Spatial distribution of motile phytoplankton in a stratified reservoir: the physical controls on patch formation

Changes in the spatial distribution of the dinoflagellate Ceratium hirundinella were observed in a stratified, medium-sized (16 km(2)) Argentinean reservoir over several days. A fluorescence profiling technique was used to identify persistent patchiness in the distribution of the dinoflagellate. A three-dimensional numerical model was used to reconcile a range of different unsteady processes and prove that the initial source of heterogeneity in the system was the vertical migration of Ceratium. Once migration established vertical heterogeneity, the dominant influence on the patch dynamics alternated between control by migration and control by mixing and transport. This led to the development of persistent horizontal patchiness. The analysis revealed that the region of the lake inhabited by Ceratium was highly predictable and from this result it was determined that physical processes (with some influence from migration) control the habitat of this dinoflagellate rather than biological/chemical gradients. When the spatial habitat of a particular phytoplankton species can be isolated in this manner, the resources available to the species can be more accurately determined by further study. The results are particularly applicable to the study of motile/buoyant plankton in aquatic systems that are periodically subject to moderate or strong wind forcing events.