Filaments help in understanding a harmful algal bloom

Ocean regions of high productivity can also be hotspots for harmful algal blooms. Understanding algae blooms in general, and the harmful ones in particular involves to delve into the biological and physical dynamics of the area, using a large number of different observations and observation tools.

The Patagonian continental shelf in the Atlantic Ocean has nutrient-rich waters where  massive phytoplankton blooms often occur, especially along the shelf-break frontal system. Two oceanographic cruises organized by researchers of CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina) in spring 2021 detected, at 10-d interval, a bloom of dinoflagellates (Amphidomataceae family), a toxin-producing plankton group, at stations 40 km apart over the outer shelf. This could be due to either a large patch remaining in the area, or to a smaller one moving from one station to the other. Another question was whether it might reach coastal areas, be advected and dispersed in the Malvinas Current, and what were the mechanisms producing such a large bloom there.

Finite-size Lyapunov exponents (FSLEs) and Lagrangian advection of virtual particles, both from altimetry data, were used to understand the physical mechanisms of the toxic algae concentration.
The FSLEs plots over the study area show a complex situtation, and highlight the stirring and heterogeneity of the oceanic waters.
In the area where the harmful algae bloom was sampled, the FSLE maps show no mesoscale fronts separating the two stations during the 10-day period between the cruises. Both stations remained within the same water mass, delimitated by two FSLEs with negative values ("ridges"). The negative FSLEs correspond to a divergence, transverse to the filaments (see Image of the Month, Sep. 2018: A buoy trapped in filaments). This kept both bloom stations within the same water mass during the period between the two samplings, thus explaining the bloom persistence there.

Altimetry made it possible to assess that the mesoscale circulation on the Patagonian shelf prevented the dispersion of a harmful algae bloom. This could be used to monitor such blooms and their dynamics.
The collection of more samples, following the water masses, could also provide with important insights on the dynamics of a large offshore algal bloom. Exploring sub-mesoscale processes should also bring additional inputs for understanding divergence, convergence, and the mixing of plankton and nutrients within and between water masses. Swot is enabling this for future events.