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Cadmium in the coastal marine environment : pathways for cadmium in oysters and using the cadmium : phosphorus ratio as an indicator of biogeochemical processes Lekhi, Priya Anka

Abstract

Pacific oysters collected on the coasts of British Columbia (BC) have shown Cd levels of around 2 μgg⁻¹ and higher (wet weight) which has resulted in the loss of some international markets. During 2004-2005, two farm sites on Vancouver Island were monitored for various parameters including the Cd concentration in oysters, dissolved and particulate Cd in seawater, temperature and salinity. One part of this study investigated the source and transfer of Cd to oysters by focusing on the role of dissolved and particulate Cd in seawater. Results show that dissolved Cd is the main source of Cd to oysters and a seasonal trend was observed for Cd in oysters, where levels were lowest during periods of warmer temperatures. Factors such as input of water masses and sediment diagenesis were found to directly affect dissolved Cd and thereby, influence the Cd levels in oysters. Particulate matter was not a significant source of Cd to oysters and actually has a negative effect. This effect is likely due to the uptake of dissolved Cd by phytoplankton and to the uptake of phytoplankton by oysters, which effectively increases the tissue mass and dilutes the Cd content in oysters. The other half of this study looked at the marine environment of these two locations using the Cd:P ratio as an indicator of many biogeochemical processes. Dissolved Cd and P, as well as particulate Cd, P and Ti were analyzed biweekly at one depth and throughout the water column three times during the year. In addition, a surface marine sediment sample was also analyzed. Cd:P is consistent in the deep open ocean, due to respiration processes but this consistency breaks down in the coastal waters. These two sites demonstrate that the coastal Cd-P ratio can be affected by phytoplankton species composition, phytoplankton abundance, sedimentary input, physical mixing of water masses with varying Cd-P ratios, and sediment diagenesis. Biological activity, highly affected by species composition and abundance, is generally the main factor affecting the Cd-P ratio, but other factors can also lead to variations of the ratio on smaller time and spatial scales.

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