Studies on environmental management of mercury (ii), chromium (vi) and zinc (ii) with respect to the impact on some arthropods and protozoans — toxicity of zinc (ii)

Abstract

In continuation of our earlier reports on the impact of heavy metals on certain annelids,^1,2 amphibian larvae³ and arthropods,⁴ we present here studies on the impact of mercury (II), chromium (VI) and zinc (II) on copepod Cyclops sp. and larvae of mosquito Aedes aegypti relative to protozoans, cladocerans, molluscs and rotifers.^5–8 The impact studies were done with the aid of static bioassays and computer aided analysis of mortality data. The Median Lethal Concentration (LC₅₀) and Safe Concentration (SC) values were calculated and compared with the corresponding values reported previously for other arthropods and protozoans and with the minimum levels of these metals allowable in drinking waters, irrigation waters and effluent outfalls as per various national and international agencies (WHO,⁹ USEPA,¹⁰ USDI,¹¹ NAS,¹² FRG,¹³ USSR,¹³ Canada,¹⁴ and ISI^15–17). The studies revealed that in many cases the prevailing standards are too liberal. The disparity between maximum allowable levels of zinc and its safe concentration appears to be specially poignant. The mosquito larvae (A. aegypti) were tolerant to zinc levels which were lethal to other arthropods and protozoans while the tolerance of the larvae for mercury and chromium was second only to Cyclops sp. and Philodina sp. This showed that while the given levels of pollution due to mercury, chromium or zinc may cause excessive mortality in several useful fish food organisms, they would be relatively safe for harmful insects like mosquitoes. Such a toxicity pattern may lead to imbalances in community structures of aquatic organisms favouring mosquito growth.