Spectral Proxy Parameter Linking Remote Sensing Reflectance and Inherent Optical Properties in Case-2 Waters

A spectral proxy parameter linking remote sensing reflectance (Rrs) and inherent optical properties (IOPs) was evaluated for six selected areas of the Baltic Sea. Measured IOPs from the individual optical datasets were used to calculate Morel’s unscaled proxy reflectance, whereas area and wavelength specific median IOPs were used as representative inputs to Monte Carlo radiative transfer simulations. Comparison of the individual proxy estimates with the corresponding area-specific Monte Carlo reference reflectance enabled determination of the spectral proxy parameter k. The results demonstrate a clear wavelength dependence and substantial area to area variability of this parameter. The purpose of the study is not to introduce a new fundamental IOPs–Rrs relationship, but to quantify how the effective scaling coefficient of a widely used proxy relationship varies spectrally and regionally in optically complex Case-2 waters. For each investigated Baltic Sea area, the spectral variability of k was approximated using a three-coefficient second-order polynomial. Area-specific parameterizations provide compact descriptions of the regional median k spectra, while a generalized formulation describes only the pooled central tendency. A radiometric evaluation was performed using 24 in situ OCP-100 spectra from the Gulf of Gdańsk (N = 96 wavelength-level pairs). Predicted and measured Rrs yielded r = 0.680, R2 = 0.462, a regression slope of 0.258, a mean bias of −0.00475 sr−1, an RMSE of 0.00590 sr−1, and a MAPE of 62.1%. Within-wavelength correlations were weak at 555 and 620 nm (r = 0.140 and 0.067, respectively). Thus, the regional parameterization preserves broad spectral ordering but underestimates absolute Rrs and does not establish strong station-level predictive skill. The results should be interpreted as a regional, configuration-dependent spectral scaling framework.