Parameterization of the light absorption properties of chromophoric dissolved organic matter in the Baltic Sea and Pomeranian Lakes
Abstract. This study presents three alternative models for estimation of absorption properties of Chromophoric Dissolved Organic Matter, aCDOM(l). For this analysis we used a database containing 556 absorption spectra measured in 2006–2009 in different regions of the Baltic Sea (open and coastal waters, the Gulf of Gdańsk and the Pomeranian Bay), at river mouths, in the Szczecin Lagoon and also in three Pomeranian lakes in Poland – Lakes Obłęskie, Łebsko and Chotkowskie. Observed variability range of the CDOM absorption coefficient at 400 nm, aCDOM(400), contained within 0.15–8.85 m−1. The variability in aCDOM(l) was parameterized with respect to three orders of magnitude variability in the chlorophyll a concentration Chla (0.7–119 mg m−3). Chlorophyll a concentration and CDOM absorption coefficient, aCDOM(400) were correlated, and statistically significant, non-linear empirical relationship between those parameters was derived (R2 = 0.83). Based on observed co-variance between these parameters, we derived two empirical mathematical models that enabled to project the CDOM absorption coefficient dynamics in natural waters and reconstruct the completed CDOM absorption spectrum in the UV and visible spectral domains. The first model used the chlorophyll a concentration as the input variable. The second model used the aCDOM(400), as the input variable. Both models were fitted to power function and the second order polynomial function was used as the exponent. Regression coefficients for derived formulas were determined for wavelengths from 240 to 700 nm at 5 nm intervals . Both approximation reflected the real shape of the absorption spectra with low uncertainty. Comparison of these approximation with other models of light absorption by CDOM proved that proposed parameterizations were better (bias from −1.45 % to 62 %, RSME from 22 % to 220 %) for estimation CDOM absorption in optically complex waters of the Baltic Sea and lakes.