Aquatic ecosystems are a large but uncertain source of greenhouse gases (GHG) to the atmosphere. Managed aquatic ecosystems may be a disproportionately large source of emissions given their co-location with human activities and the influence of artificial water level management. The Intergovernmental Panel on Climate Change (IPCC) recently published a methodology for estimating anthropogenic GHG emissions from managed aquatic ecosystems (e.g., reservoirs, canals, ditches, farm ponds) and several countries now include this source in their national GHG inventory. National GHG inventories are used to track trends in GHG emissions, monitor compliance with emission-reduction commitments under international treaties, provide an important framework for incorporating aquatic systems into climate-relevant policy discussions, and highlight the need for accurate emission estimates. In this session, we solicit presentations that address challenges and recent advances in estimating GHG emissions from managed aquatic ecosystems. We encourage contributions focusing on 1) spatial and temporal patterns in emission rates, 2) biophysical drivers of GHG production and emission, 3) models (statistical or mechanistic) for predicting aquatic GHG emissions, 4) methods for upscaling and/or downscaling emission measurements to regional and/or national scales, and 5) identification and mapping of managed aquatic ecosystems. We welcome submissions by students, early career researchers, and researchers from BIPOC, LGBTQIA+, and other marginalized identities.
Lead Organizer: Jake Beaulieu, United States Environmental Protection Agency (Beaulieu.jake@epa.gov)
Co-organizers:
Bridget Deemer, U.S. Geological Survey (bdeemer@usgs.gov)
Natalie Griffiths, Oak Ridge National Laboratory (griffithsna@ornl.gov)
Rachel Pilla, Oak Ridge National Laboratory (pillarm1@ornl.gov)
Presentations
04:00 PM
Greenhouse gas emissions of farm and residential ponds along a forest to agricultural land use gradient (7973)
Primary Presenter: Pascal Bodmer, Cornell University (pb577@cornell.edu)
Ponds are biogeochemical hotspots with significant emissions of greenhouse gases (carbon dioxide, CO2; methane; CH4) to the atmosphere. However, CO2 and CH4 emissions are highly variable among ponds, hampering accurate regional greenhouse gas emission estimates. One potential driver of emissions is land use cover, where agricultural runoff may increase emissions relative to ponds in forested catchments. Therefore, we studied 28 farm and residential ponds along a land use gradient from forest dominated to agricultural dominated catchments in upstate New York, USA, twice during summer when emissions should be highest. We measured diffusive CO2 and CH4 emissions, short-term ebullitive CH4 emissions, bottom and surface water CO2 and CH4 concentrations, along with a suite of physicochemical parameters. Our preliminary results show higher chlorophyll a, lower diffusive CO2 emissions, and lower bottom water oxygen content with increasing proportion of agricultural land use in the catchment. Ultimately, our study will identify spatial drivers at the ecosystem and landscape level which will be useful for constraining aquatic greenhouse gas emissions at regional and global scales.
04:15 PM
CLIMATE WARMING CONSTRAINS GREENHOUSE GAS EMISSIONS OF AGRICULTURAL RESERVOIRS IN THE NORTHERN GREAT PLAINS (7948)
Primary Presenter: Kerri Finlay, University of Regina (kerri.finlay@uregina.ca)
Inland waters are well known to be biogeochemical hotspots for carbon cycling, and have accordingly have been well studied in terms of quantifying rates and underlying controls of greenhouse gas emissions. The complex interplay of multiple mechanistic factors, however, can make scaling emissions rates at the landscape scale difficult, particularly for understudied hard-water ecosystems. Based on extensive research on small agricultural reservoirs in the northern Great Plains between 2017-2023, we found simple correlative relationships whereby CO2 emissions were well predicted by water-column pH, and both diffusive and ebullitive CH4 emissions were correlated with salinity. Specifically, CH4 ebullition rates were effectively zero above a water-column salinity of 0.5 ppt, and diffusive losses were constant and low (mean 3.2 + 9.5 mmol/m2/day) above that threshold, consistent with prior surveys of 100s of similar ecosystems. Importantly, ebullition rates could be predicted from water-column CH4 content (R2 = 0.56, P< 0.05). Given that the median salinity of water bodies in this region is 0.41ppt, drought conditions may constrain methane emissions and shift small water bodies from net carbon sources to sinks during dry intervals. Similarly, warmer winters are anticipated to elevate water column pH and reduce CO2 emissions. The ability to forecast greenhouse gas responses to climate variability is essential to accurately quantify the role of small reservoirs in the National Inventory Report, particularly in light of the expected 3-5oC increase in regional temperature.
