Tidal Dynamics Research Articles

Response of tidal dynamics and shear fronts to topographic changes in the Yellow River Delta

In recent years, the position of the Yellow River Estuary (YRE) entrance has changed frequently, and human activities such as land reclamation have contributed to the transformation of the deltaic topography. These combined factors have resulted in altered hydrodynamics and tidal shear fronts (TSFs) in the surrounding sea area. However, there are few studies on the characteristics of the TSFs before and after diversion, so this paper establishes a hydrodynamic model based on the Finite Volume Coastal Ocean Model (FVCOM) for the years 2005, 2014, and 2020 and analyzes the characteristics of the changes in the tidal currents and the TSFs before and after diversion and the long-term evolution trends. The results reveal that the M2 amphidromic point near the YRE shifted eastward by 4.9 km from 2005 to 2014 and migrated southeastward by 6.8 km between 2014 and 2020. Additionally, significant changes were observed in the maximum and residual currents within the active mouth (AM), the old Qing 8 (Q8) channel, the old QingShuiGou (QSG) channel, and the southeastern region. Notably, the residual currents exhibit vertical fronts with substantial current velocity differences across the slopes. After the diversion of the YRE, the northern TSFs disappeared. The TSFs in the AM gradually shifted landward, while the TSFs in the southeastern region shifted offshore. In the vertical direction, the frontal centerlines of the TSFs gradually moved offshore from top to bottom. The intensity of the TSFs at the same latitude was positively correlated with the offshore distance. Generally, steeper slopes were associated with larger bottom stress gradients, which in turn corresponded to stronger TSFs.

Land Reclamation Controls on Multi‐Centennial Estuarine Evolution

AbstractLand reclamations influence the morphodynamic evolution of estuaries and tidal basins, because an altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) will add more morphodynamic adaptation timescales. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal‐subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb and flood channel system, transitioning the estuary from a multichannel to a single channel system. We infer that the 20th‐century intensification of channel dredging and re‐alignment works accelerated rather than caused this development. The centennial‐scale observations show that the Ems estuary evolution corresponds to a land reclamation response following tidal asymmetry‐based stability theory as it moves toward a new equilibrium configuration with modified tidal flats and channels. Considering the long history of land reclamation in the Ems Estuary, it provides an analogy for expected developments in comparable tidal systems where land reclamations were recently carried out.

Internal tidal dynamics and associated processes at highly supercritical slopes in Banda Sea: Lessons from the oceanic island of Ambon, eastern Indonesia

Interactions between the deep sea and steep-sloping oceanic islands generate physical processes with potential impacts on sediment resuspension and ocean productivity. While studies have been conducted in the open oceans, those focusing on oceanic islands of the deep Banda Sea in eastern Indonesia are lacking. Here, we present the first observational evidence of vertical mechanisms (i.e. internal tidal reflection, internal hydraulic jumps) at the highly supercritical slope in outer Ambon Bay (OAB) in Ambon Island – an oceanic island in the Banda Sea. A 23-h CTD yoyo experiment combined with ADCP measurements conducted during spring flood and ebb tides demonstrated tidally-varying vertical temperature, salinity and density profiles. During spring flood tide, incoming internal tides were reflected by OAB's highly supercritical slope back to the deep sea with isopycnals and isotherms showing sharp downward plunges (downward vertical velocity = 6.5–8.3 × 10−3 m/s), and the internal tidal amplitude reaching 90–110 m. The reflection during flood tide caused seaward overturning flow at deeper depths despite the prevailing landward flow at the upper layers. During spring ebb tide, internal hydraulic jumps (upward vertical velocity = 6.1–6.48 × 10−3 m/s) occurred to rebound the downward plunges of isopycnals and isotherms as flood tide relaxed with the observed amplitude of internal tides of 90 m. We also observed weakened seaward ebb flow during the isothermal uplifting (when hydraulic jumps occurred), and subsequent intensified landward flow at the end of spring ebb tide indicating strong upslope flow when isotherms reached the maximum shoaling depths. Taken together, the observed vertical mechanisms indicate the conservation of energy at the highly supercritical slope of OAB evident by the comparable vertical velocities. An embedded turbidity-chlorophyll profiler in the CTD reveals that internal tidal activities at the highly supercritical slopes OAB may induce bottom nepheloid layers, and influence ocean productivity by regulating the vertical distribution of phytoplankton biomass.

