Comparison of the acute sensitivity to chemicals between tropical and temperate aquatic species through laboratory experiments and metadata analyses

Abstract

To protect tropical aquatic ecosystems, water quality guidelines (WQGs) are often derived based on toxicity data of chemical contaminants using temperate species as surrogates because there is a lack of such toxicity data from tropical species. Such adoptions of temperate information could result in an unknown safety margin and might lead to over- or under-protection of tropical ecosystems.

Therefore, this study systematically compared differences in chemical sensitivities between temperate and tropical aquatic species using the species sensitivity distribution (SSD) approach. Temperate and tropical saltwater species displayed different sensitivities for ten chemicals but showed similarity towards cadmium. Temperate saltwater species were more sensitive to unionised ammonia (NH3), chromium, lead, nickel and tributyltin, whereas tropical saltwater species were more sensitive to copper, mercury, zinc, pentachlorophenol and phenol. Temperate freshwater species were more sensitive to arsenic, cadmium, chromium, copper, mercury, nickel, zinc, carbaryl, chlorpyrifos, DDT, lindane and malathion than their tropical counterparts, while tropical freshwater species tended to be more sensitive to 〖NH〗_3, lead, chlordane and phenol. Extrapolation factors of two and five were recommended for deriving tropical WQGs from temperate information to protect tropical saltwater and freshwater ecosystems, respectively. The results also indicated a general declining trend in chemical sensitivities from algae to crustaceans, molluscs, worms, and then fishes for both freshwater and saltwater. Both algae and crustaceans are sensitive to diverse chemicals and hence suitable for toxicity testing.

Since temperature and pH can influence the physicochemical properties, bioavailability and thus toxicity of a chemical to aquatic organisms, the second part of this study aimed to investigate the relationships between chemical toxicity and temperature or pH. The acute median lethal or effective concentrations of 13 chemicals and 10 metals/metalloids towards various freshwater species were compared among different temperature and pH groups, respectively. The results suggested that in most cases, freshwater species exhibited the highest tolerance towards chemicals at an optimal temperature (T_opt) where the chemical toxicity was the lowest, and the chemical toxicity increased with increasing or decreasing temperature from T_opt). As reflected by SSDs at different temperatures for seven chemicals, the relationship between chemical toxicity and temperature followed the same pattern at the community level. At individual species level, relationships between metal toxicity and pH were more variable and could be classified into three models: (1) decreasing toxicity with increasing pH, (2) increasing toxicity with increasing pH, and (3) minimal toxicity at optimal pH (〖pH〗_opt), with increasing toxicity as pH increases or decreases from 〖pH〗_opt. As reflected by SSDs at different pH values for four metals, the relationship between chemical toxicity and pH conformed to the model (1) at the community level. With these conceptual models revealed in this study, it is feasible to derive site- and seasonal-specific WQGs at different temperature or pH conditions.

The present results will be useful to environmental authorities for developing WQGs and conducting risk assessment in order to safeguard valuable tropical aquatic ecosystems and unique biodiversity therein. Further laboratory and field studies will be required to validate the extrapolation factors being suggested in this study.