Computational and Analytic Perspectives on the Drift Paradox

The fact that many small aquatic and marine organisms manage to persist in their native environments in the presence of constant advection into unfavorable habitat is known as the “drift paradox.” Although advection may determine large scale biological patterns, individual behavior such as predation or vertical/horizontal migration can dominate at smaller scales. Using both computational and analytical methods to model flow in an idealized channel, we explore the extent to which biological processes can counteract physical drivers. In particular, we investigate how different zooplankton migration behaviors affect biological retention time under a variety of flow regimes and whether a combination of physical/biological regimes exists that can resolve the drift paradox, i.e., allow the zooplankton to avoid washout for time periods much greater than the hydrologic retention time. The computational model is a three-dimensional semi-implicit hydrodynamic model which is coupled with an individual-based model for zooplankton behavior, while the analytical model is a simple partial differential equation containing both advective and behavioral components. The only behavior exhibited by the zooplankton is diel vertical migration. Our studies show that the interaction of zooplankton behavior and exchange flow can significantly influence zooplankton residence time. For a channel without vegetation, the analytical methods give biological residence times that vary by at most a day from the computational results.