Spatio-temporal variability of remote sensing reflectance from MODIS imagery for water quality assessment: A case study of Northern Patagonia, Chile

The variability of phytoplankton dynamics is a crucial component of coastal biogeochemical cycles at various spatial–temporal scales. A key geophysical parameter that advances our understanding of phytoplankton dynamics in coastal zones is the remote sensing reflectance (R_rs), which can be used as a proxy for the concentrations of colored dissolved organic matter (CDOM), phytoplankton biomass, and other organic and inorganic particles. In this work, we used data from the Moderate Resolution Imaging Spectroradiometer onboard the Aqua satellite (MODIS-Aqua) to study long-term trends (2003–2021) in remote sensing reflectance (R_rs) across the inner waters of the Chilean northern Patagonia. The Chilean northern Patagonia is a major fjord/channel system receiving large freshwater discharges from continental glacial rivers loaded with terrigenous materials. These discharges significantly alter the water chemistry and its bio-optical properties. Our findings reveal that the optical complexity arising from the interplay between major freshwater sources and the marine system in the Inner Sea of Chiloé leads to a high temporal variability of R_rs, primarily linked to the seasonality in turbidity patterns. These findings have significant implications for northern Patagonia's river discharge and turbidity to climate change. They also highlight the substantial changes in water quality (Chla, turbidity, etc), triggering shifts in biophysical patterns.