We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
Widespread changes in water temperatures, salinity, alkalinity and pH have been documented in inland waters in North America, which influence ion exchange, weathering rates, chemical solubility and ...contaminant toxicity. Increasing major ion concentrations from pollution, human-accelerated weathering and saltwater intrusion contribute to multiple ecological stressors such as changing ionic strength and pH and mobilization of chemical mixtures resulting in the freshwater salinization syndrome (FSS). Here, we explore novel combinations of elements, which are transported together as chemical mixtures containing salts, nutrients and metals as a consequence of FSS. First, we show that base cation concentrations have increased in regions primarily in North America and Europe over 100 years. Second, we show interactions between specific conductance, pH, nitrate and metals using data from greater than 20 streams located in different regions of the USA. Finally, salinization experiments and routine monitoring demonstrate mobilization of chemical mixtures of cations, metals and nutrients in 10 streams draining the Washington, DC-Baltimore, MD metropolitan regions. Freshwater salinization mobilizes diverse chemical mixtures influencing drinking water quality, infrastructure corrosion, freshwater CO
concentrations and biodiversity. Most regulations currently target individual contaminants, but FSS requires managing mobilization of multiple chemical mixtures and interacting ecological stressors as consequences of freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.
Temporal trends in stream water total organic carbon (TOC) concentration and export were studied in 8 forested headwater catchments situated in eastern Finland. The Seasonal Kendall test was ...conducted to identify the trends and a mixed model regression analysis was used to describe how catchment characteristics and hydrometeorological variables (e.g. precipitation, air and stream water temperatures, and atmospheric deposition) related to the variation in the concentration and export of stream water TOC. The 8 catchments varied in size from 29 to 494
ha and in the proportion of peatland they contained, from 8 to 70%. Runoff and TOC concentration were monitored for 15–29
years (1979–2006). Trends and variation in TOC levels were analysed from annual and seasonal time series. Mean annual TOC concentration increased significantly in seven of the eight catchments. The trends were the strongest in spring and most apparent during the last decade of the study period. The slopes of the trends were generally smaller than the variation in TOC concentration between years and seasons and between catchments. The annual TOC export showed no clear trends and values were largely determined by the temporal variability in runoff. Annual runoff showed a decreasing trend in two of the eight catchments. Mean annual air and stream water temperatures showed increasing trends, most clearly seen in the summer and autumn series. According to our modeling results, stream water temperature, precipitation and peatland percentage were the most important variables explaining annual and most seasonal TOC concentrations. The atmospheric deposition of SO
4, NH
4, and NO
3 decreased significantly over the study period, but no significant link with TOC concentration was found. Precipitation was the main hydrometeorological driver of the TOC export. We concluded that stream water TOC concentrations and exports are mainly driven by catchment characteristics and hydrometeorological factors rather than trends in atmospheric acid deposition.
To better integrate lotic ecosystems into global cycles and budgets, we provide approximations of the size-distribution and areal extent of streams and rivers. One approach we used was to employ ...stream network theory combined with data on stream width. We also used detailed stream networks on 2 continents to estimate the fraction of continental area occupied by streams worldwide and corrected remote sensing stream inventories for unresolved small streams. Our estimates of global fluvial area are 485 000 to 662 000 km2 and are +30-300% of published appraisals. Moderately sized rivers (orders 5-9) seem to comprise the greatest global area, with less area covered by low and high order streams, while global stream length, and therefore the riparian interface, is dominated by 1st order streams. Rivers and streams are likely to cover 0.30-0.56% of the land surface and make contributions to global processes and greenhouse gas emissions that may be +20-200% greater than those implied by previous estimates.
Associations between catchment characteristics (altitude, slope, proportion of peatland, site fertility, total stem volume, and its distribution among the main tree species), climatic drivers ...(temperature sum, latitude, precipitation and runoff) and atmospheric deposition and concentrations and export of C, N, P and Fe in 21 unmanaged boreal catchments (0.07 to 14 km^sup 2^) were examined in data sets spanning 3 to 32 years. In the entire data base including catchments from southernmost Finland to the Arctic Circle, N and P concentrations were associated with climatic drivers and deposition, while a catchment characteristic, peatland percentage, reflecting topography and site fertility was the best predictor for TOC (total organic carbon) and total Fe concentrations and export. In a subgroup of 18 catchments located in similar climatic conditions and low N deposition in eastern Finland (<600 kg N km^sup -2^ yr^sup -1^), peatland percentage accounted for 52-75% of the export of TOC, TN (total nitrogen), TON (total organic nitrogen), NH4-N and total Fe, while slope was the best predictor of TP (total phosphorus) export (r^sup 2^ = 0.56, p < 0.001). Furthermore, among these 18 catchments TOC export accounted for as much as 95% of the variance in TON export, and 61-73% of the variance in NH4-N, total Fe and TP export. Total stem volume of the tree stands and their distribution between tree species and vegetation types were not important predictors for concentrations/export. The peatland percentage of the 21 catchments ranged widely, from 0 to 88% , with an average value of 34% as in entire Finland. Consequently, among the 21 catchments there was also a large variability both in average TOC and pH, 1.2-30 mg l^sup -1^ and 4.2-7.1, respectively. The average annual TOC export was 6,200 kg km^sup -2^ yr^sup -1^ , significantly lower values were recorded for the export of TN, TP and total Fe (130, 5 and 280 kg km^sup -2^ yr^sup -1^, respectively). N and P export was mainly in an organic form. The TIN (total inorganic N) proportion averaged 13% and elevated NO3-N export occurred only in fertile, high pH sites in southernmost Finland, where stream C/N ratio was lowest. On average, 97% of the TOC, 94% of the TN and 79% of the TP was in a dissolved form.PUBLICATION ABSTRACT
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.
Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past ...50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long‐term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy‐nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by‐products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.
The recovery of streams to atmospheric acid deposition has led to the changes in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations that have altered the stoichiometric ratios (DOC:DON) of the dissolved organic matter (DOM) pool. Certain sites have shown increasing DOC:DON trends (enrichment of the C‐rich fraction of the DOM pool), declining DOC:DON trends (enrichment of the N‐rich fraction of the DOM pool), and/or no clear trends in DOC:DON ratios. The responses vary geographically suggesting that the controls of DOM trends are at the regional to local scale.
A comprehensive cross‐biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in ...running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N‐rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0–1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean.
Plain Language Summary
Ammonium and nitrate in freshwaters have received considerable attention due to their clear ecological and health effects. A comprehensive assessment of N in freshwaters that includes DON is lacking. Including DON in studies of surface water chemistry is important because it can cause eutrophication and certain forms can be rapidly removed by microbial communities. Here, we document how elevated levels of TDN impact the concentrations and relative proportions of all three forms of dissolved N and the stoichiometry of DOM. Our results suggest that human activities fundamentally alter the composition of the dissolved nitrogen pool and the stoichiometry of DOM. Results also highlight feedbacks between the C and N cycles in freshwater ecosystems that are poorly studied.
Key Points
We synthesize a global data set of stream chemistry to examine how the composition of dissolved N and DOC:DON ratios respond to N enrichment
Under low total dissolved N concentrations, the dominate form of N is highly variable but switches to primarily inorganic forms at high TDN
With N enrichment, DOM becomes more N‐rich (lower DOC:DON ratios) while concentrations of DON are less associated with concentrations of DOC
Widespread changes in water temperatures, salinity, alkalinity and pH have been documented in inland waters in North America, which influence ion exchange, weathering rates, chemical solubility and ...contaminant toxicity. Increasing major ion concentrations from pollution, human-accelerated weathering and saltwater intrusion contribute to multiple ecological stressors such as changing ionic strength and pH and mobilization of chemical mixtures resulting in the freshwater salinization syndrome (FSS). Here, we explore novel combinations of elements, which are transported together as chemical mixtures containing salts, nutrients and metals as a consequence of FSS. First, we show that base cation concentrations have increased in regions primarily in North America and Europe over 100 years. Second, we show interactions between specific conductance, pH, nitrate and metals using data from greater than 20 streams located in different regions of the USA. Finally, salinization experiments and routine monitoring demonstrate mobilization of chemical mixtures of cations, metals and nutrients in 10 streams draining the Washington, DC–Baltimore, MD metropolitan regions. Freshwater salinization mobilizes diverse chemical mixtures influencing drinking water quality, infrastructure corrosion, freshwater CO₂ concentrations and biodiversity. Most regulations currently target individual contaminants, but FSS requires managing mobilization of multiple chemical mixtures and interacting ecological stressors as consequences of freshwater salinization.
This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.
The water chemistry of 21 outlet brooks in undisturbed, forested catchments (0.074-38 km super(2)) in Finland was monitored during three years (1997-1999) in order to estimate the background levels ...and leaching of total organic carbon (TOC), iron, nitrogen and phosphorus, and examine the relationships between catchment characteristics and brook water quality in pristine forested areas. The studied catchments are located throughout Finland except the northernmost parts of the country. The concentrations of TOC were relatively high, on average 20 mg L super(-1). Annual leaching of TOC ranged from 3000 to 10 000 kg km super(-2). The average total nitrogen concentration and annual leaching were 430 kg L super(-1) and 140 kg km super(-2), respectively. The average total phosphorus concentration and annual leaching were 15 kg L super(-1) and 5.4 kg km super(-2), respectively. On a national level temperature and discharge conditions were the most important variables for predicting total phosphorus, total nitrogen and TOC concentrations. Both total nitrogen and total phosphorus concentrationsdisplayed positive correlation with temperature, and thus the concentrations were higher in the south than in the north. Nitrogen was mainly organic and showed strong correlation with TOC. On the catchment level, both TOC and total nitrogen concentrations and export had a strongpositive correlation with the abundance of Norway spruce (Piceaabies Karsten) and a strong negative correlation with the abundance of Scots pine (Pinus sylvestris L.). Nitrate concentrations and leaching were related to average site type. The more fertile the average site type was in the catchment, the higher the nitrate concentrations and export were.
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