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 toward autotrophic conditions, altering aquatic ecosystem dynamics and POC and DIC compositions. 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 CO2 emissions from rivers, as new navigation and flood control systems are planned for future river and water resources management.
Key Points
Dissolved organic carbon composition is linked to basin forest area and wastewater inputs and shows little in‐stream biological modification
Biological processes modify dissolved inorganic carbon isotopic composition, so that it deviates from terrestrial carbon source composition
Low‐head dams and reservoirs shift fluvial systems toward autotrophic conditions, reflected in particulate organic carbon composition
Plain Language Summary
Watersheds of the Upper Mississippi River Basin drain a region of diverse land use types, ranging from heavily agricultural to forested. This study investigates links between the chemical features of carbon carried by these rivers and different land use types within their basins. These features are used to identify signals of biological activity during downstream transport, which influence how much carbon dioxide the river releases to the atmosphere. We found that certain portions of the carbon carried by the Upper Mississippi River maintain markers of land use, while other portions reflect the activity of photosynthesis and respiration. River impoundments such as the natural Lake Pepin on the Mississippi River main stem remove light‐blocking particles, thus shifting the river's balance from respiration toward photosynthesis. These results are important for understanding how rivers may respond to future changes in land use, climate, and other environmental conditions, especially given the expected global increase in river navigation structures.
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.
Soil carbon dioxide (CO2) emission (soil respiration), net CO2 exchange after photosynthetic uptake by ground-cover plants, and soil CO2 concentration versus depth below land surface were measured at ...four ages of jack pine (Pinus banksiana Lamb.) forest in central Saskatchewan. Soil respiration was smallest at a clear-cut site, largest in an 8-year-old stand, and decreased with stand age in 20-year-old and mature (60-75 years old) stands during May-September 1994 (12.1, 34.6, 31.5, and 24.9 mol C(.)m-2, respectively). Simulations of soil respiration at each stand based on continuously recorded soil temperature were within one standard deviation of measured flux for 48 of 52 measurement periods, but were 10%-30% less than linear interpolations of measured flux for the season. This was probably due to decreased soil respiration at night modeled by the temperature-flux relationships, but not documented by daytime chamber measurements. CO2 uptake by ground-cover plants ranged from 0 at the clear-cut site to 29, 25, and 9% of total growing season soil respiration at the 8-year, 20-year, and mature stands. CO2 concentrations were as great as 7150 ppmv in the upper 1 m of unsaturated zone and were proportional to measured soil respiration.
Methane exchange between the atmosphere and subalpine wetland and unsaturated soils was evaluated over a 15‐month period during 1995–1996. Four vegetation community types along a moisture gradient ...(wetland, moist‐grassy, moist‐mossy, and dry) were included in a 100 m sampling transect situated at 3200 m elevation in Rocky Mountain National Park, Colorado. Methane fluxes and soil temperature were measured during snow‐free and snow‐covered periods, and soil moisture content was measured during snow‐free periods. The range of mean measured fluxes through all seasons (a positive value represents CH4 efflux to the atmosphere) were: 0.3 to 29.2 mmol CH4 m−2 d−1 wetland area; 0.1 to 1.8 mmol CH4 m−2 d−1, moist‐grassy area; −0.04 to 0.7 mmol CH4 m−2 d−1, moist‐mossy area; and −0.6 to 0 mmol CH4m−2 d−1, dry area. Methane efflux was significantly correlated with soil temperature (5 cm) at the continuously saturated wetland area during snow‐free periods. Consumption of atmospheric methane was significantly correlated with moisture content in the upper 5 cm of soil at the dry area. A model based on the wetland flux‐temperature relationship estimated an annual methane emission of 2.53 mol CH4 m−2 from the wetland. Estimates of annual methane flux based on field measurements at the other sites were 0.12 mol CH4 m−2, moist‐grassy area; 0.03 mol CH4 m−2, moist‐mossy area; and −0.04 mol CH4 m−2, dry area. Methane fluxes during snow‐covered periods were responsible for 25, 73, 23, and 43% of the annual fluxes at the wetland, moist‐grassy, moist‐mossy, and dry sites, respectively.
