A Vertical, Time-Dependent Bio-Optical Model

Abstract

A vertical, time-dependent bio-optical model was developed to investigate the depth-dependent processes associated with the plankton distributions in the California Coastal Transition Zone (CTZ). The biological components included in the model are silicate, nitrate, ammonia two phytoplankton size fractions, copepods, doliolids, euphausiids, and a detritus pool. The wavelength-dependent attenuation of the subsurface irradiance field caused by seawater, phytoplankton, and dissolved organic matter, is incorporated into this model as a depth-dependent energy flux which balances the phytoplankton energy uptake and the kinetic energy flux (ΔT) into the water. This allows the phytoplankton to modify the vertical temperature structure which, in turn, affects the rate of the biological processes. Vertical velocities (w) along the track of simulated Lagrangian drifters, which were derived from a 3-D primitive equation circulation model simulating the flow observed within the CTZ, are used to parameterize upwelling and downwelling. Parameterization of biological processes, such as light versus photosynthesis and Zooplankton growth and fecundity rates, are based upon data obtained during the CTZ field experiments. Comparison of simulations, which were initialized with measured distributions of phytoplankton, Zooplankton, and nutrients obtained from various regions (inshore, offshore, filament, and non-filament) in the CTZ, provide insight into the role of the biological and physical processes in determining the development of the subsurface chlorophyll maximum and other related features (i.e., regions of high Zooplankton fecundity). Model verification is obtained from comparing the simulations to actual observations obtained by following a Lagrangian drifter in the CTZ during field experiments in 1988. Preliminary results suggest that the bio-optical model adequately simulates the photosynthetic processes which play a role in developing the subsurface chlorophyll maximum in the CTZ.