Riverine Suspended Particulate Matter Research Articles

Channel cross-section heterogeneity of particulate organic carbon transport in the Huanghe

Abstract. The Huanghe (Yellow River), one of the largest turbid river systems in the world, has long been recognized as a major contributor of suspended particulate matter (SPM) to the ocean. However, over the last few decades, the SPM export flux of the Huanghe has decreased over 90 % due to the high management, impacting the global export of particulate organic carbon (POC). To better constrain sources and modes of transport of POC beyond the previously investigated transportation of POC near the channel surface, SPM samples were for the first time collected over a whole channel cross-section in the lower Huanghe. Riverine SPM samples were analyzed for particle size and major element contents, as well as for POC content and dual carbon isotopes (13C and 14C). Clear vertical and lateral heterogeneities of the physical and chemical properties of SPM are observed within the river cross-section. For instance, finer SPM carry more POC in general with higher 14C activity near the surface of the right bank. Notably, we discuss how bank erosion in the alluvial plain is likely to generate lateral heterogeneity in POC composition. The Huanghe POC is millennial-aged (4020 ± 500 radiocarbon years) and dominated by organic carbon (OC) from the biosphere, while the lithospheric fraction is ca. 12 %. The mobilization of aged and refractory OC, including radiocarbon-dead biospheric OC, from deeper soil horizons of the loess–paleosol sequence through erosion in the Chinese Loess Plateau is an important mechanism contributing to fluvial POC in the Huanghe drainage basin. Altogether, anthropogenic activities can drastically change the compositions and transport dynamics of fluvial POC, consequentially altering the feedback of the source-to-sink trajectory of a river system to regional and global carbon cycles.

Quantifying uranium radio-isotope ratios in riverine suspended particulate matter: Insights into natural and anthropogenic influences in the glacial-fed river system of the NE Tibetan Plateau

The analysis of uranium isotope ratio 235U/238U in environmental media serves as a reliable method to distinguish between natural and anthropogenic sources of uranium, playing a crucial role in assessing the extent of contamination with anthropogenic uranium and disturbances in its biogeochemical cycle. In this study, we focus on the northeastern Tibetan Plateau to examine the atomic ratio of 235U and 238U in riverine suspended particulate matter (SPM) across eight glacial watersheds. Results reveal that the 235U/238U atomic ratio in the suspended load ranges from 0.007247 to 0.007437 (with an average value of 0.00727 ± 0.00003), which closely aligns with the ratio found in natural uranium (0.00725). The highest mean ratio (0.00729 ± 0.00007) is observed in the upper glacial basin of the Ningchan River. Results suggest the negligible influence of isotopically altered in relation to human nuclear activities. When considering different environmental media, such as soil, snow/cryoconite, and riverine suspended particulate matter in the study area, the 235U/238U ratio in surface soil presents the highest values, pointing to a slight enrichment of 235U. This may be attributed to the fact that soil retains the cumulative signals of uranium atmospheric deposition, including the deposition of 235U-enriched airborne particulate matter deposited after atmospheric nuclear tests carried out in the second half of the 20th century. On the contrary, riverine suspended particulate matter and glacial sediments are more influenced by the natural 235U/238U signature under modern environmental conditions. This confirms that the northeastern Tibetan Plateau is still relatively pristine with respect to biogeochemical disturbances related to human activities.

Generation of reproducible model freshwater particulate matter analogues to study the interaction with particulate contaminants

