E!ects of iron, manganese, copper and zinc enrichments on productivity and biomass

Natural plankton populations from subarctic Pacific surface waters were incubated in 7-d ex-periments with added concentrations of Fe, Mn, Cu, and Zn. Small additions of metals (0.89 nM Fe, 1.8 nM Mn, 3.9 nM Cu, and 0.75 nM Zn) were used to simulate natural perturbations in metal concentrations potentially experienced by marine plankton. Trace metal concentrations, phyto-plankton productivity, Chl a, and the species composition of phytoplankton and microzooplankton were measured over the course of the experiment. Although the controls indicated little growth, increases in phytoplankton productivity, Chl a, and cell densities were dramatic after the addition of 0.89 nM Fe, indicating that it may limit the rates of algal production in these waters. Similar increases were observed in experiments with 3.9 nM Cu added. The Cu effect is attributed to a decrease in the grazing activities of the microzooplankton (ciliates) and increases in the rates of production. Mn enrichment had its greatest effect on diatom biomass, whereas Zn enrichment had its greatest effect on other autofluorescent organisms. The extent of trace metal adsorption onto carboy walls was also evaluated. These results imply that natural systems may be affected as follows: natural levels of Fe and Cu may influence phy