Abstract
To investigate the seasonal and interannual dynamics of dissolved organic matter (DOM) in the Yangtze Estuary, surface and bottom water samples in the Yangtze Estuary and its adjacent sea were collected and characterized using fluorescence excitation-emission matrices (EEMs) and parallel factor analysis (PARAFAC) in both dry and wet seasons in 2012 and 2013. Two protein-like components and three humic-like components were identified. Three humic-like components decreased linearly with increasing salinity (r>0.90, p<0.001), suggesting their distribution could primarily be controlled by physical mixing. By contrast, two protein-like components fell below the theoretical mixing line, largely due to microbial degradation and removal during mixing. Higher concentrations of humic-like components found in 2012 could be attributed to higher freshwater discharge relative to 2013. There was a lack of systematic patterns for three humic-like components between seasons and years, probably due to variations of other factors such as sources and characteristics. Highest concentrations of fluorescent components, observed in estuarine turbidity maximum (ETM) region, could be attributed to sediment resuspension and subsequent release of DOM, supported by higher concentrations of fluorescent components in bottom water than in surface water at two stations where sediments probably resuspended. Meanwhile, photobleaching could be reflected from the changes in the ratios between fluorescence intensity (Fmax) of humic-like components and chromophoric DOM (CDOM) absorption coefficient (a355) along the salinity gradient. This study demonstrates the abundance and composition of DOM in estuaries are controlled not only by hydrological conditions, but also by its sources, characteristics and related estuarine biogeochemical processes.
Highlights
Dissolved organic matter (DOM), ubiquitous in aquatic environment, is a complex mixture of organic substances originating from a wide range of allochthonous and autochthonous sources [1]
Even though fluorescent DOM (FDOM) could be produced by microbial conversion of non-FDOM [68], the lag time between primary production by phytoplankton and FDOM production through bacterial conversion may be longer than the study period and the water retention time of the Yangtze Estuary [66]
The results suggest that the composition and transport of DOM from river to ocean would be influenced by the changing climate, especially under extreme drought and flooding
Summary
Dissolved organic matter (DOM), ubiquitous in aquatic environment, is a complex mixture of organic substances originating from a wide range of allochthonous and autochthonous sources [1]. DOM constitutes the largest reduced carbon reservoir in aquatic ecosystems [2]. Estuaries link the two largest organic carbon reservoirs (the land and the ocean), and discharge DOM and particulate organic matter (POM) through rivers [3]. Biogeochemical processes that occur in estuaries influence the transfer of DOM between these two large pools [3, 4]. Sediment resuspension, flocculation, particle adsorption and POM dissolution occur in the so-called estuarine turbidity maximum (ETM) region of many large estuaries [10,11,12,13,14,15], influencing the balance between DOM and POM pools [10, 14]
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