Aquatic fate models and risk assessment require experimental information on the potential of contaminants to interact with riverine suspended particulate matter (SPM). While for dissolved contaminants partition or sorption coefficients are used, the underlying assumption of chemical equilibrium is invalid for particulate contaminants, such as engineered nanomaterials, incidental nanoparticles, micro- or nanoplastics. Their interactions with SPM are governed by physicochemical forces between contaminant-particle and SPM surfaces. The availability of a standard SPM material is thus highly relevant for the development of reproducible test systems to evaluate the fate of particulate contaminants in aquatic systems. Finding suitable SPM analogues, however, is challenging considering the complex composition of natural SPM, which features floc-like structures comprising minerals and organic components from the molecular to the microorganism level. Complex composition comes with a heterogeneity in physicochemical surface properties, that cannot be neglected. We developed a procedure to generate SPM analogue flocs from components selected to represent the most abundant and crucial constituents of natural riverine SPM, and the process-relevant SPM surface characteristics regarding interactions with particulate contaminants. Four components, i.e., illite, hematite, quartz and tryptophan, combined at environmentally realistic mass-ratios, were associated to complex flocs. Flocculation was reproducible regarding floc size and fractal dimension, and multiple tests on floc resilience towards physical impacts (agitation, sedimentation-storage-resuspension, dilution) and hydrochemical changes (pH, electrolytes, dissolved organic matter concentration) confirmed their robustness. These reproducible, ready-to-use SPM analogue flocs will strongly support future research on emerging particulate contaminants.

In flux: Annual transport and deposition of suspended heavy metals and trace elements in the urbanised, tropical Red River Delta, Vietnam

Due to the depositional environment, river deltas are said to act as filters and sinks for pollutants. However, many deltas are also densely populated and rapidly urbanizing, creating new and increased sources of pollutants. These sources pose the risk of tipping these environments from pollution sinks to sources, to the world's oceans. We provide detailed seasonal and annual assessments of metal contaminants in riverine suspended particulate matter (SPM) across the densely populated Red River Delta (RRD), Vietnam. The global contributions of elements from the RRD are all <0.2% with many elemental fluxes <0.01%, suggesting the RRD is not a major source of elemental pollution to the ocean. However, ‘hotspots’ of metal pollution due to human activity and the impacts of tropical storm Son Tinh (July 2018) exceed both national level regulations and international measures of toxicity (e.g. enrichment factors). There is widespread ‘extreme pollution’ of Cd (enrichment factor >40) and concentrations of As higher than national regulation limits (>17 mg/Kg) at all sites other than one upstream, agricultural-dominated tributary in the dry season. These ‘hotspots’ are characterised by high inputs of organic matter (e.g. manure fertiliser and urban wastewater), which influences elemental mobility in the particulate and dissolved phases, and are potentially significant sources of pollution downstream. In addition, in the marine and fresh water mixing zone, salinity effects metal complexation with organic matter increasing metals in the particulate phase. Our calculations indicate that the delta is currently acting as a pollutant sink (as determined by high levels of pollutant deposition ∼50%). However, increased in-washing of pollutants and future projected increases in monsoon intensity, saline intrusion, and human activity could shift the delta to become a source of toxic metals. We show the importance of monitoring environmental parameters (primarily dissolved organic matter and salinity) in the RRD to assess the risk of transport and accumulation of toxic metals in the delta sediments, which can lead to net-increases in anthropogenic pollution in the coastal zone and the incorporation of toxic elements in the food chain.

Anthropogenic perturbations to the fate of terrestrial organic matter in a river-dominated marginal sea

River-dominated ocean margins (RiOMar) are major terrestrial organic carbon (OCterr) repositories that play an important role in the global carbon cycle. However, riverine inputs of sediment and associated OCterr have decreased substantially due to intensifying human activity. Thus, studying OCterr processing during transport to and within RiOMar is crucial. The East China Sea (ECS) receives reduced sediment loads from the Changjiang River (CJR), which is compounded further by the impoundment of the Three Gorges Reservoir. Here, we examined the CJR-ECS source-to-sink system to understand the fate of OCterr in a typical RiOMar. We analyzed the sedimentary properties (grain size and the specific surface area), bulk organic carbon properties (C/N ratio and δ13C), and molecular biomarkers (n-alkanes and GDGTs) of riverine suspended particulate matter (SPM) and surficial marine sediment collected in 2006 and 2018. The C/N ratio, δ13C value, and short-chain n-alkanes abundances in the riverine SPM indicate that phytoplankton productivity could be an important source of OCterr in the basin due to the reduced sediment loads. Seabed erosion occurred in the estuarine–inner shelf areas of the ECS as a result of reduced riverine inputs, which resulted in a change in the distribution of sedimentary OCterr. The reduced total organic carbon content and n-alkanes abundance are indicative of the low sedimentary OCterr content. Variations in GDGT-based indices are further manifestation of reduced sediment load, as they show increased in situ production of GDGTs. Spearman correlations indicate that the biomarker abundances were positively correlated with silt, which is in accordance with silt being easily shaped by hydrodynamic sorting. Biomarker abundances along four estuarine–inner shelf transects also exhibited pronounced alongshore or cross-shelf OCterr transport. Furthermore, the relationship between the biomarkers and sedimentary properties indicates that mineral protection plays an important role in regulating the fate of OCterr on continental margins. Therefore, variations in riverine inputs and the sedimentary environment caused by anthropogenic perturbations have affected the abundance, distribution, composition, and transport of sedimentary OCterr in the study area. These findings can be applicable to other RiOMar, as humans continue to shape riverine inputs globally.