Abstract
Northern high-latitude lakes are critical sites for carbon processing and serve as potential conduits for the emission of permafrost-derived carbon and greenhouse gases. However, the fate ...and emission pathways of permafrost carbon in these systems remain uncertain. Here, we used the natural abundance of radiocarbon to identify and trace the predominant sources of methane, carbon dioxide, dissolved inorganic and organic carbon in nine lakes within the Yukon Flats National Wildlife Refuge in interior Alaska, a discontinuous permafrost region with high landscape heterogeneity and susceptibility to climate, permafrost, and hydrological changes. We find that although Yukon Flats lakes primarily process young carbon (modern to 1290 ± 60 years before present), permafrost-derived carbon is present in some of the sampled lakes and contributes, at most, 30 ± 10% of the dissolved carbon in lake surface waters. Apportionment of young carbon and legacy carbon (carbon with radiocarbon age ⩾5000 years before present) is decoupled among the dissolved inorganic and organic carbon species, with methane showing a stronger legacy signature. Our observations suggest that permafrost-thaw-related transport of carbon through Yukon Flats lacustrine ecosystems and into the atmosphere is small, and likely regulated by surficial sediments, permafrost distribution, wildfire occurrence, or masked by contemporary carbon processes. The heterogeneity of lakes across our study area and northern landscapes more broadly cautions against using any one region (e.g. Yedoma permafrost lakes) to upscale their contribution across the pan-Arctic.
Freshwater lakes are an important component of the global carbon cycle through both organic carbon (OC) sequestration and carbon dioxide (CO2) emission. Most lakes have a net annual loss of CO2 to ...the atmosphere and substantial current evidence suggests thatbiologic mineralization of allochthonous OC maintains this flux. Because net CO2 flux to the atmosphere implies net mineralization of OC within the lake ecosystem, it is also commonly assumed that net annual CO2 emission indicates negative net ecosystem production (NEP). We explored the relationship between atmospheric CO2 emission and NEP in two lakes known to have contrasting hydrologic characteristics and net CO2 emission. We calculated NEP for calendar year 2004 using whole‐lake OC and inorganic carbon (IC) budgets, NEPOC and NEPIC, respectively, and compared the resulting values to measured annual CO2 flux from the lakes. In both lakes, NEPOC and NEPIC were positive, indicating net autotrophy. Therefore CO2 emission from these lakes was apparently not supported by mineralization of allochthonous organic material. In both lakes, hydrologic CO2 inputs, as well as CO2 evolved from net calcite precipitation, could account for the net CO2 emission. NEP calculated from diel CO2 measurements was also affected by hydrologic inputs of CO2. These results indicate that CO2 emission and positive NEP may coincide in lakes, especially in carbonate terrain, and that all potential geologic, biogeochemical, and hydrologic sources of CO2 need to be accounted for when using CO2 concentrations to infer lake NEP.
Loads and yields of dissolved and particulate organic and inorganic carbon (DOC, POC, DIC, PIC) were measured and modeled at three locations on the Yukon River (YR) and on the Tanana and Porcupine ...rivers (TR, PR) in Alaska during 2001–2005. Total YR carbon export averaged 7.8 Tg C yr−1, 30% as OC and 70% as IC. Total C yields (0.39–1.03 mol C m−2 yr−1) were proportional to water yields (139–356 mm yr−1; r2 = 0.84) at all locations. Summer DOC had an aged component (fraction modern (FM) = 0.94–0.97), except in the permafrost wetland‐dominated PR, where DOC was modern. POC had FM = 0.63–0.70. DOC had high concentration, high aromaticity, and high hydrophobic content in spring and low concentration, low aromaticity, and high hydrophilic content in winter. About half of annual DOC export occurred during spring. DIC concentration and isotopic composition were strongly affected by dissolution of suspended carbonates in glacial meltwater during summer.
