Coastal ecosystems are known for their ability to sequester organic carbon (OC), termed “blue carbon”. The molecular composition of dissolved organic matter (DOM) can affect sediment OC content; however, the impact of benthic bioturbation on DOM properties and OC storage stability is not well understood. This study examined the effects of bioturbation by fiddler crabs on DOM molecular properties and OC storage stability along the Chinese coastline. These findings indicate that crab bioturbation enhanced the release of labile molecules by 59% on average. This increase is controlled by the coupling reactions of iron and manganese minerals, and is influenced by climatic gradients. Moreover, fiddler crab bioturbation diminishes the durability of blue carbon storage, with the most significant effects observed in mangrove forests, followed by bare mudflats, tidal creek banks, and saltmarshes. These results underscore the critical role of benthic bioturbation in global blue carbon budgets.

Tributary bays, which are formed in river tributaries for reservoir construction with a relatively lentic environment, have been estimated to hold unique biogeochemical processes and to be involved in ecological issues (e.g., algal bloom). Nevertheless, the role of tributary bays in carbon cycling is unclear, precluding an accurate assessment of the carbon budgets of reservoirs. To fill this knowledge gap, we take the Three Gorges Reservoir (TGR) as an example and assess the carbon budget of a typical tributary bay (Xiangxi River, XXR) of TGR. With laboratory experiments and numerical models, we demonstrate that XXR may be an important carbon sink. This study exhibits that primary productivity enhancement (e.g., algal bloom) in tributary bays plays a critical role in CO2 sequestration and contributes significantly to carbon burial. Moreover, the organic carbon burial flux of tributary bays in TGR is estimated to be equivalent to (22 +/- 9) % of that in the mainstream, demonstrating the crucial role of tributary bays in the carbon burial of TGR. Considering the scientific debate on whether the reservoir is a carbon sink or source, we suggest that the overlooking of tributary bays might lead to a significant error in assessing the reservoir carbon budget.

The intertidal aquifer is a crucial area that significantly influences the transport of heavy metals and the health of coastal environment. In this study, we used a 3-dimensional sampling method to analyze the spatial variability of heavy metals in the groundwater of the mangrove intertidal zone in China. The pollution level of heavy metals and their release processes into coastal waters through groundwater discharge were also explored. Our findings revealed high spatial heterogeneity of heavy metals occurred along the cross-shore, parallel-shore, and depth directions. Specifically, heavy metals exhibited inconsistent variation trends between the seawater infiltrated zone and groundwater discharge zone along the cross-shore direction. Fe, Cr, Cu, Zn, and Pb exhibited a similar variation trend along the cross-shore direction, but were opposite to that of Hg and As. In addition, most heavy metals showed higher concentrations in mangrove groundwater compared to inland pond water and coastal seawater, indicating that mangrove intertidal zone played a crucial role in sequestering heavy metals. The spatial variability of heavy metals can be attributed to different exchange types and magnitudes between the surface water and groundwater, as well as the complex biogeochemical reactions. Our study also identified a slight pollution level of Zn and Hg, and high groundwater discharge-derived fluxes of Hg and As compared with other study sites in the world. In addition, this study highlights the importance of considering different groundwater end-member, especially for redox sensitive heavy metals, when determining groundwater discharge-derived fluxes into the surface water in mangrove intertidal zone.

This paper presents results of large-eddy simulations (LES) of supercritical flow in semi-filled pipes and very narrow open channels. The bulk Reynolds number and Froude number are kept the same in both cases (Re=30,100 and Fr=1.28) by matching both the hydraulic radius and bulk velocities in the semi-filled pipe and the very narrow open channel. The open channel cross-sections induce turbulence anisotropy which leads to the formation of secondary currents of significant strength. The presence of secondary currents alters both the mean flow characteristics and the Reynolds stresses distributions. Contribution of the secondary currents to the streamwise flow is negative near the free surface and positive near the walls. The pre-multiplied spectra indicate absence of VLSM in both cases.

Fiddler crabs, as coastal ecosystem engineers, play a crucial role in enhancing biodiversity and accelerating the flow of material and energy. Here we show how widespread crab burrows modify the carbon sequestration capacity of different habitats across a large climatic gradient. The process of crab burrowing results in the reallocation of sediment organic carbon and humus. Crab burrows can increase more greenhouse gases emissions compared to the sediment matrix (CO2: by 17–30%; CH4: by 49–141%). Straightforward calculations indicate that these increased emissions could offset 35–134% of sediment carbon burial in these two ecosystems. This research highlights the complex interactions between crab burrows, habitat type, and climate which reveal a potential lower carbon sink function of blue carbon ecosystems than previously expected without considering crab burrows. Bioturbation in wetlands can increase carbon dioxide and methane emissions, partially offsetting their sediment carbon burial capacity, according to a large-scale data set from sediment samples collected along the Chinese coastline and laboratory incubations.

