Geology topics

Robert G. Striegl

Publications and source records attributed to Robert G. Striegl.

At least 37 records · Page 2Linked to original sources

USGS2020

Potential impacts of mercury released from thawing permafrost

Mercury (Hg) is a naturally occurring element that bonds with organic matter and, when converted to methylmercury, is a potent neurotoxicant. Here we estimate potential future releases of Hg from thawing permafrost for low and high greenhouse gas emissions scenarios using a mechanistic model. By 2200, the high emissions scenario shows annual permafrost Hg emissions to the atmosphere comparable to current global anthropogenic emissions. By 2100, simulated Hg concentrations in the Yukon River increase by 14% for the low emissions scenario, but double for the high emissions scenario. Fish Hg concentrations do not exceed United States Environmental Protection Agency guidelines for the low emissions scenario by 2300, but for the high emissions scenario, fish in the Yukon River exceed EPA guidelines by 2050. Our results indicate minimal impacts to Hg concentrations in water and fish for the low emissions scenario and high impacts for the high emissions scenario.

Alaska↗

USGS2020

Hydrologic connectivity determines dissolved organic matter biogeochemistry in northern high-latitude lakes

Northern high‐latitude lakes are undergoing climate‐induced changes including shifts in their hydrologic connectivity with terrestrial ecosystems. How this will impact dissolved organic matter (DOM) biogeochemistry remains uncertain. We examined the drivers of DOM composition for lakes in the Yukon Flats Basin in Alaska, an arid region of low relief that is characteristic of over one‐quarter of circumpolar lake area. Utilizing the vascular plant biomarker lignin, chromophoric dissolved organic matter (CDOM), and ultrahigh‐resolution mass spectrometry, we interpreted DOM compositional changes using lake‐water stable isotope (δ 18 O‐H 2 O) composition as a proxy for lake hydrologic connectivity with the landscape. We observed a relative decrease in CDOM in more hydrologically isolated lakes (enriched δ 18 O‐H 2 O) without a corresponding decrease in dissolved organic carbon (DOC) concentration. Although DOC and CDOM were weakly correlated, a significant positive relationship between lignin and CDOM ( r 2 = 0.67) demonstrates that optical parameters are useful for estimating lignin concentration and thus vascular plant contribution to lake DOM. Indicators of allochthonous DOM, including lignin carbon normalized yields, CDOM aromaticity proxies, and relative abundances of polyphenolic and condensed aromatic compound classes, were negatively correlated with δ 18 O‐H 2 O ( r 2 > 0.45), suggesting there is little allochthonous DOM supplied to many of these hydrologically isolated lakes. We conclude that decreased lake hydrologic connectivity, driven by ongoing climate change (i.e., decreased precipitation, warming temperatures), will reduce allochthonous DOM contributions and shift lakes toward lower CDOM systems with ecosystem‐scale ramifications for heat transfer, photochemical reactions, productivity, and ultimately their biogeochemical function.

Alaska↗

USGS2019

Constraining dissolved organic matter sources and temporal variability in a model sub-Arctic lake

Circumpolar lakes comprise ~ 1.4 million km 2 of arctic and subarctic landscapes and are vulnerable to change in vegetation, permafrost distribution, and hydrological conditions in response to climate warming. However, the composition and cycling of dissolved organic matter (DOM) is poorly understood for these lakes because most are remote and unstudied. The goal of this study was to assess timescale and source controls on DOM composition in Canvasback Lake, a shallow, sub-Arctic lake in interior Alaska with similar hydrologic and geomorphic characteristics to about a quarter of circumpolar lake ecosystems. Lake dissolved organic carbon (DOC) concentration varied by as much as 16% from the mean (3.34 mg L −1 change) through diel cycles in spring 2016 to fall 2017 and was accompanied by minor changes in DOM composition. At the seasonal scale, DOC concentration increased from spring through fall to very high concentrations under ice in winter. Decreases in both condensed aromatic and polyphenolic compound classes and lignin carbon-normalized yield, plus increased relative abundance of aliphatic compounds, suggests that DOM composition shifts from a pulse of allochthonous DOM in the spring to more autochthonous under-ice. These changes highlight the seasonally-dynamic nature of DOM in circumpolar lakes that are poorly captured by single-visit lake surveys and underscores the need to measure DOM properties and fate consistently across multiple timescales (i.e. seasonally) to better constrain the role of DOM in lake processes. To further assess DOM sources, a suite of endmember leachates were compared to bulk lake DOM, indicating solely allochthonous inputs are not well reflected in lake DOM, highlighting the role of degradation processes or mixing with autochthonous sources. Thus, Canvasback Lake appears less well connected to terrestrial inputs compared to past studies of northern high-latitude lakes and does not behave as previous boreal lake models suggest.

