Near Shore Distribution of Silt and Clay, Organic Carbon, Select Trace Elements, and Seagrass in Bottom Sediments of a 0.5 kilometer Near-shore Corridor of Tampa Bay, July 2003

This study was designed to support a synthesis model that identifies correlations among near shore (depth less than 2 meters) bottom sediment properties (silt/clay, organic carbon, select trace elements) and sea grass distribution. In July 2003, the U.S. Geological Survey collected 330 surface sediment samples from stratified random sampling sites on the sea floor: half of these were located in sea grass beds most of which occur within 0.5km of the shoreline. For data analysis, Tampa Bay was divided into six sub-bays (Lower, Middle, Hillsborough, Riviera, Old, and Upper Old Tampa Bay). Average background organic carbon for the entire shoreline was 0.3 % (dry wt.). Each sub bays' shoreline ranged from 0.1 to 1.0 % (dry wt.). Silt/clay for sediments along the shoreline ranged from 0 to 100 % (dry wt.) with a mean of 16%. Selected samples (115) with the highest silt/clay percentage were used for trace element analyses including aluminum. Due to the conservative nature of aluminum, metal concentrations were normalized to aluminum. Results indicate enrichment of arsenic, cadmium, chromium, copper, nickel, lead, and zinc. Some samples with metals (arsenic, chromium, copper, nickel, and lead) within normal background concentrations were above Threshold Effect Level (upper limit of sediment contaminant concentrations which do not represent significant hazards to aquatic organisms). Percent silt/clay was approximately the same for samples inside and outside of sea grass beds. Organic carbon concentrations were less where sea grass was present.