Effects of Copper on Nitrogen Fixation and Growth of Blue-Green Algae in Natural Plankton Associations

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Abstract
Copper toxicity bioassays were conducted on six stages of the spring Aphanizomenon flos-aquae bloom in eutrophic Clear Lake, California. Major variables tested were nitrogen fixation, carbon fixation, pigments, and cell numbers. Inhibition of nitrogen fixation, carbon fixation, and pigment accumulation increased linearly between 10 and 30 μg Cu/L. Higher concentrations produced little additional toxicity. In contrast, there was a stimulatory effect of copper, especially on nitrogen fixation, at the very low level of 2 μg/L. There was no inhibitory effect of copper on nitrogen fixation without a comparable or greater effect on carbon fixation or chlorophyll a. Other algae growing with the Aphanizomenon bloom were not affected by additions of copper up to 20–30 μg Cu/L, but were affected at the 50–100 μg/L level.Key words: algae, algicide, blue-green algae, copper, cyanobacteria, metal, limiting nutrient, nitrogen fixation, photosynthesis, phytoplankton, toxicity