Divergent Drivers of Effective (K_eff) and Diffuse (K_d) Attenuation in the Baltic Sea: Quantifying the Roles of TSM and CDOM in Case 2 Waters

Shallow, turbid coastal environments (Case 2 waters) challenge optical remote sensing due to the complex, non-covariant interaction between dissolved and particulate constituents. This study quantifies the relationship between the effective (K_eff) and the diffuse attenuation coefficient of downwelling irradiance (K_d) across 14 stations in the southern Baltic Sea, representing a transition from estuarine to open coastal waters. Using K-Means clustering and Random Forest regression, we characterised Optical Water Types (OWTs) and decoupled the specific drivers of attenuation. Results indicate that K_eff consistently exceeds K_d by a factor of 2-3, with the spectral ratio (K_eff/K_d) significantly surpassing the theoretical geometric limit of 2, particularly in the 500-650 nm window. Although total suspended matter (TSM) is the primary driver for both coefficients, K_eff exhibits heightened sensitivity to coloured dissolved organic matter absorption at 440 nm (a_CDOM (440)) due to the geometric rejection in the collimated beam; in contrast, K_d remains coupled to the broad-band scattering effects of phytoplankton. We conclude that assuming a fixed geometric relationship (K_eff ≈ 2K_d) leads to systematic errors in scattering-dominated waters, and propose a robust empirical relationship (K_eff ≈ 1.71K_d + 1.44; Pseudo R² = 0.4) to improve subsurface retrievals in shallow and optically complex coastal zones.