Anthropogenic impacts on carbon and nitrogen cycling in vegetated coastal ecosystems: implications for ecosystem function and resilience
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Abstract
Vegetated coastal ecosystems, including salt marshes, seagrasses, and mangroves comprise less than 1% of the Earth’s surface, yet play a disproportionate role in global biogeochemical cycling. Additionally, they provide ecosystem services that support coastal communities and our national economy. Unfortunately, we are losing vegetated coastal ecosystems at an unprecedented rate largely due to sea level rise (SLR) and nutrient pollution. Because of their importance, a variety of resilience strategies are being employed with the goal of protecting these valuable systems. In this dissertation, I bring new understanding to how salt marsh and seagrass ecosystems sequester carbon and remove nitrogen under the current state of change. Specifically, I quantify how the salt marsh SLR resilience strategy of sediment addition, also called thin-layer placement or TLP, impacts greenhouse gas dynamics (Chapter 1) and nitrogen removal via denitrification (Chapter 2). In Chapter 3, I investigate how excess nitrogen loading and eutrophication may alter seagrass greenhouse dynamics. In Chapter 1, I demonstrate that while carbon dioxide (CO2) uptake in TLP plots was significantly higher than in reference plots, and comparable to control plots, methane (CH4) fluxes were 7 and 22 times higher in TLP plots compared to control and reference plots, respectively. Across all treatments I observed both uptake and emission of nitrous oxide (N2O), although the magnitude of these fluxes were typically much smaller than CO2 and CH4 fluxes. While the emission of CH4 and N2O partially offset CO2 uptake in the TLP plots, they remained net sinks of greenhouse gases and thus retain, at least to some degree, their net climate benefit. In Chapter 2, using an in situ isotope pairing technique I found that total denitrification (the sum of direct denitrification coming from the added isotope spike and ambient denitrification coming from the coupling of nitrification and denitrification) was ~1.5 times greater in the TLP plots than the control. While direct denitrification was dominant in the TLP plots, ambient denitrification was dominant in the control plots, accounting for ~ 60% of total denitrification. This suggests that when nitrate is available, denitrification may be enhanced under TLP. Lastly, in Chapter 3, I synthesized the existing literature on greenhouse dynamics and eutrophication impacts on seagrass ecosystems. I then hypothesize how eutrophication is likely to impact greenhouse dynamics highlighting, for the first time, how excess nitrogen loading may impact seagrass ecosystem carbon storage. I show that eutrophication decreases seagrass biomass, increases sediment organic matter quantity and lability, and decreases oxygen concentrations. In turn, I hypothesize that these changes lead to an increase in all greenhouse gas fluxes. I conclude that as eutrophication is a prevalent problem globally, it likely has a profound impact on seagrass GHG dynamics. Overall, this dissertation contributes to our understanding of how deleterious anthropogenic impacts (e.g. sea level rise and nutrient pollution) influence biogeochemical function and ecosystem services in salt marsh and seagrass systems. This work also highlights how positive action (e.g., resilience strategies such as sediment addition) can maintain or even have the potential to improve ecosystem services provided by vegetated coastal systems.
Description
2025
License
Attribution-NoDerivatives 4.0 International