Biological response to circulation driven by mean summertime winds off central Chile: A numerical model study

A coupled physical-biological model of the waters off central Chile is used to investigate the nitrogen-phytoplankton-zooplankton response to ocean circulation driven by mean summertime winds. The circulation drives the upwelling of middepth water onto the continental shelf and reaches a quasistable rate between days 40 and 60 of the simulation. High-nutrient, low-phytoplankton biomass water is upwelled at the coast, with nutrients being converted to phytoplankton within 3-10 days. A lagged response in zooplankton occurs after 6-30 days, by which time the water has been advected offshore. The magnitude and spatial distribution of phytoplankton biomass and export of organic matter off the continental shelf is sensitive to the zooplankton mortality term. For low zooplankton mortality, phytoplankton biomass on the continental shelf is limited by grazing pressure due to zooplankton, phytoplankton and zooplankton biomass remains low, and the nitrogen advected off the continental shelf in the surface waters is primarily dissolved inorganic nitrogen. When the mortality rate is increased fourfold, an approximately fourfold to fivefold increase is seen in the continental shelf phytoplankton biomass, phytoplankton productivity, and export of organic matter to the deep ocean. This dependence on zooplankton mortality illustrates the potential of top-down control of the shelf production and export of organic matter off the central Chile continental shelf. Copyright 2007 by the American Geophysical Union