An iron-based ecosystem model of the central equatorial Pacific

The central and eastern equatorial Pacific region is characterized by lower than expected phytoplankton biomass and primary production given the relat ively high ambient nitrate concentrations. These unusual conditions have sp awned several field programs and laboratory experiments to determine why th is high nitrate-low chlorophyll pattern persists in this region. To synthes ize the results from these field programs, as well as providing additional evidence in support of the iron hypothesis, we developed a one-dimensional, nine-component ecosystem model of 0 degrees N 140 degrees W. The model com ponents include two phytoplankton size fractions, two zooplankton size frac tions, two detrital size fractions, dissolved iron, nitrate, and ammonium. The model was run for 5 years (1990-1994) and was forced using an atmospher ic radiative transfer model, an ocean general circulation model (GCM), and in situ data. To our knowledge, this is the first ecosystem model at 0 degr ees N 140 degrees W to synthesize the Joint Global Ocean Flux Study Equator ial Pacific Process Study (JGOFS EqPac) data set, as well as to use both in situ and modeled physical data to drive the model. Modeled phytoplankton, zooplankton, and iron all varied on interannual timescales due to El Nino e vents. Total phytoplankton biomass increased by as much as 40% from early 1 992 (El Nino warm) to 1993 (normal). The results also indicate that the bio mass increase during a cool period is not constant for each phytoplankton c omponent, but instead the increase is most evident in the netphytoplankton (>10 mu m). Netphytoplankton increase from a low of 0.1% of the total chlor ophyll in 1992 to a high of 30% of the total in 1993. Microzooplankton graz ing rates fluctuated in response to changes in nanophytoplankton growth rat es, whereas mesozooplankton grazing was unrelated to netphytoplankton growt h rates. The magnitude and temporal variability of phytoplankton chlorophyl l agreed well with in situ data collected during 1992. Modeled primary prod uction was lower than measured during El Nino but agreed with observations during normal conditions. The low primary productivity was probably a resul t of downwelling produced by the physical model. New production was calcula ted from total and recycled iron rather than nitrate-based production and w as more variable in general and almost 3 times the nitrate-based new produc tion during non-El Nino conditions.