Comparison of hyperspectral measurements of the attenuation and scattering coefficients spectra with modeling results in the north-eastern Baltic Sea

•The power law describes particle attenuation spectra with high accuracy in the northeastern Baltic Sea.•Power law describes particle scattering spectra when Chl a concentration is low, i.e. below 5 mg m−3.•Principal component analysis enables decomposition of particle scattering spectra into mineral-type and algae-type spectrum.

The spectral variations in the attenuation and scattering coefficients measured with a hyperspectral ac-spectra (Wetlabs) instrument were analyzed from a dataset collected in the vicinity of commercial harbors on the Estonian coast of the Gulf of Finland, Baltic Sea. In total, the measured TSM concentration varied from 0.4 to 30 mg L−1 and the concentration of Chl a varied from values below the detection limit (0.05) to 23 mg m−3. The reliability of the power law describing the particle attenuation cp (λ) and scattering bp(λ) coefficients was evaluated by means of a determination coefficient (R2). The power law described the particle attenuation spectra with high accuracy (R2 > 0.67), giving the dataset an average cp (λ) slope of 1.3. In the case of particle scattering coefficients, the power law did not represent the whole dataset. Depending on a particular spectrum, the R2 varied from 0 to 1.0 and the slope varied from 1.15 to −0.56. Decomposition of bp(λ) into dominant modes using principal component analyses resulted in the first principal mode accounting for the power law dependence of bp(λ), i.e. the “mineral-type” spectrum, and the second and third mode representing the characteristic bp(λ) of dominant algal particles, i.e. the “algae-type” spectrum. From our dataset we estimated that if Chl a concentration is above 10 mg m−3 or below 5 mg m−3 then most likely the “algae-type” or the “mineral-type” spectrum is dominant, respectively. There was strong linear relationship (R2 > 0.92) between TSM concentration and cp(555) and bp(555),irrespective of the dominant shape of the particle scattering spectra. The estimated TSM-specific attenuation and scattering coefficients at 555 nm were 0.8 m2 g−1 and 0.68 m2 g−1, respectively. Corresponding values for water samples with a dominant “mineral-type” spectrum were 0.85 m2 g−1 and 0.73 m2 g−1, respectively and for water samples with a dominant “algae-type” spectrum were 0.64 m2 g−1 and 0.52 m2 g−1, respectively.