Phytoplankton diversity and community structure affected by oceanic dispersal and mesoscale turbulence

We explore the role of oceanic dispersal in setting patterns of phytoplankton diversity, with emphasis on the role of mesoscale turbulence, using numerical simulations that resolve mesoscale eddies and a diverse set of phytoplankton types. The model suggests that dispersal of phytoplankton by oceanic transport processes increases phytoplankton diversity at the local scale of O(10–100) km (α-diversity), extends the range of many phytoplankton types, and decreases the ability of rare types to persist in isolated areas. As a consequence, phytoplanktonic assemblages are modified and diversity decreases at the regional scale of O(1000) km (γ-diversity). By progressively accounting for different classes of motion, we show that the increase of α-diversity ensues from vertical mixing of the organisms, dispersal by mean lateral currents, and in slightly larger proportion, dispersal due to eddies. With the progressive inclusion of mechanisms of dispersal, the community becomes dominated by a smaller number of types but with larger degree of coexistence, in larger home range areas. From a resource competition perspective, physical transport can reduce the effective concentration R* of a limiting resource R, thus allowing more types to become equally fit. In addition, mixing of nearby populations allows coexistence of types with unequal fitness. The simulations suggest that mesoscale turbulence plays a particular role, concomitantly providing a means for different phytoplankton types to achieve comparable fitness and extending the exclusion time scale for less competitive types.