Limited capacity of suspended particulate matter in the Yangtze River Estuary and Hangzhou Bay to carry phosphorus into coastal seas

The Yangtze River carries high fluxes of suspended particulate matter (SPM) and associated particulate phosphorus into the Yangtze River Estuary (YRE), but primary production in coastal seas is phosphorus-limited. Understanding the kinetics of exchangeable inorganic phosphorus (Ex-iP) released from SPM in response to different salinities and SPM concentrations is the key to demonstrating their capacity to carry bioavailable phosphorus into coastal seas. We conducted field investigations and set up a series of incubation experiments with SPM collected from marine and freshwater settings under controlled conditions, with biotic activities inhibited.Our incubation experiments showed that initial rise in salinity substantially increases the efficiency and rates of Ex-iP release, leading to an early release of majority of Ex-iP from riverine SPM when encountering saline waters, but high salinity may result in desorption. Concentrations of released DIP are linearly related to SPM concentrations, but concentrations of DIP in micromoles per gram of SPM are negatively logarithmically related to SPM concentrations, which help prevent SPM from full release of Ex-iP at the river mouth. Release of Ex-iP is insufficient to buffer DIP in the YRE. The contribution of Ex-iP from SPM decline rapidly with the attenuation of SPM concentration, and becomes negligible offshore. Extremely high loads of SPM in Hangzhou Bay allows it to retain some capacity to carry Ex-iP offshore. High concentrations of SPM in a turbid estuary are more important in reducing the removal of DIP by phytoplankton uptake through restriction of light penetration, as the SPM possesses a very limited capacity to carry Ex-iP to coastal seas. However, such high concentrations might be a key contributor to the regeneration of remineralization of organic phosphorus in an estuary.

Terrestrial inputs govern spatial distribution of polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB) in an Arctic fjord system (Isfjorden, Svalbard).

Considerable amounts of previously deposited persistent organic pollutants (POPs) are stored in the Arctic cryosphere. Transport of freshwater and terrestrial material to the Arctic Ocean is increasing due to ongoing climate change and the impact this has on POPs in marine receiving systems is unknown This study has investigated how secondary sources of POPs from land influence the occurrence and fate of POPs in an Arctic coastal marine system. Passive sampling of water and sampling of riverine suspended particulate matter (SPM) and marine sediments for analysis of polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB) was carried out in rivers and their receiving fjords in Isfjorden system in Svalbard. Riverine SPM had low contaminant concentrations (<level of detection-28pg/g dw ΣPCB14, 16-100pg/g dw HCB) compared to outer marine sediments 630-880pg/g dw ΣPCB14, 530-770pg/g dw HCB). There was a strong spatial gradient in sediment PCB and HCB concentrations with lowest concentrations in river estuaries and in front of marine-terminating glaciers and increasing concentrations toward the outer fjord. This suggests that rather than leading to increased concentrations, inputs of SPM from land lead to a dilution of contaminant concentrations in nearshore sediments. Preliminary estimates of SPM:water activity ratios suggest that terrestrial particles (with low contaminant concentrations) may have the potential to act as sorbents of dissolved contaminants in the coastal water column, with implications for bioavailability of POPs to the marine food web. There is concern that ongoing increases in fluxes of freshwater, sediments and associated terrestrial material (including contaminants) from land to the Arctic Ocean will lead to increased mobilization and transport of POPs to coastal ecosystems. However, the results of this study indicate that on Svalbard, inputs from land may in fact have the opposite effect, leading to reduced concentrations in coastal sediments and waters.

