The anticipated environmental impacts caused by the presence of oyster culture operations are similar to those of mussel culture, as both bivalves can be grown in suspended or bottom culture. The presence of oyster rafts has been shown to alter thecharacteristics of the sediments below the farms (Hayakawa et al, 2001) although the effect of such  a change in sediment character was not considered. The use of a model by Chapelle et al (2000) illustrated that in an enclosed Mediterranean lagoon, an oyster farm led to increased ammonium concentrations, and decreased concentrations of phytoplankton, zooplankton and oxygen. The increase in ammonium was attributed to direct excretion by the oysters, with decreases in plankton and oxygen due to filtration and respiration of the oysters. As has been found in other studies, sedimentation was greater in the area of the farm, leading to higher levels of organic matter. Using the model, Chapelle et al (2000) found that by halving the oyster biomass, there was an increase in phytoplankton and zooplankton. They also illustrated a change in the dominance and succession of both phytoplankton and zooplankton and this is likely to be due to a shift from oyster predation to zooplankton predation on the phytoplankton. An alteration in phytoplankton and zooplankton concentrations represents a shift in the trophic balance within the ecosystem and as

such may result in a failure to comply with criteria 1 of the MSC’ s Principle 2.

Cultivation of oysters, namely Crassostrea gigas, has been shown to affect the macrobenthic community. De Grave et al (1998) took samples from a trestle culture site, from both beneath the trestles and the access lanes, and compared the results to samples from a control site. The diversity of organisms beneath the trestles was lower than the control site, with lower numbers of individuals and a lower number of species. The samples from the access lanes, however, showed an increased diversity when compared to the control site. De Grave  et al  (1998) concluded that the pre sence of oyster trestles, whilst not increasing the organic content of the sediment, induced a slight shift in total species and displaced some species. It was also noted that the trestles a ct ed as a refuge for mobile scavengers such as Carcinus maenas and Paleamon serratus as few were found on open sand compared to larger numbers beneath the trestles. Results from this study suggest that although there is a minor effect on the benthic community structure, there is still a stable community present and when compared to other aquaculture systems the effects are negligible. A decrease in biological diversity could constitute a failure to comply with the second criteria of Principle 2. Although it should be noted that the area under the trestles which exhibi ts a decreased biodiversity is only a small area of a wider ecosystem, and if the ecosystem were considered to include the areas outside of the farm then the effect is likely to be negligible. Providing that culture sites do not dominate an ecosystem, the n the second criteria could be met and Principle 2 complied with.

A major impact of oyster culture on the environment is the use of Carbaryl, a broad spectrum pesticide, which is used in the USA to control burrowing shrimppopulations. There are two species of burrowing shrimp, Neotrypaea californiensis and Upogebia pugettensis, which burrow into the mud beneath oyster reefs in the Pacific Northwest region of the USA (Dumbauld et al, 2006). The Carbaryl is added to the sediment every 6 years as part of ground cultivation plot preparation (Simenstad & Fresh, 1995). The use of Carbaryl can potentially alter the community

structure and trophic web and could result in a failure to comply with Principle 2 criteria.

There are also positive impacts of cultured oysters, as with other bivalve mollusc culture, which  are often overlooked. Oysters are efficient filters of the sea and can remove heavy metals, organic matter, suspended solids and phytoplankton, which can be especially useful in heavily polluted waters and eutrophic  conditions . Oyster reefs have been shown to effectively remove faecal coliform bacteria and chlorophyll a (in the form of phytoplankton) from the water column, and form an important part of the nutrient cycle releasing ammonium into the environment (Cressman et al, 2003). It should be noted that oyster beds were once much more prolific (Keiser  et  al , 1998) and as such the ir re-introduction into estuaries could be an effective way of resetting the balance of these ecosystems by removing urban and agricultural waste entering the marine environment through the rivers and runoff. An example of how prolific oyster beds were in previous times is illustrated in Chesapeake Bay, USA. The estimated biomass of oysters occurring in the bay pre-1870 was 188 x 10 6  kg (dry weight) and the oysters could filter the entire volume of water within the bay in 3- 6 days. The biomass in 1988 was calculated at 1.9 x 10 6 kg (dry weight) and the turnover time had increased dramatically to 325 days (Newell, 1988). Th e dramatic decrease in oyster biomass has been attributed to continued overexploitation by capture fisheries and by the pressures of disease. The result of the dramatic decrease has been the formation of an anoxic layer below the pycnocline during the summer months. The re-introduction of oysters would help to reverse these effects through their role as a major benthic-pelagic coupler (Mann, 2000), and is an example of how the enhancement of shellfish fisheries and shellfish culture can improve the natura l environment and stabilise endangered ecosystems.

Oysters have also been shown to successfully reduce the organic load in aquaculture effluents. The Sydney Rock oyster, Saccostrea commercialis, reduces the total suspended solid (TSS) content of shrimp pond effluent to 49% of initial levels when stocked at high density (Jones & Preston, 1999). Such studies show promise for the poly -culture of oysters with other commercially cultured species, reducing the environmental impact of the operation and increasing the economic gains. The use of pearl oysters to remove heavy metals  has been investigated and illustrates a method of aquaculture to improve the environment whilst producing an economically viable product without the concerns over human consumption (Gi fford  et al, 2004). The production of pearl oysters could comply with the criteria for  Principle 2, although

whether the flesh would be edible is not the focus of this report and would be dependant on the level of pollution in the locality of the culture site.