Meilian Chen

Dynamic exchanges between DOM and POM pools in coastal and inland aquatic ecosystems: A review.

Dynamic exchanges between dissolved organic matter (DOM) and particulate organic matter (POM) plays a critical role in organic carbon cycling in coastal and inland aquatic ecosystems, interactions with aquatic organisms, mobility and bioavailability of pollutants, among many other ecological and geochemical phenomena. Although DOM-POM exchange processes have been widely studied from different aspects, little to no effort has been made to date to provide a comprehensive, mechanistic, and micro-spatial schema for understanding various exchange processes occurring in different aquatic ecosystems in a unified way. The phenomena occurring between DOM and POM were explained here with the homogeneous and heterogeneous mechanisms. In the homogeneous mechanism, the participating components are only organic matter (OM) constituents themselves with aggregation and dissolution involved, whereas OM is associated with other components such as minerals and particulate colloids in the heterogeneous counterpart. Besides the generally concerned processes of aggregation/dissolution and adsorption/desorption, other ecological factors such as sunlight and organisms can also participate in DOM-POM exchanges through altering the chemical nature of OM. Despite the limitation of current analytical technologies, many unknown and/or unquantified processes need to be identified to unravel the complicated exchanges of OM between its dissolved and particulate states. Based on the review of several previous mathematical models, we proposed a unified conceptual model to describe all major dynamic exchange mechanisms on the basis of exergy theory. More knowledge of dynamic DOM-POM exchanges is warranted to overcome the potential problems arising from a simple division of OM into dissolved versus particulate states and to further develop more sophisticated mathematic models.

Effects of dissolved organic matter (DOM) sources and nature of solid extraction sorbent on recoverable DOM composition: Implication into potential lability of different compound groups.

AbstractsNoting the source-dependent properties of dissolved organic matter (DOM), this study explored the recoverable compounds by solid phase extraction (SPE) of two common sorbents (C18 and PPL) eluted with methanol solvent for contrasting DOM sources via fluorescence excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Fresh algae and leaf litter extracts DOM, one riverine DOM, and one upstream lacustrine DOM were selected for the comparison. C18 sorbent was generally found to extract more diverse molecular formula, relatively higher molecular weight, and more heteroatomic DOM compounds within the studied mass range than PPL sorbent except for the leaf litter extract. Even with the same sorbent, the main molecular features of the two end member DOM were distributed on different sides of the axes of a multivariate ordination, indicating the source-dependent characteristics of the recoverable compounds by the sorbents. In addition, further examination of the molecular formula uniquely present in the two end members and the upstream lake DOM suggested that proteinaceous, tannin-like, and heteroatomic DOM constituents might be potential compound groups which are labile and easily degraded during their mobilization into downstream watershed. This study provides new insights into the sorbent selectivity of DOM from diverse sources and potential lability of various compound groups.

Production of fluorescent dissolved organic matter in Arctic Ocean sediments

Little is known about the production of fluorescent dissolved organic matter (FDOM) in the anoxic oceanic sediments. In this study, sediment pore waters were sampled from four different sites in the Chukchi-East Siberian Seas area to examine the bulk dissolved organic carbon (DOC) and their optical properties. The production of FDOM, coupled with the increase of nutrients, was observed above the sulfate-methane-transition-zone (SMTZ). The presence of FDOM was concurrent with sulfate reduction and increased alkalinity (R2 > 0.96, p < 0.0001), suggesting a link to organic matter degradation. This inference was supported by the positive correlation (R2 > 0.95, p < 0.0001) between the net production of FDOM and the modeled degradation rates of particulate organic carbon sulfate reduction. The production of FDOM was more pronounced in a shallow shelf site S1 with a total net production ranging from 17.9 to 62.3 RU for different FDOM components above the SMTZ depth of ca. 4.1 mbsf, which presumably underwent more accumulation of particulate organic matter than the other three deeper sites. The sediments were generally found to be the sources of CDOM and FDOM to the overlying water column, unearthing a channel of generally bio-refractory and pre-aged DOM to the oceans.