Alaska↗

USGS2019

Negligible cycling of terrestrial carbon in many lakes of the arid circumpolar landscape

High-latitude environments store nearly half of the planet’s below-ground organic carbon (OC), mostly in perennially frozen permafrost soils. Climatic changes drive increased export of terrestrial OC into many aquatic networks, yet the role that circumpolar lakes play in mineralizing this carbon is unclear. Here we directly evaluate ecosystem-scale OC cycling for lakes of interior Alaska. This arid, low-relief lake landscape is representative of over a quarter of total northern circumpolar lake area, but is greatly under-represented in current studies. Contrary to projections based on work in other regions, the studied lakes had a negligible role in mineralizing terrestrial carbon; they received little OC from ancient permafrost soils, and had small net contribution to the watershed carbon balance. Instead, most lakes recycled large quantities of internally derived carbon fixed from atmospheric CO 2 , underscoring their importance as critical sites for material and energy provision to regional food webs. Our findings deviate from the prevailing paradigm that northern lakes are hotspots of terrestrial OC processing. The shallow and hydrologically disconnected nature of lakes in many arid circumpolar landscapes isolates them from terrestrial carbon processing under current climatic conditions.

Nature Geoscience↗

USGS2018

Ice wedge degradation and stabilization impacts water budgets and nutrient cycling in Arctic trough ponds

Trough ponds are ubiquitous features of Arctic landscapes and an important component of freshwater aquatic ecosystems. Permafrost thaw causes ground subsidence, creating depressions that gather water, creating ponds. Permafrost thaw also releases solutes and nutrients, which may fertilize these newly formed ponds. We measured water budget elements and chloride, ammonium, and dissolved organic nitrogen (DON) across a chronosequence of trough ponds representing different stages of ice wedge degradation and stabilization. We developed a coupled hydrologic and biogeochemical model to explore how ice wedge degradation affects hydrology and nutrient availability in trough ponds in the advanced degradation stages (DAs), which are characterized by deep troughs with warmer temperatures relative to the other stages. DAs experienced greater evaporation than the other stages, and subsurface inflows entered the DAs from a wide area. Chloride accumulated in the ponds with time since thaw, implying that subsurface fluxes are delivering solutes from the thawing permafrost. Ammonium accumulated at high rates in the initial degradation stage and was seasonally depleted over the summer in all degradation stages. Ammonium trends in the DAs were consistent with high concentration inflows and in‐pond assimilation at rates between 0.37 and 2.0 mg N m −2 day −1 . Seasonal DON trends indicated that the accumulation of recalcitrant organic matter may eventually limit aquatic ecosystem production and foster pond infilling. These results provide direct evidence of nutrient release from thawing permafrost and the utilization of these nutrients by Arctic trough pond ecosystems and highlight infilling as a mechanism by which Arctic surface waters may be lost

Journal of Geophysical Research: Biogeosciences↗

USGS2018

Assessing historical and projected carbon balance of Alaska: A synthesis of results and policy/management implications

