Sediment Resuspension Research Articles

Statistical characterization of erosion and sediment transport mechanics in shallow tidal environments – Part 1: Erosion dynamics

Abstract. Reliable descriptions of erosion events are foundational to effective frameworks relevant to the fate of tidal landscape evolution. Besides the rhythmic, predictable action of tidal currents, erosion in shallow tidal environments is strongly influenced by the stochastic wave-induced bottom shear stress (BSS), mainly responsible for sediment resuspension on tidal flats. However, the absence of sufficiently long, measured time series of BSS prevents a direct analysis of the combined tide- and wave-driven erosion dynamics and its proper representation in long-term morphodynamic models. Here we test the hypothesis of describing erosion dynamics in shallow tidal environments as a Poisson process by analysing, with the peak-over-threshold theory, the BSS time series computed using a fully coupled, bi-dimensional numerical model. We perform this analysis on the Venice Lagoon, Italy, taking advantage of several historical surveys done in the last 4 centuries, which allow us to investigate the effects of morphological modifications on spatial and temporal erosion patterns. Our analysis suggests that erosion events on intertidal flats can effectively be modelled as a marked Poisson process in different morphological configurations because the interarrival times, durations, and intensities of the over-threshold exceedances are always well described by exponentially distributed random variables. The resulting statistical characterization allows a straightforward computation of morphological indicators, such as the erosion work, and paves the way for a novel synthetic, yet reliable, approach for the long-term morphodynamic modelling of tidal environments.

Statistical characterization of erosion and sediment transport mechanics in shallow tidal environments – Part 2: Suspended sediment dynamics

Abstract. A proper understanding of sediment resuspension and transport processes is key to the morphodynamics of shallow tidal environments. However, a complete spatial and temporal coverage of suspended sediment concentration (SSC) to describe these processes is hardly available, preventing the effective representation of depositional dynamics in long-term modelling approaches. Through aiming to couple erosion and deposition dynamics in a unique synthetic theoretical framework, we here investigate SSC dynamics, following a similar approach to that adopted for erosion (D'Alpaos et al., 2024). The analysis with the peak-over-threshold theory of SSC time series computed using a fully coupled, bi-dimensional model allows us to identify interarrival times, intensities, and durations of over-threshold events and test the hypothesis of modelling SSC dynamics as a Poisson process. The effects of morphological modifications on spatial and temporal SSC patterns are investigated in the Venice Lagoon, for which several historical configurations in the last 4 centuries are available. Our results show that, similar to erosion events, SSC can be modelled as a marked Poisson process in the intertidal flats for all the analysed morphological lagoon configurations because exponentially distributed random variables describe over-threshold events well. Although erosion and resuspension are intimately intertwined, erosion alone does not suffice to describe also SSC because of the non-local dynamics due to advection and dispersion processes. The statistical characterization of SSC events completes the framework introduced for erosion mechanics, and together, they represent a promising tool to generate synthetic, yet realistic, time series of shear stress and SSC for the long-term modelling of tidal environments.

Summer recreational boating impacts on erosion, turbidity, and phosphorus levels in Canadian freshwater lakes

Increases in boat traffic over time can present a risk to the integrity of aquatic ecosystems. In addition to severe environmental degradation or boat-related disturbances, boat wave’s kinetic energy can induce a cumulative impact on freshwater ecosystems. However, seldom data report the impacts of boat waves on shoreline erosion and physico-chemical properties on freshwater lakes. In this study, we monitored shoreline erosion, turbidity, and total phosphorus levels. Wave-induced shoreline erosion was measured through shoreline rebar pin excavation across five lakes in southern Quebec. Sediment resuspension was measured through turbidity sampling in Lake Massawippi over three years. Total phosphorus was also monitored for potential impacts of sediment resuspension. Our study did not detect significant shoreline erosion. However, water turbidity was positively correlated with boat traffic and wind speed and negatively with the littoral slope angle. Moreover, water total phosphorus levels were positively correlated with turbidity. Thus, despite a lack of detectible impact on the shoreline, our results suggest that boat waves can have an impact on freshwater lakes with an increase in sediment resuspension and phosphorus availability. Management agencies could therefore benefit in the long-term from implementing or reinforcing policies aiming to minimize the impact of boats on sediment resuspension.

