Catchment Influence on the Seasonal Dynamics of Dissolved Organic Matter (DOM) Optical Properties in Small Lakes

Dissolved organic matter (DOM) plays an important role in biogeochemical cycling and water quality in lake ecosystems, and its composition is strongly influenced by catchment characteristics and anthropogenic pressure. Understanding seasonal variability in DOM is therefore important for assessing ecological processes in small lakes. The aim was to assess the optical properties and seasonal variability of DOM in small (<50 ha) lakes differing in morphometry and catchment characteristics. Samples were collected in spring, summer and autumn. The quality of the DOM was assessed using UV–Vis spectroscopy and synchronous scan fluorescence spectra (SSF), in addition to measurements of nutrients and basic physicochemical parameters. UV–Vis indices (SUVA270, a*355 and E250:E365) revealed a steady increase in aromaticity, chromophore content and degree of DOM humification throughout the season. In landscape lakes, fluorescence parameters followed a typical seasonal pattern: shifting from protein-like fractions in the spring to fulvic acid-like fractions in the summer and autumn. A different pattern of optical changes was observed in a lake subject to anthropogenic pressure, where a marked increase in protein-like fluorescence was recorded in the autumn, indicating an influx of fresh, reactive organic matter (DOM). These results demonstrate that optical properties of DOM are sensitive indicators of seasonal variability and differences among lakes with contrasting environmental characteristics. The combined optical approach provides useful information on changes in DOM quality and composition and may support the monitoring of ecological processes in small lake ecosystems.