity across these ecosystems is key for the advancement of knowledge‐based management of natural ecosystems. Here, we take advantage of existing and extensive long‐term monitoring of the terrestrial food web in the Norwegian Arctic to propose how both new and existing freshwater monitoring can be integrated with terrestrial monitoring to develop a unifying design for adaptive monitoring of low‐Arctic catchments. Specifically, we suggest that relevant cross‐ecosystem linkages between freshwater and terrestrial ecosystems should be integrated in long‐term monitoring, and we outline the methods and timing for how to do this and use the aquatic systems as sentinels for detecting climate‐driven changes. While the plan we present here is tailored for the Norwegian low‐Arctic, it provides an approach that can be more broadly applied (or modified) to monitor many terrestrial–aquatic meta‐ecosystems subjected to impact of climate change around the world.

Fluvial silicon (Si) plays a critical role in controlling primary production, water quality, and carbon sequestration through supporting freshwater and marine diatom communities. Geological, biogeochemical, and hydrological processes, as well as climate and land use, dictate the amount of Si exported by streams. Understanding Si regimes-the seasonal patterns of Si concentrations-can help identify processes driving Si export. We analyzed Si concentrations from over 200 stream sites across the Northern Hemisphere to establish distinct Si regimes and evaluated how often sites moved among regimes over their period of record. We observed five distinct regimes across diverse stream sites, with nearly 60% of sites exhibiting multiple regime types over time. Our results indicate greater spatial and interannual variability in Si seasonality than previously recognized and highlight the need to characterize the watershed and climate variables that affect Si cycling across diverse ecosystems.

Climate change is altering patterns of precipitation, cryosphere thaw, and land-ocean influxes, affecting understudied Arctic estuarine tidal flats. These transitional zones between terrestrial and marine systems are hotspots for biogeochemical cycling, often driven by microbial processes. We investigated surface sediment bacterial community composition and function from May to September along a river-intertidal-subtidal-fjord gradient. We paired metabarcoding of in situ communities with in vitro carbon-source utilization assays. Bacterial communities differed in space and time, alongside varying environmental conditions driven by local seasonal processes and riverine inputs, with salinity emerging as the dominant structuring factor. Terrestrial and riverine taxa were found throughout the system, likely transported with runoff. In vitro assays revealed sediment bacteria utilized a broader range of organic matter substrates when incubated in fresh and brackish water compared to marine water. These results highlight the importance of salinity for ecosystem processes in these dynamic tidal flats, with the highest potential for utilization of terrestrially derived organic matter likely limited to tidal flat areas (and times) where sediments are permeated by freshwater. Our results demonstrate that intertidal flats must be included in future studies on impacts of increased riverine discharge and transport of terrestrial organic matter on coastal carbon cycling in a warming Arctic.

Climate warming causes shorter winters and changes in ice and snow cover in subarctic lakes, highlighting the need to better understand under-ice ecosystem functioning. The plankton community in a subarctic, oligotrophic lake was studied throughout the ice-covered season, focusing on lipid dynamics and life history traits in two actively overwintering copepods, Cyclops scutifer and Eudiaptomus graciloides. Whereas C. scutifer was overwintering in C-IV to C-V stage, E. graciloides reproduced under ice cover. Both species had accumulated lipids prior to ice-on and showed a substantial decrease in total lipid content throughout the ice-covered period: E. graciloides (60%-38% dw) and C. scutifer (73%-33% dw). Polyunsaturated fatty acids of algal origin were highest in E. graciloides and declined strongly in both species. Stearidonic acid (18:4n-3) content in E. graciloides was particularly high and decreased rapidly during the study period by 50%, probably due to reproduction. The copepods differed in feeding behavior, with the omnivore C. scutifer continuing to accumulate lipids until January, whereas the herbivorous E. graciloides accumulated lipids from under-ice primary production during the last months of ice-cover. Our findings emphasize the importance of lipid accumulation and utilization for actively overwintering copepods irrespective of the timing of their reproduction.

