Patchwork: Mesocosm Tank Studies on Marine Plankton Patchiness

The 10 m deep Tower Tank of the Aquatron Laboratory at Dalhousie University is a controlled-environment marine research facility which provides an experimental space scale particularly appropriate for studies in plankton ecology. Two recent projects in the 117 m³tank have addressed a key problem in plankton patchiness, namely the behavioural responses of zooplankton interacting with patches of phytoplankton. The first project studied the extent to which the normal diurnal migration pattern of copepods may be modified by the presence of a discrete layer of phytoplankton in the water column. The tank was vertically divided to provide a simultaneous control, and sampled at 2 m depth intervals with a series of multiple-inlet samplers. The vertical migration pattern of some copepod species was significantly modified by the presence of the phytoplankton layer. The second project investigated the swimming behaviour of individual euphausiids, larger planktonic-micronektonic organisms capable of wide-ranging vertical and horizontal swimming excursions. A dual-camera, split-screen video system was used to provide three-dimensional spatial coordinates of individual organisms over time, in order to determine the behavioural responses which enable euphauslids to locate and remain in food patches. The results indicated that euphausiids were able to locate rapidly a phytoplankton patch, that their swimming speed increased in the patch and that they were able to remain in the patch by turning at the patch boundary. The mesocosm tank has thus provided some new insights into plankton behaviour which could not have been obtained from studies in either "beakers or bays" (Strickland,1967).