Modeling nutrient and plankton processes in the California coastal transition zone: 3. Lagrangian drifters

Two types of numerical Lagrangian drifter experiments were conducted, using a set of increasingly complex and sophisticated models, to investigate the processes associated with the plankton distributions in the California coastal transition zone (CTZ). The first experiment used a one‐dimensional (1‐D; vertical) time‐dependent physical‐bio‐optical model, which contained a nine‐component food web. Vertical velocities, along the track of simulated Lagrangian drifters, derived from a three‐dimensional (3‐D), primitive equation circulation model developed to simulate the flow observed within the CTZ, were used to parameterize the upwelling and downwelling processes. The second experiment used 880 simulated Lagrangian drifters from a 3‐D primitive equation circulation model which was coupled to the same food web and bio‐optical model used in the first experiment. Parameterization of the biological processes in both experiments were based upon data obtained during the CTZ field experiments. Comparison of simulations with data provided insight into the role of the biological and physical processes in determining the development of the subsurface chlorophyll maximum and other related features. In both studies, the vertical velocities experienced by a simulated Lagrangian drifter as it was advected offshore while entrained within a filament played a major role in determining the depth to which the euphotic zone and the chlorophyll maximum developed. Also, as the drifters moved offshore, the food web changed from a coastal, neritic food web to an offshore, oligotrophic food web due to the decrease in nutrient availability. The temporal development of the food web constituents following the simulated drifters was dependent upon the environment to which the drifter was exposed. For example, the amount of time upwelled or downwelled and the initial location in the CTZ region greatly affected the development of the food web.