Leveraging Lagrangian drifters to validate satellite particulate backscatter estimates and unravel ocean bio-physical dynamics

The concentration of particulate matter is a critical ocean variable for understanding biogeochemical processes across diverse spatial and temporal scales. It plays a key role in refining ocean productivity estimates, which are essential for constraining coupled physical-biogeochemical numerical models. However, direct measurements are often challenging to obtain. A reliable alternative is the optical backscattering coefficient of marine particles (b_bp), which serves as a robust proxy for particulate matter concentration and can be estimated from space. Traditionally, most in-situ multi-spectral b_bp measurements are conducted using ship-based or moored systems, limiting their spatial and temporal coverage.

To address these limitations, we have integrated bio-optical sensors into Lagrangian Surface Velocity Programme (SVP) drifters, resulting in the Backscatter-Optical (BO)-SVP drifter. These systems enable continuous data collection in challenging marine environments by adopting a water-following approach and high-frequency sampling. Such measurements can validate satellite estimates, bridge observational gaps when satellite data is unavailable and capture small-scale variability that cannot be resolved by satellite observations or other sampling strategies. By crossing multiple satellite pixels within a single day, these drifters significantly enhance satellite validation efforts to ocean color missions, such as Sentinel-3/OLCI and PACE/OCI.  Furthermore, BO-SVP drifters offer a unique perspective for studying surface bio-physical dynamics critical to ocean ecosystem functioning, spanning a continuum of spatial (sub-mesoscale to basin scale) and temporal (hours to weeks) resolutions.

Here, we detail the integration of a commercially available multispectral optical backscatter sensor into an SVP drifter to perform near-surface b_bp measurements. The collected data demonstrated high reliability across a range of environmental conditions, showing strong agreement with independent datasets. These results highlight the potential for deploying a global network of BO-SVP drifters, offering new opportunities to monitor and understand the world’s oceans.

The high-frequency observations obtained from BO-SVP drifters could be impactful across ongoing  and future hyperspectral ocean color satellite missions (e.g., NASA GLIMR, ESA Sentinel Next Generation, and ESA CHIME), and lidar mission (e.g., ASI CALIGOLA).  In the next future, multiple deployments are planned in the Mediterranean Sea and on a global scale through the support of the INSPIRE and ITINERIS projects. These deployments will facilitate measurements of ocean processes across broad and fine spatial and temporal scales. Some of these deployments will be coordinated with BGC-Argo floats and other autonomous platforms, providing complementary surface and subsurface data at multiple temporal, spatial, and spectral scales. These efforts are expected to yield new insights into oceanic ecosystem functioning by enabling more comprehensive assessments of biogeochemical cycles, plankton dynamics, and carbon fluxes.