The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termed DSi regime, mediates the timing of DSi delivery to downstream waters and thus governs river biogeochemical function and aquatic community condition. Previous work identified five distinct DSi regimes across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple DSi regimes over time. Several potential drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, and yet the large-scale spatiotemporal controls on DSi regimes have not been identified. We evaluate the role of environmental variables on the behavior of DSi regimes in nearly 200 rivers across the Northern Hemisphere using random forest models. Our models aim to elucidate the controls that give rise to (a) average DSi regime behavior, (b) interannual variability in DSi regime behavior (i.e., Annual DSi regime), and (c) controls on DSi regime shape (i.e., minimum and maximum DSi concentrations). Average DSi regime behavior across the period of record was classified accurately 59% of the time, whereas Annual DSi regime behavior was classified accurately 80% of the time. Climate and primary productivity variables were important in predicting Average DSi regime behavior, whereas climate and hydrologic variables were important in predicting Annual DSi regime behavior. Median nitrogen and phosphorus concentrations were important drivers of minimum and maximum DSi concentrations, indicating that these macronutrients may be important for seasonal DSi drawdown and rebound. Our findings demonstrate that fluctuations in climate, hydrology, and nutrient availability of rivers shape the temporal availability of fluvial DSi. The amount of dissolved silicon (DSi) in rivers is an important control on numerous ecological and biogeochemical processes, such as types of algae that bloom and rates of carbon sequestration. Compared to our knowledge of other nutrients, such as nitrogen and phosphorus, we have limited understanding of what controls the timing and concentration of DSi in rivers. Previous work identified five distinct seasonal patterns of DSi concentrations in rivers across the Northern Hemisphere; here we look at the environmental variables that control these seasonal patterns. We found that rivers often have one to five seasonal patterns over time due to interannual shifts in temperature, evapotranspiration, and streamflow. In addition, we found that the average shape of the seasonal pattern for a given river, specifically minimum and maximum DSi concentrations, was related to nitrogen (N) and phosphorus (P) concentrations, highlighting linkages between N, P, and DSi cycling in rivers. This work identifies why river DSi concentrations exhibit both within and between year variability, highlighting that temperature, streamflow, and nutrient availability control the timing of river DSi availability for biological uptake. Seasonal variations in annual riverine dissolved silica concentrations (DSi regime) were correctly classified 80% of the time Climate and primary productivity emerge as the most important drivers in differentiating among average DSi regimes Median nitrogen and phosphorus concentrations strongly predicted minimum and maximum DSi concentration, regardless of regime type

There is an urgent need to understand and address the risks associated with a warming climate for ecosystems and societies in the Arctic and sub-Arctic regions. There are major gaps in our understanding of the complex effects of climate change-including extreme events, cascading impacts across ecosystems, and the underlying socioecological dynamics and feedbacks-all of which need collaborative efforts to be resolved. Here, we present results where climate scientists, ecologists, social scientists, and practitioners were asked to identify the most urgent research needs for understanding climate change impacts and to identify the actions for reducing future risks in catchment areas in the Norwegian High North, a region that encompasses both Arctic and sub-Arctic climates in northern Norway. From a list of 77 questions, our panel of 19 scientists and practitioners identified 15 research needs that should be urgently addressed. We particularly urge researchers to investigate cross-ecosystem impacts and the socioecological feedbacks that could amplify or reduce risks for society.

