Dispersal, eddies, and the diversity of marine phytoplankton

We examined the role of physical dispersal in regulating patterns of diversity of marine phytoplankton in the context of global ocean simulations at eddy-permitting and coarse resolutions. Swifter current speeds, faster dispersal, and increased environmental variability in the higher-resolution model enhanced local diversity almost everywhere. In the numerical simulations, each resolved phytoplankton type was characterized as “locally adapted” at any geographical location (i.e., having net local biological production and physical export) or “immigrant” (i.e., net local biological loss but a population sustained by immigration via physical transport). Immigrants accounted for a higher fraction of the total diversity in the equatorial and subtropical regions, where the exclusion timescale is long relative to the physical transport between “provinces.” Hotspots of diversity were associated with western boundary currents and coastal upwelling regions. The former had high locally adapted diversity within the core of the current system, maintained by confluence of upstream populations and the induction of nutrient resources, as well as environmental variability associated with mesoscale eddies. Downstream of strong nutrient sources, convergence of populations led to immigrant-dominated diversity. The numerical simulations provide testable predictions of patterns in diversity and hypotheses regarding the mechanisms that control them. Molecular approaches to characterizing diversity in microbial populations will provide a means to test these hypotheses.