Oceanic Frontal Divergence Alters Phytoplankton Competition and Distribution

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. Plain Language Summary Oceanic phytoplankton populations, which play an essential role in regulating the carbon and oxygen in our atmosphere and oceans, are shaped by their fluid environment. Ocean currents can alter the diversity and distribution of these populations. However, little is known about the ecological impact of submesoscale flows (1–10 km), which are challenging to observe and model but have a number of interesting features, including increased variability and vertical motion. Here we combine an unprecedented high‐resolution spatial survey of closely related cyanobacterial populations with simulations and theory. We find that previously underappreciated submesoscale flows can drive significant local and regional changes in community composition and biodiversity. Key Points Unexplained genetic variability is observed among phytoplankton populations at a front in the Mediterranean Sea A theoretical relationship is derived for the influence of horizontal velocity divergence on population dynamics Submesoscale velocity divergence is proposed as a mechanism for generating genetic variability in frontal regions