Seasonal Variations in Trace Metal Concentrations and Growth Factors in Bivalve Molluscs from New Zealand and Oman as Indicators of Marine Pollution

Abstract

The research described in this thesis investigated the use of seasonal profiles of trace metal concentrations and growth factors in three species of bivalve molluscs from two different geographic regions as a more accurate means of assessing trace metal and other pollutant inputs into the coastal marine environment than single period measurements. The unifying theme of these investigations was the use of the "Mussel Watch" strategy to obtain seasonal profiles of trace metal concentrations in these species. The research results comprising this thesis are presented in three case studies. The first two case studies compared seasonal trace metal concentration profiles and the results of scope for growth measurements in two species of indigenous New Zealand mussels Mytilus edulis aoteanus Powell and Perna canaliculus (Gmelin) in order to define the geographic extent and possible biological effects of a known source of sewage and trace metal contamination in Wellington Harbour, New Zealand. In the third case study, seasonal variation in the concentrations of eleven trace metals and the condition index of the rock oyster Saccostrea cuccullata Born, were used to detect trace metal inputs from previously suspected sources along the coast of the Sultanate of Oman and to assess the potential effect of exposure to petroleum hydrocarbon pollution on trace metal content and condition in this species. In Case Study 1, seasonal trace metal data were used to distinguish natural sources of Cr, Fe, Mn and V input from the input of these and other metals associated with effluent discharged from a combined domestic and industrial sewage outfall. Consistently high levels of some elements, especially Ag, Cu, Pb and Zn in both mussel species, defined areas exposed to sewage effluent and the relative degree of this exposure. Seasonal data also indicated that trace metal concentrations in mussels further than 100 m from the outfall were below statutory standards for human consumption. Seasonal condition index estimates revealed a difference in the condition cycles of M. edulis aotenaus and P. canaliculus and that variations in trace metal concentrations were not related to reproductive state. Mussel populations monitored for trace metal concentrations described in Case Study 1, were also tested for Scope for Growth (SFG) to evaluate the biological impact of sewage and trace metal contamination on these two representative species. The results of these measurements, presented in Case Study 2, indicate that the SFG of both mussel species is adversely affected only at sites within 750 m. of the sewage outfall and that the SFG of M. edulis aoteanus from two sites may actually be enhanced by exposure to the sewage effluent. These results show that the zone of adverse biological impact is confined to within a limited radius of the outfall. The results of SFG measurements of P. canaliculus transplanted along a concentration gradient within the effluent field clearly indicate that the physiological condition of this species is impaired by exposure to concentrated effluent. However, the adverse effects of almost continuous exposure to the effluent were negligible further than 600 m. from the outfall. The results of this study also demostrate the influence of body-size/age on SFG and trace metal content of both native and transplanted P, canaliculus and that the way in which size affects the SFG and trace metal content of this species may be related to prior exposure to sources of sewage and trace metal contamination and/or the environment from which they were collected. In Case Study 3, seasonal data confirmed regional trends noted in earlier, single period Surveys, and identified two major sources of trace metal contamination along the Omani coast. Seasonal trace metal profiles indicate that most of the metals analysed vary by a factor of between 1.5 and 5 with time at any one location, and that the patterns of variation for some elements are similar in oysters from sites within the same region, but vary in oysters from different geographic and/or oceanographic regions during the same period. Biological measurements and condition estimates indicate that seasonal variations in the content of some elements in S. cuccullata are related to changes in body weight, which in turn varies with the reproductive state of this species. The data suggest that S. cuccullata spawns only once a year and that peak spawning periods coincide with periods when sea surface temperatures are between 26 and 28 °C. Neither trace metal content nor condition of S. cuccullata was influenced by exposure to petroleum hydrocarbon contamination, which suggests that those metals which are found in relatively high concentrations in crude oils are not available for accumulation by this species. Therefore, petroleum hydrocarbon contamination in ambient waters, or in S. cuccullata, cannot be detected by monitoring the concentrations of the elements tested in this species.