1. Environmental Quality Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 10300 Baltimore Avenue, Beltsville, Maryland 20705; Southeast Watershed Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Tifton, Georgia 31793; Tropical Research and Education Center, University of Florida, Homestead, Florida 33031; and National Park Service, Biscayne National Park, Homestead, Florida 33033