Inter-annual depth-dependent toxicity and bioaccumulation of cadmium in marine benthic protist communities

The toxicity and bioaccumulation of cadmium in a marine benthic protist community were examined at different depths within the sediment. For this purpose, sediment-water microcosms with 1000 žg Cd dm-3 of the pollutant were used in two assays. The addition of cadmium caused a significant reduction in protist density, number of species and biomass. There was also a decrease in these three parameters with depth. During the treatment the density of protist groups was strongly depth-dependent. The dominant groups of protists at the different depths during the assay were also considered. The most dominant protist group in terms of density were the heterotrophic flagellates, both in the control and in the treatment with cadmium. In the treatments with cadmium, these were followed by ciliates and by dinoflagellates in both assays. In the control, all protist groups were present during the assay, whereas in the treatments with cadmium, autotrophic flagellates, diatoms and sarcodines were found in reduced proportion or not at all. Cadmium bioaccumulation increased towards the end of the assay. At any time during the assay, the proportion of cadmium bioaccumulated was an increasing function of depth.