Bioaccumulation of 65Zn by the Sydney rock oyster (Saccostrea glomerata) from dissolved and particulate phases

Oysters and other similar bivalves are popular ‘biomonitors’ or ‘sentinels’ of the environment and are commonly used to assess the health of marine ecosystems. It has been generally accepted that uptake and bioaccumulation in bivalves is influenced predominantly by dietary ingestion of contaminated particles as well as from dissolved sources. An organism of relevance to Australian ecosystems is the Sydney rock oyster (SRO; Saccostrea glomerata), an intertidal, suspension filter-feeder commonly found on the coasts and estuaries of Victoria, New South Wales, and Queensland, Australia. Farmed SRO organisms were used for a 2 month mesocosm study where specimens were exposed to a gradient of resuspended sediment loads and sediment-bound trace metal concentrations. The results indicated poor correlations between SRO tissue metal concentrations and either sediment metal concentration or resuspended volume, with the greatest bioaccumulation being observed in the control tank containing no sediment. These results suggested that SRO metal bioaccumulation was driven primarily from a dissolved source. To investigate this further, a radiotracer study using the gamma-emitting radioisotope 65Zn was conducted, in which SRO organisms were exposed to the dissolved 65Zn radioisotope at three concentrations (5, 25 and 50 µg/L) for 4 days, followed by 20 days of depuration. Dietary assimilation of Zn was examined through pulse-chase experiments where SRO specimens were fed either 65Zn labelled fine-fraction sediments in suspension, or algae. The outcome of this experiment was to outline the relative importance of dissolved, sedimentary, and algal metal sources, and conclusively determine the primary uptake pathway for metal bioaccumulation. The resulting data were used to establish uptake and efflux rate constants from dissolved sources, and assimilation efficiencies from the dietary sources, which were then incorporated into a biodynamic accumulation model. The results of this study are discussed in the context of the use of bivalves as indicators of sediment quality