An ecological study of the effects of offshore dredge material disposal with special reference to hard-bottom habitats in the eastern Gulf of Mexico.

Abstract

The present study was conducted at the request of the Manatee County Chamber of Commerce to evaluate the effects of offshore disposal of sediments dredged from Bayboro Harbor, St. Petersburg, Florida. Data collected included sediment grain size analysis at 25 stations, sediment chemistry (heavy metal content) at several sites, bottom currents, bathymetry, and diver transect observations including qualitative sediment analysis, benthic species identification and photographic surveys. Sediment grain size analysis indicated a considerable increase in sediment silt content as compared to results of a study previous to dredge material disposal (Taylor,1979). Elevated silt content was detectable as far as 0.5 miles north and south of the designated disposal site. Concentrations of zinc, lead, and cadmium in sediments at the center of the disposal site were higher than the 96 hour -LC50 (50% lethal concentration) values for several standard marine bioassay animals. Diver observation at 23 stations revealed the presence of partially buried and completely buried hard-bottom habitats within the boundaries of the disposal site. Living hard-bottom communities, including hard corals, soft corals, and sponges were observed on all sides of the disposal site. The. effects of siltation on reef animals were visible up to one mile from the border of the disposal site.