Effects of pH , phosphate and ammonia on the rate of uptake of nitrate and ammonia by freshwater phytoplankton

Nitrogen-15 tracers were used to study the effects of pH and phosphate on the rate of nitrate and ammonia uptake by phytoplankton in two lakes in Oklahoma. It is shown that, with one exception, addition of phosphate above ambient levels (7.6 ug per litre), did not significantly affect uptake of either nitrate or ammonia. The high pH of lake water observed during 1974 suggests that dissolved inorganic carbon was a limiting factor, but laboratory experiments on varying pH did not appear to have a significant effect. A correlation analysis indicated that ammonia had an inhibitory effect on nitrate uptake.