We summarize the results of a recent interagency assessment of land carbon dynamics in Alaska, in which carbon dynamics were estimated for all major terrestrial and aquatic ecosystems for the historical period (1950–2009) and a projection period (2010–2099). Between 1950 and 2009, upland and wetland (i.e., terrestrial) ecosystems of the state gained 0.4 Tg C/yr (0.1% of net primary production, NPP), resulting in a cumulative greenhouse gas radiative forcing of 1.68 × 10 −3 W/m 2 . The change in carbon storage is spatially variable with the region of the Northwest Boreal Landscape Conservation Cooperative (LCC) losing carbon because of fire disturbance. The combined carbon transport via various pathways through inland aquatic ecosystems of Alaska was estimated to be 41.3 Tg C/yr (17% of terrestrial NPP). During the projection period (2010–2099), carbon storage of terrestrial ecosystems of Alaska was projected to increase (22.5–70.0 Tg C/yr), primarily because of NPP increases of 10–30% associated with responses to rising atmospheric CO 2 , increased nitrogen cycling, and longer growing seasons. Although carbon emissions to the atmosphere from wildfire and wetland CH 4 were projected to increase for all of the climate projections, the increases in NPP more than compensated for those losses at the statewide level. Carbon dynamics of terrestrial ecosystems continue to warm the climate for four of the six future projections and cool the climate for only one of the projections. The attribution analyses we conducted indicated that the response of NPP in terrestrial ecosystems to rising atmospheric CO 2 (~5% per 100 ppmv CO 2 ) saturates as CO 2 increases (between approximately +150 and +450 ppmv among projections). This response, along with the expectation that permafrost thaw would be much greater and release large quantities of permafrost carbon after 2100, suggests that projected carbon gains in terrestrial ecosystems of Alaska may not be sustained. From a national perspective, inclusion of all of Alaska in greenhouse gas inventory reports would ensure better accounting of the overall greenhouse gas balance of the nation and provide a foundation for considering mitigation activities in areas that are accessible enough to support substantive deployment.

Alaska↗

USGS2018

Limited nitrate retention capacity in the Upper Mississippi River

The Mississippi River and other large rivers have the potential to regulate nitrogen export from terrestrial landscapes, and thus mitigate eutrophication in downstream aquatic ecosystems. In large rivers, human-constructed impoundments and connected backwaters may facilitate nitrogen removal; however, the capacity of these features is poorly quantified and incompletely incorporated into model frameworks. Using a high-resolution and spatially intensive sampling technique, we assessed the contribution of individual navigation pools, as well as impounded open waters and backwater wetlands within them, to overall nitrate retention by mapping the entire length (1370 km) of the Upper Mississippi River (UMR) main channel. Based on this single spatial survey of water chemistry, the river appeared to act primarily as a passive nitrate transporter, retaining only 12.5% of the incoming load, most of which occurred in the upper 150 km of the river, which includes the largest and only naturally impounded reach of the river. Although reservoirs typically are nitrogen sinks, our data indicate that UMR dams do not impede river flows to the extent necessary to promote substantial changes in water residence times and subsequent nitrogen removal. Backwaters routinely had lower nitrate concentrations than the main channel, but their limited hydrologic connectivity to the through-flowing river channel constrained their influence on downstream export. As a whole, the UMR did not remove a substantial proportion of its nitrate load despite optimal N removal conditions, numerous impoundments, and the presence of extensive backwater habitats. These results suggest that efforts to reduce delivery of nitrogen to the Gulf of Mexico should emphasize mitigation strategies that target upland nutrient sources rather than relying on removal within the Mississippi River.

Upper Mississippi River↗

USGS2018

Dissolved organic carbon and nitrogen release from boreal Holocene permafrost and seasonally frozen soils of Alaska

