Spatial and temporal variations in trace metal concentrations in sediments and oysters from Galveston Bay, Texas
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For the work described in this thesis, about one hundred sediment samples were collected on seven sampling trips from 42 locations in Galveston Bay and adjacent areas, including Trinity Bay, East Bay and Gulf of Mexico beaches on Galveston Island. Nearly one thousand oysters (Crassostrea virgincia) were collected at 16 sites in conjunction with sediment sampling on four trips during 1992-1993. Total and non-residual metals were determined in the <63 Ým size fractions of sediments, and total metals in the whole soft parts of individual oysters. Both media were analyzed for Fe, Ag, As, Cd, Cu, Ni, Pb and Zn, whereas Al and Mn were analyzed in sediments only. Trace metal concentrations in fine grained fractions of Galveston Bay sediments suggested that they are complex in mineralogy, especially nearshore sediments, with variable Al and Fe concentrations both spatially and temporally. Trace metals were highly enriched in the fine fractions of nearshore sediments. Metal concentrations in open-water sediments were relatively constant and were similar to those reported for other bays and estuaries. Anthropogenic fractions of trace metals varied both spatially and temporally. Physical conditions (waves, tides) are important factors causing variation of metal concentrations at nearshore sites. The redox condition in surface sediments may account for some of the variability in sediments at open-water sites. Significant variations were observed in metal concentrations in oysters, both spatially and temporally. Differences in metal concentrations were a factor of 5 or more for some metals from site to site, or from time to time at a given site. Statistically, Zn was found in anomalously high concentrations at several sites, mainly along the west side of Galveston Bay. Isolate high values of individual metals were found scattered throughout the Bay, however only one site (GBSL, near Swan Lake in lower Galveston Bay) was observed to have significantly high concentrations for several metals (Ag, Cu, Pb and Zn). Comparing metal concentrations in oysters and in sediments suggested that sediment bound trace metals may control some metal concentrations in oysters (Ag, As, Pb and Zn). Apparently, salinity played a major role in determining bioavailability of Cu and possibly other metals (Ag, Zn). However, all the reasons for the observed large changes in metal concentrations in oysters over time could not be documented in this study.