The central and eastern equatorial Pacific has recently been the focus of intensive field and laboratory experiments to understand the processes that control phytoplankton productivity and biomass. Research efforts have been concentrated in this region because of three related phenomena: a) the equatorial Pacific is the largest natural source of carbon dioxide to the atmosphere; b) the ecosystem experiences large variability due to El Nino events; and c) this is a high nutrient-low chlorophyll region of the world's oceans. The first part of this dissertation is a historical study of physical-biological interactions in the tropical Pacific. The second part addresses a one-dimensional ecosystem model that was developed for 0N 140W. The model includes two phytoplankton size fractions, two zooplankton size fractions, iron, nitrate, ammonium, and two detrital size fractions and is forced with a combination of in situ and modeled physical parameters. The model was run for 5 years starting in 1990 to encompass both El Nino and normal conditions.

A stability analysis of the model was performed on a simplified version of the nine-component model to determine if model oscillations were a function of the model parameter space or resulted from outside forcing functions. The model's equilibrium solution was non-oscillatory, even when the system was perturbed from equilibrium. Within the ecosystem, nanoplankton biomass always dominated netplankton, regardless of initial conditions. Microzooplankton grazing, however, regulated nanoplankton biomass and made iron available for netplankton growth and subsequent biomass accumulation.

During the 5 year run, modeled phytoplankton, zooplankton, and iron all varied on interannual time scales due to El Nino events. Total chlorophyll concentrations increase by as much as 40% from early 1992 (El Nino warm) to 1993 (normal). The ecosystem structure also changes with netplankton increasing from a low of 0.1% of the total chlorophyll in 1992 to a high of 30% of the total in 1993. Microzooplankton grazing was coupled to nanoplankton growth, whereas macrozooplankton grazing was independent of netplankton growth. The magnitude and temporal variability of phytoplankton chlorophyll agreed well with in situ data collected during 1992. Modeled primary production was lower than measured during El Nino, but agreed with observations during normal conditions. New production was calculated from total and recycled iron rather than nitrate-base production, and was more variable in general and much higher than the nitrate-based calculations during non-El Nino conditions.