Spectroscopic and molecular characterization of humic substances ( HS) from soils and sediments in a watershed: comparative study of HS chemical fractions and the origins

Optical properties and molecular composition of humic substances (HS) can provide valuable information on the sources and the history of the associated biogeochemical processes. In this study, many well-known spectral and molecular characteristics were examined in eight different HS samples, which were extracted from soils and sediments located in a forested watershed, via two advanced tools including fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC) and high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Two humic-like (C1 and C2) and one protein-like (C3) components were identified from EEM-PARAFAC. Irrespective of the origins, humic acid (HA) fractions were distinguished from fulvic acid (FA) fractions by the HS characteristics of specific UV absorbance (SUVA), the number of formulas, maximum fluorescence intensities of C1 and C2, condensed aromatics, tannins, and CHON, CHOS, and CHONS classes. In contrast, only five HS indices, including C3 intensity, H%, modified aromatic index (AImod), the percentages of carbohydrates, and unsaturated hydrocarbons, were found to be significant factors in discriminating between the two HS origins (i.e., soils and sediments). The ordination of the Bray-Curtis dissimilarity matrix further confirmed that the HS chemical fraction (i.e., HA or FA) was the more important factor to determine the measured HS characteristics than the HS origin. Our results provided an in-depth insight into the chemical and structural heterogeneity of bulk HS, which could be even beyond the differences observed along the two HS origins. This study also delivers a cautious message that the two operationally defined HS chemical fractions should be carefully considered in tracking the origins of different HS samples.

Molecular diversity of riverine alkaline-extractable sediment organic matter and its linkages with spectral indicators and molecular size distributions

Few studies have been conducted to examine the spatial heterogeneity of riverine sediment organic matter (SOM) at the molecular level. The present study explored the chemical and molecular heterogeneity of alkaline-extractable SOM from riverine sediments via multiple analytical tools including molecular composition, absorption and fluorescence spectra, and molecular size distributions. The riverine SOM revealed complex and diverse characteristics, exhibiting a great number of non-redundant formulas and high spatial variations. The molecular diversity was more pronounced for the sediments affected by a higher degree of anthropogenic activities. Unlike the cases of aquatic dissolved organic matter, highly-unsaturated structures with oxygen (HUSO) of SOM were more associated with the spectral and size features of humic-like (or terrestrial) substances than aromatic molecules were, cautioning the interpretation of the SOM molecules responsible for apparent indicators. Noting that a higher detection rate (DR) produces fewer common molecules, the common molecules of 23 different SOMs were determined at a reasonable DR value of 0.35, which accounted for a small portion (5.8%) of all detected molecules. They were mainly CHO compounds (>98%), which positively correlated with spectral indicators of biological production. Despite the low abundance, however, the ratios of aromatic to aliphatic substances could be indexed to classify the common molecules into several geochemical molecular groups with different degrees of the associations with the apparent spectral and size indicators.

Dynamics of dissolved organic matter in riverine sediments affected by weir impoundments: Production, benthic flux, and environmental implications

In order to understand the characteristics and dynamics of dissolved organic matter (DOM) in the sediment of rivers affected by impoundments, we examined the vertical profiles and the benthic fluxes of DOM in four different core sediments located at upstream sites of weirs in major rivers of South Korea. In three out of four sites, exponential accumulation of dissolved organic carbon (DOC) with depth was observed with the signature of seasonal variability. Except for the site displaying a below-detection limit of Fe(II), the general accumulation trends of DOC with depth was concurrent with the increases of Fe(II) and NH4+ and the decrease of PO43-, signifying a close linkage of the DOM dynamics with anaerobic respiration via iron reduction, an important early diagenesis pathway. The estimated benthic fluxes from the cores revealed that the sediments likely serve as DOC, chromophoric DOM (CDOM), and fluorescent DOM (FDOM) sources to the overlying water. The benthic effluxes based on DOC were comparable to the ranges previously reported in lake and coastal areas, and those of CDOM and FDOM showed even higher levels. These findings imply that impoundment-affected river systems would change the DOM composition of the overlying water, ultimately influencing the subsequent water treatment processes such as disinfection byproducts production and membrane fouling. A simple mass balance model indicated that the impoundment-affected river sediments may operate as a net carbon sink in the environments due to a greater extent of sedimentation compared to the estimated benthic efflux and sediment biological respiration.

