Optical Properties of Dissolved Organic Matter and Controlling Factors in Dianchi Lake Waters

Abstract

Characterization of dissolved organic matter (DOM) is useful in understanding environment quality and carbon cycling in the lake system. In this study, the fluorescence of DOM, major ions, and nutrients in water were investigated to understand the sources and the transformation of DOM in Dianchi Lake, the sixth largest freshwater lake in China. The dissolved organic carbon content in water above the deposition layer was higher than 5 mg C∙L−1 but lower than that in pore water. Two primary components of humic (C1) and protein-like components (C2) were identified using parallel factor (PARAFAC) modeling on sample fluorescence spectra. Organic components were related to mineral structures, and encapsulation of bacterial or algal cells into particulates could be disintegrated to release DOM. The aromaticity and the hydrophobicity of optical properties were regulated by percentages of chromophores (CDOM) of DOM in surface water, whereas by percentages of fluorophores (FDOM) in DOM in pore water, the underlying water layer was defined as a belt of transition. The molecular weight enhanced with percentages of C1 in CDOM increased in water above the sediment layer and the pore water at the northern lake site, but molecular weight attenuated with percentages decreased in pore water at the southern lake site. DOM not only originated from particulate decomposition but also derived from internal transformation among different, dissolved organic molecules. Small molecules were aggregated into larger ones, and, conversely, large molecules decomposed into small sizes. Another speculation is that dissolved molecules adsorbed or were encapsulated into particulates or were degraded and released into dissolved phases. The precise factors regulated composition, structure, and spectral properties of dissolved organic matter in the Dianchi Lake. This study highlights that sources of DOM and transformation mechanisms in the lake water could be correlated with nutrients and primary geochemical factors for mobility and distribution in different water compartments.

Spatial Variations of Trace Metals and Their Complexation Behavior with DOM in the Water of Dianchi Lake, China

“…Temperature in pore water was on average 17.3 • C, which was 2.8 • C higher than that in surface and underlying waters. Water above deposition layer was weakly basic with pH ranging from 7.12-9.07, while there was a narrower range of 7.24-8.48 in pore water [24]. The milliequivalent percentages of Ca 2+ and Na + in cations were more than 61%, especially at inlets that the proportion was 74.3%, while the proportion values of Mg 2+ and K + were lower.…”

“…Sediments were sampled with the sediment column sampler to extract the solid core at DC-2 and DC-9 sampling sites in the central lake area (Figure 1). Sediments were collected with a sediment cylindrical sampler to extract the solid core at DC-2 of the northern lake and DC-9 of the southern lake [24]. Sediments were sliced at 3-cm intervals above 9-cm sedimentation and every 5-cm as the slicing unit from 9-cm downward to 49-cm.…”

“…Slope ratio of S 275-295 /S 350-400 (S R ) has been shown to be sensitive to characterizing chromophore dissolved organic matter (CDOM) in natural water, with lower values indicative of DOM higher molecular weight, greater aromaticity and vascular plant input [17,24]. UV-Vis absorbance spectra (200-600 nm, slit 0.5 nm) were recorded using a double beam UV-T9 CS spectrophotometer (T9, Persee Company, Beijing, China) on 1-cm quartz cuvette, the ultrapure water being used as reference solution.…”

“…Public Health 2019, 16, 4919 3 of 20and beyond the 1970 s. Most of all, it provides an important water source for around 6 million residents in the Kunming City. In a semi-enclosed state for a long time, DC is faced with increasingly eutrophication and heavy metal pollution due to sewage input, which is not sufficiently exchangeable with clean water in time, and these environmental issues have become more severe [22][23][24]. The deterioration of the water quality has pervasively threatened the water quality of people living around the lake in recent years [23,24].…”

“…In a semi-enclosed state for a long time, DC is faced with increasingly eutrophication and heavy metal pollution due to sewage input, which is not sufficiently exchangeable with clean water in time, and these environmental issues have become more severe [22][23][24]. The deterioration of the water quality has pervasively threatened the water quality of people living around the lake in recent years [23,24]. In order to explore the inherent mechanisms of removal or fixation, research in the aspect of nutrients and organic matter intermediating trace metals' mobility during their biogeochemical cycles could be an effective approach, but it is scarcely reported.…”