Carbon and nitrogen cycling in vegetated coastal ecosystems

Coastal ecosystems comprise a relatively small area of the ocean, yet they play a disproportionate role in greenhouse gas (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) and nutrient cycling. Vegetated coastal ecosystems (e.g., mangroves, salt marshes, and seagrasses) are key drivers of coastal greenhouse gas and nutrient cycling because of their environmental characteristics (e.g., shallow depths, organic matter rich sediments, etc.). My dissertation addresses the role of vegetated coastal ecosystems in greenhouse gas budgets and biogeochemical cycling. In Chapter 1, I conducted a meta-analysis to quantify the global emissions of CH4 from mangrove, salt marsh, and seagrass ecosystems. Here I show that mangrove ecosystems contribute the most CH4 out of these vegetated areas to the global marine CH4 budget. Further, while a well-known negative relationship between salinity and CH4 fluxes exists for salt marshes globally, this relationship does not hold for mangrove or seagrass meadows, suggesting that other environmental drivers are more important for predicting CH4 fluxes in these ecosystems. In Chapter 2, I present in situ fluxes of CH4 and N2O across the sediment-water interface as well as air-sea fluxes in seagrass meadows and adjacent non-vegetated sediments in two temperate coastal lagoons. Here I demonstrate that seagrass meadows can be sources or sinks of CH4 and that N2O uptake can enhance carbon sequestration in seagrass meadows by ~10%. In Chapter 3, I quantify fluxes of dissolved inorganic carbon, nitrogen, and phosphorous across the sediment-water interface in seagrass meadows and adjacent non-vegetated sediments in the same two coastal lagoons. I found that both seagrass and non-vegetated sediments exhibited dissolved inorganic carbon emission and denitrification, and that dissolved inorganic phosphorous fluxes varied by site and not with vegetation presence. This dissertation highlights the dynamic role coastal ecosystems play in biogeochemical cycling and the importance of vegetated coastal ecosystems in coastal greenhouse gas budgets.2024-10-03T00:00:00