  Ecological interactions among phytoplankton occur in a moving fluid
environment. Oceanic flows can modulate the competition and coexistence between
phytoplankton populations, which in turn can affect ecosystem function and
biogeochemical cycling. We explore the impact of submesoscale velocity
gradients on phytoplankton ecology using observations, simulations, and theory.
Observations reveal that the relative abundance of Synechoccocus oligotypes
varies on 1--10 km scales at an ocean front with submesoscale velocity
gradients at the same scale. Simulations in realistic flow fields demonstrate
that regions of divergence in the horizontal flow field can substantially
modify ecological competition and dispersal on timescales of hours to days.
Regions of positive (negative) divergence provide an advantage (disadvantage)
to local populations, resulting in up to ~20% variation in community
composition in our model. We propose that submesoscale divergence is a
plausible contributor to observed taxonomic variability at oceanic fronts, and
can lead to regional variability in community composition.
