A theoretical study of phytoplankton growth and nutrient distribution in the Pacific Ocean off the northwestern U.S. coast

Abstract

Abstract A numerical two-dimensional (time and depth) model of primary production is used to identify and quantify the physical and biological processes that govern the spring and summer variations in the vertical distribution of phytoplankton and nutrients in the offshore zone. The study focuses on the subsurface chlorophyll maximum. The model consists of coupled integro-partial differential equations expressing conservation of chlorophyll a , nitrate-nitrogen, and ammonium-nitrogen. The dominant vertical transport process is assumed to be turbulent mixing (and also sinking in the case of algal cells). Biological processes considered are gross algal production, algal respiration, zooplankton, and selective uptake and regeneration of nitrogen. The long-term simulation over spring and summer represents the main features of the biological and chemical data. According to the model, the development and subsequent deepening of the subsurface chlorophyll maximum is attributable to in situ production and differential sinking associated with nutrient depletion. The vertical distribution of nutrients in summer is a consequence of biological processes. The rate of development of the subsurface chlorophyll maximum is determined in part by zooplankton grazing.