Laodong Guo

University of Wisconsin-Milwaukee, School of Freshwater Sciences, Faculty Member

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Dr. Guo obtained his PhD degree in Chemical Oceanography from Texas A&M University and his MS degree in marine chemistry from Xiamen University, China. He did his postdoc in the Laboratory for Oceanographic and Environmental Research at Texas A&M University at Galveston. Dr. Guo was a research faculty at University of Alaska Fairbanks between 2000-2005, a faculty member in the Department of Marine Science at University of Southern Mississippi between 2005-2012. He has been a professor in the School of Freshwater Sciences at the University of Wisconsin-Milwaukee since 2012.
Phone: 414-382-1742
Address: 600 E Greenfield Ave.
Milwaukee, WI 53204, USA

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Edaphic Conditions Regulate Denitrification Directly and Indirectly by Altering Denitrifier Abundance in Wetlands along the Han River, China

Environmental Science & Technology, May 3, 2017

Riparian wetlands play a critical role in retaining nitrogen (N) from upland runoff and improving... more Riparian wetlands play a critical role in retaining nitrogen (N) from upland runoff and improving river water quality, mainly through biological processes such as soil denitrification. However, the relative contribution of abiotic and biotic factors to riparian denitrification capacity remains elusive. Here we report the spatio-temporal dynamics of potential and unamended soil denitrification rates in 20 wetlands along the Han River, an important water source in central China. We also quantified the abundance of soil denitrifying microorganisms using nirK and nirS genes. Results showed that soil denitrification rates were significantly different between riparian and reservoir shoreline wetlands, but not between mountain and lowland wetlands. In addition, soil denitrification rates showed strong seasonality, with higher values in August (summer) and April (spring) but lower values in January (winter). The potential and unamended denitrification rates were positively correlated with edaphic conditions (moisture and carbon concentration), denitrifier abundance, and plant species richness. Path analysis further revealed that edaphic conditions could regulate denitrification rates both directly and indirectly through their effects on denitrifier abundance. Our findings highlight that not only environmental factors, but also biotic factors including denitrifying microorganisms and standing vegetation, play an important role in regulating denitrification rate and N removal capacity in riparian wetlands.

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Intriguing changes in molecular size and composition of dissolved organic matter induced by microbial degradation and self-assembly

Water Research, May 1, 2018

Dissolved organic matter (DOM) is ubiquitous and plays an important role in regulating water qual... more Dissolved organic matter (DOM) is ubiquitous and plays an important role in regulating water quality, ecological function, and the fate and transport of trace elements and pollutants in aquatic environments. Both the colloidal precursors (i.e. <1 kDa) and bulk DOM collected from a freshwater estuary were incubated in the dark for 21 days to examine dynamic changes in molecular size and composition induced by microbial degradation and self-assembly. Results showed that the concentrations of total organic carbon, carbohydrates, and protein-like substances decreased by 11-30% during incubation, while those of humic-and fulvic-like substances remained relatively constant, indicating humic substances are more resistant to microbial utilization compared to carbohydrates and protein-like DOM. Despite the different extents in decline, these DOM components had a similar transformation pathway from the <1 kDa to colloids (1 kDa-0.45 µm) and further to microparticles (>0.45 µm). Overall, carbohydrates and protein-like substances, especially the high molecular weight components, were preferentially decomposed by microorganisms whereas humic-and fulvic-like DOM components significantly coagulated through abiotic self-assembly. The contrasting degradation/transformation pathways between the humic-like and protein-like substances along the size continuum, as also characterized by flow field-flow fractionation analysis, demonstrated that the dynamic transformation and degradation of DOM is regulated by both molecular size and organic composition. This finding provides new insights into the biogeochemical cycling pathways of heterogeneous DOM and its environmental fate and ecological role in aquatic systems.

