Toxicity to sedimentdwelling organisms

Bioavailability and toxicity to benthic organisms of trace metals in aquatic sediments is governed, on one hand, by the sediment properties as well as by the chemistry of pore-water and overlying water and, on the other hand, by the physiology and feeding behaviour of the benthic organisms. Lately, several studies have been conducted to test the usefulness of the SEM/AVS concept to assess the bioavailability of trace metals in sediments under more or less realistic field conditions.

In one major study, field sediments from four different aquatic habitats were investigated and then spiked with zinc (400 mg Zn/kg and 1,200 mg Zn/kg added) and returned to the original s ites to allow benthic fauna to colonize the Zn-enriched sediments. It was found that the total Zn concentration in the sediments did not show any relationship with adverse effects on benthos. No chronic toxicity was observed in sediments with SEM/AVS ratios <2. When the excess SEM was below 147 fimol/g organic carbon, no chronic toxicity was observed. On the other hand, Zn-spiked sediments with SEM/AVS ratios >8 or SEM >580 fimol/g organic carbon were found to be toxic. The authors concluded that zinc bioavailability and toxicity in freshwater sediments can be accurately predicted by the SEM/A VS model if the sediment's content of organic carbon is taken into account. However, the general applicability of this conclusion remains to be proven, because of the somewhat artificial conditions created in the sediments by spiking with great amounts of Zn and because no assessment of the vertical gradients of redox potential, AVS or dissolved sulphides from the sediment surface and downwards was made. The most plausible deviation from the accuracy of the model's predictions is that - in the real world -sediment-associated metals seem to have a lower bioavailability and toxicity than what is predicted by the model. Thus, the model seems to be a good predictor of non-toxicity of trace metals in sediments, but may not be fully reliable in predicting that a certain sediment is toxic.

Undisturbed sediment cores from metal-polluted and reference sites were used in the second major study being reviewed. These intact sediment cores, with oxygen-rich water continuously flowing over the sediment surface, were incubated in the laboratory for several months. In this period, the change in concentrations of AVS, dissolved sulphides and trace metals was followed at different depths in the sediment core, in the pore-water, the overlying water and in the tissues of invertebrate fauna being introduced to provide bioturbation in the sediment surface. The concentration of AVS declined during incubation with oxygen-rich water, but only in the top 01.0 cm sediment layer. Below this level, both dissolved sulphides and AVS remained in high concentrations. Only Zn decreased in concentration in the sediment surface, but not Cu and some other trace metals forming sulphides with low solubility.

It was also demonstrated that in spite of several months of oxygenation of the originally anaerobic sediments, in which only the top cm turned aerobic, they exhibited a low toxicity in terms of mortality and embryonic malformations to the introduced amphipods. With regard to the relevance of the SEM/AVS model for predicting trace metal bioavailability to the amphipods, the authors found that the model was useful particularly for Cd, Pb and Zn, while the bioavailability of Ni, Cu and Hg was less accurately predicted, although the non-toxicity of all studied metals was well predicted. Pore-water metal concentrations had a low influence on the metal body burden of exposed amphipods, which led to the conclusion that a major route for metal uptake in the amphipods was not directly from the pore-

water, but via the food chain. However, whether the metals bioaccumulated via the food really produced any toxic responses in the amphipods could not be shown; it was rather speculated by the authors that the low toxicity of the sediment was mainly due, not to the presence of bioavailable metals but to the presence of toxic sulfides.

0 0

Post a comment