Long-term trends and mesoscale eddies impact on hyperspectral particulate absorption coefficients in the South China Sea

Documenting the variations of hyperspectral particulate absorption coefficients is essential for understanding how light is propagated underwater and thereby estimating marine primary production. Using an extensive in-situ dataset (2004–2018), this study investigates the spatial, vertical, and long-term trends of hyperspectral particulate absorption coefficients in the South China Sea (SCS). The dataset comprises 608 profiles (spanning depths from surface to 100 m) and includes measurements of the spectral absorption coefficients for total particulates, phytoplankton, and non-algal fractions between 2004–2018. Our results indicate that particulate absorption coefficients at 443nm (chlorophyll-a peak) exhibit a clear decreasing gradient from the coastal to open ocean, varying by two orders of magnitude (from ~0.005 m-1 to ~0.5 m-1), with maxima typically observed in the subsurface layer in the open ocean. The influence of mesoscale eddies on particulate absorption was subtle at the surface but became more pronounced with depth, where both the magnitude and spectral shape differed increasingly between anticyclonic and cyclonic eddies. The observational results reveal an interdecadal shift in particulate absorption coefficients around 2009, characterized by a non-significant increasing trend from 2004 to 2009 (Slope=0.0042 m-1 yr-1, P=0.12) followed by a significant decline from 2009 to 2018 (Slope=-0.0016 m-1 yr-1, P=0.05) at 75 m depth. This variability shows a marginally significant negative correlation with the Pacific Decadal Oscillation (R=-0.52, P=0.07). This assessment of hyperspectral particulate absorption coefficients establishes a foundation to refine ocean-color products in the SCS.