CARBON DYNAMICS UPSTREAM AND DOWNSTREAM OF A SALT MARSH TIDAL RESTRICTION, DRAKES ISLAND MARSH, WELLS, MAINE

As the state of Maine strives for carbon neutrality by 2045, the restoration of salt marsh and other “blue carbon” ecosystems has become an important part of the conversation. Tidal restrictions across salt marshes are common and can result in reduced carbon uptake and storage, and increased greenhouse gas emissions. The aim of this study is to better understand the impact of tidal restrictions on salt marsh carbon dynamics. This study compares sediment core carbon analyses, monthly LICOR greenhouse gas flux data, sedimentation, vegetation, and stream hydrology from upstream and downstream of the Drakes Island Road tidal restriction on Drakes Island Marsh in Wells, ME. Currently, a tide gate sits below the road crossing, and initial analysis of stream channel water levels indicates reduced tidal amplitude upstream of the restriction. The average carbon density in the upper 80 cm of the sediment cores was not significantly different on either side of the restriction with values of 0.049 ± 0.008 gC/cm³ (n= 51) and 0.051 ± 0.013 gC/cm³ (n=49) downstream and upstream of the tidal restriction, respectively. Average methane fluxes across four field days appear to be slightly higher upstream [average flux of 6.0 ± 7.1 nmol/m²s (n= 24)] than downstream of the tidal restriction [average flux of 1.6 ± 2.4 nmol/m²s (n= 33)], likely due to variations in pore water salinity and biogeochemical cycling in the soils and/or vegetation type. Average CO₂ fluxes over four field days were -3.8 ± 5.9 µmol/m²s (n= 24) upstream and -2.8 ± 4.3 µmol/m²s (n= 33) downstream, and indicate no significant difference between the two sections of the marsh. Preliminary annual carbon sequestration rates were similar on both sides of the tidal restriction with 138 ± 23 gC/m² downstream and 187 ± 48 gC/m² upstream of the tidal restriction. The data from this study suggest that the tidal restriction at Drakes Island Rd may have an impact on carbon cycling (e.g. methane emissions) within the marsh. Continued work creating a preliminary/partial carbon budget for the system will help to better understand how carbon is moving in and out of the system on each side of the restriction.