Stimulation of phytoplankton growth by mixtures of phosphate, nitrate, and organic chelators

An in situ nutrient enrichment experiment in which mixed treatments of nitrate, phosphate, and EDTA were applied to natural lake phytoplankton communities is discussed. Changes in community productivity and species composition in response to the treatments revealed strong interactions among the components of the treatment mixture. On the community level, phosphate exerted a stimulatory effect that was reduced by EDTA, enhanced by nitrate, and enhanced even more by nitrate and EDTA together. Examination of 15 individual species revealed that the treatment effects were highly variable from species to species. Seven of the 15 species were stimulated by the nutrient treatments, and the growth patterns of 5 of these accounted for essentially all of the features of the productivity response patterns. The eight other species either failed to respond to any treatment or declined in response to containment or treatments. Phosphate was the key substance in all of the positive treatment effects, and its omission from the treatment mixture essentially eliminated all growth responses.