Implications of increased intertidal inundation on seagrass net primary production

Seagrass meadows are productive and functionally important seafloor habitats that are declining in many parts of the world under pressure from human activities. Seabed light availability is inversely related to water depth, thus sea level rise associated with global warming may impinge upon seagrass productivity and its contributions to coastal ecosystems. Across natural variation in seagrass bed depth driven by tidal oscillations and distance from shore, we quantified seafloor oxygen exchange every 15 min for three days and nights at sites in New Zealand using aquatic eddy covariance to better understand the implications of sea level rise on shallow seagrass. Four sites of differing mean depth (1.5–4.0 m deep at high tide) were net producers of oxygen when photosynthetically active radiation at the seabed was >87.5 μmol photons m−2 s−1, with net oxygen production >4 mmol m−2 h−1 at all sites during peak sunlight hours. The effect of light attenuation on dissolved oxygen production was evident when comparing fluxes in the morning (low tide) and evening (high tide), with relatively similar incident light levels arriving at water surface at these times of day. The shallowest site was expected to receive the greatest amount of light at the seabed and be most productive. However, the second shallowest site was the most productive, due to higher seagrass cover and clearer water (less light attenuation in the water column). Higher cover of seagrass likely reduced sediment resuspension while increasing photosynthetic capacity (i.e., more photosynthetic biomass). The existence of turbid fringes of water near the upper shore and climate-related shifts in suspended sediment concentrations may interfere with predictions of shoreward migration of seagrass meadows, which are already complicated by interactions with upper intertidal vegetation (e.g., mangroves) and human built structures (e.g., seawalls).

The Interaction Between Internal Solitary Waves and Submarine Canyons

Internal solitary waves (ISWs) occur in stratified seawater and are characterized by substantial amplitudes and short periods. ISWs engender significant interactions between seawater and seafloor topography. They resuspend seabed sediments in various ways, and the submarine canyon acts as a main channel for transporting the suspended sediments from the shallow sea to the deep sea. Among the seas worldwide where ISWs are prevalent, the South China Sea exhibits the largest amplitude of ISWs, and the ISWs directly interact with the seafloor topography near the Dongsha area in the northern South China Sea. Through the analysis of the water profile and high-resolution multi-beam detection data in the vicinity of the Dongsha area, it was found that the submarine canyon exerts a great impact on the sediment resuspension and the bottom nepheloid layer formation induced by ISWs. The observation results show that the concentration of suspended particulate matter within the submarine canyon is remarkably higher than that outside the canyon. The concentration at the bottom of the canyon can reach twice that of the outside regions, forming a vast bottom nepheloid layer. In addition, with the presence of the submarine canyon, the suspended matter carried by the ISWs accumulates in the water column above the canyon, leading to a higher concentration above the canyon. This study deepens the understanding of the effect of ISWs on sediment resuspension in submarine canyons.

Selective Sorting and Degradation of Permafrost Organic Matter in the Nearshore Zone of Herschel Island (Yukon, Canada)

