Content and composition of natural organic matter in water of Lake Pitkjärv and mire feeding Kuke River (Estonia)

The seasonal and spatial changes of content and composition of dissolved organic carbon in water flowing out from bog were studied. Water was sampled from the Kuke River flowing out from Koigi Bog (eastern Saaremaa, Estonia) and in comparison the water samples from the bog lake (Lake Pitkjärv) were analyzed regularly during 18 months. Absorptions of water samples were measured at 250, 254, 285, 365 and 436 nm on UV–Vis spectrophotometer and pH, conductivity and concentration of dissolved organic carbon (DOC) were measured. Specific ultraviolet absorptions (SUVA) and the ratios of absorptions were calculated to characterize the nature of dissolved organic carbons. The significant changes of the concentration of DOC were determined in water sampled from the Kuke River during the dry days, but it was much steadier during rest sampling time. Analysis of water from the Lake Pitkjärv showed relatively high content of macromolecular organic compounds rich in aromatics (SUVA254 = 4.58 L mg−1 m−1), whereas water samples from the Kuke River contained predominantly more homogeneous low-molecular-weight materials poor in aromatics (SUVA254 = 2.96 L mg−1 m−1). Calculated values of SUVA285 confirmed that organic carbon in river water can be also originated from the decomposition of root litter. The values of ratio of UVA250/A365 for water samples from Kuke River indicated the fulvic nature of DOC as conformed by the SUVA254 and SUVA285 values. The values of A254/A436 for river were higher than 14 indicating that the organic matter in water from lake and river contained lots of UV absorbing functional groups.