Alm, M. B.1, DiTullio, G. R.1, Rue, E. L.2, & Bruland, K. W.2 1Grice Marine Laboratory, University of Charleston, Charleston, SC 29412 USA; 2Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA 95064 USAUpwelling off the coast of Peru produces some of the most productive oceanic waters in the world. High macronutrient (nitrate, phosphate, and silicate) concentrations from upwelled Peruvian water fuel high primary production rates. This increased primary production leads to high fisheries yields. Recent evidence from our research suggests, however, that iron limitation can limit photosynthetic production even along the Peruvian coastline. To assess the relative importance of Fe‐limitation on algal growth rates along the Peru coastline, we monitored the photosynthetic efficiency of photosystem II (Fv /Fm) using a fast repetition rate fluorometer as an indicator of algal physiological state. Vertical profiles at selected stations and underway surface Fv /Fm measurements were performed on transects perpendicular to the coastline. We also monitored hydrography, phytoplankton biomass (chl a), species composition (HPLC pigments), and concentrations of both macronutrients and Fe during August/September 2000. In high macronutrient regions, a positive correlation was observed between iron concentrations and Fv /Fm. The usefulness for utilizing Fv /Fm as a diagnostic indicator for Fe‐limitation in the Peruvian upwelling zone will be discussed.