Spectral Absorption Characteristics and Phytoplankton Dynamics Across Optical Water Types: Evaluating Sentinel-2 and Sentinel-3 Phytoplankton Absorption Retrieval Accuracy in Boreal Lakes

Accurate detection of chlorophyll-a (Chl-a) is critical for monitoring water quality in inland waters, where high concentrations of coloured dissolved organic matter (CDOM) complicate retrieval process. Reliable Chl-a estimation depends on the precise determination of the phytoplankton absorption coefficient (aph). This study evaluates Chl-a detection from in situ aph measurements and assesses the accuracy of phytoplankton absorption retrieval from Sentinel-2/MSI (S2) and Sentinel-3/OLCI (S3) using the Case-2-Regional-Coast-Colour (C2RCC) processor across diverse optical water types (OWTs) in boreal lakes. OWTs were classified based on remote sensing reflectance features, representing Clear, Moderate, Turbid, Very Turbid, and Brown conditions. CDOM absorption strongly influenced the underwater light field, particularly in Brown and Turbid waters. Linear relationships between aph and Chl-a were generally strong across OWTs, with improved relationships in the red spectral region (670 nm). Satellite-derived apig estimates showed a weak relationship with in situ data (R2 = 0.26–0.45). Both sensors overestimated small aph values, while S3 underestimated larger ones. S2 underestimated aph in Clear and Brown OWTs, with median absolute percentage differences near 100% for all OWTs. These findings emphasize the challenges posed by bio-optical complexity in boreal lakes and highlight the need for OWT-specific algorithms to improve satellite-based absorption and Chl-a retrieval accuracy.