The reproductive rates of 21 species of marine phytoplankton were measured in media in which free zinc, manganese, and iron ion activities were controlled at different levels using EDTA-trace metal ion buffer systems. In general, the reproductive rates of neritic species were limited by zinc activities below 1O-11.5 M, while those of oceanic species were either not limited or only slightly limited at the lowest zinc activity attained in the experiment, ca. lo-l3 M. The reproductive rates of oceanic coccolithophores were either not limited or only slightly limited by the lowest manganese ion activity attained, ca. 10-l' M, but those of a neritic coccolithophore and all diatoms, both neritic and oceanic, were limited below a manganese activity of 10-l" M. Neritic species had reduced reproductive rates in media containing clod7 M iron while oceanic species reproduced at maximal or close to maximal rates in the media with the lowest iron concentrations, ca. IO+' M. The habitat-related patterns in zinc, manganese, and iron requirements of oceanic and neritic species are consistent with the oceanic-neritic distributions of concentrations of these metals. This similarity in requirement and distributional patterns provides evidence that Zn, Mn, and Fe availability have been important selective forces on marine phytoplankton populations and communities. -The nutrients most frequently considered to limit the reproductive rates of phytoplankton in the ocean are the macronutrients nitrogen, phosphorus, and silicon. The possibility that trace metal micronutrients can be of significance in the ecology of phytoplankton has been considered occasionally (Harvey 1947 have for several reasons. Extensive contamination of water samples analyzed by trace metal chemists (Boyle and Edmond 1975; Bruland et al. 1978 Bruland et al. , 1979 and used for bioassays has, in the past, eliminated much of the evidence for trace metal limitation. It is only within the last few years that reasonably uncontaminated samples have been available for accurate measurement of trace metal levels in the ocean (Boyle et al. 1977; Bruland et al. 1978; Klinkhammer and Bender 1980;Bruland 1980). Also, only recently has methodology been developed to quantify relationships between phytoplankton reproductive rate and low free ion activities of trace metals. This methodology, proposed by Hutner et al. (1950), involves the use of trace metal ion activity buffers (combinations of synthetic chelators and trace metal salts), which permit quantification and control of extremely low activities of trace metal ions. These buffers also have 1182