Drivers of new optical water types in southwestern Patagonian fjords derived from ship-borne radiometry

Abstract. This research aimed to take a step closer to a holistic assessment of the Optical Water Types (OWT) of the southwestern Patagonian fjords and channels. Using a participatory science approach, we collected highfrequency data from a Ferry Box system and TriOS radiometers on the Yaghán Ferry between 2022 and 2023, as it travelled from Punta Arenas to Puerto Williams, across the Chilean Patagonian fjords and channels. By combining ERA5 meteorological data with on-site water quality measurements such as turbidity, Chl-a, and CDOM, we identified the main factors driving nine unique OWTs, which, for the first time in Patagonia, were grouped using in situ measured reflectance data.

Importantly, this study does not aim to predict outcomes; instead, it focuses on statistically separating these OWTs and identifying their specific water-quality features as well as meteorological and potential land use forcings. To test the reliability of these groups, we used a Random Forest classifier to identify the main factors and key parameters for each water type. The results show that runoff, CDOM, and Chl-a are generally the main drivers of OWT differences, though each group has its own subtle influences, as shown by One-vs-Rest analysis. We found that working closely with community partners greatly improves our ability to monitor complex systems, such as the Patagonian fjords and channels. This research encourages the creation of inclusive, broad monitoring strategies that recognise the close connection between people and the environment, helping us better understand and protect these valuable areas.