In frame of the SAREDA (SArgassum Evolving Distribution in the Atlantic) project, the Mediterranean Institute of Oceanology (MIO), in collaboration with LIS, HYGEOS and AERIS/ICARE, has set up a system for monitoring Sargassum in the Atlantic Ocean using spatial data from NASA’s MODIS instrument. The objective of this project is the operationalization of a processing chain to provide a new product for the remote sensing of sargassum and to establish a climatology. This project was financed by TOSCA/IRD and ANR (FORESEA).
Thanks to its privileged access to MODIS products and its experience in processing of these data, the AERIS/ICARE Data and Services Centre joined forces with MOI for an efficient prototyping of this new product.
Sargassum: harmful algae that proliferates in abundance
Made up of small branched branches a few tens of centimetres long, equipped with floats, the sargassum drift on the surface of the ocean and come to entangle with each other to form fairly dense clusters. These clusters generally take the form of long lines aligned in the wind rows, but can also form more compact aggregations of several tens to several hundreds metres. On a larger scale, these structures are organized into filaments which can extend over several tens of kilometres, visible by satellite. Their thickness below the surface can reach several metres. They proliferate in tropical waters from the Caribbean and West Indies up to West Africa. The phenomenon has increased and shifted over the past decade. The sargassum then come ashore on the beaches covering them with a thick brownish layer beyond sight and undermining local tourism. Moreover, their degradation produces hydrogen sulphide which is a toxic gas at high concentrations.
These algae have thus become a real curse impacting fishing, health, tourism and the economy.
Since 2017, the Mediterranean Institute of Oceanology (MIO) has been interested in understanding the phenomenon. Scientists are studying the biology and ecology of these algae. They are seeking to understand the origin of their proliferation and their significant displacements. Only observation from space makes it possible to describe these processes through systematic observation on a large scale and over long periods. Several satellites make it possible to observe Sargassum from space thanks to instrumental characteristics revealing the presence of plants in the ocean.
Other scientific initiatives exist on Sargassum:
- CLS is developing and implementing an automated service based on a combination of satellite data (Sentinel-2, Sentinel-3, MODIS) and a drift model.
- The University of South Florida produces daily maps to monitor these algae, particularly in the Caribbean, the West Indies and also in the Atlantic basin.
- The Mediterranean Institute of Oceanology led a Sargassum expedition in 2017.
- BRGM, in partnership with ADEME and the DEALs of Guadeloupe and Martinique, has developed observation tools on the coast using autonomous cameras.
The SAREDA project
Within the SAREDA project, the MIO has joined forces with the ICARE data and services centre of the AERIS cluster, which specialises in the massive exploitation of Earth observation space data. Together they produce daily and monthly maps of Sargassum in their area of presence: Gulf of Mexico, Caribbean Sea and Atlantic. The aim of this project is to produce a complete history of the biomasses of these seaweeds from the archive [2000-2020] of data from the MODIS instrument flying aboard NASA’s Terra and Aqua satellites. These data provided by ICARE will be used for modelling and abundance forecasting within the framework of the ANR FORESEA project. An operational production in near-real time (D+3) is also planned via a transfer in progress to the Space Meteorology Centre of Météo France. The ODATIS Ocean cluster will take over to ensure the management and distribution of the data sets as well as the sharing, backup and cross-disciplinary uses.
This project is an excellent example of coordination and sharing of resources and skills between the different clusters of the DATA TERRA Research Infrastructure.
Image 3 and 4: Map of the occurrence of Sargassum for the period from 31 December 2020 to 19 January 2021 and the trajectory of the boats during their northward journey on 19 January 2021.
A project highlighted during the Vendée Globe
Skippers who sail the Atlantic Ocean are familiar with Sargassum and the embarrassment it represents. The floating clusters are so dense that it is difficult for a sailboat to cross them. The sargassas get caught in the protruding parts of the sailboat: the keel, foils, rudders, to the point of considerably reducing the speed of the boat or even immobilizing it, forcing the skipper to maneuver or even to go backwards. The sargassas can also block the propeller of the hydro generator, making it unusable.This phenomenon is reported in particular by Kévin Escoffier, a French skipper who had to abandon the Vendée Globe due to a major waterway in his boat: “I have had a lot of flying fish since yesterday, but mostly sargassas. I’ve got plenty of them on deck and in the cockpit. My hydro generator is feathering because of sargassum. I didn’t remember there were any around here, I’m very surprised, normally it’s more along the islands of the Caribbean arc. The cockpit is full of them! »
All the skippers of the 2020 edition of the Vendée Globe had the surprise of encountering large quantities of Sargassum off the coast of Cape Verde in mid-November 2020 while heading south. MIO and AERIS/ICARE then set up a system capable of automatically producing maps of sargassum in the equatorial Atlantic based on the MODIS observations of the previous days. These maps were made available to the Vendée Globe participants to facilitate their northward journey. Although sailing in an area more favorable for the return trip, the skippers once again experienced the presence of Sargassum on their route.