04:30 PM
IDIOSYNCRATIC PHENOLOGY OF GREENHOUSE GAS EMISSIONS IN A MEDITERRANEAN RESERVOIR (8127)
Primary Presenter: Isabel Reche, Universidad de Granada (ireche@ugr.es)
Extreme hydrological and thermal regimes characterize the Mediterranean biome and can significantly impact the phenology of greenhouse gas (GHG) emissions in reservoirs. Our study examined the seasonal changes in GHG emissions of a shallow, eutrophic, hardwater reservoir in Spain. We observed distinctive seasonal patterns for each gas. CH4 emissions substantially increased during stratification, reaching more than 1200 mg C m-2 day-1 predominantly by ebullition. This dynamic was influenced predominantly by the rise of water temperature, gross primary production, and the drop-in reservoir mean depth. N2O emissions mirrored CH4's seasonal trend, significantly correlating to water temperature, wind speed, and net primary production with maximum values of 671 µg N m-2 day-1. The CO2 emissions ranged from 12 to 567 mg C m-2 day-1 and decreased during stratification. These emissions displayed a quadratic, rather than a linear relationship with water temperature -an unexpected deviation from CH4 and N2O emission patterns- not reported previously and likely associated with the coupling between intracellular calcite formation and photosynthesis. We found the maximum climatic forcing during the summer due to the CH4 and N2O emissions. This investigation highlights the need to integrate these idiosyncratic patterns of Mediterranean hardwater reservoirs into GHG emissions models, enhancing the prediction of global GHG emissions in the global change era.
04:45 PM
Methane and CO2 emissions from four Pacific NW US reservoirs with contrasting water management regimes (8354)
Primary Presenter: John Harrison, Washington State University (john_harrison@wsu.edu)
Dams and reservoirs are ubiquitous and widely used to improve human well-being, resulting in their rapid increase in number and influence. They are also major players in biogeochemical and ecosystem dynamics both at regional and global scales. One key unintended consequence of dam construction and operation is the increased emission of carbon-based greenhouse gases (GHGs) to the atmosphere. Recent synthesis efforts have suggested that collectively, at the global scale, reservoirs account for approximately 1.5% of all anthropogenic GHG emissions and 5% of anthropogenic methane (CH4) emissions. These estimates are based on the best available information (i.e., in-situ flux estimates), but there is considerable uncertainty inherent in these estimates. Reasons for this uncertainty include both substantial spatial and temporal flux variation, and a bias toward summer month measurements. Summer measurements often exclude ebullition fluxes, which can be an important pathway for CH4 emissions. Additionally, until recently, there has been a comparative lack of reservoir GHG flux estimates from temperate zone systems. This is because most of the effort to constrain these fluxes has occurred in tropical and boreal systems. Here we report results from a first complete year of bimonthly GHG flux measurements collected from 4 Columbia River Basin reservoirs undergoing contrasting management regimes (2 run of river reservoirs and 2 storage reservoirs). We employed low-cost, CH4 and CO2 sensors fitted to floating flux chambers to collect more than 400 individual flux estimates from our study systems. There was considerable spatial and seasonal variability in GHG fluxes both within and among systems. Despite this variability, surface CH4 concentrations were consistently supersaturated, even in cold oligotrophic conditions. All study reservoirs were net CH4 emitters year-round (CH4 flux rates 1-140 mg CH4-C m-2 d-1), with highest fluxes during summer and lowest fluxes during winter. In contrast, CO2 fluxes did not demonstrate a consistent seasonal pattern and appeared to be more strongly influenced by surface chlorophyll a (Chl a) than water temperatures, with negative CO2 fluxes observed during times (and in systems) with high surface Chl a.
