Abstract
The Pascagoula River Estuary contains one of the most expansive marsh ecosystems on the Mississippi Gulf Coast. The marsh provides essential economic and ecological services by supporting fisheries, filtering impurities and sediments from estuarine waters, and reducing storm surge impacts to upland areas. To assess spatial changes in marsh extent over the past two decades, we classified land cover at 3 meters GSD using four multispectral aerial image datasets collected between 1996 and 2018, with the goals of 1) quantifying changes in marsh extent, and 2) identifying areas of retreat and expansion at the lower marsh boundary. Using these classified maps, we measured changes in the areal extent of marsh, woodland, water, and unvegetated areas, and used linear regression to quantify land cover trends over time. We also measured movement of the marsh edge along channels, lakes, and the marine boundary of the marsh by calculating transect lengths at 50-meter intervals perpendicular to baselines derived from National Hydrography Dataset features. Linear regression indicated a total rate of marsh loss of 24.4 ha/yr (r^2=0.83), and a rate of 13.0 ha/yr (r^2=0.99) when only interactions between marsh and water or unvegetated areas were considered. Mean channel width increased by 5.1 m, and the marsh edge retreated by a mean 7.0 m along areas bordering the Mississippi Sound and a mean 7.0 m along lakes and other nonlinear hydrological features within the estuary, with positive and negative changes in transect length seen in each category. As conversion from marsh to woodland land cover has also resulted in net marsh loss, maintenance of the lower marsh boundary is one key factor in slowing future losses in marsh extent. These findings may be used to target areas for restoration and to examine the conditions which have allowed expansion to occur at the marsh edge.
