Date of Award

Fall 2020

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Wilfred M Wollheim

Second Advisor

William H McDowell

Third Advisor

Ruth K Varner

Abstract

Fluvial wetlands, wetlands connected to streams and rivers, can act as buffers in headwaters to limit nitrogen (N) from reaching downstream coastal ecosystems and causing problems, such as coastal eutrophication and loss of habitat. However, as significant hotspots for N removal, fluvial wetland dominated streams are also natural sources of greenhouse gas (GHG) to the atmosphere and contribute to global climate change. With ongoing changes to the flow regime from increased climate variability and intensification of storm events, as well as landscape development, the ability for fluvial wetland dominated streams to regulate downstream N fluxes may decline and come at a greater cost of GHG emissions. To better understand these tradeoffs, I investigated storm influence on nitrate (NO3-) removal and GHG evasion along two fluvial wetland dominated flow paths with differing nutrient inputs (high vs. low) in an urbanizing coastal watershed in New England. Results suggest that flow paths with abundant fluvial wetlands are able to remove most NO3- (median NO3--N removal = 95%) over a wide range of flow conditions. Due to their substantial demand for NO3-, fluvial wetland dominated streams were greater sinks of NO3- than upstream channels. Although emissions by fluvial wetland dominated reaches are much larger than those by channels when total area is considered, fluvial wetland dominated streams were found to emit lower GHG compared to channelized streams on a per unit area basis. After storms during heightened flow conditions, the flow paths maintained high NO3- removal but showed tendencies for greater GHG evasion, as areal GHG evasion by wetland dominated streams increased on average by more than 19,000 mg m-2 d-1 for carbon dioxide (CO2), 49 mg m-2 d-1 for methane (CH4), and 0.15 mg m-2 d-1 for nitrous oxide (N2O) over an order of magnitude change in discharge. Thus, as climate variability intensifies, we can expect to see pulses in GHG emissions along whole flow paths. However, GHG evasion by wetland dominated streams did not increase in association with higher nutrient loads. Ultimately, the ability for fluvial wetland dominated streams to effectively remove NO3- from surface water flow paths draining higher N inputs does not come at the expense of greater GHG emissions beyond those that naturally occur. Understanding these tradeoffs in river networks is important for improving the management of coastal watersheds and predicting how diverse fluvial systems will respond as N loading increases in a changing climate.

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