Source and flux of methane released from Corpus Christi coastal area

Methane (CH4) is the second most abundant greenhouse gas after carbon dioxide (CO2), with an atmospheric warming potential of approximately 28-34 times and 84-86 times that of CO2 over 100 years and 20 years, respectively (IPCC 2014). Coastal areas, particularly vegetated coastal ecosystems, including seagrass meadows, salt marshes, and mangroves, referred to as major reservoirs of blue carbon, have a high potential for the emission of CH4 and other greenhouse gases to the atmosphere due to their high capability of carbon storage. Hence, the effect of coastal areas on global warming is based on the balance between carbon deposition and greenhouse gas emissions. Located at the interface of land and ocean, coastal areas are more vulnerable to the consequences of human activities and climate change, e.g., global warming, eutrophication, sea-level rise, and hypoxia, which could influence carbon sequestration and emissions of greenhouse gases, including CH4, as feedback. In this project, we determined the sources and fluxes of CH4 released from the Corpus Christi coastal area to the atmosphere, explored the mechanisms influencing CH4 emissions, and estimated the contributions of natural processes and anthropogenic activities to the local atmospheric CH4 budget. Water and air samples were collected from Corpus Christi Bay and Nueces Bay, Upper Laguna Madre, and Aransas Bay from June 2018 to May 2021 to determine dissolved CH4 concentrations and CH4 fluxes at the sea-air interface. We also measured CH4 fluxes at the sediment-water interface using the porewater CH4 profile and incubation experiments. Half-year continuous atmospheric CH4 monitoring was performed at Ingleside, Texas, in 2021 to determine the impact of anthropogenic CH4 emissions on the residential community. We also applied aircraft observations over the Ingleside industrial area in August 2021 to measure industrial emissions of CH4. We observed highly spatial and temporal variations in CH4 emissions from these estuaries. The annual sea-air CH4 flux was largest at Upper Laguna Madre and lowest at Aransas Bay. Such discrepancies could be attributed to the distribution of different environmental characteristics, i.e., seagrass, mangroves, channels, and open bays in each estuary. Tidal processes, amplitude (spring and neap), and topographic characteristics are crucial factors controlling CH4 cycling in mangrove estuaries. Dissolved CH4 concentrations in creeks were higher during ebbs due to the export of CH4 from inside mangroves and porewater tidal pumping. During floods, CH4 concentrations in water were dependent on the balance between CH4 input from sediment and bay water dilution. Elevated CH4 concentrations in spring tides compared with neap tides could be attributed to additional CH4 emissions from upper intertidal sediment. The annual CH4 emissions offset approximately 0.17% of local organic carbon deposits, indicating that these estuarine mangrove creeks are a weak CH4 source. In seagrass meadows, both diurnal and long-term variations showed a tight relationship between CH4 concentration and dissolved oxygen, dissolved inorganic carbon and pH, which was driven by photosynthesis and respiration of the seagrass ecosystem. Photosynthetic oxygen transported by seagrass to sediment played a significant role in reducing CH4 production and transport. Seasonal variations in CH4 concentrations in seagrass meadows coincided with seagrass growth patterns, indicating a possible plant mediation of CH4 emissions from sediment to water. The CH4 emissions were estimated to offset 1.4%~2.2% of the blue carbon deposited in local seagrass meadows. This study reported the largest CH4 emissions from global seagrass meadows to date. In comparison with mangrove creeks, seagrass meadows were a more significant CH4 source. Further analyses showed that tidal processes could largely decrease CH4 emissions, by exporting CH4 from mangroves to open bays during ebbs and diluting CH4 concentrations by bay water during floods. It implied a strategy of applying tidal processes in coastal wetland restoration. Extreme weather was found to enhance CH4 emissions in these coastal areas. Higher CH4 concentrations were observed after the extreme cold event in February 2021, which was related to more organic carbon deposits induced by the high mortality of mangrove forests during the extremely cold days. The larger riverine discharge in November 2018 caused by heavy precipitation delivered more CH4 from freshwater to coastal water, temporarily enhancing CH4 concentrations and sea-air CH4 flux in the Nueces Bay. Daily CH4 flux was highest in the channel at Upper Laguna Madre, suggesting that the disturbance of vegetated sediment could severely enhance CH4 emissions in comparison with less vegetated sediment. In addition, maritime transportation and gas pipeline leakage are direct anthropogenic CH4 sources in these estuaries, which have been widely overlooked. The long term monitoring at a fixed station and aircraft observations at Ingleside estimated that one third of the CH4 input to the residential community was related to fugitive emissions during crude oil loading and offloading operations. At least 11% of CH4 fluxes corresponded to emissions from Ingleside industrial areas, much larger than the emissions reported to EPA in previous years. The overall anthropogenic emissions, including from large facilities in industrial areas and fugitive sources during maritime operations and from sediment disturbance, have been largely underestimated. The estimated CH4 budget in Corpus Christi coastal area was at least 6.3×109 g/yr, among which 90% was from anthropogenic sources and 10% from natural sources. It suggested that anthropogenic emissions contributed the majority to local atmospheric CH4 levels.