The increased export of terrestrial dissolved organic matter (terrDOM) to coastal marine ecosystems may affect local filter feeders and the local food web via the altered uptake of organic material and associated contaminants. To compare terrDOM to marine DOM (marDOM) as contaminant vectors to coastal biota, we exposed blue mussels (Mytilus sp.) to the different DOM types in combination with teflubenzuron, a widely applied lipophilic aquaculture medicine targeting salmon lice (Lepeophtheirus salmonis). A 16-day exposure of the blue mussels to DOM and teflubenzuron was followed by a depuration phase of 20 days without teflubenzuron. We calculated teflubenzuron adsorption rates and bioaccumulation factors (BAF) using a Bayesian model, expecting teflubenzuron uptake to be greater with terrDOM than marDOM due to the higher prevalence of large amphipathic humic acids in terrDOM. Humic acids have strong absorption properties and are able to envelope lipophilic molecules. Thus, humic acids can function as an efficient contaminant vector when taken up by filter feeders. Although there were varying degrees of overlap, the mussels tended to accumulate higher amounts of teflubenzuron in the DOM treatments than in the seawater control (bioaccumulation factor [BAF] in seawater: median 106 L/kg; 2.5 %-97.5 % percentile: 69-160 L/kg). Contrary to expectations, mussels exposed to marDOM showed a trend toward more bioaccumulation of teflubenzuron than those exposed to terrDOM (BAF marine 144 L/kg; 102-221 L/kg versus BAF terrestrial: 121 L/kg; 82-186 L/kg). The highest teflubenzuron accumulation was observed with the 50:50 mixture of marDOM and terrDOM (BAF mix: 165 L/kg; 117-244 L/kg). The slight difference in DOM-type accumulation rates observed in this experiment-especially the accumulation rate of terrDOM compared to that of the seawater-only treatment type-was not considered environmentally relevant. Further studies are necessary to see if the observed trends transfer to complex environmental systems.

Ecosystems are shaped by physical, chemical, and biological drivers, which affect the quality and quantity of basal energy sources, with impacts that cascade to higher trophic levels. In coastal, shelf, and marine habitats, terrestrial-derived organic matter (ter-OM) can be a key driver of ecosystem structure and function. Climate change is expected to alter land-ocean connectivity in many regions, with a broad range of potential consequences for impacted ecosystems, particularly in the coastal zone. The benthic compartment is an important link between the large organic carbon pools stored on land and the marine environment. At the same time, the macrofauna plays a key role in the processing, biological uptake, and fate of ter-OM in the aquatic environment, with implications for coastal ecosystem functioning, benthic-pelagic coupling, carbon burial, and biogeochemical cycles. However, information about relationships between land-ocean connectivity (including ter-OM loads) and coastal benthic community responses remains spread across disciplines, and a broad perspective on the potential impacts of a changing climate is still missing. Here, we explore the interplay between benthic macrofaunal communities and ter-OM through a paired narrative and research weaving analysis, which combines systematic mapping and bibliometric analysis. The review describes the past development and status of the research field as well as the lack of information in some geographical regions and habitats worldwide. We highlight the role of macrofauna in carbon cycling and the growing evidence that ter-OM plays a key role in the structure and function of benthic communities, not strictly limited to estuarine habitats. Climate change poses challenges for the prediction of future ter-OM fluxes and potential macrofauna responses to this additional stressor, thus requiring new methodological approaches (e.g., multimarker approaches for OM characterization) and long-term monitoring programs across different habitats and spatiotemporal scales.

Mercury (Hg) is a serious concern for aquatic ecosystems because it may biomagnify to harmful concentrations within food webs and consequently end up in humans that eat fish. However, the trophic transfer of mercury through the aquatic food web may be impacted by several factors related to network complexity and the ecology of the species present. The present study addresses the interplay between trophic ecology and mercury contamination in the fish communities of two lakes in a pollution-impacted subarctic watercourse, exploring the role of both horizontal (feeding habitat) and vertical (trophic position) food web characteristics as drivers for the Hg contamination in fish. The lakes are located in the upper and lower parts of the watercourse, with the lower site located closer to, and downstream from, the main pollution source. The lakes have complex fish communities dominated by coregonids (polymorphic whitefish and invasive vendace) and several piscivorous species. Analyses of habitat use, stomach contents, and stable isotope signatures (δ15 N, δ13 C) revealed similar food web structures in the two lakes except for a few differences chiefly related to ecological effects of the invasive vendace. The piscivores had higher Hg concentrations than invertebrate-feeding fish. Concentrations increased with size and age for the piscivores and vendace, whereas habitat differences were of minor importance. Most fish species showed significant differences in Hg concentrations between the lakes, the highest values typically found in the downstream site where the biomagnification rate also was higher. Mercury levels in piscivorous fish included concentrations that exceed health authorization limits, with possible negative implications for fishing and human consumption. Our findings accentuate the importance of acquiring detailed knowledge of the drivers that can magnify Hg concentrations in fish and how these may vary within and among aquatic systems, to provide a scientific basis for adequate management strategies. Environ Toxicol Chem 2023;42:873-887. © 2023 SETAC.

