Assessing the Hyperspectral Advantage: A First Evaluation of PACE/OCI for Chlorophyll-a Retrieval in Southern Portuguese Coast

Satellite-derived products are essential for monitoring aquatic ecosystems. This study presents a first assessment of the capabilities of the hyperspectral Ocean Colour Instrument (OCI) onboard of NASA’s recently launched Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) satellite. OCI delivers spectral observations from 340 to 895 nm with bandwidth at 5 nm resolution and spectral steps of 2.5 nm sampling, marking a major advance in ocean colour monitoring. Because the launch is recent and the premises very encouraging, an early assessment of its performance both against existing sensors and in-situ data in complex coastal waters is of great interest for validating its capabilities. This work is focused on the coastal waters of southern Portugal, analysing three sampling areas off Sagres, Portimão and Armona Island. PACE/OCI Level-2 chlorophyll-a (Chl-a) products from multiple dates between February 2024 and July 2025 were extracted and compared with coincident opportunistic in-situ Chl-a measurements collected from water samples across  various CIMA projects. To contextualize the performance of the new hyperspectral sensor, a parallel comparison was conducted using standard reduced resolution Level-2 Chl-a products (CHL_OC4Me, CHL_NN) from the multispectral Sentinel-3 Ocean and Land Colour Instrument (S-3/OLCI), which has a comparable spatial resolution with PACE/OCI, approximately 1.2 km. An array of statistical metrics (e.g. Median Absolute Percentage, MdAPD, and Coefficient of Determination, R²) was applied to evaluate the agreement between satellite-derived and in-situ Chl-a concentrations. Preliminary results, based on the available matchup dataset, indicate that PACE/OCI estimations show a closer alignment with in-situ measurements compared to those from S-3/OLCI in the study region, as evidenced by the higher R² and lower log-transformed MdAPD. This improved performance suggests that the hyperspectral capability of OCI sharpens the algorithmic quantification of the phytoplankton signal, using NASA’s combined OCx and Colour Index approach, within the optically complex conditions characteristic of coastal and oceanic waters. Despite the limited temporal coverage (the PACE was launched in the first months of the 2024) and the scarce number of matchups between the opportunistic in-situ data and the corresponding satellite data, this early analysis underscores the promising potential of PACE/OCI for delivering more accurate Chl-a estimates in marine coastal environments. The study provides a first overview on the potential of PACE/OCI in Portuguese coastal waters, and in general on the feasibility of hyperspectral observation, for future research and applications of ocean colour data in contexts like regional coastal management and Blue Economy sectors.