Iron Nutrition of Phytoplankton and Its Possible Importance in the Ecology of Ocean Regions with High Nutrient and Low Biomass

Abstract

IN SOME REGIONS of the oceans, namely the North and Equatorial Pacific and the Antarctic, the surface waters contain relatively high concen- trations of nutrients (e.g., P04 > 1 uM) yet only a low biomass (chlorophyll < 0.5 ug/L). On the basis of measurements of very low iron concen- trations (<0,1 nM) and observations of increased growth on Fe enrichments, J. Martin and co- workers have proposed that iron limits primary production in these waters (Martin, 1991, this is- sue). Other authors have proposed alternate hy- potheses chiefly on the basis of grazing (Miller et al., 1991, this issue) and light limitation (Mitchell et al., 1991). Here we examine the "iron hypoth- esis" in light of what we know about the iron physiology of marine phytoplankton and its re- lation to iron chemistry.