Evaluation of the Suspended Particulate Matter Content in the Coastal Zone of the Northeastern Black Sea Based on Relative Transparency

Abstract—Based on the materials of 20 expeditions conducted by the Shirshov Institute of Oceanology (IO RAS) in various areas of the coastal zone of the Russian sector of the Black Sea from 1999 to 2021, empirical equations of the relationship between the relative transparency (Secchi depth, Zd) and mass concentrations of the total suspended particulate (SPM), suspended mineral (SMM), and suspended organic matter (SOM) fractions (Cspm, Csmm, and Csom, respectively) were obtained. The Zd values were recorded in the range from 0.1 to 21 m; SMM concentrations, from 0.11 to 104 mg/L; SOM, from 0.09 to 2.0 mg/L; and chlorophyll a (Ca), from 0.03 to 4.4 µm/L. The relative abundance of SOM in SPM ranged from 1.1 to 48.7%. The calculation error for these equations is estimated. The determination coefficients r² = 0.85 (for logCspm and logCsmm) and 0.72 (for logCsom). The influence of the SOM concentration on the accuracy in calculating Ssmm and chlorophyll a concentration on the accuracy in calculating Csom was revealed. It is concluded the empirical formulas can be used with respect to the entire dataset to calculate the concentrations of these SPM fractions in the entire studied water area. It is proposed to consider the method for estimating the content of SPM fractions using the obtained empirical equations as a regional variant of passive sounding of the upper layer of the sea with a Secchi disk.