AbstractErosion of permafrost coasts due to climate warming releases large quantities of organic carbon (OC) into the Arctic Ocean. While burial of permafrost OC in marine sediments potentially limits degradation, resuspension of sediments in the nearshore zone potentially enhances degradation and greenhouse gas production, adding to the “permafrost carbon feedback.” Recent studies, focusing on bulk sediments, suggest that permafrost OC derived from coastal erosion is predominantly deposited close to shore. However, bulk approaches disregard sorting processes in the coastal zone, which strongly influence the OC distribution and fate. We studied soils and sediments along a transect from the fast‐eroding shoreline of Herschel Island—Qikiqtaruk (Yukon, Canada) to a depositional basin offshore. Sample material was fractionated by density (1.8 g cm−3) and size (63 μm), separating loose OC from mineral‐associated OC. Each fraction was analyzed for element content (TOC, TN), carbon isotopes (δ13C, Δ14C), molecular biomarkers (n‐alkanes, n‐alkanoic acids, lignin phenols, cutin acids), and mineral surface area. The OC partitioning between fractions changes considerably along the transect, highlighting the importance of hydrodynamic sorting in the nearshore zone. Additionally, OC and biomarker loadings decrease along the land‐ocean transect, indicating significant loss of OC during transport. However, molecular proxies for degradation show contrasting trends, suggesting that OC losses are not always well reflected in its degradation state. This study, using fraction partitioning that crosses land‐ocean boundaries in a way not done before, aids to disentangle sorting processes from degradation patterns, and provides quantitative insight into losses of thawed and eroded permafrost OC.

Modeling the inhibition effect of recreating submerged plants habitat on sediment resuspension in Lake Gehu

Modeling the inhibition effect of recreating submerged plants habitat on sediment resuspension in Lake Gehu

On the Effects of Turbulence Modulation Driven by Suspended Sediment Stratification in Emergent Rigid Canopies

AbstractAquatic vegetation has the potential to increase suspended sediment capture while also increasing sediment resuspension and bedload transport. Suspended sediment can induce density stratification, which modulates the turbulence in the water column. We derive a Rouse‐based formulation for suspended sediment concentration (SSC) including the effect of sediment‐induced density stratification. We perform Large Eddy Simulations of vegetated and non‐vegetated channels to explicitly highlight the effect of stratification on SSC profiles. We found that the impact of stratification is dominant in the near‐bed region within the bottom boundary layer, affecting both sediment resuspension and bedload transport. Stratification reduces the likelihood of both dominant sweep and ejection events in the near the bed region which may affect sediment entrainment and bedload transport. Modifications to existing models of sediment entrainment and bedload transport are suggested to account for the effects of sediment induced stratification in vegetated and non‐vegetated channels.

Spatial and temporal dynamics of near-bottom dissolved oxygen in the central basin of Lake Erie

Abstract Eutrophication continues in Lake Erie and low oxygen concentration remains a concern in the central basin of the lake. Summertime dissolved oxygen concentrations can be hypoxic (low dissolved oxygen) and anoxic (dissolved oxygen <1 mg l-1) in the hypolimnion. We examined the spatial and temporal patterns of hypoxia in the central basin along a ∼ 26 km west-east station transect in the western portion of the central basin (depth gradient from 11.4 m in the west to 20 m in the east). Water column properties were monitored using moored instruments (dissolved oxygen, temperature, turbidity, Chlorophyll a fluorescence) and instrument profiling during cruises in 2008 and 2009. Hypoxia was examined using a biologically relevant value of 40% dissolved oxygen saturation (i.e. ∼4 mg l-1) and 25% dissolved oxygen saturation (i.e. ∼2.5 mg l-1) as used by government agencies. Our goals were to determine the onset and location of hypoxia, as well as the frequency of hypoxic events. We observed differences in the spatial and temporal patterns between the two years, related potentially to different water levels, stronger winds, and a smaller hypolimnion and lower thermocline depth in 2009. Near-bottom hypoxia occurred in the east at the end of June and extended westward by the end of July 2008 using 40% saturation; in early July to mid-August using 25% saturation. The onset of hypoxia (40% sat) occurred earlier in the west in 2009 but was similar to 2008 using 25% saturation. Hypoxia was not static, rather there were a total of ∼100 events of both levels of hypoxia, which were of different duration, noted across the transect in both years. Both the frequency and duration of hypoxic events (> 1 min) were higher and longer in 2008, perhaps related to water circulation and the resuspension of bottom sediment by synoptic-scale storms, which coincided with low oxygen events. Understanding the spatial and temporal patterns of hypoxia provides insight into their effects on habitat quality as well as biogeochemical processes in benthic and hypolimnetic environments in Lake Erie.

