Derived Effective (Keff) Versus Scalar (K0) Attenuation in the Baltic Sea: Characterising Spectral Divergence and Physical Drivers

The optical complexity of shallow Case 2 waters challenges remote sensing accuracy due to the non-linear behaviour of optically active constituents. This study evaluates the spectral divergence between the target-derived effective attenuation (Keff) and the ambient scalar attenuation coefficient (K0) across 12 Baltic Sea locations. Using hyperspectral radiometry and K-Means clustering, three optical water types (OWTs) were identified. We demonstrate that the historical static approximation based on the diffuse attenuation coefficient (Keff ≈ 2Kd) is systematically biased in scattering-dominated environments. Our empirical results yielded a regional relationship of Keff = 2.33K0 (R2 = 0.65); however, residual analysis reveals that linear multipliers fail to capture non-linear light decay. Random Forest regression identified total suspended matter (TSM) as the primary driver of Keff variance (28.0%), confirming that “geometric rejection” of scattered photons artificially inflates signal loss in turbid waters. This divergence is most pronounced in the 500–650 nm range, where low absorption facilitates multiple scattering events. We conclude that active remote sensing requires a sensor-fusion approach, utilising passive OWT classification to dynamically parameterise active attenuation models.