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Molecular size-dependent abundance and composition of dissolved organic matter in river, lake and sea waters

Water Research, Jun 1, 2017

Dissolved organic matter (DOM) is ubiquitous in natural waters. The ecological role and environme... more Dissolved organic matter (DOM) is ubiquitous in natural waters. The ecological role and environmental fate of DOM are highly related to the chemical composition and size distribution. To evaluate sizedependent DOM quantity and quality, water samples were collected from river, lake, and coastal marine environments and size fractionated through a series of micro-and ultra-filtrations with different membranes having different pore-sizes/cutoffs, including 0.7, 0.4, and 0.2 mm and 100, 10, 3, and 1 kDa. Abundance of dissolved organic carbon, total carbohydrates, chromophoric and fluorescent components in the filtrates decreased consistently with decreasing filter/membrane cutoffs, but with a rapid decline when the filter cutoff reached 3 kDa, showing an evident size-dependent DOM abundance and composition. About 70% of carbohydrates and 90% of humic-and protein-like components were measured in the <3 kDa fraction in freshwater samples, but these percentages were higher in the seawater sample. Spectroscopic properties of DOM, such as specific ultraviolet absorbance, spectral slope, and biological and humification indices also varied significantly with membrane cutoffs. In addition, different ultrafiltration membranes with the same manufacture-rated cutoff also gave rise to different DOM retention efficiencies and thus different colloidal abundances and size spectra. Thus, the sizedependent DOM properties were related to both sample types and membranes used. Our results here provide not only baseline data for filter pore-size selection when exploring DOM ecological and environmental roles, but also new insights into better understanding the physical definition of DOM and its size continuum in quantity and quality in aquatic environments.

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Dissolved and particulate organic carbon dynamics in the lower Changjiang River on timescales from seasonal to decades: Response to climate and human impacts

Journal of Marine Systems, Mar 1, 2023

$Research paper thumbnail of Specific PhytOC fractions in rice straw and consequent implications for potential of phytolith carbon sequestration in global paddy fields$

Specific PhytOC fractions in rice straw and consequent implications for potential of phytolith carbon sequestration in global paddy fields

Science of The Total Environment, 2023

Chemical Speciation of Phosphorus in Arctic Sediments

Abstracts of Papers of The American Chemical Society, 2009

The bio- and thermal lability of dissolved organic matter as revealed by high-resolution mass spectrometry and thermal chemical analyses

Marine Chemistry, Mar 1, 2023

Isotope Composition of Organic Matter in Seawater

CRC Press eBooks, Jun 9, 2009

A scientific synthesis and assessment of the Arctic Carbon Cycle

Eos, Transactions American Geophysical Union, Jun 26, 2007

ABSTRACT

Composition and lability of riverine dissolved organic matter: Insights from thermal slicing ramped pyrolysis GC–MS, amino acid, and stable isotope analyses

Organic Geochemistry, Nov 1, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The efficiency of sequential extraction of phosphorus in soil and sediment: insights from the oxygen isotope ratio of phosphate

Journal of Soils and Sediments, Dec 10, 2019

Purpose Phosphorus (P) occurs in different chemical forms in soil and sediment, and they play dif... more Purpose Phosphorus (P) occurs in different chemical forms in soil and sediment, and they play different ecological and environmental roles. Thus, separation and quantification of different P pools are essential to the understanding of P dynamics in the environment. Over the past decades, different sequential extraction methods have been used concurrently. However, systematic evaluation of different extraction methods is lacking, making choice of methods and comparisons between results difficult. Materials and methods We compared the oxygen isotope compositions (δ 18 O P ) of different phosphate pools that were extracted by two major extraction methods, Hedley (Hedley et al., Soil Sci Soc Am J 46:970-976, 1982 ) and SEDEX methods (Ruttenberg, Limnol Oceanogr 37:1460-1482, 1992 ), for both a soil and an estuarine sediment sample. Additionally, Ag 3 PO 4 and KH 2 PO 4 standards were used to test the fidelity of the extraction and purification procedure. The purity of the precipitated Ag 3 PO 4 was also examined. Results and discussion The δ 18 O P values have large differences among phosphate pools for the soil extracted by the Hedley method and for the sediment extracted by the SEDEX method. The δ 18 O P values of Ag 3 PO 4 and KH 2 PO 4 standards after extraction, purification, and/or re-precipitation are similar to their original δ 18 O P values. The XRD patterns of the precipitated Ag 3 PO 4 are identical to that of Ag 3 PO 4 standard. The residual carbon and nitrogen contents in the precipitated Ag 3 PO 4 are low. Conclusions The Hedley method has good efficiency for separating isotopically distinct P i pools (different sources/bioavailability) in the soil samples, while the SEDEX method has good efficiency for separating isotopically distinct P i pools (different sources/bioavailability) in sediments that are more marine in character such as for the estuarine sample. The extraction and purification processes do not alter δ 18 O P , and the precipitated Ag 3 PO 4 has high purity.

