Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecology and Evolution 8 (2018): 4958-4966, doi:10.1002/ece3.3955.Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha−1 year−1) and unfertilized (12 kg N ha−1 year−1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO2 uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N2O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO2 and N2O and decreased CH4 emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N2O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH4 emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N

Transient coastal landscapes : rising sea level threatens salt marshes

© The Author(s), 2018. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Science of The Total Environment 640-641 (2018): 1148-1156, doi:10.1016/j.scitotenv.2018.05.235.Salt marshes are important coastal environments that provide key ecological services. As sea level rise has accelerated globally, concerns about the ability of salt marshes to survive submergence are increasing. Previous estimates of likely survival of salt marshes were based on ratios of sea level rise to marsh platform accretion. Here we took advantage of an unusual, long-term (1979-2015), spatially detailed comparison of changes in a representative New England salt marsh to provide an empirical estimate of habitat losses based on actual measurements. We show prominent changes in habitat mosaic within the marsh, consistent and coincident with increased submergence and coastal erosion. Model results suggest that at current rates of sea level rise, marsh platform accretion, habitat loss, and with the limitation of the widespread “coastal squeeze”, the entire ecosystem might disappear by the beginning of the next century, a fate that might be likely for many salt marshes elsewhere.Ivan Valiela and Elizabeth Elmstrom were supported by Woods Hole Sea Grant, NOAA grant no. NA14OAR4170074. Javier Lloret was supported by a Rosenthal Postdoctoral Fellowship Award from the Marine Biological Laboratory, and by a Northeast Climate Science Center Fellowship. Tynan Bowyer was supported by a Metcalf Research Fellowship of the University of Chicago. David Remsen was supported by MBL Cox and Bernstein funds.2020-06-0