Model for partitioning the light absorption coefficient of seawater within the near-UV and visible spectral region into phytoplankton and non-phytoplankton components

Improving models to separate phytoplankton from non-phytoplankton light absorption in seawater is essential for advancing ocean biogeochemical and ecosystem studies via optical remote sensing. We present a novel model, to our knowledge, referred to as ANW, that partitions the spectral non-water absorption coefficient, a nw ( λ ), into phytoplankton, a ph ( λ ), and non-phytoplankton, a dg ( λ ), components where the latter combines non-algal particulate and colored dissolved organic matter contributions. To accommodate significant variations in complex spectral shape and magnitude of a dg ( λ ) and a ph ( λ ) from the near-UV (NUV) through the visible (VIS) spectral range across diverse aquatic environments, this model avoids restrictive assumptions by utilizing a combination of a spectral shape library for a dg ( λ ) within the NUV–VIS spectral range (350–700 nm, 1 nm interval) and stacked inequality constraints for a ph ( λ ) within the VIS range. This approach allows for wide, realistic variations in both a dg ( λ ) and a ph ( λ ). The spectral shape library and inequality constraint bounds were established using a comprehensive dataset of absorption measurements satisfying strict method-related and data quality criteria. The ANW model requires at least seven spectral values of a nw ( λ ) from the VIS range as input; however, the core objective is to use hyperspectral input (350–700 nm at ≤5nm intervals), which offers the most scientific utility. While a ph ( λ ) is retrieved at input wavelengths, the model produces a consistent hyperspectral a dg ( λ ) output (350–700 nm, 1 nm interval). The ANW model shows strong performance when evaluated against field measurements that meet strict quality criteria. Comparison of optimal model solutions and measured values for a dg ( λ ) shows negligible or small median bias (within a few percent) and a median percent difference below 10% for most of the spectrum. For a ph ( λ ), the median bias remains within ±10% and the median difference <20% over significant portions of the spectrum, with best statistics observed in the blue and red spectral regions of phytoplankton absorption maxima.