IRON AND GRAZING CONSTRAINTS ON PRIMARY PRODUCTION IN THE CENTRAL EQUATORIAL PACIFIC - AN EQPAC SYNTHESIS

Recent studies in the central equatorial Pacific allow a comprehensive assessment of phytoplankton regulation in a high-nutrient, low-chloro phyll (HNLC) ecosystem. Elemental iron enters the euphotic zone princi pally via upwelling and is present at concentrations (less than or equ al to 30 pM) wen below the estimated half-saturation constant (120 pM) for the large cells that bloom with iron enrichment. In addition, the meridional trend in quantum yield of photosynthesis suggests that eve n the dominant small phytoplankton are held below their physiological potential by iron deficiency. Grazing by microzooplankton dominates ph ytoplankton losses, accounting for virtually all of the measured phyto plankton production during El Nino conditions and similar to 66% durin g normal upwelling conditions, with mesozooplankton grazing and latera l advection closing the balance. Nitrate uptake is strongly correlated with the pigment biomass of diatoms, which increase in relative abund ance during normal upwelling conditions. Nonetheless, the f-ratio rema ins low (0.07-0.12) under all conditions. Iron budgets are consistent with the notions that new production is determined by the rate of new iron input to the system while total production depends on efficient i ron recycling by grazers. Although the limiting substrates differ, the interactions of resource limitation and grazing in HNLC regions are c onceptually similar to the generally accepted view for oligotrophic su btropical regions. In both systems, small dominant phytoplankton grow at rapid, but usually less than physiologically maximal, rates; they a re cropped to low stable abundances by microzooplankton; and their sus tained high rates of growth depend on the remineralized by-products of grazing.