Coastal wetland soils are important sites of carbon burial that can mitigate the intensity of carbon-induced climate change. However, soil microbial processes produce potent greenhouse gases, such as carbon dioxide and methane, which can then be released into the atmosphere and offset some of the climate benefit provided by carbon burial. A great deal is known about the influence of seasons, tides, and salinity on salt marsh greenhouse gas emissions, but little effort has been devoted to determining their responses to natural and anthropogenic disturbances. In this study, we specifically evaluate how fiddler crab (Uca longisignalis) bioturbation and oil pollution affect the fluxes of carbon dioxide and methane from Louisiana salt marsh soil. This study included three burrow treatments (no burrow, artificial burrow, and crab-made burrow) and four oil treatments with oil concentrations of 0 mg cm-2, 0.85 mg cm-2, 8.52 mg cm-2, and 25.55 mg cm-2. Soil microcosms were incubated for five days after which carbon dioxide and methane fluxes were measured using a field gas analyzer and the microcosms were extruded to quantify burrow size and depth. Oil concentration did not affect carbon dioxide fluxes, but methane fluxes were significantly lower in the high oil treatment than in the other treatments. We found a linear relationship between the mass of burrowed material and the carbon dioxide flux, but burrow size did not influence the methane flux. Instead, we determined in a follow-up experiment that burrows of any size were sufficient to drive an enhanced methane flux. Our study demonstrates the potential of both natural and anthropogenic disturbances to alter salt marsh soil greenhouse gas fluxes, but more work is required to determine their influence on the ecosystem scale.

Fiddler crab burrowing and oil pollution alter greenhouse gas fluxes from salt marsh soil

Charles A. Schutte; Adrianna Grow; Scott Jones; Brian Roberts

Wetlands are globally important sites for carbon sequestration accounting for 44 million metric tons of carbon per year and can also be large sources of methane (CH4) to the atmosphere. Sequestration occurs via a net flux of carbon dioxide (CO2) into the soil as a result of photosynthesis exceeding respiration and through direct deposition of organic carbon onto marsh platforms. Much of our current knowledge of marsh carbon fluxes is based on sampling during windows of anticipated maximal photosynthesis limiting our understanding of diel variation in these important fluxes  In this study, we took advantage of a new Spartina alterniflora marsh mesocosm (2.7m diameter) facility in which we were able to maintain constant water levels in 4 marshes over a diel cycle, allowing us to decouple variation due to diel patterns from flooding regime seen in natural marshes. We measured CO2 and CH4 fluxes in 3 types of static chambers (light and dark plant chambers and a dark soil chamber located between stems) in each of 4 marshes at 7 time points over a diel cycle. Simultaneously, we measured photosynthetic yield on a leaf from five stems in each gas flux plot at the same frequency. There was essentially no net CO2 emission in early morning and late afternoon (despite the sun being up), high net influxes into the soil during the midday and net emission to the atmosphere at night. In term of photosynthetic parameters, the slope of the response in electron transport rate (ETR) with increasing light (alpha) was higher during day than night but didn’t vary throughout the illuminated period. In contrast, maximum electron transport rate (ETRmax) followed a very similar pattern to that observed for CO2 fluxes. These results have important implications for scaling discrete measures of gas fluxes into wetland carbon cycling and sequestration models.

Diel variation in carbon fluxes and photosynthetic efficiency in salt marsh ecosystems

Brian J. Roberts; Scott Jones; Herbert Leavett; Ryann Rossi; Charles Schutte

The tidal Hudson River Estuary (HRE) receives significant inputs of readily dissolvable carbon and nitrogen from wastewater treatment concentrated in urban areas. The largest urban metros, New York City (NYC) and Albany are located at the terminal ends of the tidal HRE with varied levels of salinity as well as terrestrial and anthropogenic inputs found in intervening waters. Over the course of ten cruises, we quantified carbon dioxide (CO2) and methane (CH4) surface concentrations in parallel with biogeochemical parameters including anthropogenic indicators throughout the tidal Hudson River, its embayments, and tributaries. Additionally, efflux values were calculated from mid-channel sites utilizing a series of meteorological towers. Greatest surface concentrations were found in urban embayments, likely to be sewage delivery areas, with diminishing concentrations observed at urban followed by less developed mid-channel sites. CH4 and CO2 surface concentrations were also correlated with salinity, oxygen saturation, fecal indicator bacteria, and temperature with multiple regression analyses producing models with high predictive power. The HRE was found to be both a CO2 and CH4 source for every site and almost all (>99%) sampling dates. The greatest combined effluxes (37 - 289 mg C m-2 day-1) were quantified in close proximity to NYC and Albany. Conversely, the lowest combined effluxes (14.3 - 140 mg C m-2 day) were quantified in suburban/rural regions. If climate warming potential is considered, the ratio of efflux between urban: suburban/rural was due to (77%) CH4 efflux. However, large variation in efflux values were driven by pervasive variability in windspeed data which obfuscated potential differences in urban vs. nondeveloped regions of the HRE. The magnitude of elevated CH4 and CO2 surface concentrations/efflux observed here can be used to evaluate the potential climate impact coastal mega cities have on estuaries. 

Anthropogenic inputs enhance CH4 and CO2 values in the Hudson River Estuary

Brian A. Brigham; Jeffrey A. Bird; Andrew R. Juhl; Angel D. Montero; Gregory D. O'Mullan