Seasonal Variations in Molecular Size of Chromophoric Dissolved Organic Matter From the Lower Changjiang (Yangtze) River

Large world rivers and their dissolved organic matter (DOM) fluxes could regulate ecosystem function and biogeochemical processes in coastal marine environments. Knowledge about the seasonal variations in composition and molecular size of DOM remains scarce but is important to a better understanding of the role of river fluxes. Monthly time-series (July 2018 to June 2019) surface water samples were collected from the lower Changjiang (Yangtze) River, one of the largest world rivers, for the measurements of both dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) to elucidate seasonal changes in their abundance, optical properties, and molecular size distributions among the <1 kDa, 1–3 kDa, 3–10 kDa, and 10 kDa–0.7 μm size fractions. On average, the <1 kDa low-molecular weight DOM made up about two thirds of the bulk CDOM, leaving the other one third in the 1 kDa–0.7 μm size fraction. Optical properties and molecular size of DOM transported by the Changjiang River had an evident seasonality. In general, DOM exported from the Changjiang River can be characterized as lower DOC, lower aromaticity, and lower colloidal abundance compared to other world rivers. Together with available literature data, our results show that world rivers with higher/lower DOC concentrations and higher/lower aromaticity also contain more/less colloidal (>1 kDa) DOM. Our year-long time-series study provides additional valuable baseline data for a better understanding of seasonal changes in the abundance, composition, and molecular weight distribution of DOM from the lower Changjiang River and the potential impacts on the biogeochemical cycling of DOM in the adjacent estuarine, coastal, and shelf areas.