Background/Question/Methods 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 acquired over a period of approximately 7 decades (1940-2007) to quantitatively determine changes in seagrass (predominantly Halodule wrightii Ascherson) coverage on Horn and Petit Bois islands, Mississippi, USA. 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. Brightness density slicing (ENVI v.4.2) was used to produce thematic classes which represented seagrass beds and sandy bottom. Total seagrass coverage was computed with an average error of 10 percent relative to visual recognition of seagrass beds in the images.

Results/Conclusions From 1940 to 2007, seagrass coverage on Horn Island declined dramatically from 201 ha to 30 ha (570 percent) while coverage on Petit Bois declined from 56 ha to 14 ha (300 percent).  Most of this loss occurred during 1940-1985 as relative sea level (sea level rise plus coastal subsidence) increased by an estimated 8 cm. Seagrass beds which exhibited a wide meadow-like appearance in the 1940 images have degenerated into thin bands that tend to occur in protected areas adjacent to small sand ridges.  This corresponded with a decrease in the total area of shallow-water habitat required for seagrass growth and overall declines in island land area. Results indicate that declines in seagrass coverage may be linked with a reduced sediment supply.