Transplanted oysters and resident mud crabs as biomonitors in Spillway Creek

Abstract

Transplanted oysters and resident mud crabs were used as biomonitoring tools to assess spatial differences in fluoride and metal (aluminium (AI), arsenic (As), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), nickel (Ni), selenium (Se) and zinc (Zn)) concentrations in Spillway Creek. Oysters obtained from an oyster lease from an uncontaminated area were deployed for a three month period at four sites within Spillway Creek and two external control sites. After retrieval, oysters (soft tissue) were analysed for concentrations of fluoride and nine metals. Between site comparisons included oysters from the oyster lease. Mud crabs were collected from the same sites over a two week period immediately prior to oyster retrieval and assessed for the presence of rust spot shell disease. Hepatopancreas (liver) and muscle tissues of mud crabs were also analysed for fluoride and metal concentrations and between site comparisons made. Concentrations of fluoride and metals in oysters and mud crabs were also compared to food guidelines. The mud crab results were also compared to the findings of a previous study 'Fluoride and metals in Spillway Creek Crustacea' (Andersen et al., 2001). Analyses of water metal and fluoride concentrations in Spillway Creek were also undertaken by BSL on one occasion. Despite elevated concentrations of fluoride in water samples closer to the discharge channel there were no between site differences in fluoride accumulations in oysters. Concentrations were, however, elevated in mud crab muscle from mud crabs closer to the discharge channel compared to sites near the mouth of Spillway Creek and the external reference sites, althoug.lJ. the site separation was not statistically significant. A similar trend of fluoride accumulation in mud crab muscle was evident in the previous study. Although mean concentrations in mud crab hepatopancreas tended to be more elevated in the Spillway Creek sites these were not significantly different to reference sites. Concentrations in mud crab muscle were not at such a level as to pose a human health risk from the consumption of mud crab meat. Nickel was elevated in Spillway oysters closer to the discharge suggesting exposure to bioavailable nickel, however a reverse trend of accumulation was evident for copper, zinc and to a lesser extent iron. Lilly Island oysters often had some of the highest metal concentrations, with oysters from the lease area often the lowest. Wild Cattle appeared to be a suitable control site. There also appeared to be an accumulation of selenium in mud crabs closer to the discharge channel. Apart from selenium there appeared to be no site trend for metal accumulations in mud crab tissue, which was similar to the fmdings of the previous study. Metal concentrations in mud crabs or oysters were not outside the boundaries of current food guidelines. There also appeared to be no relationship between water metal concentrations sampled on one occasion and biota metal concentrations. Water concentrations in Spillway Creek are known to be variable and the findings highlight the benefits of biologically monitoring, representing average ambient bioavailable contaminant concentrations over a time period. The differences in fluoride and metal concentrations in oysters and mud crabs in this study are likely to be due to a combination of their accumulation strategies and the nature of the exposure being pulse rather than continuous