Tracing zinc sources with Zn isotope of fluvial suspended particulate matter in Zhujiang River, southwest China

The distinct isotopic composition of metallic elements observed in different sources and reservoirs provides an important perspective on tracing the source, transportation, and circulation of metal pollutants. As an essential trace element for life, Zinc (Zn) will show strong toxicity when its concentration reaches a certain limit. The riverine suspended particulate matter (SPM) is the common occurrence state of Zn. However, few studies have examined the sources and isotope of Zn in SPM. To examine the origins of Zn in SPM, the Zn isotope (δ66Zn) and Zn contents of SPM from 22 sites in the Zhujiang River were investigated and the relative contributions of potential sources to particulate Zn were also calculated. The results indicate that the Zn concentrations have a large variation from 49 to 733 mg kg−1 and a relatively higher enrichment level (EFZn, up to 20.8). The percentage of Zn transportation in the form of SPM ranges between 9% and 97% (average 83%) in the Zhujiang River. The roughly negative correlation is reflected between EFZn and δ66Zn (−0.11‰ to 0.41‰). Based on isotopic mass balance, the contribution of the natural and anthropogenic sources was calculated. The relative proportion of particulate Zn derived from anthropogenic inputs is 30.2%, while the natural source contributes 69.8% Zn to the fluvial SPM. These results further indicate that Zn is controlled by both the natural and anthropogenic inputs. Moreover, the preliminary application of Zn isotope shown in this study highlights the metallic isotope can be an effective indicator of the riverine eco-environmental pollution.

Dynamics of phosphorus composition in suspended particulate matter from a turbid eutrophic shallow lake (Lake Chaohu, China): Implications for phosphorus cycling and management

Particulate phosphorus (P) dominates the total P (TP) content in lacustrine water columns and is a primary source of dissolved P in turbid eutrophic shallow lakes. However, the spatiotemporal variability of P compositions in suspended particulate matter (SPM) remains poorly understood. In this study, we applied chemical extraction and solution 31P nuclear magnetic resonance (31P NMR) to assess the seasonal variations of SPM P compositions from a shallow turbid lake (Lake Chaohu, China) and its main river tributaries. P fractionation analysis indicated that mobile P (the sum of labile-P, iron-bound P, and organic P) accounted for >60% of the TP in SPM and showed high spatiotemporal variability throughout the year-long field investigation. In most seasons, riverine SPM (in urban rivers or rivers with high flow) contained a higher mobile P content than that of the lake and was therefore a dominant source of lacustrine mobile particulate P. Solution 31P NMR identified five types of P compounds in SPM, with highest contributions from orthophosphate. Organic P components and concentrations showed high seasonal variability, and elevated p values occurred during the summer algal bloom. The correlation analysis between organic and inorganic P fractions inferred the possible degradation of organic P into reactive inorganic components of SPM. Consequently, biological or chemical processes would further transform the labile inorganic P into soluble reactive phosphorus, which is readily utilized by lacustrine algae. Our results suggest that the labile forms of P in SPM were highly dynamic and significantly contributed to the eutrophication of the turbid shallow lake.

Occurrence and partitioning behavior of perfluoroalkyl acids in wastewater effluent discharging into the Long Island Sound

