Water Quality

Water quality is a worldwide issue affecting human health, ecosystems’ integrity, food production, industry, and recreation. Satellites offer a cost-effective solution to monitor water quality at a global scale through the measurement of spectral water reflectance, commonly referred to as water colour. Atmospheric correction is a critical step for retrieving water colour from space-borne images. Through the Polymer atmospheric correction algorithm, HYGEOS has acquired recognized expertise in water colour retrieval for monitoring the biological activity of the oceans, coastal and inland waters.

HYGEOS contributes to European operational water quality products

Since an independent intercomparison exercise, performed in the ESA Ocean Colour Climate Change Initiative project (OC-CCI), has confirmed that Polymer was the most efficient algorithm to retrieve accurate marine reflectances, it has been implemented in the OC-CCI operational chain to process data from SeaStar/SeaWiFs, Aqua/MODIS, Envisat/MERIS, Suomi/VIIRS and Sentinel-3/OLCI sensors in order to generate timeseries of remote sensing reflectances at sea surface, from 1997 to the present. The OC-CCI processing chain is now operated in the Copernicus Climate Change Service (C3S) and in the Copernicus Marine Environment Monitoring Service (CMEMS) to provide global daily and monthly estimates of ocean surface chlorophyll-a concentration. As the most prevalent photosynthetic green pigment in marine photosynthetic organisms, chlorophyll-a is an indicator of the phytoplankton abundance, and therefore the base of the marine food web. Polymer is also implemented in the near-real-time Sentinel-3/OLCI processing chain of CMEMS generating regional chlorophyll-a concentration products over the Baltic Sea.

Chlorophyll-a concentration in sea water in December 2023.

Chlorophyll-a concentration (mg/m3) in sea water in December 2023. From the MyOcean PRO visualization tool

HYGEOS is also involved in the Global component of the Copernicus Land Monitoring Service (CLMS). Indeed, Polymer is implemented in the near-real-time operational processing chains to retrieve the Lake Water Quality products suite at 300m resolution from the Sentinel-3/OLCI reflectance and at 100m resolution from the Sentinel-2/MSI. This suite of products includes the turbidity of the lake which varies seasonally both with the discharge of rivers and growth of phytoplankton, the trophic state index as an indicator of the productivity of a lake in terms of phytoplankton and the lake surface reflectance which describes the apparent colour of the water body. More than 4260 lakes all around the world are covered.

Water quality of Lake Vänern, Sweden, on 11 April 2018 expressed as trophic state index (left) and turbidity (right) provided by the Copernicus Global Land Service

Tropic State Index (left) and Turbidity (right) of Lake Vänern, Sweden, on 11 April 2018 as provided by the Copernicus Global Land Service.

HYGEOS has developed for EUMETSAT a Bright Pixel Correction for the Sentinel-3/OLCI sensor. This correction, as well as a totally revised Clear Water Atmospheric Correction algorithm with new aerosol modelling, detection and new capabilities regarding aerosol layer height retrieval, is now included in the robust and versatile Ocean Colour Standard Atmospheric Correction module (OC-SAC) which is being integrated into the OLCI operational processor of EUMETSAT in order to deliver the Collection 4 of Copernicus Ocean Colour products. Always looking for improvement, HYGEOS has developed, using Polymer as a baseline, an alternative atmospheric correction scheme (SACSO), using a spectral matching approach and including an uncertainty propagation scheme, to improve the quality of the OLCI water reflectance, in particular in presence of absorbing aerosols. Polymer, as well as the alternative SACSO processor, are both integrated into a modular Ocean Colour prototype processor deployment and validation framework. HYGEOS is also developing a daily Photosynthetically Available Radiation (PAR) product from Sentinel-3/OLCI sensor which would extend the list of operational Level 2 Ocean Color products distributed by EUMETSAT.

HYGEOS also works on the ocean colour products derived from geostationary satellites operated by EUMETSAT. HYGEOS contributes to develop prototype processing for retrieval of ocean colour products, such as water turbidity and chlorophyll concentrations, from SEVIRI on MSG and the upcoming FCI on MTG for open ocean, coastal and inland water coverage. The SEVIRI and FCI processing will be embedded in the multi-mission modules of the EUMETSAT’s existing ocean colour prototype processor with the goal to raise the ocean colour from European geostationary satellites to an operational level.

HYGEOS acts for the Sargassum monitoring

Thanks to its experience on the exploitation of geostationary imagery, HYGEOS has developed an Sargassum detection algorithm from the Advanced Baseline Imager (ABI) on GOES-16 geostationary platform covering the Caribbean area. With its 10-minute temporal resolution, ABI allows to extend the satellite coverage to the whole day. HYGEOS algorithm is now integrated into SAMTool, the operational Sargassum detection system operated by CLS, in complement of ocean colour products derived from the combination of 6 polar orbiting satellites. Output are delivered to Meteo-France for its mandate of monitoring Sargassum in the Caribbean and French Guyana and to the French Ocean Data Information and Service.

Sargasses dérivant au large de la Martinique

Sargassums off French Antilles

In the continuity, HYGEOS and CLS participate in an extension of the CMEMS through its OC-TAC by developing a new set of Sargassum data products derived from multiple satellite observations: wide swath sensors (Aqua/MODIS, Sentinel-3/OLCI), high spatial resolution sensors (Sentinel-2/MSI, Landsat-8/OLI) and high temporal resolution with geostationary satellites (GOES-16/ABI). These products will contribute to develop a new ecosystem of downstream applications aiming to react to the massive landings of sargassum seaweed that occurred periodically on the shorelines of the Tropical Atlantic Basin, causing devastating effects on local economies.

Continuous research topics

HYGEOS is involved in many research projects to remove atmosphere contribution before retrieving water quality indicators, including for coastal and inland waters, whose optical properties (extreme diffusion and absorption) are different from those of the open ocean. Recent activities target transitional waters with particular focus on adjacency and bathymetry effects (bottom visibility), which are also taken into account to retrieve state-of-the-art water reflectance close to the shoreline in order to improve the estimate of Essential Biodiversity Variables.