On the role of biological dynamics in plankton patchiness at the mesoscale: An example from the eastern North Atlantic Ocean

In situations where the direct physical forcing of biological processes, through nutrient supply or light limitation of phytoplankton, does not show significant spatial variability, it is demonstrated that the effects of the mesoscale circulation and population dynamics on plankton patchiness are in principle, separable. In this case, the physical stirring and mixing control the size and shape of structures seen in plankton distributions, whilst the biological dynamics controls the size of the populations within those structures. This is demonstrated using field observations and a simple process model. The field data, which come from mesoscale surveys of a region (16-20W, 46-49N) in the eastern North Atlantic during April-May 1997, provide simultaneous continuous measurements of hydrography, phytoplankton and three size classes of zooplankton with the same spatio-temporal resolution and sampling. The lack of significant spatial variability in the physical forcing arises due to the absence of any strong frontal features in the area and to the fact that storm mixing maintained nutrient levels above limiting values throughout the surveys. When the observed biological parameters are plotted in phase space, a close relationship between constituents of the ecosystem is revealed that is missed by other data analysis techniques. In particular, this approach provides an explanation of how zooplankton and phytoplankton can be simultaneously positively and negatively correlated at different, sites in a region and yet still tightly coupled. It is further demonstrated that the much-maligned standard feature of surveying plankton patchiness at the mesoscale-asynopticity in sampling-actually assists in the extraction of the phase space relationships.