The inadequacy of Daphnia in regulatory aquatic toxicity testing.

Background/Question/Methods

Under the Toxic Substances Control Act, the Environmental Protection Agency (EPA) has the power to require testing of chemical substances in order to regulate exposure to chemicals and set acceptable environmental concentrations. Acute toxicity tests (LC₅₀) are highly criticized for being unable to capture the complexity of ecological interactions. Despite this, Daphnia (Crustacea: Branchiopoda) are utilized in these tests as the primary model organism for aquatic toxicity testing. Daphnia have many appealing attributes for toxicity testing, and are commonly thought to be sensitive to most contaminant exposures. While Daphnia species have been found to be relatively sensitive, there has yet to be a thorough analysis of how sensitive Daphnia are in comparison to other taxa. This study is the first to exhaustively examine the relative sensitivity of Daphnia to other taxonomic groups in each of the major contaminant categories. We examined over 110,000 toxicity studies in EPA’s AQUIRE database to obtain LC₅₀ values for over 7000 species. Using species sensitivity distributions, we calculated the relative sensitivity of taxonomic groups for 25 taxonomic classes in 29 chemical categories.

Results/Conclusions

Of the 29 chemical categories analyzed, 19 contained taxa significantly more sensitive to chemical exposure than Daphnia. Within the 19 chemical groups exhibiting taxa more sensitive than Daphnia, nine groups had a single taxon that was more sensitive to exposure. On average, these taxa were five times more sensitive to the chemicals. More interestingly, ten chemical categories revealed several taxa more sensitive than Daphnia. In four of these categories, Daphnia were actually the least sensitive aquatic taxon, the most interesting being the triazine group that contains the commonly used herbicide atrazine. By using the EPA’s own data, we suggest alternative model organisms be utilized for particular chemical categories. These data also reveal the issues of limiting regulatory decisions to a few representative species and we use our data to encourage the use of more species in more complex settings.