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

Experiments were conducted in the equatorial Pacific Ocean to assess t he role of Fe and grazing in regulating use of NO3- by the phytoplankt on community. Nitrate uptake rates in situ were slow because NH4+ conc entrations were inhibitory and because phytoplankton biomass was kept low by grazing. When feeding of grazers was artificially suppressed, p hytoplankton net growth rate increased, biomass accumulated, and NO3- was consumed. Rapid rates of Fe uptake [40 mumol Fe (g Chl a)-1 h-1] d ecreased by an order of magnitude in 1-2 d after Fe was added, demonst rating that these rates were under physiological regulation and were e levated in response to low Fe concentrations. Addition of Fe increased carbon uptake and the short-term N-specific NO3- uptake rate by 2-9 t imes. These physiological stimulations were confined to large phytopla nkton (> 3 mum), which thus must have been Fe-limited in situ. NO3- up take rate and biomass of small phytoplankton were unaffected by Fe enr ichment. The results thus suggest that the low biomass, NO3--rich cond ition of the equatorial Pacific Ocean exists because low Fe concentrat ions limit use of NO3- by large phytoplankton and favor growth of smal l phytoplankton that are grazed efficiently and use NH4+ preferentiall y.