Elucidating the Hidden Nonconservative Behavior of DOM in Large River‐Dominated Estuarine and Coastal Environments

Journal Of Geophysical Research: Oceans, Jun 1, 2019

Understanding the mixing behavior and biogeochemical cycling of heterogeneous dissolved organic m... more Understanding the mixing behavior and biogeochemical cycling of heterogeneous dissolved organic matter (DOM) at the land-ocean interface has been a major challenge, especially in large river-dominated estuarine and coastal environments with multiple and complex end-members of riverwater and seawater. Significant correlations and thus an "apparent" conservative behavior are generally observed for the bulk DOM when plotting concentration-salinity relationships, but the relatively small deviations from the apparent conservative behavior are difficult to be quantified. In July 2016, the flood in the Changjiang River basin caused the Changjiang Diluted Water (CDW) extended widely to the western East China Sea (ECS), making it a natural laboratory to examine the river-sea mixing behavior of riverine DOM and other optical properties. Here, a new biogeochemical approach was established to elucidate their hidden non-conservative behavior in the study area. Comparisons in the bulk DOC and the three chromophoric DOM (CDOM) properties among 13 pairs of same-salinity patches (with similar salinities but quite different distances from the river mouth) revealed that the ECS shelf was a significant sink for all the three CDOM properties. The CDW surface branches transporting southwards tended to serve as a sink for the bulk DOC while the ones transporting northwards were less a sink or even a source. The different removal extents of various biogeochemical constituents can also be calculated from the same-salinity patch comparisons. Our results obtained by the new approach provide detailed magnitudes of sink or source for different DOM properties and their in situ removal/production extents, giving new insights into biogeochemical processes in large river-dominated estuarine and coastal environments.

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Preferential removal of dissolved carbohydrates during estuarine mixing in the Bay of Saint Louis in the northern Gulf of Mexico

Marine Chemistry, Apr 1, 2010

Water samples were collected from the Bay of Saint Louis (BSL) estuary in the northern Gulf of Me... more Water samples were collected from the Bay of Saint Louis (BSL) estuary in the northern Gulf of Mexico to investigate the abundance, distribution, and mixing behavior of dissolved organic carbon (DOC) and total dissolved carbohydrates (TCHO). Laboratory experiments were also conducted to examine the removal mechanisms of DOC and TCHO during estuarine mixing. Both DOC and TCHO concentrations decreased with increasing salinity, showing a non-conservative behavior in the BSL with considerable removal of DOC (43%) and TCHO (63%). The ratio of TCHO to DOC varied from 0.31 in riverwater to ∼ 0.10 in coastal waters, decreasing with increasing salinity within the estuary. The TCHO was higher in the N 1 kDa colloidal fraction, ranging from 53% to 73% as quantified using an ultrafiltration permeation model compared to the bulk DOC that had a colloidal fraction of 40-63%. Coastal waters also had a slightly elevated CHO/DOC ratio in the colloidal phase, suggesting in situ production of CHO. Results from laboratory experiments showed that physical mixing resulted in a removal of 5-10% TCHO, while microbial processes accounted for a removal of up to 25-30% of TCHO during incubation experiments, suggesting that microbial degradation was the primary mechanism responsible for the removal of CHO in the BSL. However, physical removal combined with microbial degradation accounted for only 30-40% of the removal of DOC and TCHO in the BSL, indicating that the removal might be overestimated and/or other processes are also important in the removal of DOC and CHO in the estuarine environment. Both field observations and laboratory experiments demonstrated consistently greater removal of CHO compared to bulk DOC, supporting our hypothesis that carbohydrates are a more active organic component and could be preferentially removed during estuarine mixing.

