Cadmium toxicity to diatom communities assessed in freshwater microcosms

In situ diatom surveys conducted on the river Riou-Mort, a cadmium and zinc-polluted tributary of the river Lot (South West France), have revealed shifts in biofilm settlement and diatom community structure. The present experimental study is aimed at determining dissolved cadmium toxicity to biofilms and diatom assemblages. Diatom communities sampled from the field (at a clean site on the river Riou-Mort) were inoculated in experimental units consisting of replicate artificial streams, supplied with water contaminated by different levels of dissolved cadmium (0, 10 and 100µgCd/L), at a temperature of 15°C in order to prevent the development of filamentous algae. Glass substrates immersed in the artificial streams were sampled after a 1-, 2-, 4- and 6-week exposure. The response of biofilm characteristics to metal contamination was described through dry weight, ash-free dry matter, chlorophyll a and cadmium content. Diatom communities were assessed by quantitative countings and qualitative identifications down to the species level. Teratological valves of diatoms were also enumerated. There was a positive correlation between cadmium accumulation in the biofilm and dissolved cadmium concentrations and duration of exposure. Biofilm settlement was affected by high cadmium concentrations. Dry weight and ash-free dry weight were similar in the biofilms grown under 0 and 10 µgCd/L, and were significantly higher than those measured in the samples exposed to a 100µg/L contamination. We also observed for all stages of settlement a reduction of diatom densities correlated to the highest cadmium contamination, compared to control units and low cadmium concentrations.