Prediction of the Turbidity Distribution Characteristics in a Semi-Enclosed Estuary Based on the Machine Learning

This study addresses the critical challenge of predicting sediment behavior in a semi-enclosed estuary, where the interplay between artificial freshwater discharge and seawater significantly impacts turbidity. Such environments are characterized by complex hydrodynamic interactions that lead to cycles of sediment settling and resuspension, influenced by tidal forces. To tackle this problem, we employed machine learning, leveraging its capability to analyze and predict complex non-linear phenomena. Our approach involved extensive transect observations conducted over two years, encompassing 11 ebb tide and 9 flood tide cycles. These observations were crucial for training the machine learning model, ensuring it captured the nuanced dynamics of sediment behavior under varying hydrodynamic conditions. The necessity of this research lies in its potential to enhance our understanding of sediment dynamics in estuaries, a vital aspect for environmental management and engineering projects. The findings demonstrate a promising alignment between the machine learning model’s predictions and the theoretically assumed sediment behavior, highlighting the model’s effectiveness in deciphering and predicting turbidity patterns in these challenging environments.

Influence of a small submarine canyon on biogenic matter export flux in the lower St. Lawrence Estuary, eastern Canada

Abstract. Submarine canyons enhance shelf–slope sediment exchange and influence hydrodynamic processes, with consequences for biogeochemical cycles. This work documents variations in the vertical export of biogenic matter on the northern shore of the lower St. Lawrence Estuary (LSLE, Quebec, eastern Canada), which is characterized by the presence of an active submarine canyon system. A total of three moorings were deployed from November 2020 to September 2021. One nearshore mooring (PDMc) was deployed in the main axis of the Pointe-des-Monts (PDM) canyon system and was equipped with an acoustic Doppler current profiler (ADCP), and two moorings equipped with sediment traps were deployed in the distal PDM canyon system (PDM-154, PDM-224) and offshore Baie-Comeau (BC-133). The ADCP data revealed the occurrence of a minor sediment remobilization event (December 2020) and a small turbidity current (February 2021) in the canyon. Concurrent elevated fluxes of total particulate matter, particulate organic carbon, particulate nitrogen, and chloropigments showed that these events left a signature in sediment traps PDM-154 and PDM-224 located > 2.6 km further offshore by enhancing lateral dispersion of resuspended sediments. The composition of diatom and dinoflagellate assemblages was similar in the canyon system and offshore BC, but the diatom bloom occurred 2 weeks earlier (in mid-April) at the PDM site. A bloom of the potentially toxic diatom Pseudo-nitzschia seriata was also observed during the second half of September 2021 at the BC site. Annual diatom and dinoflagellate fluxes were almost 2 times lower at the PDM site than at the BC site, possibly due to differences in riverine input and the structure of the water column, as well as increased sediment input and resuspension at the PDM site, leading to limited light availability. This study notably helps identify the relationship between near-bed canyon processes and biogenic matter export in the water column, thereby directly influencing the ecosystem offshore PDM. The study period further covered an anomalously nearly ice-free winter, and thus, in the context of climate change, it provides valuable insight into future trends of biogenic matter export in the LSLE.

