Abstract:
Tropical freshwater ecosystems are increasingly influenced by chemical stressors including heavy metals posing
threats to biodiversity. Adequate ecotoxicity data are not available for native tropical freshwater species for
deriving water quality guidelines and for conducting ecological risk assessments. Objectives of this study were (i)
to generate freshwater ecotoxicity data for cadmium (Cd) and copper (Cu) for tropical temperature exposures
using standard laboratory bioassays with selected freshwater species and (ii) to derive ecotoxicity thresholds
(protection concentrations, PC) for tropical freshwater life based on ‘tropical temperature-specific exposure’
ecotoxicity data. Estimated final chronic toxicity values of the six species tested in the study indicate that the
most sensitive species was the crustacean, Moina macrocopa for both metals while the algae Chlorella vulgaris and
the plant Lemna perpusilla showed highest tolerance to Cd and Cu respectively. Tropical temperature-specific
exposure (25–30 ◦C) was used as the decision criterion for deriving ecotoxicity thresholds of Cd and Cu for
protection of tropical freshwater life based on species sensitivity distribution analysis of the final chronic toxicity
data sets which included published toxicity data of selected species in addition to the six species tested in this
study. The derived PC99, PC95, PC90 and PC80 values for protection of tropical freshwater life under chronic
exposure are 0.5, 1.2, 1.9 and 3.5 μg/L for Cd and 0.34, 0.84, 1.4 and 2.6 μg/L for Cu respectively. These derived
threshold chronic values (PC99 and PC95) indicate that the established freshwater quality guidelines based on
temperate species for Cu may not provide sufficient protection of the freshwater species in the tropics while the
available freshwater guidelines for Cd would provide adequate protection for the tropical freshwater species. The
tropical freshwater ecotoxicity thresholds derived in this study may be used with some caution as reference
points for site specific ecological risk assessments in the tropics.