Permafrost (perennially frozen) soils store vast amounts of organic carbon (C) and nitrogen (N) that are vulnerable to mobilization as dissolved organic carbon (DOC) and dissolved organic and inorganic nitrogen (DON, DIN) upon thaw. Such releases will affect the biogeochemistry of permafrost regions, yet little is known about the chemical composition and source variability of active-layer (seasonally frozen) and permafrost soil DOC, DON and DIN. We quantified DOC, total dissolved N (TDN), DON, and DIN leachate yields from deep active-layer and near-surface boreal Holocene permafrost soils in interior Alaska varying in soil C and N content and radiocarbon age to determine potential release upon thaw. Soil cores were collected at three sites distributed across the Alaska boreal region in late winter, cut in 15 cm thick sections, and deep active-layer and shallow permafrost sections were thawed and leached. Leachates were analyzed for DOC, TDN, nitrate (NO 3 − ), and ammonium (NH 4 + ) concentrations, dissolved organic matter optical properties, and DOC biodegradability. Soils were analyzed for C, N, and radiocarbon ( 14 C) content. Soil DOC, TDN, DON, and DIN yields increased linearly with soil C and N content, and decreased with increasing radiocarbon age. These relationships were significantly different for active-layer and permafrost soils such that for a given soil C or N content, or radiocarbon age, permafrost soils released more DOC and TDN (mostly as DON) per gram soil than active-layer soils. Permafrost soil DOC biodegradability was significantly correlated with soil Δ 14 C and DOM optical properties. Our results demonstrate that near-surface Holocene permafrost soils preserve greater relative potential DOC and TDN yields than overlying seasonally frozen soils that are exposed to annual leaching and decomposition. While many factors control the fate of DOC and TDN, the greater relative yields from newly thawed Holocene permafrost soils will have the largest potential impact in areas dominated by organic-rich soils.

Alaska↗

USGS2018

Permafrost stores a globally significant amount of mercury

Changing climate in northern regions is causing permafrost to thaw with major implications for the global mercury (Hg) cycle. We estimated Hg in permafrost regions based on in situ measurements of sediment total mercury (STHg), soil organic carbon (SOC), and the Hg to carbon ratio (R HgC ) combined with maps of soil carbon. We measured a median STHg of 43 ± 30 ng Hg g soil −1 and a median R HgC of 1.6 ± 0.9 μg Hg g C −1 , consistent with published results of STHg for tundra soils and 11,000 measurements from 4,926 temperate, nonpermafrost sites in North America and Eurasia. We estimate that the Northern Hemisphere permafrost regions contain 1,656 ± 962 Gg Hg, of which 793 ± 461 Gg Hg is frozen in permafrost. Permafrost soils store nearly twice as much Hg as all other soils, the ocean, and the atmosphere combined, and this Hg is vulnerable to release as permafrost thaws over the next century. Existing estimates greatly underestimate Hg in permafrost soils, indicating a need to reevaluate the role of the Arctic regions in the global Hg cycle.

Geophysical Research Letters↗

USGS2018

Variation in soil carbon dioxide efflux at two spatial scales in a topographically complex boreal forest

Carbon dynamics of high-latitude regions are an important and highly uncertain component of global carbon budgets, and efforts to constrain estimates of soil-atmosphere carbon exchange in these regions are contingent on accurate representations of spatial and temporal variability in carbon fluxes. This study explores spatial and temporal variability in soilatmosphere carbon dynamics at both fine and coarse spatial scales in a high-elevation, permafrost-dominated boreal black spruce forest. We evaluate the importance of landscape-level investigations of soil-atmosphere carbon dynamics by characterizing seasonal trends in soil-atmosphere carbon exchange, describing soil temperature-moisture-respiration relations, and quantifying temporal and spatial variability at two spatial scales: the plot scale (0–5 m) and the landscape scale (500–1000 m). Plot-scale spatial variability (average variation on a given measurement day) in soil CO 2 efflux ranged from a coefficient of variation (CV) of 0.25 to 0.69, and plot-scale temporal variability (average variation of plots across measurement days) in efflux ranged from a CV of 0.19 to 0.36. Landscape-scale spatial and temporal variability in efflux was represented by a CV of 0.40 and 0.31, respectively, indicating that plot-scale spatial variability in soil respiration is as great as landscape-scale spatial variability at this site. While soil respiration was related to soil temperature at both the plot- and landscape scale, landscape-level descriptions of soil moisture were necessary to define soil respiration-moisture relations. Soil moisture variability was also integral to explaining temporal variability in soil respiration. Our results have important implications for research efforts in high-latitude regions where remote study sites make landscape-scale field campaigns challenging.

