Peripheral freshwater deltaic wetlands are hotspots of methane flux in the coastal zone

Methane (CH4) emissions are low in the coastal zone due to a higher redox poise, related to sulfate reduction. However, river deltas are a potential source of CH4 flux in coastal zones globally, due to fresh condition and high primary production. The goal of this studywas to seasonally measure CH(4)flux at three different geomorphic settings (newly forming island, river channel bottom and established freshwater marsh) within the Wax Lake Delta, Louisiana, USA. CH4 flux rates were 386 +/- 327 mg C m(-2) d(-1) in March and 2859 +/- 1286 mg C m(-2) d(-1) in June at the freshwater marsh site. At the island site, CH4 flux was significantly smaller at 7.94 +/- 3.57 mg C m(-2) d(-1) in March and 215 +/- 153 mg C m(-2) d(-1) in June while at adjacent river channel bottom site, CH4 flux was lowest at 2.49 +/- 3.38 mg C m(-2) d(-1) in March and 19.5 +/- 1.12 mg C m(-2) d(-1) in June at the air-water interface. CH4 emission rates show significant spatial heterogeneity with rates up to two orders of magnitude greater at themarsh site at the periphery of the delta, related to greater soil total C. Therefore regions within the active delta do not provide a significant source of methane, due to a lack of soil C, despite freshwater conditions. However, marshes at the periphery within the halo of fresh water, populated with established plant communities can be significant hotspots of CH4 emissions, despite their location within the coastal zone. (c) 2021 Elsevier B.V. All rights reserved.