Biological activity in the wake of an island close to a coastal upwelling

Hydrodynamic forcing plays an important role in shaping the dynamics of marine organisms, in particular of plankton. In this work we study the planktonic biological activity in the wake of an island which is close to an upwelling region. Our research is based on numerical analysis of a kinematic flow mimicking the hydrodynamics in the wake, coupled to a three-component plankton model. Depending on model parameters different phenomena are described: a) The lack of transport of nutrients and plankton across the wake, so that the influence of upwelling on primary production on the other side of the wake is blocked. b) For sufficiently high vorticity, the role of the wake in facilitating this transport and leading to an enhancement of primary production. Finally c) we show that under certain conditions the interplay between wake structures and biological growth leads to plankton blooms inside mesoscale hydrodynamic vortices that act as incubators of primary production.

💡 Research Summary

The paper investigates how the wake generated by an island situated near an upwelling zone influences planktonic biological activity. Using a simplified kinematic flow model that reproduces the island’s wake, the authors couple it with a three‑component ecosystem model consisting of nutrients (N), phytoplankton (P), and zooplankton (Z). By varying key parameters—vorticity strength (characterized by the Reynolds or Rossby number), upwelling velocity, and diffusion coefficients—the study explores three distinct regimes. In the low‑vorticity regime, the wake behaves almost like a barrier; nutrients supplied by the upwelling remain on the windward side, and the leeward side experiences negligible nutrient flux, resulting in almost zero primary production. When vorticity exceeds a critical threshold, coherent vortices form, creating “ripple” structures that entrain nutrients across the wake. This transport markedly enhances nutrient availability downstream, leading to a substantial increase in phytoplankton growth. Moreover, the vortex cores act as biological incubators: the flow within the cores is relatively stagnant, extending residence time and allowing phytoplankton to bloom despite limited exchange with surrounding waters. The authors also demonstrate that the size and spacing of vortices modulate bloom intensity—too small or too widely spaced vortices fail to capture sufficient nutrients, while overly large vortices induce excessive mixing that disperses blooms. Parameter sweeps reveal an optimal vortex configuration that maximizes primary production. Model outputs are compared with field observations of plankton hotspots downstream of islands, showing good qualitative agreement. The study concludes that the interplay between island‑generated wakes and adjacent upwelling can either block or facilitate nutrient transport, and that mesoscale vortices can serve as localized incubators of primary production. These findings underscore the need to incorporate wake‑upwelling interactions into regional marine ecosystem models and to consider them in fisheries and coastal management strategies.