Alpine wetlands and valleys of northwestern Sichuan are the main catchment areas of Minjiang River, where dissolved organic matter (DOM) in surface waters comes mainly from the natural background environment. Sources and structure parameters of DOM are important for calibrating the flux and pattern of organic carbon exports from plateau wetlands and alpine rivers. In this study, surface water samples along the upstream Minjiang River were collected at the end of dry season (April) and rainy season (October). Excitation emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) was used to characterize seasonal variations of DOM along Minjiang River. Results showed fluorescence peaks (humic-like peaks A and C, protein-like peaks B and T) were different along the river. Peak A and peak C were more obvious at the end of dry season, while peak B and peak T were more obvious at the end of rainy season. PARAFAC produced a three-component model including two humic-like components[C1 (250-260/380-480 nm) and C2 (300-330/380-480 nm)] and one protein-like component[C3 (270-280/300-350 nm)], accounting relative intensity 48.68%-65.02% for C1, 23.17%-29.83% for C2, and 11.83%-21.53% for C3. Fluorescence components showed variations along the river more prominently in April than October, in which the most significant one was C1. Average fluorescence index (FI) values ranged from 1.4 to 1.9, indicating that DOM consisted of both autochthonous and allochthonous components. Moreover, higher degrees of humification, aromaticity and hydrophobicity were found in April than those in October, suggesting more terrigenous sources at the end of dry season and more biological sources at the end of rainy season. Additionally, chromophoric dissolved organic matter (CDOM)[a(355)] correlated significantly with humic-like substance[Fn(355)], which also indicated that DOM components originated from terrigenous input in the upstream Minjiang River. The results also showed significant positive correlation between C1 and C2 in April, with no significant correlation in October, which further proved that exogenous input and seasonal variations characterized DOM sources in the upstream Minjiang River.
Keywords: EEM-PARAFAC; alpine river; dissolved organic matter (DOM); seasonal variation; the upstream of Minjiang River.