Effects of copper on production of periphyton, nitrogen fixation and processing of leaf litter in a Sierra Nevada, California, stream

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SUMMARY. Production of periphyton, nitrogen fixation and processing of leaf litter were examined in an oligotrophic Sierra Nevada stream and the responses of these processes to copper (2.5, 5 and 10μg 1‐1 CuT [total filtrable copper]; approximately 12, 25 and 50 ng 1‐1 Cu2+) were determined. Autotrophic and total production were estimated from 3‐week accumulations of biomass on artificial substrates. Mean autotrophic production in the control ranged from 0.22 to 0.58 mg C m‐2 h‐1 in summer‐autumn 1979, but declined to 0.08–0.28 mg C m 2 h‐1 after peak discharge in summer 1980, apparently due to phosphorus‐limited growth. Total production in the control ranged from 0.30 to 0.82 mg C m‐2 h ‐1 in summer‐autumn 1979 and from 0.16 to 0,68 mg C m ‐2 h ‐1 in 1980. Mean autotrophic productivity, estimated by l4C‐bicarbonate uptake in daylight, ranged from 0.30 to 2.8 mg C m‐2 h‐1. Autotrophic productivity was reduced by 57–81% at 2.5μg 1‐1 CuT, 55–96% at 5μg 1‐1 CuT, and 81–100% at 10μg 1‐1 CUT, Heterotrophic productivity (based on dark 35S‐sulphate uptake) was inhibited to a lesser extent (28–63% at 2.5μg 1‐1 CuT, 24–84% at 5μg 1‐1 CuT, and 67–92% at 10μg 1‐1 CuT), The inhibition of autotrophic and heterotrophic productivity persisted through the year of exposure. Production in stream sections previously exposed to 2.5 and 5μg 1‐1CuT increased to control levels within 4 weeks after dosing, but remained depressed for more than 7 weeks after exposure to 10μg 1‐1 CuT. The specific rate of photosynthesis (mg C mg chlorophyll a‐1 h‐1) of mature periphyton communities declined at all test concentrations of copper, but the rate for periphyton on newly‐colonized surfaces did not change. The species composition of benthic algae shifted during exposure to an assemblage more tolerant of copper. Achrtanthes minutissima and Fragilaria crotonensis were the primary replacement species on newly‐colonized surfaces. The nitrogenase activity of blue‐green algae was low. with controls ranging from 2.4 to 12 nmol C2H2 m‐2 h‐1. Nitrogenase activity was inhibited during the initial weeks of exposure by 5 and 10μg 1‐1 CuT. However, after 9 months of exposure, control and copper‐treated sections did not differ. The rate of processing of leaf litter, estimated by microbial respiration and nutrient quality of litter of resident riparian woodland taxa, was inhibited at all test concentrations of copper.