Assessing spatial and seasonal variations of dissolved organic matter by spectroscopic analysis in a stratified estuary: Yura River Estuary, Japan

Dissolved organic matter (DOM) discharged from terrestrial areas may play an important role in global carbon dynamics as well as in estuarine and coastal primary and heterotrophic productivity. Dynamics of DOM in estuarine areas may be affected by various biological processes, such as production of autochthonous DOM and degradation of DOM. A spectroscopic method, which is a three-dimensional excitation emission matrix (EEM) fluorescence analysis has been applied to evaluate the qualitative variation of DOM. Parallel factor (PARAFAC) analysis is a newly introduced statistical modeling method to quantitatively decompose EEMs into individual fluorescent component. Combination of EEMs and PARAFAC analysis is a useful tool for assessing the dynamics of DOM. However our knowledge about the dynamics of individual PARAFAC component in esturine environment is still limited. The Yura River estuary is a typical microtidal estuary, which flows into the Sea of Japan. The estuary is characterized by the seasonal salt-wedge intrusion: In summer and autumn, seawater intrudes into the river approximately 20 km upstream from the river mouth due to the generally low water discharge during the season except for storm-induced sudden increase. In contrast, due to the high precipitation and snowmelt, water discharge is higher and the seawater rarely intrudes into the river in winter and early spring. The main objective of this study is to assess the spatial and seasonal variations of individual fluorophore in the stratified estuarine environment by using EEM-PARAFAC method. Six sampling stations were set between the mouth of the Yura River and the location about 16 km upstream. Twenty sampling stations were set at 5, 10, 20 and 30 m depths in the coastal area. Sampling and observations were carried out seasonally from July 2010 to July 2011. Measurement of vertical profiles of salinity, temperature and chlorophyll fluorescence was conducted by a CTD profiler at each station. We collected surface water samples from all 26 stations, and water samples from different depths at 6 stations located in the river and at 4 coastal stations along the extension of the river. Concentrations of DOC were determined with a total organic carbon analyzer. EEM was measured by using a fluorescent spectrometer. After EEM data were standardized by the Raman correction, several peaks are extracted by the PARAFAC analysis. Our study shows a great variability of DOM in terms of quantity and quality. DOC concentration tended to be high when the water discharge was high during the snowmelt season and in the summer when there was a storm-induced increase in water discharge. Two different terrestrial humic-like components (component 1 and 2) were identified by the PARAFAC analysis and the ratio between the fluorescent intensities of two components was greatly varied seasonally. Our study highlights the applicability of EEM-PARAFAC method to investigate DOM dynamics in esturine environments. This study was conducted as a part of the Kibunka Project, FSERC, at Kyoto University.