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Colored dissolved organic matter dynamics across the shelf-basin interface in the western Arctic Ocean

Journal of Geophysical Research, May 19, 2007

Spatial variations in the concentration and nature of colored dissolved organic matter (CDOM) in ... more Spatial variations in the concentration and nature of colored dissolved organic matter (CDOM) in the western Arctic Ocean were examined by three-dimensional excitation/emission matrix (3-D EEM) spectroscopy. CDOM profiles showed distinctive features well correlated with hydrographic characteristics. CDOM fluorescence was particularly high at depths between 40 and 200 m (up to 3 fluorescence units (Fl.U.)) in both Chukchi Sea and Beaufort Sea transects. Penetration of the high CDOM signal, formed on the shelves, into the Canada Basin was confined to the upper halocline layer (salinity of $33.1). This layer had distinctive 3-D EEM fingerprints in fluorescence spectra, showing a marked terrestrial humic signature. The presence of CDOM in the halocline layer likely resulted from two main processes: the brine rejection during sea ice formation and transport across the sediment-water interface during early diagenesis. Despite the high primary productivity in the Chukchi shelf, CDOM contribution from in situ production seemed to have little influence on the overall CDOM distributions in the study area.

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Speciation and fluxes of nutrients (N, P, Si) from the upper Yukon River

Global Biogeochemical Cycles, Mar 1, 2004

Water samples were collected from the Yukon River near the Stevens Village Station from May to Se... more Water samples were collected from the Yukon River near the Stevens Village Station from May to September 2002 and analyzed for nutrients (N, P, and Si) in dissolved, particulate, organic, and inorganic forms to examine temporal variations in nutrient concentrations, fluxes, and phase partitioning. Both NO 3 and PO 4 concentrations in the Yukon River were much lower than those of world rivers, with an average concentration of 2.43 ± 0.63 mM-N and 0.053 ± 0.040 mM-P, respectively. Si(OH) 4 concentrations were more comparable to those of world rivers, with an average concentration of 82 ± 21 mM-Si. Integrated annual fluxes were 2.4 Â 10 8 mole-NO 3 , 3.4 Â 10 6 mole-PO 4 , and 8.7 Â 10 9 mole-Si(OH) 4 , respectively. Nutrient discharge during the river ice open season contributed 73 to 95% of the annual flux depending on nutrient species. Within the total N pool transported by the Yukon River, dissolved inorganic N comprised 7 ± 4% and particulate N made up 25 ± 10%, while dissolved organic N (DON) was the dominant N species (with an average of 67 ± 10%). In contrast, P was predominantly partitioned in the particulate phase (with an average of 94 ± 6%), leaving 4 ± 5% of the total P in the dissolved organic phase and $2 ± 1% in the dissolved inorganic phase. The partitioning of N and P indicates that the transformation between dissolved and particulate or inorganic and organic phases may play a critical role in controlling the flux of bioavailable nutrients and thus the nutrient dynamics in the Yukon River Basin and its coastal region. Nutrient specific fluxes normalized to drainage area in the Yukon River Basin were 0.57 mmole/m 2 /yr for NO 3 , 0.012 mmole/m 2 /yr for PO 4 , and $19 mmole/m 2 /yr for Si(OH) 4 , respectively. The relatively low specific fluxes of NO 3 and PO 4 in the Yukon River Basin reflect its pristine status or little anthropogenic influence, whereas cold climate in the Arctic/subarctic region may be responsible for its lower Si(OH) 4 specific flux, in agreement with a general trend of increasing Si specific flux with decreasing latitude in global river systems. A warming climate and thus deeper permafrost active layer in the Yukon River watershed would likely enhance the export flux of nutrients into the Bering Sea.

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Comparisons in Molecular Weight Distributions and Size-Dependent Optical Properties Among Model and Reference Natural Dissolved Organic Matter

Social Science Research Network, 2022

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

Journal Of Geophysical Research: Biogeosciences, Aug 1, 2021

Large world rivers and their dissolved organic matter (DOM) fluxes could regulate ecosystem funct... more 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.

Variations in the isotopic composition of particulate organic carbon and their relation with carbon dynamics in the western Arctic Ocean