Perfluoroalkyl acids (PFAAs) were measured in aqueous and suspended particulate matter (SPM) fractions in the final effluents from 12 wastewater treatment facilities located around the Connecticut shoreline. Aqueous phase concentrations ranged from 53 to 198 ng/L for ∑PFAAs with ≤7 perfluorinated carbons (CF2) and 2–73 ng/L for >7 CF2 PFAAs. Predominant PFAAs associated with effluent derived SPM were perfluorodecanoic acid and perflurorooctane sulfonic acid, detected in 48% and 52% of samples in concentrations ranging from <LOQ–1770 ng/g and <LOQ–2750 ng/g respectively. Based on the range of concentrations detected and the average flow of final effluent to the Long Island Sound (LIS), average total annual PFAA mass loads from wastewater treatment facilities to the LIS is estimated in the range of 70–315 kg/year, with 4–100 kg/year consisting of >7 CF2 PFAAs. Partitioning coefficients (log KOC) derived for effluent water and SPM phases (4.2 ± 0.3, 4.4 ± 0.4, 5.1 ± 0.2 and 5.3 ± 0.2 for PFOA, PFNA PFDA and PFUnA; 4.5 ± 0.2 and 5.2 ± 0.2 for PFOS and PFHsX respectively) were found to be of similar magnitude to aeration tank particles, though 0.5 to 2 log units greater than sludge solids and to natural system particulates including riverine SPM, estuarine SPM and sediments. Results from this study suggest that effluent derived suspended particulate matter could be an effective vector in the transport of long-chained PFAAs through wastewater treatment into receiving waters, and a potential vector to the local food chain.

Constraining the origin of recently deposited particles using natural radionuclides 7Be and 234Thex in deltaic sediments

7Be and 234Thex activities were determined in sediment cores off the Rhône River mouth (Gulf of Lions), in order to trace the initial transport and deposition of riverine suspended particulate matter (SPM) and evaluate the impact of flood events through 7 cruises carried out between 2007 and 2008. Consistently high 7Be and 234Thex inventories of 2000–3000 mBq cm−2 and 3000–5000 mBq cm−2, respectively, were observed within a ~5 km radius off the Rhône River mouth. Their spatial distributions showed a gradual decrease with increasing distance from the Rhône River mouth, and the decrease in 7Be was more pronounced than that of 234Thex, indicating that recent riverine SPM is rapidly deposited in the area located near the river mouth. This area is also characterized by high accumulation rates determined using 137Cs or 210Pbex. Both 7Be and 234Thex inventories increased in 2008 compared to 2007, and are correlated to the cumulated SPM flux for normal and flood discharge. Moreover, the 7Be/234Thex inventory ratio appears to be a potential tracer to identify the dominant influence of recently deposited particles between terrestrial and marine waters. This ratio provides an effective tool to assess river and marine influence: Zone I at a distance inferior to 3.0 km, with 7Be/234Thex inventory ratio over 0.50 (surface area near river mouth ~ 7 km2) is dominated by riverine influence; in contrast, Zone III at a distance superior to 8.5 km, with 7Be/234Thex inventory ratio less than 0.10 (surface area off river mouth beyond 150 km2) is predominantly under a marine influence. In between, an intermediate area (Zone II at a distance between 3.0 and 8.5 km, with 7Be/234Thex inventory ratios between 0.10 and 0.50) displays a mixed influence. This zoning could help in further understanding the spreading of particle-reactive contaminants and its initial sedimentary deposition in the Gulf of Lions.

Nitrogen exchange across the sediment-water interface after dredging: The influence of contaminated riverine suspended particulate matter

Dredging has been widely implemented in shallow lakes to reduce internal nitrogen (N) loading. The suspended particulate matter (SPM) coming from polluted rivers usually contains high levels of N and ultimately deposits on the dredged sediment surfaces near the river mouth. To study the influence of the riverine SPM on N exchange across the sediment-water interface (SWI) after dredging, a 360-day experiment was carried out comparing un-dredged and dredged sediments from Lake Chaohu, China. Dredged treatments showed a significant increase (p < 0.01) in total N concentrations in the sediments, while the deposition of SPM had little influence on labile NH4+-N concentrations. In addition, NH4+-N concentrations in pore-water and NH4+-N fluxes were significantly lower in dredged than in un-dredged sediments, despite the deposition of SPM. The oxygen production rates and the oxygen penetration depth in the dredged sediments were both higher than those in the un-dredged sediments. The increase of Nitrospira in dredged sediments was consistent with their decreased NH4+-N concentrations and fluxes across the SWI. Therefore, the oxidizing condition, increased oxygen production/consumption rates and Nitrospira relative abundance across the SWI were believed to be correlated with the low N exchange rates in dredged sediments. Dredging for reducing internal N loading in a river mouth area is therefore feasible, although the influence of the riverine SPM should be taken into account when aiming to achieve a long-term internal N loading reduction.

