Unveiling optical and phytoplankton variability in the northwestern Iberian Peninsula using in-situ radiometry and the Water Colour Simulator – WASI

The seasonal and spatial variability of optical and phytoplankton-related properties was investigated in Ría de Arousa and Ría de Vigo, two large embayments within the northwestern Iberian upwelling system. Over one year, quasi-monthly in-situ sampling was conducted to measure remote sensing reflectance ( R rs ) alongside biophysical variables. Apparent and inherent optical properties (AOPs and IOPs, respectively), including absorption by phytoplankton, detritus, and coloured dissolved organic matter (CDOM), were analysed. Phytoplankton group composition (cyanobacteria, dinoflagellates, diatoms, cryptophytes, and green algae) and IOPs were retrieved using the Water Colour Simulator (WASI), applied to the R rs dataset (380, 395, 412, 443, 465, 490, 510, 555, 670, 694, 710 nm). Model outputs were evaluated against in-situ chlorophyll-a concentrations (Chl-a), demonstrating satisfactory agreement. R rs (λ) spectra exhibited low reflectance in blue bands (<490 nm) associated with strong absorption by water constituents, and elevated reflectance in green-red bands (>490 nm), consistent with optically complex coastal waters. Phytoplankton was identified as the dominant optical constituent, followed by CDOM, whereas detritus showed the lowest absorption at 443 nm. Ría de Vigo was characterised by greater variability in R rs and Chl-a, likely reflecting its more confined hydrodynamics and enhanced local retention processes. Seasonal shifts were observed with increased microplankton contribution during summer in Ría de Arousa and higher variability in phytoplankton size structure in Ría de Vigo. These findings highlight the capability of WASI to resolve bio-optical variability and community dynamics in estuarine waters, supporting its application for aquaculture management and ecosystem monitoring in optically complex coastal systems.