EXAMINING THE DISAPPEARING SALT MARSH “SPITS” IN A TIDAL SUBESTUARY OF MOBILE BAY: RESULTS FROM PISTON CORE ANALYSES, HISTORIC SHORELINE COMPARISONS, AND WAVE CLIMATE MONITORING

As sea level has risen throughout the Holocene due to glacial retreat, the sinuous meanders and freshwater marshes of Fowl River, a local tidal river that discharges to Mobile Bay, have gradually been inundated and replaced by a broad basin surrounded by salt marshes. Although geomorphic evolution is natural, anthropogenic stressors including vessel traffic and changes in land use may accelerate marsh loss. Here we investigate changes in net marsh extent and accretion in Fowl River using marsh sediment cores and shoreline comparisons from historical aerial photography. Loss on ignition, grain size characterization, and radiocarbon dating were employed to detect accretion changes over the past millennium. Cesium-137/lead-210 analyses are currently underway to characterize recent accretion over the last century. In addition, wave monitoring was conducted to evaluate changes in wave climate attributed to vessel traffic. Results thus far demonstrate no readily apparent temporal (i.e. depth) trends in organic carbon or grain size; however, there were spatial differences. Cores collected on upstream sides of marsh spits tended to have less organic carbon (5 to 35 %) and more sand (40 to 80 %) than cores collected on downstream sides (30 to 65 % organic carbon and 5 to 25 % sand), perhaps reflecting remnant fluvial geomorphology. Despite spatial differences in sediment composition, linear net marsh accretion from core radiocarbon dates were similar and averaged 1.15 mm yr^-1 (σ = 0.13 mm yr^-1). Regression of depth versus radiocarbon age yielded a net accretion from 590 to 880 BP of 1.0 mm yr^-1 (R² = 0.61) which approximates subsidence; however, the intercept of the regression is 85 mm which is below the recent eustatic, post-industrial revolution sea level rise. Shoreline comparisons further confirm the disappearance of marsh in Fowl River, and wave monitoring revealed a unique wave climate with that majority of the wave energy occurring between sunrise and sunset (i.e. vessel traffic). These results suggest that the Fowl River marshes are not accreting at a rate sufficient to keep pace with current sea level rise and are at a continued risk of outright collapse.