USING SUBMERSIBLE FLUORIMETER TO STUDY SEASONAL DYNAMICS OF CHLOROPHYLL “a” IN THE SOUTHEASTERN PART OF THE BALTIC SEA

Abstract

Chlorophyll “a” concentration is a parameter used in environmental monitoring. The aim of this study was to identify seasonal dependencies between chlorophyll “a” values measured by directly method (extractively, spectrophotometrically) and by submersible fluorimeter and to improve them by taking into account other water parameters. The study was done during five cruises from 2023 to 2025 in the Russian sector of the southeastern part of the Baltic Sea. A comparison of submersible fluorimeter data with direct measurements revealed a weak correlation, particularly during periods of active phytoplankton growth (spring and summer) in the photic layer, due to underestimation of the submersible fluorimeter sensor values. Introducing turbidity and temperature variables into the equations allowed the determination coefficients to be nearly doubled for the warm period. In November and December, the correlation remained at the same level, which is due to initially low chlorophyll “a” values in this period. The obtained data highlight the need to clarify the regional uncertainty of measurements obtained using the submersible fluorimeter for future use in monitoring the Baltic Sea.