Effects of iron, manganese, copper, and zinc enrichments on productivity and biomass in the subarctic Pacific

Natural plankton populations from subarctic Pacific surface waters were incubated in 7‐d experiments 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, phytoplankton 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 phytoplankton productivity and trophic structure in open‐ocean, high‐nutrient, low‐biomass systems; rates of net production are not limited by one micronutrient alone. Because of the effects of adsorption and complexation, experiments must be carefully monitored for free vs. total metal concentrations, and short‐term incubations (1 d) may not be affected dramatically by small perturbations in trace metal micronutrients.