Use of in situ structural and functional variables of phytoplankton of the river Ganga for assessment of heavy metal toxicity.

Toxicity of Cd and Zn on autotrophic index, pigment diversity. 14C uptake, and in situ nitrogenase activity of phytoplankton of the river Ganga has been studied for the first time in India using CEPEX enclosures. Maximum reduction in algal population was noted at 8.0 micrograms ml-1 Cd followed by 8.0 micrograms ml-1 Zn. Complete elimination of two and four species was observed respectively at 4.0 and 8.0 micrograms ml-1 Cd and Zn. The filamentous forms showed greater tolerance against Cd and Zn, whereas unicellular forms were more sensitive to test metals used. Bacillariophytes in general depicted greater sensitivity for both the metals. A concentration-dependent metal-specific increase in autotrophic index and pigment diversity of phytoplankton was noted for Cd and Zn. Inhibition of carbon and nitrogen fixation was, however, concentration dependent and metal specific. Looking at the sensitivity of 14CO2 uptake rather than other variables, we recommend the employment of this parameter for assessment of heavy metal toxicity in an aquatic ecosystem.