Reynolds-averaged Navier-Stokes simulation of nearshore Langmuir circulation and the formation of oil-particle aggregates

Langmuir turbulence in the inner shelf coastal ocean is characterized by Langmuir circulation (LC) or cells that can span the full depth of unstratified water columns. A Reynolds-averaged Navier-Stokes (RANS) simulation strategy resolving full-depth LC coupled with an oil-particle aggregate (OPA) formation model is introduced. It is seen that full-depth LC generated by wind and waves under storm conditions can result in sediment resuspension and oil droplet entrainment (in the case of an oil spill) and subsequent mixing between these two, leading to significant OPA formation. This conclusion comes from a simulation in which various classes of oil droplets (with diameters ranging between 40 and 140 μm) were released at the surface, each class initialized with a 0.1 kg m−3 concentration. The full-depth LC led to most of the oil becoming trapped within OPAs in the first 5 min. The majority of the larger oil droplets were quickly aggregated with sediments near surface, whereas the smaller oil droplets were first submerged by the downwelling limbs of the Langmuir cells, and eventually aggregated with sediment while being carried upwards by the upwelling limbs. The OPAs conglomerated in the form of clouds transported by the action of the cells, with the heavier OPAs eventually settling within the upwelling limb of the cells while slowly depositing to the bed over time.

The coupled effect of sediment resuspension and microbiota on phosphorus release and transformation in a simulated aquatic ecosystem

Plant microbial involvement in phosphorus (P) release and transformation during resuspension events in bottom aquatic systems remains poorly understood. Laboratory experiments were used in this study to explore the release and transformation of P from sediments and the microbiota involved. Dissolved and labile P were obtained by high-resolution dialysis and the diffusive gradients in thin films (DGT) technique, respectively. The bacterial community was investigated using a scanning electron microscope and 16S rRNA high throughput sequencing technique. Finally, the diffusion fluxes dynamics of phosphorus were calculated using the DGT-induced fluxes in sediments and soils (DIFS) model. These results showed that sustained disturbance can balance dissolved P concentrations in the resuspension layer (overlying water) and enhance the stability of P transformation and release processes. Notably, the bacterial community within the deeper sediment layer (Below 10 mm sediment depth) significantly influences P interception. Acidobacteria and Neobacteria were identified as the key bacterial genera responsible for phosphorus adsorption of plant roots at speeds of 150 rad/min (R150). In addition, as the sediment depths increased, a slower decrease in R curves fitted with the DIFS model, which indicated a continuous resupply process from solid phase to pore water, especially below R150. This study provides theoretical references for managing shallow lakes with frequent sediment resuspension.

The effects of wind-wave disturbances on sediment resuspension and phosphate release in Lake Chao

As a typical shallow lake with a wind-generated flow, the resuspension state of sediment and phosphorus release under wind field disturbance plays an important role in controlling lake eutrophication in Lake Chao. In this study, we proposed a combination of experimental analysis of dynamic disturbances, wind-wave disturbance shear stress calculation, and model simulation (experimental-calculative-modeling) to quantitatively investigate the effects of wind-wave disturbances on the resuspension state of Lake Chao bottom sediment and phosphorus release and distribution. The results showed that the release rate of phosphorus from the Lake Chao bottom sediment was affected by the wind field and bottom sediment content, which varied significantly spatially and showed some difference between different seasons. Under the condition of sufficient water body disturbance, the substrate in the Western Lake area of Lake Chao mainly adsorbed phosphate from the water body, while the substrate in the Central Lake area and the Eastern Lake area adsorbed phosphate along with the release. The magnitude of the phosphorus release rate due to sediment resuspension was mainly affected by wind speed, and the distribution of phosphorus content was influenced by the circulation generated by different dominant wind directions. The wind-wave disturbances have a significant effect on the spatial and temporal distribution of phosphorus in Lake Chao, and the proposed experimental-calculative-modeling ensemble can provide relevant technical support for the study of water pollution control strategies and comprehensive remediation and management of Lake Chao.

