Environmental drivers of greenhouse gas dynamics in temperate and tropical wetlands

Abstract

Wetlands play important roles in carbon cycling, contributing to both carbon storage and carbon emissions. Surface waters in wetlands are particularly important sources of methane (CH4) and can also function as net emitters of carbon dioxide (CO2) due to unique hydrologic and biogeochemical conditions in wetlands. Wetland carbon cycling can have high variability compared to other freshwater ecosystems because of the many underlying processes influencing the production, consumption, and transport of carbon. However, wetlands are frequently overlooked in studies of aquatic ecosystems due to their complexity. In addition, some wetland ecosystems have been difficult to incorporate into global wetland studies such as small, headwater wetlands and tropical wetlands. Due to unique local conditions, such as dynamic hydrology and warmer temperatures, headwater and tropical wetland ecosystems could potentially have larger contributions to carbon emissions than other wetland ecosystems. I used a combination of synoptic sampling, high-frequency sensors, and experimental manipulations to address questions about the spatiotemporal variability and environmental drivers of CO2 and CH4 in understudied wetland ecosystems. In small, headwater wetlands in the Delmarva Peninsula, I sampled 20 wetlands, which varied in their geomorphic features (size, perimeter:area, etc.) and characterized the spatial and temporal variability of CO2 and CH4 over the course of two years. I also used paired oxygen and CO2 high-frequency sensor data and estimated rates of aerobic metabolism to further identify dominant biogeochemical processes influencing carbon cycling in these wetlands during a one-year period. In freshwater coastal wetlands in Puerto Rico, I sampled seven sites over four years and performed an experimental manipulation using seawater to test the influence of salinization on CO2 and CH4 production. Wetlands across both Delmarva and Puerto Rico study sites were supersaturated in CO2 and CH4 with respect to the atmosphere, highlighting the role of wetland surface waters as likely sources of CO2 and CH4 to the atmosphere. Wetlands in Puerto Rico had particularly high CH4 concentrations, reaching up to 93μM. In both regions, spatial and temporal variability in CO2 and CH4 concentrations was high, particularly for CH4. In Delmarva, both gases were more spatially variable than temporally while in Puerto Rico, CO2 was more variable temporally and CH4 was more variable spatially. At the landscape scale, smaller wetlands with higher perimeter:area ratios in Delmarva had higher CO2 and CH4 concentrations. Larger wetlands also had higher rates of aerobic metabolism, while the smaller wetlands showed more influences of groundwater and anaerobic respiration. Comparing wetlands with other freshwater ecosystems globally, CO2 in Delmarva wetlands was more variable than in lakes and rivers. In Puerto Rico, CH4 varied across regions but CO2 did not vary spatially. In addition, increasing salinization decreased CH4 but increased CO2, with differences in the magnitude of response across wetland sites. This work highlights how geomorphologic characteristics such as size, govern patterns of CO2 and CH4 and their dominant processes in small, headwater wetlands while other local factors such as underlying geology could influence the spatial variability of CH4 in tropical coastal wetlands. Temporally, we find that CO2 increases with temperature and CH4 increases with decreasing oxygen in both regions. Ultimately, understanding the variability of CO2 and CH4 in wetlands, and how these might change with changing environmental conditions and across different wetland types, will continue to be critical to understanding the current and future role of wetlands in the global carbon cycle.