Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2008Marine ecosystems reflect the physical structure of their environment and the biological
processes they carry out. This leads to spatial heterogeneity and temporal variability, some
of which is imposed externally and some of which emerges from the ecological mechanisms
themselves. The main focus of this thesis is on the formation of spatial patterns in
the distribution of zooplankton arising from social interactions between individuals. In the
Southern Ocean, krill often assemble in swarms and schools, the dynamics of which have
important ecological consequences. Mathematical and numerical models are employed
to study the interplay of biological and physical processes that contribute to the observed
patchiness.
The evolution of social behavior is simulated in a theoretical framework that includes
zooplankton population dynamics, swimming behavior, and some aspects of the variability
inherent to fluid environments. First, I formulate a model of resource utilization by
a stage-structured predator population with density-dependent reproduction. Second, I incorporate
the predator-prey dynamics into a spatially-explicit model, in which aggregations
develop spontaneously as a result of linear instability of the uniform distribution. In this
idealized ecosystem, benefits related to the local abundance of mates are offset by the cost
of having to share resources with other group members. Third, I derive a weakly nonlinear
approximation for the steady-state distributions of predator and prey biomass that
captures the spatial patterns driven by social tendencies. Fourth, I simulate the schooling
behavior of zooplankton in a variable environment; when turbulent flows generate patchiness
in the resource field, schools can forage more efficiently than individuals. Taken
together, these chapters demonstrate that aggregation/ schooling can indeed be the favored
behavior when (i) reproduction (or other survival measures) increases with density in part
of the range and (ii) mixing of prey into patches is rapid enough to offset the depletion.
In the final two chapters, I consider sources of temporal variability in marine ecosystems.
External perturbations amplified by nonlinear ecological interactions induce transient excursions away from equilibrium; in predator-prey dynamics the amplitude and duration of
these transients are controlled by biological processes such as growth and mortality. In the
Southern Ocean, large-scale winds associated with ENSO and the Southern Annular Mode
cause convective mixing, which in turn drives air-sea fluxes of carbon dioxide and oxygen.
Whether driven by stochastic fluctuations or by climatic phenomena, variability of the biogeochemical/physical environment has implications for ecosystem dynamics.Funding was provided by the Academic Programs Office of the MIT-WHOI Joint Program,
an Ocean Ventures Fund Award, an Anonymous Ys Endowed Science Fellowship, and by
NSF grants OCE-0221369 and OCE-336839