A mathematical exploration of phytoplankton blooms in the North Atlantic

Cook, L. PamelaOliver, Matthew J.Phytoplankton are the base of the marine food web. They are also responsible for much of the oxygen we breathe and they remove carbon dioxide from the atmosphere. The cause of seasonal phytoplankton blooms in the ocean is a debated topic. One hypothesis is that blooms are initiated when seasonally changing environmental conditions disrupt the balance in the predator-prey relationship between zooplankton and phytoplankton. This dissertation follows up on this notion with a Nutrient-Phytoplankton-Zooplankton (NPZ) model incorporating diffusion and depth-dependent coefficients. Full spatiotemporal solutions of this coupled reaction-diffusion system are computed. An explanation of the bloom process in this model is presented that involves a saddle point transient equilibrium state. The saddle point bloom process is demonstrated with an ordinary differential equations NPZ model with time dependent forcing to imitate seasonally oscillating solar radiation. This process is illustrated by an animation (movie1_ODE.avi). The details from this analysis inform the bloom process in the reaction-diffusion NPZ model for which the equilibria must be determined computationally. The bloom process in the reaction-diffusion NPZ model is illustrated by another animation (movie2_PDE.avi). The reaction-diffusion NPZ model, incorporated with seasonal solar radiation and mixed layer depth data, simulates blooms with better timing than the ordinary differential equations model but still leaves much to be desired. However, results from models that simulate blooms more accurately show signs of the saddle point bloom process described in this dissertation. The saddle point bloom mechanism described here could be the mechanism by which the seasonal disruption in ecological balance initiates a high-latitude marine phytoplankton bloom, like that in the North Atlantic Ocean.University of Delaware, Department of Mathematical SciencesPh.D