Deep-sea Research Part Ii-topical Studies in Oceanography, Dec 1, 2012

The relation between the dissolved carbon dioxide (CO 2 ) and the stable isotopic composition of ... more The relation between the dissolved carbon dioxide (CO 2 ) and the stable isotopic composition of particulate organic matter in the water column has not been well quantified, but this information could help provide a better understanding of carbon dynamics in a warmer Arctic Ocean. The stable carbon isotopic composition of suspended particulate organic carbon (d 13 C POC ) in the surface waters of the western Arctic Ocean was measured during July-September 2003, to evaluate the spatial variability of d 13 C POC and its key controlling factors. Values of d 13 C POC fell within the range of À 28.5% to À 21.1%, with an average of À 24.5 7 2.3%. The spatial variability of d 13 C POC showed a general decreasing trend from shallow waters in the continental shelf toward the deeper, colder waters in the basin. A negative correlation between d 13 C POC and the dissolved CO 2 concentration in surface waters was observed, indicating that carbon isotopic fractionation during photosynthesis was largely dependent on the dissolved CO 2 concentration. Compared to the solubility pump, biological processes may play a more important role in determining the distribution and variation of d 13 C POC in the western Arctic Ocean during summer. The coupled relationship between CO 2 concentration and stable isotopic composition of particulate organic matter has the potential to be used for reconstruction of sea-surface CO 2 changes in the past, provided a quantitative relationship of d 13 C between POC and sediments can be established.

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Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria

Arctic, 2003

Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from a... more Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from aboard the R/V Xue Long during August 1999. Activity concentrations of dissolved and particulate 234 Th were measured using beta counting techniques to quantify the scavenging and residence time of 234 Th and organic carbon export fluxes. Primary production (PP) and bacterial production were also determined in the study areas through in situ incubation experiments. Significant 234 Th scavenging was observed in the upper 100 m of the water column in both study areas, with up to 40% of 234 Th deficit found at Bering Sea stations and ~15% of 234 Th deficit at the Canada Basin station. Measured PP decreased from ~12.5 µmol C/m 3 /h in surface water to near zero at ~100 m depth, with an integrated PP of 3.83 mmol C/m 2 /d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol C/m 2 /d, which is up to 52% of the integrated PP. Particulate organic carbon (POC) export fluxes derived from 234 Th/ 238 U disequilibrium were ~1 mmol C/m 2 /d in the Canada Basin and ~10 mmol C/m 2 /d in the Bering Sea, with fluxes in the latter area being 5 to 10 times higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of 234 Th-derived POC export to primary production) of 0.26 for the Canada Basin and 0.7 for the Bering Sea. The higher ThE ratios in the study areas suggest a decoupling of production and particulate export in the high-latitude ocean. Ratios of POC to particulate 234 Th (µmol C/dpm) decreased consistently with increasing depth, suggesting that organic carbon is preferentially remineralized relative to 234 Th. Interestingly, the profile of particulate 234 Th in the Canada Basin showed a unique characteristic: particulate 234 Th activities increased with increasing depth, suggesting a continuous scavenging of 234 Th and a rapid settling rate of the particles.

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Contrasting effects of photochemical and microbial degradation on Cu(II) binding with fluorescent DOM from different origins

Environmental Pollution, Aug 1, 2018

Effects of photochemical and microbial degradation on variations in composition and molecular-siz... more Effects of photochemical and microbial degradation on variations in composition and molecular-size of dissolved organic matter (DOM) from different sources (algal and soil) and the subsequent influence on Cu(II) binding were investigated using UVeVis, fluorescence excitation-emission matrices coupled with parallel factor analysis, flow field-flow fractionation (FlFFF), and metal titration. The degradation processes resulted in an initial rapid decline in the bulk dissolved organic carbon and chromophoric and fluorescent DOM components, followed by a small or little decrease. Specifically, photochemical reaction decreased the aromaticity, humification and apparent molecular weights of all DOM samples, whereas a reverse trend was observed during microbial degradation. The FlFFF fractograms revealed that coagulation of both protein-and humic-like DOM induced an increase in molecular weights for algal-DOM, while the molecular weight enhancement for allochthonous soil samples was mainly attributed to the self-assembly of humic-like components. The Cu(II) binding capacity of algal-derived humic-like and fulvic-like DOM consistently increased during photo-and bio-degradation, while the soil-derived DOM exhibited a slight decline in Cu(II) binding capacity during photo-degradation but a substantial increase during microbial degradation, indicating source-and degradation-dependent metal binding heterogeneities. Pearson correlation analysis demonstrated that the Cu(II) binding potential was mostly related with aromaticity and molecular size for allochthonous soil-derived DOM, but was regulated by both DOM properties and specific degradation processes for autochthonous algal-derived DOM. This study highlighted the coupling role of inherent DOM properties and external environmental processes in regulating metal binding, and provided new insights into metal-DOM interactions and the behavior and fate of DOM-bound metals in aquatic environments.

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