Vertical distribution and population dynamics of copepods by dynamic optimization

Vertical distribution and population dynamics of copepods are viewed as a consequence of individual maximization of reproductive value (RV). RV for all individuals of all size classes (stages) and conditions (physiological states) is found by dynamic programming, and specifies a trajectory of optimal habitats in time and space. The optimal habitat is found by balancing the risk of predation and growth. Predation risk from visually searching planktivores is included as a mechanistic submodel, and growth is a function of individual size, food concentration, temperature, and energetic costs of migratory behaviour. The optimal policy followed by single individuals eventually gives rise to the population dynamics, based on individual mortality and reproduction rates. The model focuses on the role of temperature, predators, and food resources on dynamics and distribution, and shows that food can affect predation risk through both physiological and physical mechanisms. In fact, increased food concentration may influence dynamics more through reduced predation than through increased growth, because the planktivores' searching efficiency is very sensitive to increased turbidity. This effect is suggested as a potentially important factor in the survival of planktonic organisms susceptible to visually searching predators, and may be most beneficial to macrozooplankton and fish larvae. The optimal copepod response (vertical migration) to increased density of planktivores is to seek less risky habitats, and therefore the predation risk of copepods is a non-linear function of planktivore density. The model suggests that optimal diel migration intensity is changed with food density from no migration at low food levels, reaches a maximum at intermediate levels, but is reduced again at high algal concentrations.