Monitoring the Influence of the Mesoscale Ocean Dynamics on Phytoplanktonic Plumes around the Marquesas Islands Using Multi-Satellite Missions

Angelina Cassianides,Christophe Maes,Thomas Gorgues,Xavier Carton,Elodie Martinez

doi:10.3390/rs12162520

Abstract

The Marquesas islands are a place of strong phytoplanktonic enhancement, whose original mechanisms have not been explained yet. Several mechanisms such as current−bathymetry interactions or island run-off can fertilize waters in the immediate vicinity or downstream of the islands, allowing phytoplankton enhancement. Here, we took the opportunity of an oceanographic cruise carried out at the end of 2018, to combine in situ and satellite observations to investigate two phytoplanktonic blooms occurring north and south of the archipelago. First, Lagrangian diagnostics show that both chlorophyll-a concentrations (Chl) plumes are advected from the islands. Second, the use of Finite-size Lyaponov Exponent and frontogenesis diagnostics reveal how the Chl plumes are shaped by the passage of a mesoscale cyclonic eddy in the south and by a converging front and finer-scale dynamic activity in the north. Our results based on these observations provide clues to the hypothesis of a fertilization from the islands themselves allowing phytoplankton to thrive. They also highlight the role of advection to disperse and shape the Chl plumes in two regions with contrasting dynamical regimes.

Highlights

In the open ocean, usually limited in nutrients, phytoplanktonic enhancement can be locally observed downstream from islands and may cause the local development of marine ecosystems
The strong phytoplanktonic enhancement occurring in the south of the archipelago showed a Chl plume with a vortex shape extending south-westward from Fatu Hiva over more than 200 km, and concentrations higher than 0.3 mg·m−3 (Figure 1a)
The present study aimed to investigate some physical mechanisms likely to be involved in these two blooms

Summary

Introduction

Usually limited in nutrients, phytoplanktonic enhancement can be locally observed downstream from islands and may cause the local development of marine ecosystems. This enhancement has been referred as the Island Mass Effect (IME, [1]). Several mechanisms induced by the islands can enhance nutrient concentrations allowing phytoplankton to thrive. Isopycnal shoaling through coastal upwelling can drive vertical transport of nutrient-rich deep-waters [2]. Interactions between currents and bathymetry can drive vertical transport of nutrient-rich waters offshore of the islands, through wind-shear convergent/divergent frontal regions, eddies and downstream mixing [5,6,7]

Methods

Results

Discussion

Conclusion