A model of plankton dynamics for the coastal upwelling system of Monterey Bay, California

A seven-box plankton model for the mixed layer of the open ocean (Fasham et al., 1990, Journal of Marine Research 48, 591–639) was modified and applied to the coastal upwelling ecosystem of Monterey Bay, California. Upwelling velocities, photosynthetically active radiation and changes in mixed-layer depth drove the model. The observed long-term mean (1989–1995) seasonal cycles of nitrate concentration, primary production and phytoplankton concentration within the mixed layer were reproduced. Bacteria and zooplankton biomass, and ammonium concentration were within the same order of magnitude as observations. Modeled dissolved organic nitrogen (labile) values were an order of magnitude lower than observations (total). New production estimates were high, leading to an annual f-ratio of 0.84, typical of productive coastal upwelling systems. Export production was much lower, and horizontal export by upwelling-induced advection was the dominant loss term for phytoplankton primary production. It exceeded sinking and grazing and the flux of zooplankton-derived detritus throughout most of the year. The model suggests that a large proportion of the daily phytoplankton growth is ungrazed in the mixed layer, and is advected downstream and offshore of the production site.