In March 2025, the OBSEA cabled observatory recorded a short but intense microphytoplankton bloom, providing a clear example of how high-frequency observations can reveal rapid ecosystem dynamics.

After being re-deployed in February following a maintenance period, the CytoSub instrument (developed by CytoBuoy) resumed its high-resolution imaging of phytoplankton communities as part of the ANERIS project.

Thanks to an hourly sampling strategy, CytoSub detected a sudden increase in microphytoplankton concentrations—from approximately 1,000 cells/L on the morning of March 18th to a peak of 20,000 cells/L by the afternoon, mostly composed of diatom colonies. By March 21st, values had returned to baseline levels.

Based on the environmental conditions, this transient bloom was likely driven by water currents generated by recent easterly wind events, which may have transported the diatom colonies into OBSEA’s monitoring area.

The images and data are the result of a collaborative effort between OBSEA (UPC), CytoBuoy, and the Laboratoire d’Océanographie de Villefranche (LOV, Sorbonne Université / CNRS), specialists in plankton imaging. The protocol optimization and data analysis were carried out by Clémentine Gallot and Melilotus Thyssen from MIO Marseille (AMU / CNRS), with classification and visualization using the Ecotaxa platform.

The accompanying graph of Microphytoplankton >20µm abundance clearly illustrates the rapid onset and decline of the bloom over the course of five days.

This event highlights the value of automated, high-frequency observations in detecting short-term biological phenomena that might be missed with traditional sampling.

Partners involved:
 ANERIS Project, OBSEA Observatory, Universitat Politècnica de Catalunya (UPC), CytoBuoy, Institut de Ciències del Mar (ICM-CSIC), Laboratoire d’Océanographie de Villefranche (LOV – CNRS/SU), EMSO ERIC, MIO Marseille (AMU/CNRS)

 

 

The graph illustrates the rapid onset and decline of the bloom over the course of five days

Cell images by CytoSub

Deployment of Cytosub at OBSEA

 

Photo Credit:

Cell images by CytoSub; data analysis by MIO Marseille

OBSEA

 

Author: Ikram Bghiel, OBSEA – UPC