Iron limitation of phytoplankton photosynthesis in the equatorial Pacific Ocean

Abstract

THE surface waters of the equatorial Pacific have unusually high nitrate and phosphate concentrations, but relatively low phyto-plankton biomass1–3. This 'high nitrate, low chlorophyll' (HNLC)4 phenomenon has been ascribed to 'top-down' grazing pressure by herbivores, which prevent the phytoplankton from fully utilizing the available nutrients5. In the late 1980s, however, Martin and co-workers proposed that iron, which is delivered to the remote open ocean in aeolean dust6, is the key factor limiting the standing crop of phytoplankton in HNLC areas7,8. Using a sensitive fluor-escence method9, we have followed changes in photochemical energy conversion efficiency9–10 of the natural phytoplankton com-munity both before and after artificial enrichment with iron of a small area (7.5 x 7.5 km) of the equatorial Pacific Ocean11. Our results show that iron limits phytoplankton photosynthesis in all size classes in this region by impairing intrinsic photochemical energy conversion, thereby supporting the hypothesis of physiologi-cal ('bottom up') limitation by this element.