TOXICITY OF SELECTED PESTICIDES TO LAKE PHYTOPLANKTON MEASURED USING PHOTOSYNTHETIC INHIBITION COMPARED TO MAXIMAL UPTAKE RATES OF PHOSPHATE AND AMMONIUM

The toxicity of 16 pesticides to the energy-producing process of photosynthesis was determined from inhibition or depression of the assimilation rate of [C-14]bicarbonate by natural plankton samples collected from Jack's Lake, a mesotrophic lake on the edge of the Canadian Shield 200 km northeast of Toronto, Ontario. Measurements were made al increasing concentrations of pesticide while samples were exposed to optimal light. At the same time, in the same 300-ml bottle, measurements of maximal uptake rates for phosphate and ammonium were made using radiotracer and stable isotope methods. In this way the more commonly used screening technique of photosynthetic inhibition could be compared to the energy-demanding processes of phosphate and ammonium uptake. Phosphate assimilation was inhibited at much higher herbicide concentrations than carbon fixation, whereas ammonium assimilation was inhibited at intermediate concentrations. Herbicides that specifically inhibit photosynthesis were most toxic to carbon uptake, while those herbicides affecting other cellular metabolic processes had a greater influence on phosphate and ammonium assimilation. No relationship was found among pesticide toxicity and molecular weight, solubility, or log K-ow.