Distribution of suspended and dissolved matter and its absorption properties in the Gulf of Gdańsk (Baltic Sea) in the summer season

Analyses of the spatial and interannual variability of suspended particulate matter (SPM) and chromophoric dissolved organic matter (CDOM) concentrations were carried out in the waters of the Gulf of Gdańsk in the summer season in 2016–2020. Additionally, the absorption properties of those substances were analyzed. The inflowing waters of the Vistula River are characterized by high concentrations of SPM, chlorophyll a (Tchla) and CDOM, significantly affecting the absorption properties in the mixture of river and sea waters. The high contribution of particulate organic matter, POM, in the total SPM in the Gulf of Gdańsk (80% on average) indicates high phytoplankton productivity in the summer. High concentrations of pigments characteristic of different size classes of algae and cyanobacteria (fucoxanthin and zeaxanthin, chlorophyll b, peridinin and alloxanthin) were recorded, being markers of diatoms, cyanobacteria, green algae, dinophytes and cryptophytes, respectively. Analysis of interannual variability showed changes of SPM, Tchla and CDOM concentrations, depending on volume and direction of the river inflow and weather conditions. The composition of individual pigments changed year to year in a mosaic (heterogeneous) manner. The average contribution of CDOM, phytoplankton and detritus in the total light absorption was determined (at 443 nm – 50%, 34% and 16%, and at 675 nm – 41%, 54% and 5%). Spatial and temporal variability of the light absorption coefficients by suspended particles and CDOM in sea water was examined, and the relationships between the individual light absorption coefficients by sea water components were determined as a function of the dependence on SPM and Tchla.