Arctic, Antarctic, and Alpine Research↗

USGS2018

Size, age, renewal, and discharge of groundwater carbon

Groundwater carbon (C) supply to lakes and streams is important to understanding the role of inland waters in global and regional cycles and in the functioning of aquatic ecosystems. We provide new estimates of the size and discharge of the groundwater C pool using data from a broad survey of groundwater C, information on the depth distribution of groundwater, and data on groundwater age. About 0.25 × 10 6 km 3 of the 8 × 10 6 km 3 of groundwater resource is within 100 m of the surface and 4.2 × 10 6 km 3 is above 2000 m. Ages show an average groundwater turnover time of 10 yr at 25 m, 350 yr at 100 m, increasing to about 100 000 yr at 600 m. Global groundwater discharge is 16 000 km 3 yr −1 ; >16% of precipitation passes through groundwater. Groundwater dissolved organic C (DOC) can be high in shallow groundwater but stabilizes at ~2–4 mg L −1 at 100 m. Average groundwater dissolved inorganic C (DIC) is ~30–43 mg L −1 . Groundwater C content to 2000 m is ~145 Pg, about the same as all marine sediments and about one-sixth that of the surface ocean. Groundwater C discharge to continental waters is 0.68 Pg yr −1 , or 3.4 times that estimated from river base-flow and submarine groundwater discharge. This discharge is 68 times previous estimates, implying a total C flux from land of 3.6 Pg yr −1 ; 80% of discharge occurs from above 40 m and 99% from the upper 100 m.

Inland Waters↗

USGS2018

Inland waters

1. The total flux of carbon—which includes gaseous emissions, lateral flux, and burial—from inland waters across the conterminous United States (CONUS) and Alaska is 193 teragrams of carbon (Tg C) per year. The dominant pathway for carbon movement out of inland waters is the emission of carbon dioxide gas across water surfaces of streams, rivers, and lakes (110.1 Tg C per year), a flux not identified in the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007). Second to gaseous emissions are the lateral fluxes of carbon through rivers to coastal environments (59.8 Tg C per year). Total carbon burial in lakes and reservoirs represents the smallest flux for CONUS and Alaska (22.5 Tg C per year) (medium confidence). 2. Based on estimates presented herein, the carbon flux from inland waters is now understood to be four times larger than estimates presented in SOCCR1. The total flux of carbon from inland waters across North America is estimated to be 507 Tg C per year based on a modeling approach that integrates high-resolution U.S. data and continental-scale estimates of water area, discharge, and carbon emissions. This estimate represents a weighted average of 24 grams of carbon per m2 per year of continental area exported and removed through inland waters in North America (low confidence). 3. Future research can address critical knowledge gaps and uncertainties related to inland water carbon fluxes. This chapter, for example, does not include methane emissions, which cannot be calculated as precisely as other carbon fluxes because of significant data gaps. Key to reducing uncertainties in estimated carbon fluxes is increased temporal resolution of carbon concentration and discharge sampling to provide better representations of storms and other extreme events for estimates of total inland water carbon fluxes. Improved spatial resolution of sampling also could potentially highlight anthropogenic influences on the quantity and quality of carbon fluxes in inland waters and provide information for land-use planning and management of water resources. Finally, uncertainties could likely be reduced if the community of scientists working in inland waters establishes and adopts standard measurement techniques and protocols similar to those maintained through collaborative efforts of the International Ocean Carbon Coordination Project and relevant governmental agencies from participating nations.

Book chapter↗

USGS2017

Spatial variability of CO2 concentrations and biogeochemistry in the Lower Columbia River

Carbon dioxide (CO 2 ) emissions from rivers and other inland waters are thought to be a major component of regional and global carbon cycling. In large managed rivers such as the Columbia River, contemporary ecosystem changes such as damming, nutrient enrichment, and increased water residence times may lead to reduced CO 2 concentrations (and emissions) due to increased primary production, as has been shown in another large North American river (Upper Mississippi). In this work, spatial patterns of water quality, including dissolved CO 2 concentrations, were assessed in the Lower Columbia River (LCR) and major tributaries using underway measurements from a small research vessel during July 2016. We observed near-equilibrium CO 2 conditions and overall weak supersaturation of CO 2 in the main channel (average 133.8% saturation) and tributaries. We observed only weak correlations between CO 2 saturation, chlorophyll a fluorescence, and turbidity, thus not strongly supporting our hypothesis of primary productivity controls. In general, the LCR was clear (low turbidity, mean = 1.48 FNU) and had low chlorophyll fluorescence (mean = 0.177 RFU) during the sampling period. As a whole, the LCR was homogeneous with respect to biogeochemical conditions and showed low spatial variability at >100 km scales. Overall, we find that the LCR is likely a weak summertime source of CO 2 to the atmosphere, in line with findings from other altered rivers such as the Upper Mississippi.