Estimating lake‐atmosphere CO2 exchange Anderson, Dean E.; Striegl, Robert G.; Stannard, David I. ...
Limnology and oceanography,
June 1999, Letnik:
44, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Lake‐atmosphere CO2 flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake‐water CO2 storage and ...with flux predictions from boundary‐layer and surface‐renewal models. Over a 3‐yr period, lake‐atmosphere exchanges of CO2 were measured over 5 weeks in spring, summer, and fall. Observed springtime CO2 efflux was large (2.3–2.7 umol m‐2 s‐1) immediately after lake‐thaw. That efflux decreased exponentially with time to less than 0.2 umol m‐2 s−1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO2 concentrations, lake CO2 storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (−0.17 μmol m‐2 s1) to the lake's surface, while late fall flux was trendless and smaller (−0.0021 μmol m‐2 s−1). Large springtime efflux afforded an opportunity to make direct measurement of lake‐atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air‐water gas‐transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake‐water storage change in CO2, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary‐layer and surface‐renewal models. Comparison of measured and modeled transfer velocities for CO2, along with measured and modeled cumulative CO2 flux, indicates that in most instances the surface‐renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary‐layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.
Aquatic ecosystems are important components of landscape carbon budgets. In lake‐rich landscapes, both lakes and streams may be important sources of carbon gases (CO2 and CH4) to the atmosphere, but ...the processes that control gas concentrations and emissions in these interconnected landscapes have not been adequately addressed. We use multiple data sets that vary in their spatial and temporal extent during 2001–2012 to investigate the carbon gas source strength of streams in a lake‐rich landscape and to determine the contribution of lakes, metabolism, and groundwater to stream CO2 and CH4. We show that streams emit roughly the same mass of CO2 (23.4 Gg C yr−1; 0.49 mol CO2 m−2 d−1) as lakes at a regional scale (27 Gg C yr−1) and that stream CH4 emissions (189 Mg C yr−1; 8.46 mmol CH4 m−2 d−1) are an important component of the regional greenhouse gas balance. Gas transfer velocity variability (range = 0.34 to 13.5 m d−1) contributed to the variability of gas flux in this landscape. Groundwater inputs and in‐stream metabolism control stream gas supersaturation at the landscape scale, while carbon cycling in lakes and deep groundwaters does not control downstream gas emissions. Our results indicate the need to consider connectivity of all aquatic ecosystems (lakes, streams, wetlands, and groundwater) in lake‐rich landscapes and their connections with the terrestrial environment in order to understand the full nature of the carbon cycle.
Key Points
Stream emissions nearly equal lake emissions
Groundwater and stream metabolism support CO2 flux
Lakes do not significantly affect stream gases
We measured mercury (Hg) concentrations and calculated export and yield from the Yukon River Basin (YRB) to quantify Hg flux from a large, permafrost-dominated, high-latitude watershed. Exports of Hg ...averaged 4400 kg Hg yr–1. The average annual yield for the YRB during the study period was 5.17 μg m–2 yr–1, which is 3–32 times more than Hg yields reported for 8 other major northern hemisphere river basins. The vast majority (90%) of Hg export is associated with particulates. Half of the annual export of Hg occurred during the spring with about 80% of 34 samples exceeding the U.S. EPA Hg standard for adverse chronic effects to biota. Dissolved and particulate organic carbon exports explained 81% and 50%, respectively, of the variance in Hg exports, and both were significantly (p < 0.001) correlated with water discharge. Recent measurements indicate that permafrost contains a substantial reservoir of Hg. Consequently, climate warming will likely accelerate the mobilization of Hg from thawing permafrost increasing the export of organic carbon associated Hg and thus potentially exacerbating the production of bioavailable methylmercury from permafrost-dominated northern river basins.
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