Gregg¹, Watson W., Paul Ginoux², Paul S. Schopf³ and Nancy W. Casey⁴

¹Laboratory for Hydrospheric Processes, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, E-mail: watson.w.gregg@nasa.gov, ²GEST, University of Maryland Baltimore County, ³Climate Dynamics Program, School for Computational Sciences, George Mason University, and ⁴Science Systems and Applications, Inc.

Phytoplankton group distributions simulated in a global three-dimensional ocean biogeochemical model

The JGOFS program and NASA ocean color satellites have provided a wealth of data that can be used to test and validate models of ocean biogeochemistry. A coupled three-dimensional general circulation, biogeochemical, and radiative model of the global oceans was validated using these in situ data sources and satellite data sets. Biogeochemical processes in the model were determined from the influences of circulation and turbulence dynamics, irradiance availability, and the interactions among four phytoplankton functional groups (diatoms, chlorophytes, cyanobacteria, and coccolithophores) and four nutrients (nitrate, ammonium, silica, and dissolved iron).

Basin scale model chlorophyll seasonal distributions were statistically positively correlated with SeaWiFS chlorophyll in all 12 oceanographic basins (P < 0.05). The global mean difference was 3.9% (model higher than SeaWiFS). The four phytoplankton groups were initialized as homogeneous and equal distributions throughout the model domain. After 26 years of simulation, they arrived at reasonable distributions throughout the global oceans: diatoms predominated high latitudes, coastal, and equatorial upwelling areas, cyanobacteria predominated the mid-ocean gyres, and chlorophytes and coccolithophores represented transitional assemblages.

While the overall patterns of phytoplankton functional group distributions exhibited broad qualitative agreement with in situ data, quantitative comparisons were mixed. Three of the four phytoplankton groups exhibited statistically significant correspondence across basins. Diatoms did not. Some basins exhibited excellent correspondence, while most showed moderate agreement. The results are encouraging for a first attempt at simulating functional groups in a global coupled three-dimensional model but many issues remain.