Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems

TL;DR

This study compares surface mixing and biological activity across four Eastern Boundary Upwelling Systems using satellite data, revealing an inverse relationship between surface stirring and chlorophyll levels and suggesting a complex vertical response of phytoplankton to turbulence.

Contribution

It provides a comparative analysis of surface stirring and biological activity in four upwelling systems, highlighting the inverse correlation and vertical effects on phytoplankton response.

Findings

Negative correlation between surface mixing and chlorophyll stocks

Seasonal variability in surface stirring

Potential shift in phytoplankton response to turbulence

Abstract

Eastern Boundary Upwelling Systems (EBUS) are characterized by a high productivity of plankton associated with large commercial fisheries, thus playing key biological and socio-economical roles. The aim of this work is to make a comparative study of these four upwelling systems focussing on their surface stirring, using the Finite Size Lyapunov Exponents (FSLEs), and their biological activity, based on satellite data. First, the spatial distribution of horizontal mixing is analysed from time averages and from probability density functions of FSLEs. Then we studied the temporal variability of surface stirring focussing on the annual and seasonal cycle. There is a global negative correlation between surface horizontal mixing and chlorophyll standing stocks over the four areas. To try to better understand this inverse relationship, we consider the vertical dimension by looking at the Ekman-transport and vertical velocities. We suggest the possibility of a changing response of the phytoplankton to sub/mesoscale turbulence, from a negative effect in the very productive coastal areas to a positive one in the open ocean.