Interactions of riverine suspended particulate matter with phosphorus inactivation agents across sediment-water interface and the implications for eutrophic lake restoration

Phosphorus (P) inactivation agents (PIAs) have been widely used in lakes for eutrophic management. However, the interactions of riverine suspended particulate matter (SPM) with PIAs have not been elucidated to the point of complete confidence in the success of PIA application to eutrophic lakes. In this study, a long-term field incubation experiment was carried out to investigate the effects of riverine SPM on sediment internal P management. Fluorescence excitation emission matrix coupled with parallel factor (EEM–PARAFAC) was used to characterize dissolved organic matter (DOM) derived from SPM/sediment, and to study its complexation with La (III) and Ca (II) contained in two PIAs. The analysis identified one protein-and two humic-like components in these samples, which show variable quenching effects by La (III) and Ca (II). The continuous settling of SPM can bury PIAs deeper into the sediment layers and while increasing the mobile P content in surface sediments. This consequently made an increase of P fluxes and surface sediment reactivity, which was witnessed by the increased labile P concentration measured by diffusive gradient in film (DGT). However, the binding capacity of PIAs towards sediment mobile P was kept in a buried layer during the six months’ field experiment. This study indicates that the influence mechanism of riverine SPM on PIAs was through a decrease of the reactive sorption sites of PIAs by complexing with DOM and continuously increasing the mobile P content in surface sediment thereby largely surpassing their maximum P sorption capacity. Repeated dosing at scheduled times would be better for turbid eutrophic lake remediation with high input of riverine SPM in long-term management.

Microplastics en route: Field measurements in the Dutch river delta and Amsterdam canals, wastewater treatment plants, North Sea sediments and biota

Environmental contamination by plastic particles, also known as ‘microplastics’, brings synthetic materials that are non-degradable and biologically incompatible into contact with ecosystems. In this paper we present concentration data for this emerging contaminant in wastewater treatment plants (WWTPs) and freshwater and marine systems, reflecting the routes via which these particles can travel and the ecosystems they potentially impact along their path. Raw sewage influents, effluents and sewage sludge from seven municipal WWTPs in the Netherlands contained mean particle concentrations of 68–910L−1, 51–81L−1 and 510–760kg−1 wet weight (ww), respectively (particle sizes between 10 and 5000μm). Even after treatment, wastewater constitutes a source of microplastic pollution of surface waters, and via biosolids applications in farming and forestry, plastic retained in sewage sludge can be transferred to terrestrial environments. The WWTPs investigated here had a mean microplastics retention efficiency of 72% (s.d. 61%) in the sewage sludge. In the receiving waters of treated and untreated wastewaters, we detected high microplastic levels in riverine suspended particulate matter (1400–4900kg−1 dry weight (dw)) from the Rhine and Meuse rivers. Amsterdam canal water sampled at different urban locations contained microplastic concentrations (48–187L−1), similar to those observed in wastewater that is emitted from sewage treatment facilities in the area. At least partial settling of the particles occurs in freshwater as well, as indicated by microplastics in urban canal sediments (<68 to 10,500particleskg−1dw). Microplastics in suspension in the water column have the potential to be discharged into the sea with other riverine suspended particulates. We report microplastic concentrations from 100 up to 3600particleskg−1 dry sediment collected at 15 locations along the Dutch North Sea coast. The high microplastic enrichment in marine sediments compared to most literature data for seawater at the surface supports the hypothesis of a seabed sink for these materials. Marine species are heavily exposed to plastic particles. Body residues between 10 and 100particlesg−1dw were measured in benthic macroinvertebrate species inhabiting the Dutch North Sea coast: filter-feeding mussels and oysters (species for human consumption) as well as other consumers in the marine food chain.