A numerical model is developed to investigate metal release from estuarine sediments. The model includes three‐dimensional (3D) large‐eddy simulation of water above the sediment‐water interface, 3D advective and diffusive transport within sediments by both physical and biological processes (bioturbation and bioirrigation), and biogeochemical processes within the sediment including reduction of electron acceptors, acid‐base reactions, and metal sorption and precipitation. The model was applied to explore the influence of overlying water chemistry and sediment chemical, physical and biological processes on metal release from sediments. Overlying water conditions (pH, salinity, oxygen saturation) may vary diurnally due to tidal cycling and control the short‐term release of metals, while over longer times (months), metals are controlled by long‐term averages of sediment convective processes (groundwater upwelling, hyporheic exchange and bioirrigation). Metal release is significantly enhanced when there is bioroughness and bioirrigation due to local oxidation of surficial sediments.

Large-eddy simulations (LES) of supercritical flow in a straight-wall, open-channel contraction of 6° and contraction ratio of 2:1 are performed. The LES code solves the filtered Navier-Stokes equations for two-phase flows (water-air) and employs the level-set method. The simulation was validated by replicating a previously reported experiment. Contours of the time-averaged velocities indicate that the flow loses energy and momentum in the contracting channel. Further, secondary currents in the contraction are redistributing momentum and are responsible for local up-and down-flows. The turbulent kinetic energy reaches very high values at the entrance of the contraction, mainly contributed by the streamwise normal stress. The flow contains coherent turbulence structures which are responsible for carrying low-momentum from the bed and the water surface towards the channel centre. Flow deceleration results in significant turbulence anisotropy in the contracted section. It is shown that mainly pressure drag contributes to the energy loss in the contraction.

水利工程的建设和运行将导致水沙发生变化、河床发生演变。这些变化还引起了河流中水、沙、营养盐之间的分配关系发生变化,从而导致床面生物膜、浮游动植物、底栖动物以及水生植物等相应随之改变。针对这些变化的系统研究,本文提出了生态河流动力学的概念及理论框架,梳理了生态河流动力学研究中涉及的多个方面的研究进展,包括水沙输移及微地形演变的物理过程、营养盐和污染物质随泥沙输移的化学过程、以及水体中及床面处各类生物过程对于水沙输移及微地形演变的响应等,探讨了水沙输移与河流生物化学过程之间的相互耦合关系,进而提出了生态河流动力学研究所面临的多学科交叉、多尺度耦合等方面的挑战。

Knowledge of the relationship between sediment motion and flow conditions is fundamental to our understanding of three-dimensional sediment dune development in river and coastal environments. In this study, numerical simulations were performed on a mobile flat sand bed. The simulation results provide important insights into the coupling between migrating bedforms and turbulent stratified flow in the open channel. The formation of micro sand waves can be divided into three stages. First, the initial defects appear on the bed at the beginning of the process and are closely correlated with the instantaneous flow velocity just before the bed is destabilized. Second, the defects in areas of high instantaneous flow velocity are washed away, while the defects in areas of low instantaneous flow velocity grow in length and height due to sediment deposition. Finally, a constant wake zone where sediment continues to accumulate forms downstream of the micro sand wave. Despite the formation of micro waves, the near-bed flow velocity and turbulent structures play important roles as sand passes from upstream dune crests to downstream ones. The high flow velocity breaks O-shaped dune crests and drives excess sand to the downstream dune crests. The near-bed vortices usually occur at the stoss sides of the dunes, and most are elongated in the spanwise direction.

In this paper the effects of boulder concentration on hydrodynamics and local and reach‐averaged sediment transport properties with a flow over submerged boulder arrays are investigated. Four numerical simulations are performed in which the boulders' streamwise spacings are varied. Statistics of near‐bed velocity, Reynolds shear stresses, and turbulent events are collected and used to predict bed load transport rates. The results demonstrate that the presence of boulders at various interboulder spacings altered the flow field in their vicinity causing (1) flow deceleration, wake formation, and vortex shedding; (2) enhanced outward and inward interaction turbulence events downstream of the boulders; and (3) a redistribution of the local bed shear stress around the boulder consisting of pockets of high and low bed shear stresses. The spatial variety of the predicted bed load transport rate qs based on local bed shear stress is visualized and is shown to depend greatly on the boulder concentration. Quantitative bed load transport calculations demonstrate that the reach‐averaged bed load transport rate may be overestimated by up to 25 times when including the form‐drag‐generated shear stress of the immobile boulders in the chosen bed load formula. Further, the reach‐averaged bed load transport rate may be underestimated by 11% if the local variability of the bed shear stress is not accounted for. Finally, it is shown that for the small‐spaced boulder array, the bed load transport rates should no longer be predicted using a normal distribution with standard deviation of the shear stress distribution σ.