Modeling Analysis of Physical Transport and Swimming Behaviors Determining Plankton Distributions

An experimental study and modeling analysis was performed to determine the relative importance of physical transport versus swimming behavior in determining the distribution of zooplankton and their retention in patches of high phytoplankton density. The experiment involved establishing, maintaining and sampling a natural plankton patch in a ∼1-km long tank (3.3 m deep by 7.3 m wide). Natural plankton containing a mixture of Chesapeake Bay algal species dominated by the calanoid copepod Acartia hudsonica were inoculated into the lighted center of the tank within a plastic bag such that the plankton patch was held in place. The patch was grown (via lighting and nutrient additions) to a relatively high density as compared to the surrounding water. After balancing the water density throughout the tank, the patch was released by removing the plastic bag. The physical regime was such that the patch was sheared. Copepod distributions were sampled and modeled, including physical transport and various swimming behaviors, to determine if the observed movements could be accounted for purely by physical transport, or whether the animals needed to swim either vertically or horizontally to account for the changing densilies in space and lime. Modeling results indicated that younger copepod stages moved purely by physical transport and randomly-directed swimming, but that there was evidence of directed movement by older copepods. Vertical migration behavior, coupled with physical transport and randomized foraging behavior, was sufficient to explain the distribution of older stage copepods; oriented horizontal swimming behaviors were not needed to account for distributions. Such directed movements, coupled with physical transport, are important determinants of plankton distributions, both for holo-zooplankton and larval fishes. Additional species and conditions need to be evaluated to fully understand the roles of directed swimming behaviors and physical transport in determining plankton distributions.