THE EQUATORIAL PACIFIC-OCEAN - GRAZER-CONTROLLED PHYTOPLANKTON POPULATIONS IN AN IRON-LIMITED ECOSYSTEM

Experiments were conducted in the equatorial Pacific Ocean to assess the role of Fe and grazing in regulating use of NO3- by the phytoplankton community. Nitrate uptake rates in situ were slow because NH4+ concentrations were inhibitory and because phytoplankton biomass was kept low by grazing. When feeding of grazers was artificially suppressed, phytoplankton net growth rate increased, biomass accumulated, and NO3- was consumed. Rapid rates of Fe uptake [40 mumol Fe (g Chl a)-1 h-1] decreased by an order of magnitude in 1-2 d after Fe was added, demonstrating that these rates were under physiological regulation and were elevated in response to low Fe concentrations. Addition of Fe increased carbon uptake and the short-term N-specific NO3- uptake rate by 2-9 times. These physiological stimulations were confined to large phytoplankton (> 3 mum), which thus must have been Fe-limited in situ. NO3- uptake rate and biomass of small phytoplankton were unaffected by Fe enrichment. The results thus suggest that the low biomass, NO3--rich condition of the equatorial Pacific Ocean exists because low Fe concentrations limit use of NO3- by large phytoplankton and favor growth of small phytoplankton that are grazed efficiently and use NH4+ preferentially.