Mensurative and Manipulative Experiments Test Effects of Global Change Drivers on Greenhouse Gas Fluxes in Coastal Marshes

Abstract

For centuries, coastal marshes have been subjected to anthropogenic stressors. Great expanses of coastal marshes were drained and filled to make way for development, and those that remained were diked and ditched, encroached upon by upland development, and used for agricultural purposes such as livestock grazing. Today, as the values and services coastal marshes provide to human society are understood, marshes are protected from direct degradation. However, especially in developed and densely populated estuaries such as Narragansett Bay, coastal marshes are subject to impacts including nutrient pollution and introduction of invasive species. Global climate change and associated sea level rise further threaten coastal ecosystems. As marsh vegetation community structure, biogeochemistry, and microbial and faunal assemblages shift in response to anthropogenic impacts and global change, ecosystem function is likely to be altered as well. Since coastal marshes provide highly valued services such as coastline protection, wildlife habitat, nitrogen (N) transformations and carbon (C) sequestration, understanding the outcomes of these functional shifts is an important research concern. Of particular interest is the potential for impacts to coastal marshes’ important ecosystem service of C sequestration, since perturbations to this function could result in climate change-exacerbating feedbacks. Coastal marshes are such effective C sinks due to their high productivity and associated carbon dioxide (CO2) uptake, slow decomposition, and minimal emission of climate-altering greenhouse gases (GHGs). However, emission of GHGs may be stimulated by several of the global change drivers coastal marshes face. These potential drivers include N pollution, which can stimulate emission of the potent GHG nitrous oxide (N2O) from coastal marshes, and invasion of the aggressive introduced grass Phragmites australis, which may stimulate emission of methane (CH4). Testing how these impacts may interact to alter fluxes of GHGs in coastal marshes is important for a clear understanding of the role that coastal marshes play in global climate and whether this role is likely to be affected by a changing climate. Very recently, development of novel technologies for measuring GHG concentrations in situ in real time have made simultaneous measurement of the GHGs CO2, CH4, and N2O a possibility, and have opened the door to experiments that will improve understanding of coastal marsh GHG flux dynamics and their response to changes to the coastal marsh ecosystem. The objective of the research projects presented in this dissertation was to elucidate responses of coastal marsh GHG fluxes to drivers of global change including climate change, N