A time- and depth-dependent, physical-bio-optical model was developed for the California coastal transition zone (CTZ) with the overall objective of understanding and quantifying the processes that contribute to the vertical and temporal development of nutrient and plankton distributions in the CTZ. The model food web components included silicate, nitrate, ammonium, two phytoplankton size fractions, copepods, doliolids, euphausiids, and a detritus pool. The wavelength-dependent subsurface irradiance field was attenuated by seawater and phytoplankton pigments. The one-dimensional (1-D) model adequately simulated the development and maintenance of a subsurface chlorophyll maximum in different regions within the CTZ. An analysis of the individual terms in the model governing equations revealed that phytoplankton in situ growth was primarily responsible for the creation and maintenance of the subsurface chlorophyll maximum at both coastal and oceanic regions in the CTZ. The depth to which the maximum in situ growth occurred was controlled by the combined effect of light and nutrient limitation. Also, the simulated bio-optical fields demonstrated the effect of nonlinear couplings between food web components and the subsurface irradiance field on vertical biological distributions. In particular, the e-folding scale of the subsurface photosynthetically available radiation (PAR) was influenced by the level of zooplankton grazing. ¿ American Geophysical Union 1996