Columbia River↗

USGS2017

Biological and land use controls on the isotopic composition of aquatic carbon in the Upper Mississippi River Basin

Riverine ecosystems receive organic matter (OM) from terrestrial sources, internally produce new OM, and biogeochemically cycle and modify organic and inorganic carbon. Major gaps remain in the understanding of the relationships between carbon sources and processing in river systems. Here we synthesize isotopic, elemental, and molecular properties of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the Upper Mississippi River (UMR) system above Wabasha, MN, including the main stem Mississippi River and its four major tributaries (Minnesota, upper Mississippi, St. Croix, and Chippewa Rivers). Our goal was to elucidate how biological processing modifies the chemical and isotopic composition of aquatic carbon pools during transport downstream in a large river system with natural and man-made impoundments. Relationships between land cover and DOC carbon-isotope composition, absorbance, and hydrophobic acid content indicate that DOC retains terrestrial carbon source information, while the terrestrial POC signal is largely replaced by autochthonous organic matter, and DIC integrates the influence of in-stream photosynthesis and respiration of organic matter. The UMR is slightly heterotrophic throughout the year, but pools formed by low-head navigation dams and natural impoundments promote a shift towards autotrophic conditions, altering aquatic ecosystem dynamics and POC and DIC composition. Such changes likely occur in all major river systems affected by low-head dams and need to be incorporated into our understanding of inland water carbon dynamics and processes controlling CO 2 emissions from rivers, as new navigation and flood control systems are planned for future river and water resources management.

Upper Mississippi River Basin↗

USGS2017

Inland waters and their role in the carbon cycle of Alaska

The magnitude of Alaska (AK) inland waters carbon (C) fluxes is likely to change in the future due to amplified climate warming impacts on the hydrology and biogeochemical processes in high latitude regions. Although current estimates of major aquatic C fluxes represent an essential baseline against which future change can be compared, a comprehensive assessment for AK has not yet been completed. To address this gap, we combined available data sets and applied consistent methodologies to estimate river lateral C export to the coast, river and lake carbon dioxide (CO 2 ) and methane (CH 4 ) emissions, and C burial in lakes for the six major hydrologic regions in the state. Estimated total aquatic C flux for AK was 41 Tg C/yr. Major components of this total flux, in Tg C/yr, were 18 for river lateral export, 17 for river CO 2 emissions, and 8 for lake CO 2 emissions. Lake C burial offset these fluxes by 2 Tg C/yr. River and lake CH 4 emissions were 0.03 and 0.10 Tg C/yr, respectively. The Southeast and South central regions had the highest temperature, precipitation, terrestrial net primary productivity (NPP), and C yields (fluxes normalized to land area) were 77 and 42 g C·m −2 ·yr −1 , respectively. Lake CO 2 emissions represented over half of the total aquatic flux from the Southwest (37 g C·m −2 ·yr −1 ). The North Slope, Northwest, and Yukon regions had lesser yields (11, 15, and 17 g C·m 2 ·yr −1 ), but these estimates may be the most vulnerable to future climate change, because of the heightened sensitivity of arctic and boreal ecosystems to intensified warming. Total aquatic C yield for AK was 27 g C·m −2 ·yr −1 , which represented 16% of the estimated terrestrial NPP. Freshwater ecosystems represent a significant conduit for C loss, and a more comprehensive view of land-water-atmosphere interactions is necessary to predict future climate change impacts on the Alaskan ecosystem C balance.