05:00 PM
RESULTS FROM A NATIONAL SCALE SURVEY OF METHANE AND CARBON DIOXIDE EMISSIONS FROM US RESERVOIRS (8295)
Primary Presenter: Jake Beaulieu, United States Environmental Protection Agency (Beaulieu.Jake@epa.gov)
The US Environmental Protection Agency has included reservoirs in the annual Inventory of US Greenhouse Gas Emissions and Sinks since 2022. US reservoirs are estimated to emit 30.6 kt of methane per year, making them the seventh largest anthropogenic methane source in the country. This estimate is based on Intergovernmental Panel on Climate Change default emission factors, however, and it is unclear how well these emission factors represent US systems. To improve the Inventory estimate, we executed a survey of methane and carbon dioxide emissions from 108 US reservoirs. The reservoirs were selected using a spatially balanced survey design and were widely distributed across the conterminous US. Ebullition and diffusion emissions were measured at 15-25 locations in each reservoir during a 48-hour period between June 1 and September 15, 2020-2023. Approximately 60% of the reservoirs were a carbon dioxide source during the survey. All reservoirs were a source of methane, with a mean emission rate of 134 mg CH4 m-2 d-1 (Inter Quartile Range (IQR): 18-110 mg CH4 m-2 d-1), which is similar to the average global flux from lakes and reservoirs (156 mg CH4 m-2 d-1), but includes some of the highest diffusive emissions reported in the literature (maximum reservoir-wide average of 2808 mg CH4 m-2 d-1). On average, diffusion composed 75% of total emissions (IQR: 57–89%), with ebullition constituting the balance. In addition to aiding inventory efforts, this dataset will improve our understanding of both inter and intra-reservoir spatial patterns in greenhouse gas fluxes.
05:15 PM
Reducing Uncertainty in Upscaled CH4 Ebullition Estimates from Reservoirs (8452)
Primary Presenter: Anna Cardall, Brigham Young University (anna.cardall@gmail.com)
Methane (CH4), a potent greenhouse gas, is produced in freshwater ecosystems by microbial activity in the sediment. Estimates of CH4 emissions from freshwaters, including hydropower reservoirs, are highly uncertain, often reflecting annualization or upscaling from snapshot sampling events during the summer when water levels are high. Some of the uncertainty in these estimates exists because CH4 emissions, particularly via the ebullition (bubbling) pathway, are highly variable in space and time. CH4 ebullition occurs primarily in the shallow areas of reservoirs where lower hydrostatic pressure and oxidation potential allow CH4-rich bubbles to escape to the surface. However, reservoir-wide estimates of CH4 ebullition often do not account for the changing size of the ebullition zone as water levels fluctuate on multiple temporal scales. Additionally, estimates often do not account for the temporal variations in CH4 production due to physiochemical parameters such as temperature and eutrophication. In a case study of Douglas Reservoir, TN, USA, we used a monthly sampling regime from March 2023 to February 2024 to capture temporal variation in CH4 ebullitive fluxes and the size of the ebullition zone. We also used a comprehensive survey design that incorporated reservoir bathymetry to reduce spatial uncertainty. We found that upscaled estimates of CH4 ebullition that accounted for spatial and temporal variation were significantly lower than estimates that assumed a constant summertime flux and full pool elevation. These results indicate that incorporating sub-annual variations in flux and reservoir bathymetry into reservoir-wide estimates of CH4 ebullitive emissions is important to reducing the uncertainty in greenhouse gas emissions estimates from hydropower reservoirs.
SS08B - Advances in Estimating Greenhouse Gas Emissions from Managed Aquatic Ecosystems
Description
Time: 4:00 PM
Date: 3/6/2024
Room: Hall of Ideas I