Trophic transfer of metals and OCP’s in organisms from a warm temperate and a subtropical intertidal rocky shore
Globally, of all the services and functions that ecological systems provide to human welfare, ± 63 % comes from the marine domain. Pollution of the ocean by means of metals and organochlorine pesticides pose a great threat to the biodiversity of intertidal rocky shores, especially sedentary filter-feeders since they are well known to accumulate a wide range of metals in their soft tissues, as well as to consumers of these species. Past research focused on the use of bio-indicator species to determine concentrations of compounds found within the environment. This hinders the comparison of uptake and transfer between species and results in a knowledge gap in terms of most intertidal rocky shore food webs and species. By examining sites that are as near as possible to a natural ecosystem, the degree of uptake and transfer can be determined. In this study two hypotheses were tested at Tsitsikamma and Sheffield Beach: 1) stable isotope signatures will differentiate the food web structures between the intertidal zones of two temperature based biogeographic regions; and 2) a stable isotope approach can be applied to demonstrate the transfer of inorganic (metals) and organic (OCPs) substances from one trophic level to another in the intertidal zone of these two sites. The aims of this study were: 1) to compare to the two intertidal food webs using stable isotope signatures and to determine whether there are any differences in food webs between the two biogeographic regions; 2) to determine a food chain of species that directly influence each other; 3) to determine the bio-accumulation of both metals and OCPs in selected organisms from the above mentioned intertidal zones; and 4) to determine the degree of trophic transfer of these substances in the intertidal ecosystems to allow for comparison of the biomagnification potential of metals and OCPs between the two biogeographic regions. These aims were achieved through stable isotope analysis of carbon and nitrogen isotope signatures in a variety of organisms from the intertidal rocky shores, as well as ICP-MS and GC/µECD analysis for metals and OCPs, respectively. From the stable isotope signatures, a food web was established for the species that have been collected at Tsitsikamma and Sheffield Beach, as well as a food chain for trophic transfer of compounds. The results were then compared between sites for the species that were collected from both intertidal shores, and a correlation between compounds and the trophic structures of the two regions were determined. Tsitsikamma is located in the Eastern Cape Province on the south coast of South African and is one of the oldest Marine Protected Areas in Africa. It is located within the warm temperate biogeographic region and the Agulhas Current is the prominent ocean current that influences it. Sheffield Beach is a small KwaZulu-Natal coastal town and a well-known tourist location. It has no formal protection but houses some of the southern-most hard corals in the southern hemisphere. Sheffield Beach falls in the Benguela Current within the subtropical biogeographic region. Both of the sites are considered to be pristine with little to no pollution in the vicinity. Stable isotope ratios were determined for both of the locations, where the warm temperate Tsitsikamma site revealed a nutrient enriched food web and a carbon increase, compared to the subtropical Sheffield Beach site. This can be attributed to the different ocean currents associated with the specific stretch of coastline, as well as upwelling and nutrients, temperature, and freshwater input. The metal concentrations detected in the present study correlates with past research within the Tsitsikamma section. Ulva lactuca, Perna perna, Parvulastra exigua and Actinia sp. were collected at both sites and were used to determine significant differences between the two biogeographic regions. For P. exigua, all of the elements indicated significant differences, while concentrations in P. perna, had the least significant differences. A positive regression was calculated for all of the elements at Tsitsikamma but only for Cu, Zn, As, Se and Cd at Sheffield Beach. The sea surface temperatures are predicted to have great influence on the patterns observed. From the concentrations detected at Tsitsikamma there was an obvious trend of magnification from primary producer to secondary consumer and then a decrease from secondary consumer to tertiary consumer. This can further be explained by the feeding habits of the specific species, where tertiary consumers consisted of omnivorous organisms that depict a variety of prey source concentrations rather than the direct magnification of just one secondary consumer species. Organochlorine pesticides were also analysed in selected species. At Tsitsikamma a wider variety of compounds were detected when compared to those at Sheffield Beach. In most cases the concentrations were also higher at Tsitsikamma compared to Sheffield Beach. With regards to the trophic magnification factor calculated for OCPs, total HCH, total DDx and total Chlordane were determined for each trophic group in order to establish the overall degree of magnification that occurs. All of the compounds did however show biodilution rather than magnification. From the results obtained during the study, magnification of compounds within the marine environment under consideration is rather the exception than a general trend. Both of the above stated hypotheses set for the study has been accepted based on the results obtained from each chapter. The stable isotope analysis distinguished between the biogeographic regions based on species sampled, and a trophic structure for each of the regions were established. The second hypothesis was also successfully applied with regards to demonstrating the transfer of inorganic and organic substances through the established trophic structure.