Seasonal Variations in Proton Binding Characteristics of Dissolved Organic Matter Isolated from the Southwest Baltic Sea

The physicochemical characteristics
of dissolved organic matter
(DOM) strongly influence its interactions with inorganic species such
as protons and trace elements in natural waters. We collected water
samples at Boknis Eck, a time series station in the Baltic Sea with
a low exposure to freshwater inputs, to investigate how seasonal fluctuations
impact the proton binding properties of the isolated DOM. We used
potentiometric titrations to assess the binding properties of solid-phase
extracted DOM (SPE–DOM) over a seasonal cycle. We report and
critically analyze the first NICA parameters estimates of carboxylic-like
and phenolic-like sites for brackish water SPE–DOM. The total
amount of functional groups (QmaxH,tot) showed no seasonal fluctuations and an average value of 136 ±
5.2 mmol·mol C–1. The average proton affinity
(logKH) and binding site heterogeneity
(m) showed a relatively minor variability for samples
obtained between April and September, when the water remained stratified.
These results contribute to a better understanding of the ion binding
characteristics of DOM in natural brackish waters.