LAUSR

Abstract Coastal oceans play a pivotal role in fisheries production and global biogeochemical cycles, making large-scale monitoring an essential task. The advent of modern remote sensors, such as OLCI (Ocean Land Colour Instrument), on board the Sentinel-3A satellite, has made it possible to obtain oceanic biogeochemical products at higher spatial (300 m) and temporal (daily) resolutions than previously possible. However, validating the Sentinel-3A retrievals for coastal waters is an ongoing effort. Using a regional in situ dataset from British Columbia (BC) and Southeast Alaska (SEA) coastal waters, we evaluated the performance of OLCI Sentinel-3A in retrieving remote-sensing reflectance (Rrs) and biophysical products, including total suspended matter (TSM), chlorophyll-a concentration (Chl-a) and coloured dissolved organic matter (CDOM). The OLCI data were processed through a spectral optimization-based algorithm (POLYMER) and a neural net-based (NN) algorithm (C2RCC), including the original (C2RCC v1) and the alternative version (altNN or v2), in which the neural network was trained with extended ranges to cope with larger dynamic range for high backscatter waters. The processors' performance was evaluated through match-up analysis using data from southern BC, as well as expected ranges and seasonal trends for northern BC and SEA. Multimetric statistical analyses demonstrated that POLYMER provided the best overall performance for TSM and Chl-a retrievals, with the Chl-a product improved by the use of the so-called “Case-2” flag. Despite the relative outperformance of POLYMER-derived products, with low systematic biases, the relative percent differences are still high (80–100%) and should be acknowledged in future analyses when using these data. CDOM, only retrieved here from NN approaches, was better estimated using the alternative version of C2RCC. The best performing approaches were used to evaluate Level-3 composites for northern BC and SEA waters. The observed spatial and seasonal trends compared favourably with those reported in the literature, including highlighting more productive areas (e.g. west coast of Vancouver Island), and the important interactions between riverine systems and adjacent coastal waters (e.g., high TSM and CDOM loads near the Skeena and Nass rivers). This study demonstrated the benefits of OLCI Sentinel-3A to investigate complex coastal ecosystems and provides a robust evaluation of OLCI performance and a framework for future observational and process-oriented studies.