Temporal occurrence and sources of persistent organic pollutants in suspended particulate matter from the most heavily polluted river mouth of Lake Chaohu, China

The Nanfei River is by many measures the most heavily polluted tributary to Lake Chaohu. In this study, the temporal occurrence and sources of four classes of persistent organic pollutants (POPs), including polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs), in suspended particulate matter (SPM) from the river mouth were investigated monthly during 2014. Results show that concentrations of all four POPs in SPM were higher than those in the sediment of Lake Chaohu. PBDEs (26.7 ng g−1 dry weight (dw)) were originated mainly from commercial deca-BDE mixtures. PCB concentrations (1.336 ng g−1 dw) were lower than those of sediments in many other water bodies worldwide. PAHs (2597 ng g−1 dw) and OCPs (57.38 ng g−1 dw) were the most common POPs. PAHs mainly had high molecular weights and originated from pyrolytic sources, with a small proportion of petrogenic origin. The predominant OCPs were DDTs, heptachlorepoxides, dieldrin, hexachlorocyclohexanes, hexachlorobenzene, and chlordanes. Most OCPs originated from historical use, except lindane. Generally, of all the POPs studied, those of primary ecological concern should be acenaphthene, fluorene, DDTs, and chlordanes. Higher concentrations of POPs were detected during winter and spring than in summer and autumn, probably because of the high river flow during the rainy season. The high concentration of POPs in the riverine SPM and the fractionation of POPs in the water and SPM of the river should be a focal point in the future study of Lake Chaohu.

Metal Ions and Hydroperoxide Content: Main Drivers of Coastal Lipid Autoxidation in Riverine Suspended Particulate Matter and Higher Plant Debris

Autoxidation is a complex abiotic degradation process, and while it has long been known and well studied in biological compounds, it has been widely overlooked in environmental samples and as a part of environmental processes. With recent observations showing the magnitude of the involvement of autoxidation in coastal environments, it has become critical to better understand how and why this degradative process takes place. At the riverine/marine interface, recent findings evidenced a spike in autoxidation rates upon the arrival of suspended particulate matter in seawater. In this study, we aimed at identifying autoxidation-favoring factors in vitro by analyzing suspended particulate matter incubated under different conditions. If metal ions have long been known to induce autoxidation in biological systems, we show that they indeed induce autoxidation in particulate matter incubated in water, but also that the content in photochemically-produced hydroperoxides in suspended particulate matter is crucial to the induction of its autoxidation in water.

Effects of riverine suspended particulate matter on the post-dredging increase in internal phosphorus loading across the sediment-water interface

Dredging is frequently used in the river mouths of eutrophic lakes to reduce internal phosphorus (P) loading from the sediment. However, the accumulation of P-adsorbed suspended particulate matter (SPM) from the inflowing rivers negatively affects the post-dredging sediment-water interface and ultimately increases internal P loading. Here, a 360-d experiment was carried out to investigate the influence of riverine SPM on the efficacy of dredging in reducing internal P loading. SPM was added to dredged and undredged sediments collected from the confluence area of Lake Chaohu. Several parameters related to internal P loading, including oxygen profile, soluble reactive P, and ferrous iron across the sediment-water interface, organic matter, alkaline phosphatase activity, and P fractions, were measured throughout the experimental period. The results showed that the P content (especially mobile P) in the sediment increased to the pre-dredging level with the accumulation of SPM in the dredged sediment. In addition, the P flux across the sediment-water interface increased with the accumulation of SPM. Several characteristics of SPM, including high organic matter content, mobile P, high activity of alkaline phosphatase, and high biological activity, were considered correlated with the post-dredging increase in internal P loading. Overall, this study showed that the heavily contaminated riverine SPM regulates the long-term efficacy of dredging as a nutrient management option in the confluence area. Management is needed to avoid or reduce this phenomenon during dredging projects of this nature.

Effects of riverine suspended particulate matter on post-dredging metal re-contamination across the sediment-water interface.