High abundance of protein-like fluorescence in the Amerasian Basin of Arctic Ocean: Potential implication of a fall phytoplankton bloom

The seawater samples from the Chukchi and East Siberian Seas were collected along a shelf-slope-basin gradient and analyzed for chromophoric and fluorescent DOM (i.e., CDOM and FDOM, respectively). Unexpected high protein-like FDOM (0.35±0.40 and 0.24±0.34 RU for peaks B and T, respectively) levels were identified, which corresponded to 1-2 orders of magnitude higher than those documented by previous reports. This unique phenomenon could be attributed to a fall phytoplankton bloom. The seawater chl-a data, estimated from in situ fluorescence measurements and satellite remote sensing data, showed the subsurface chl-a maximum of up to 1.52mgm-3 at ~25-70m depths and the surface monthly average values (August 2015) up to 0.55 to 0.71mgm-3, which fall in the range of ~0.5-2.0mgm-3 during fall phytoplankton blooms in this area. Meanwhile, the depth profile of DOM parameters revealed subsurface maxima of protein-like fluorescence peaks along the shelf-slope gradient. The positive correlations between the protein-like peaks and biological index implied the lateral transport of DOM and nutrients from the shelf to the slope and basin. Despite still being a largely ice-covered environment, potential shifts in the ecosystem appear to make progress in response to changing climate in the Arctic Ocean.

Biological early diagenesis and insolation-paced paleoproductivity signified in deep core sediment organic matter

The dynamics of a large stock of organic matter contained in deep sediments of marginal seas plays pivotal role in global carbon cycle, yet it is poorly constrained. Here, dissolved organic matter (DOM) in sediments was investigated for core sediment up to ~240 meters deep in the East/Japan Sea. The upper downcore profile (≤118 mbsf, or meters below seafloor) at a non-chimney site (U1) featured the exponential production of dissolved organic carbon (DOC) and optically active DOM with time in the pore water above sulfate-methane-transition-zone (SMTZ), concurrent with the increases of nutrients and alkalinity, and the reduction of sulfate. Such depth profiles signify a biological pathway of the DOM production during the early diagenesis of particulate organic matter presumably dominated by sulfate reduction. Below the SMTZ, an insolation-paced oscillation of DOM in a ~405-Kyr cycle of orbital eccentricity was observed at site U1, implying astronomically paced paleoproductivity stimulated by light availability. Furthermore, DOM dynamics of the deep sediments were likely governed by intensive humification as revealed by the less pronounced protein-like fluorescence and the lower H/C and O/C ratios below SMTZ among 15,281 formulas identified. Our findings here provide novel insights into organic matter dynamics in deep sediments.

Exploring pore water biogeochemical characteristics as environmental monitoring proxies for a CO2 storage project in Pohang Basin, South Korea

Biogeochemical parameters of pore waters, including dissolved organic matter, nutrients, sulfate, alkalinity, and chloride are explored as convenient and sensitive proxies to monitor the CO2 geological storage sites. Five sites for a CO2 storage project in the Pohang Basin of the East Sea in South Korea were investigated for the pre-injection biogeochemical conditions of these sites. Higher dissolved organic carbon (~36 mg L-1), chromophoric and fluorescent dissolved organic matter, nutrients, and alkalinity were observed in a fluvially affected acoustic blanking site with geological faults. A general increasing downcore trend of measured DOM parameters, nutrients, and alkalinity with depth was found at the acoustic blanking site affected by riverine runoff with significant correlations among the parameters (R2: ~0.4-0.8), highlighting the impact of geological features and external inputs on the downcore biogeochemical properties. The results presented in this study suggest that DOM could be utilized as a robust and complementary biogeochemical parameter.