Increased export of terrestrially derived dissolved organic matter (DOM) to coastal marine ecosystems may affect local filter feeders and the associated local food web via altered uptake of organic material and associated contaminants. To assess terrestrial derived DOM (terrDOM), compared to marine derived DOM (marDOM) as a contaminant vector to coastal biota, we exposed blue mussels (Mytilus sp.) to different types of DOM in combination with teflubenzuron. Teflubenzuron is a widely applied lipophilic aquaculture medicine targeting salmon lice (Lepeophtheirus salmonis). A 16-day exposure of the blue mussels to DOM and teflubenzuron was followed by depuration phase of 20 days without teflubenzuron. We calculated teflubenzuron adsorption rates as well as bioaccumulation factors (BAF) using a Bayesian model. We expected teflubenzuron uptake to be greater with terrDOM due to the higher prevalence of large amphipathic humic acids in terrDOM. Humic acids may result in strong absorption and potential for envelopment of lipophilic molecules, and thus could function as an efficient contaminant vector when taken up by filter feeders. In all DOM treatments, mussels showed a slight tendency towards accumulating higher amounts of teflubenzuron than the seawater control, although to varying degree overlapping with seawater (bioaccumulation factor (BAF) in seawater: median 106 L/kg; 2.5%–97.5% percentile: 69–160 L/kg). Contrary to expectations, mussels exposed to marDOM apparently bioaccumulated more teflubenzuron than those exposed to terrDOM (BAF marine 144 L/kg; 102–221 L/kg versus BAF terrestrial: 121 L/kg; 82–186 L/kg). The highest teflubenzuron accumulation was however observed for the 50:50 mixture of marDOM and terrDOM (BAF mix: 165 L/kg; 117–244 L/kg). The small difference in accumulation rates observed between terrDOM and the seawater only treatment type is likely not environmentally relevant, thus the effect of terrDOM alone as a vector for teflubenzuron seems negligible.

Climate change driven increases in permafrost thaw and terrestrial runoff are expected to facilitate the mobilization and transport of mercury (Hg) from catchment soils to coastal areas in the Arctic, potentially increasing Hg exposure of marine food webs. The main aim of this study was to determine the impacts of seasonal riverine inputs on land-ocean Hg transport, zooplankton diet and Hg bioaccumulation in an Arctic estuary (Adventfjorden, Svalbard). The Adventelva River was a source of dissolved and particulate Hg to Adventfjorden, especially in June and July during the river's main discharge period. Stable isotope and fatty acid analyses suggest that zooplankton diet varied seasonally with diatoms dominating during the spring phytoplankton bloom in May and with increasing contributions of dinoflagellates in the summer months. In addition, there was evidence of increased terrestrial carbon utilization by zooplankton in June and July, when terrestrial particles contributed substantially to the particulate organic matter pool. Total (TotHg) and methyl Hg (MeHg) concentrations in zooplankton increased from April to August related to increased exposure to riverine inputs, and to shifts in zooplankton diet and community structure. Longer and warmer summer seasons will probably increase riverine runoff and thus Hg exposure to Arctic zooplankton.

While over 99% of coastal arctic rivers drain small catchments, future projections of land-ocean fluxes are based on data from large rivers. We encourage inclusion of and increased focus on smaller catchments to support representative assessments of arctic ecosystem change.