Legacy contaminant trends in the Great Lakes uncovered by the wildlife environmental quality index

Since the 1970s, wildlife managers have prioritized the recovery of Great Lakes ecosystems from contamination by Persistent Organic Pollutants (POPs). Monitoring and quantifying the region's recovery is challenged by the diversity of legacy contaminants in the environment and the lack of benchmarks for their potential biological effects. We address this gap by introducing the Wildlife Environmental Quality Index (WEQI) based on prior water and sediment quality indices. The tool summarizes, in a single score, the exposure of wildlife to harmful levels of multiple contaminants – with harmful levels set by published guidelines for protecting piscivorous wildlife from biological impacts. We applied the new index to a combined Canadian and American dataset of Herring Gull (Larus argentatus) egg data to elucidate trends in wildlife for eight legacy industrial pollutants and insecticides in the Great Lakes. Environmental quality of the Great Lakes region (as indexed by WEQI) improved by 18% between 2002 and 2017. Improvement came from reductions in both the scope of contamination (the number of guideline-exceeding contaminants) and its amplitude (the average size of guideline exceedances) at bird colonies. But recovery was unequal among lakes, with Lake Erie showing no improvement at one extreme. Weakly- or non-recovering lakes (Erie, Ontario, Huron) were marked by inconsistent improvement in scope and amplitude, likely due to ongoing loading, sediment resuspension and other stressors reported elsewhere. Fast-recovering lakes (Superior and Michigan), meanwhile, improved in both scope and amplitude. Contrasting trends and contaminant profiles (e.g., exceedances of PCBs versus DDTs) highlight the importance of lake-specific management for equalizing recoveries. Lower environmental quality at American than Canadian colonies, particularly in Lake Huron, further suggest uneven success in – and opportunities for – the binational management of wildlife exposure to legacy contaminants.

Air‐Sea Gas Exchange and Its Potential Influence on the Regional Fate of Polycyclic Aromatic Hydrocarbons in the East China Marginal Sea

AbstractTo investigate the air‐sea gas exchange and its potential influence on the regional fate of polycyclic aromatic hydrocarbons (PAHs) in the East China Marginal Seas (ECMS), which consist of the East China Sea (ECS) and Yellow Sea (YS), we collected air and surface seawater samples of this area in the summer 2018 and winter 2019, respectively. Generally, PAHs underwent a strong volatilization process in the ECMS in both summer and winter. Good correlations between wind speed and the magnitude of air‐sea gas exchange were found for low molecular weight PAHs, suggesting the rate of their air‐sea gas exchange was influenced by the static stability of overlying atmosphere. However, such an influence for high molecular weight PAHs was constrained by their low Henry's law constant. Dissolved concentration of PAHs in surface seawater was another key factor regulating their air‐sea gas exchange, which not only influenced the rate of air‐sea gas exchange but also was involved in the exchange direction. Higher air‐sea gas exchange fluxes of PAHs in winter were attributed to their increasing dissolved concentrations in seawater during this season. A mass conservation analysis revealed a huge volatilization loss of PAHs from seawater to atmosphere, suggesting air‐sea gas exchange might be a key process to modulate the distribution and occurrence of PAHs in the ECS and YS. Such a loss of PAHs in seawater might be compensated by the sediment resuspension, which implied that the sedimentary deposit could serve as a secondary source of PAHs in seawater and overlying atmosphere.

Investigation of ship-induced hydrodynamics and sediment resuspension in a restricted waterway

The movement of the ship-generated waves on restricted shallow waterways creates complex river dynamics. The irregular bathymetry increases the bed shear stress in the waterway, causing significant sediment resuspension. In order to investigate the impact of the ship-generated waves on the sediment resuspension, field measurements were performed at the Falta stretch of the Hooghly river, India (National Waterway 1). The present study applied the acoustic backscattering intensity of a horizontally-looking ADCP to quantify the sediment resuspension generated by ship waves. Based on the low-pass filter, the primary and secondary waves were separated. Based on the analysis, the low-frequency primary waves are less significant compared to the high-frequency secondary waves. The correlation study suggested that the ship-induced sediment resuspension is highly influenced by irregular bathymetry, river geometry, depth Froude number and ship speed. Finally, two new empirical models were proposed to estimate the sediment resuspension due to ship-generated waves by using correlation study and regression analysis.