Alaska↗

USGS2017

Spatial heterogeneity of within-stream methane concentrations

Streams, rivers, and other freshwater features may be significant sources of CH 4 to the atmosphere. However, high spatial and temporal variabilities hinder our ability to understand the underlying processes of CH 4 production and delivery to streams and also challenge the use of scaling approaches across large areas. We studied a stream having high geomorphic variability to assess the underlying scale of CH 4 spatial variability and to examine whether the physical structure of a stream can explain the variation in surface CH 4 . A combination of high-resolution CH 4 mapping, a survey of groundwater CH 4 concentrations, quantitative analysis of methanogen DNA, and sediment CH 4 production potentials illustrates the spatial and geomorphic controls on CH 4 emissions to the atmosphere. We observed significant spatial clustering with high CH 4 concentrations in organic-rich stream reaches and lake transitions. These sites were also enriched in the methane-producing mcrA gene and had highest CH 4 production rates in the laboratory. In contrast, mineral-rich reaches had significantly lower concentrations and had lesser abundances of mcrA . Strong relationships between CH 4 and the physical structure of this aquatic system, along with high spatial variability, suggest that future investigations will benefit from viewing streams as landscapes, as opposed to ecosystems simply embedded in larger terrestrial mosaics. In light of such high spatial variability, we recommend that future workers evaluate stream networks first by using similar spatial tools in order to build effective sampling programs.

Journal of Geophysical Research G: Biogeosciences↗

USGS2017

Carbonate buffering and metabolic controls on carbon dioxide in rivers

Multiple processes support the significant efflux of carbon dioxide (CO 2 ) from rivers and streams. Attribution of CO 2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO 2 production is closely related to O 2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO 2 and O 2 concentrations and other water quality parameters were analyzed in two data sets: a synoptic field study and nationwide water quality monitoring data. CO 2 and O 2 concentrations were strongly negatively correlated in both data sets ( ρ = −0.67 and ρ = −0.63, respectively), although the correlations were weaker in high-alkalinity environments. In nearly all samples, the molar oversaturation of CO 2 was a larger magnitude than molar O 2 undersaturation. We used a dynamically coupled O 2 CO 2 model to show that lags in CO 2 air-water equilibration are a likely cause of this phenomenon. Lags in CO 2 equilibration also impart landscape-scale differences in the behavior of CO 2 between high- and low-alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO 2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO 2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO 2 dynamics are primary steps in understanding the sources and magnitude of CO 2 oversaturation in rivers and streams.

Global Biogeochemical Cycles↗

USGS2016

Spatial and temporal patterns of dissolved organic matter quantity and quality in the Mississippi River Basin, 1997–2013

Recent studies have found insignificant or decreasing trends in time-series dissolved organic carbon (DOC) datasets, questioning the assumption that long-term DOC concentrations in surface waters are increasing in response to anthropogenic forcing, including climate change, land use, and atmospheric acid deposition. We used the weighted regressions on time, discharge, and season (WRTDS) model to estimate annual flow-normalized concentrations and fluxes to determine if changes in DOC quantity and quality signal anthropogenic forcing at 10 locations in the Mississippi River Basin. Despite increases in agriculture and urban development throughout the basin, net increases in DOC concentration and flux were significant at only 3 of 10 sites from 1997 to 2013 and ranged between −3.5% to +18% and −0.1 to 19%, respectively. Positive shifts in DOC quality, characterized by increasing specific ultraviolet absorbance at 254 nm, ranged between +8% and +45%, but only occurred at one of the sites with significant DOC quantity increases. Basinwide reductions in atmospheric sulfate deposition did not result in large increases in DOC either, likely because of the high buffering capacity of the soil. Hydroclimatic factors including annual discharge, precipitation, and temperature did not significantly change during the 17-year timespan of this study, which contrasts with results from previous studies showing significant increases in precipitation and discharge over a century time scale. Our study also contrasts with those from smaller catchments, which have shown stronger DOC responses to climate, land use, and acidic deposition. This temporal and spatial analysis indicated that there was a potential change in DOC sources in the Mississippi River Basin between 1997 and 2013. However, the overall magnitude of DOC trends was not large, and the pattern in quantity and quality increases for the 10 study sites was not consistent throughout the basin.

Hydrological Processes↗