Environmental dredging is often used in river mouth areas to remove heavy metals. However, following dredging, high levels of metal-adsorbed suspended particulate matter (SPM) originating from polluted inflowing rivers might adversely affect the sediment-water interface (SWI). Here, we conducted a 360-day-long experiment investigating whether the riverine SPM adversely affects dredging outcome in a bay area of Lake Chaohu, China. We found that the heavy metal concentrations in the post-dredging surface sediment increased to pre-dredging levels for all metals studied (As, Cd, Cr, Cu, Ni, Pb, and Zn) after the addition of SPM. In addition, the increased concentrations were mostly detected in the relatively bioavailable non-residual fractions. Of the metals studied, the rate of increase was the greatest for Zn and Cd (482.98% and 261.07%, respectively), mostly in the weak acid extractable fraction. These results were probably due to certain characteristics of SPM (fine grain size, and high concentrations of organic matter and heavy metals) and the good oxic conditions of the SWI. Furthermore, As was the only metal for which we observed an increasing trend of diffusive flux across the SWI. However, the flux was still significantly lower than that measured before dredging. In conclusion, the quantity and character of riverine metal-adsorbed SPM affect metal re-contamination across the post-dredging SWI, and this information should be incorporated into the management schemes of dredging projects dedicated to reducing metal contamination in similar areas.

Oxidation products of betulin: New tracers of abiotic degradation of higher plant material in the environment

In order to fill the need for specific and stable tracers for monitoring the degradative state of particulate organic matter (OM), betulin and its degradation products were selected to trace biotic and abiotic degradation processes affecting terrestrial higher plant-derived OM in riverine environments. Samples of Quercus ilex leaves and suspended particulate matter were collected from the Marseille Luminy area and the Rhône River, respectively, and analyzed in order to validate the tracer potential of betulin oxidation products identified during in vitro simulations. Three degradation products were selected as tracers: lup-20(30)-ene-3β,28,29-triol, lupan-20-one-3β,28-diol and the 20R and 20S epimers of 3β,28-dihydroxy-lupan-29-oic acid. They were deemed sufficiently stable for tracing the different degradative processes in aquatic systems affecting OM of terrestrial higher plant origin. All were found in riverine suspended particulate matter (SPM), evidencing the advanced degradation state of riverine particulate OM (POM), as well as the importance of autoxidation in the degradation. Lup-20(30)-ene-3β,28,29-triol and lupan-20-one-3β,28-diol were also found in senescent leaves of Q. ilex, attesting to the involvement of photo- and autoxidation in the degradation of plant leaves. Alongside existing tracers, these compounds provide a better insight into the degradation state of riverine OM, as well as into the degradative processes at play, a knowledge that will be a necessary basis for further studies of the degradative state of particulate marine OM and sediments.

Impact of riverine suspended particulate matter on the branched glycerol dialkyl glycerol tetraether composition of lakes: The outflow of the Selenga River in Lake Baikal (Russia)

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids found in several environments, including soils, rivers and lakes, whose distribution varies with temperature and pH, although this dependence is apparently not the same for the different environments. Mixing of brGDGT sources may thus complicate palaeoenvironmental reconstruction. The extent to which brGDGTs in a lake outflow reflect the distribution delivered by upstream rivers was studied for Lake Baikal (Russia), one of the largest freshwater lakes worldwide. Fifteen brGDGTs in suspended particulate matter (SPM) of the Selenga River and its outflow from the lake were quantified. The river and lake SPM had rather different distributions. The riverine distribution was still apparent in the SPM of the lake surface water 5km from the river mouth, but shifts in the distribution were already apparent in the SPM of the surface water after 1km. Based on the brGDGT distributions of the SPM of the Selenga outflow and that of the lake, conservative mixing between the river and the lake brGDGT distributions could not fully explain the observed shifts in distributions. Both preferential degradation and in situ production of brGDGTs in the surface and, especially, bottom water of the river outflow were potentially responsible. This implies that a riverine lipid distribution delivered to a lake can be modified prior to being transported downstream. The lacustrine brGDGT distribution, that possibly could have reflected a mixture of mountainous and Selenga River SPM, was not recognized in downstream Yenisei River SPM. The watershed of Lake Baikal thus does not seem to contribute to the brGDGTs transported to the marine system. As many large rivers have major lakes in their watershed, this has implications for palaeoclimate reconstruction from river fan sediments globally.