Characteristics of dissolved organic matter in the inflows and in the outflow of Lake Võrtsjärv, Estonia

UV–Vis spectroscopy was applied to characterize the nature of dissolved organic matter (DOM) in the inflows and in the outflow of the large and shallow temperate Lake Võrtsjärv and to determine its temporal and spatial dynamics in 2008–2010. The concentration of dissolved organic carbon (DOC) was influenced by water temperature, precipitation, stream gradient, discharge, and percentage of peatland in the catchment area and its mean annual values by rivers varied from 9.2 to 27.7 mg L−1. Higher DOC values in the outflow in summer/autumn corresponded to periods of intensive phytoplankton development whereas lower values were observed in winter or early spring when biological processes are at a standstill. The results of specific ultraviolet absorption at 254 nm (SUVA254) and 285 nm (SUVA285) showed domination of fulvic acids in the rivers. According to our spectroscopic results, the aromaticity of DOM increased with the increasing percentage of peatland and forest and with the decreasing percentage of arable land in the river catchments.