Decadal-scale changes in seagrass coverage on the Mississippi barrier islands, northern Gulf of Mexico

Abstract

Seagrasses provide habitat for a wide variety of aquatic species, and reduce coastal erosion via sediment stabilization. The areal coverage of seagrass beds on the Mississippi barrier islands appears to have declined substantially since the 1960s. However, difficulties in sampling benthic vegetation, differences among previous studies in survey methods, and a lack of data on year-to-year variation in coverage have impeded quantitative study of this decline. Thus, the goal of the present study was to utilize historical remote sensing data to quantitatively determine changes in seagrass (predominantly Halodule wrightii Ascherson) coverage on the Mississippi barrier islands. Analyses were limited to imagery acquired during late summer through autumn, when seagrass canopies remained fully developed and water turbidity was low. Given these conditions, seagrass beds were readily identifiable in older black-and-white to color aerial photographs as well as in more recent multi-spectral imagery.On Horn, the largest Mississippi island, seagrass coverage declined from 77 ha in 1940 to 19 ha in 1971. Coverage had returned to its 1940 value by 2006, but declined again in 2007. Seagrass coverage on Petit Bois, the most rapidly-migrating island, declined from 54 ha in 1940 to an approximately stable 8-19 ha from 1952 through 2007. Petit Bois was the only island for which decline in seagrass coverage corresponded with decline in island land area. On the smallest island, East Ship, seagrass coverage remained nearly constant at 16-19 ha from 1963-2007 despite dramatic reductions in land area during Hurricanes Camille (1969) and Katrina (2005). On West Ship, coverage dropped to zero in 2003 but increased by 2007 to slightly exceed its 1975 value of 18 ha. Data for Cat Island extended only from 2003-2007. Seagrass coverage more than tripled during this period, from 22 ha in 2003 to 71 ha in 2007. Protection from trawling may have been a significant factor in the post-1995 recovery of seagrass on Horn Island. The absence of a similar recovery on Petit Bois may be linked with its rapid westward migration in combination with a nearly 40% reduction in land area from 1940-2007. There was no discernable negative impact of Hurricane Camille or Hurricane Katrina on seagrass coverage. Methodological differences (e.g., mapping potential seagrass habitat rather than existing seagrass beds) are important in explaining the dramatic decline in seagrass coverage that is apparent when recent data are compared with results of earlier surveys.