Salihoglu, Baris, and Eileen E. Hofmann

Center for Coastal Physical Oceanography, Old Dominion University, Norfolk VA, USA 23508, Tel: 757-683 5557, Fax: 757-683 5550, E-mail: baris@ccpo.odu.edu

A one-dimensional model of lower trophic level interactions in the equatorial Pacific

A multi-component ecosystem model is developed to investigate the biological and physical interactions in the equatorial Pacific Ocean at 140°W and simulation results are verified using observations from 4 U.S. JGOFS EqPac cruises. Autotrophic growth is represented by five algal groups of phytoplankton, which have light and nutrient utilization characteristics that reflect those of Prochlorococcus, Synechococcus, Chromophycota and Diatom species. The model is forced by seasonal changes in spectral light, temperature, and water column mixing. Simulations with the model are designed to investigate mechanisms that lead to seasonal shifts in phytoplankton species composition. The simulated vertical and time-dependent distributions show shifts in the phytoplankton species composition that are functions of depth and season. The biomass of the extreme light-inhibited Prochlorococcus becomes highest (~0.6 μm) in deeper waters in response to decreasing light intensity. During La-Niña periods, the simulated distributions of the large algal groups, Chromophycota (~2.5 μm) and Diatoms (~15 μm), double in biomass (from 0.5 to 1.0 μmol C/l and from 0.1 to 0.2 μmol C/l, respectively), relative to what is obtained during El-Niño periods. The smaller algal groups, Prochlorococcus and Synechococcus, are not as strongly affected by the change between La-Niña and El-Niño periods. Increase in nutrient concentrations during La-Niña period is the main factor that causes the increase in biomass of larger algal groups. The simulation results also show that all algal groups are affected by high frequency physical processes, such as tropical instability waves and internal gravity waves.