Tidal Variability of Phytoplankton Distribution in the Highly Turbid Changjiang River Estuary: Mechanisms and Implications

AbstractPhytoplankton distribution in macrotidal estuaries varies strongly due to highly dynamic conditions, contributing to the complexity of aquatic environments and marine ecosystems. Here the Changjiang River Estuary was analyzed as an example to explore this issue, based on field data collected through multiple cruises (including one covering a complete spring‐neap tidal cycle) and a fully coupled hydrodynamics‐sediment‐ecosystem numerical model. The composite analysis of all cruise data clearly showed that the chlorophyll concentrations were relatively low on lunar days 14–17 and 28–2, which corresponded to spring tides. Mechanistic analysis indicated that during spring tides, the river plume front retracts upstream with enhanced vertical mixing. This process restricts the transport of fluvial nutrients while increasing sediment resuspension, and therefore inhibiting net phytoplankton growth. During non‐spring tides, sediment resuspension was reduced but the river plume was further extended with slowed tidal currents, which enhanced the onshore light availability and offshore nutrient conditions and hence stimulated phytoplankton blooms in a larger area. The onshore shifting trend of the chlorophyll maximum region could last till the following neap tide with improved onshore light conditions and activate more conservative nutrients. The non‐conservation of nutrients augments the uncertainty of biomass variations during non‐spring periods. Besides in the spring‐neap tidal cycle, the phytoplankton variability between the perigean and apogean spring tides was also revealed. This study contributes to an improved understanding of intertidal variations of phytoplankton bloom and underlying physical‐sedimentological‐biological coupling mechanisms in tidal estuaries.

Tidally‐Modulated Turbidity and Density Fluctuations in a Rotationally‐Modified Gravity Current

AbstractRotationally‐modified gravity currents are known to occur on the ocean bottom in many locations around the world. Although their time scales are long enough that the Coriolis force is important in their dynamics, tidal forcing may also play a role in intermediate‐term variability by periodically enhancing or diminishing the flow and bottom friction, although the importance and implications of this variability has not previously been studied. Here we investigate hourly‐to‐tidal variations in the turbidity and density of intermittent gravity currents associated with deep‐water renewal events in the Strait of Georgia, Canada. We find that features such as the height and magnitude of the gravity current's nose, as well as bottom stress, undergo tidal variations, and that sediment resuspension largely occurs at the times of greatest stress at peak flood, with highest turbidities occurring in the hours following that peak. Larger particles soon settle again, but smaller ones can remain suspended. In addition, there is a significant tidally‐driven lateral advection of the gravity current, which results in density fluctuations in observations at a fixed location.

Unveiling microplastic distribution and interactions in the benthic layer of the Yangtze River Estuary and East China Sea

Microplastics (MPs), recognized as an emerging global environmental concern, have been extensively detected worldwide, with specific attention directed towards the Yangtze River Estuary (YRE) and East China Sea (ECS) regions. Despite their critical research significance, there remains a knowledge gap concerning the distribution of MPs in the benthic layer within this area, particularly regarding interactions governing their occurrence. Here we illuminate the distribution of MPs within the benthic layer and unravel the intricate interplay between bottom water and sediment in the YRE and ECS. We find that MPs are notably more abundant in bottom water, ranging from 8 to 175 times higher than in surface water. These MPs predominantly consist of polyester fibers, exhibit a size range between 0.5 and 5.0 mm, and display distinct coloration. Co-occurrence network analysis and Principal Coordinate Analysis confirm a robust correlation between MPs in bottom water and sediment, signifying the pivotal role of bottom water in mediating the distribution and transportation of MPs within the benthic layer. Furthermore, a positive correlation between MPs in sediment and bottom water turbidity underscores the impact of surface sediment resuspension and upwelling on MPs distribution. This study clarifies the intricate interactions within the benthic layer and highlights the crucial role of bottom water as a mediator in the vertical distribution of MPs, advancing our understanding of the “source-to-sink” transport processes governing MPs within water-sediment systems.