Impact of human-induced alterations in bathymetry and geometry on tidal and salt dynamics: Thresholds for morphological dimensions in a convergent estuary

Estuary morphology changes significantly with intensive human intervention, resulting in dynamic structural variations in the estuary. However, little is known about the threshold sizes of estuary morphology that ensure water security. A two-dimensional width-averaged semi-analytical model is applied to explore the impacts of human-induced alterations in bathymetry and geometry on tidal and salt dynamics in Lingdingyang Bay (LDYB). Salt flux (Fs) is positively correlated with tidal flux (Ft) and negatively correlated with tidal energy flux (Fte). Greater geometric convergence reduces Ft and increase Fte. Increased water depth increases Ft and Fte. Thus, both greater geometric convergence and increased bathymetry enhance tidal hydrodynamics. Saltwater transport is weakened by greater geometric convergence and enhanced by increasing water depth. Sensitivity experiments are conducted using maximum water level and salt intrusion length as the criterion. Thresholds for morphological sizes in the LDYB are found to be a convergence ratio (convergence length over mouth width,LcB0) of 0.83–0.88 and a combined depth factor (α) of 1.04–1.07. Moreover, transition of the estuarine morphology from an exponential convergent shape to a prismatic shape had a limited effect on tidal hydrodynamics when the convergent length rate (e-folding convergence length over total convergence length,LeLc) is >0.61. These results notice that human activities, such as reclamation and tidal channel dredging, exacerbate the risk of salt intrusion and should be applied with caution.

Long-term alien marsh grass in China brings high carbon uptake capacity but cannot sustain high-temperature weather

Coastal wetlands, crucial in the global carbon cycle, face increasing challenges brought by extreme climate events, particularly high temperatures above plant tolerance thresholds. These conditions often exert great impact on plant, thereby potentially reducing overall ecosystem productivity. However, it has been observed that alien species, typically exhibiting higher productivity compared to native plant. Would plant invasion offset the loss of productivity caused by high-temperature events at ecosystem scale? In this study, we utilized data from 2020 to 2023 in China's Yangtze Estuary to investigate the responses of Spartina alterniflora (alien) and Phragmites australis (native) to high-temperature stress. Our results demonstrate that though the alien vegetation exhibits higher productivity before high temperature events, it experiences significant declines during high temperatures. In average, net ecosystem productivity (NEP) and gross primary productivity (GPP) of alien plant drops by 21.03% overall, with a notable 29.59% reduction during Neap tide. In contrast, native vegetation maintains a more stable productivity profile under the same conditions. Spring tide alleviate the negative impact of high temperatures on the alien vegetation, exhibiting a distinct environmental buffering effect. Photosynthetic photon flux density emerged as a crucial factor driving productivity, yet its effectiveness was moderated by aerodynamic conductance for heat transfer (Ga_h). Through the application of the Michaelis-Menten model, we confirmed that both species maintain similar maximum light utilization efficiencies, yet native vegetation demonstrates greater resilience to thermal stress. Additionally, we observed a 33.82% overestimation in productivity by the vegetation photosynthesis model (VPM) under high temperatures, emphasizing the need to refine how Ga_h impacts are integrated, particularly when comparing the resilience of native and alien species. We emphasize necessity of incorporating canopy structure factors into ecological models and underscore the importance of maintaining tidal dynamics for coastal wetland management.

Can We Protect China's Northern Coast Wetlands From Tidal Dynamics by Restoring Native Species After Spartina alterniflora Is Eradicated? A Model‐Based Analysis

ABSTRACTIn recent years, the Chinese government has implemented a series of initiatives with the objective of eradicating the invasive Spartina alterniflora along the entire coastline. This has given rise to concerns about the potential for subsequent coastal erosion and salt marsh shrinkage. This study introduces a novel contribution in the form of a model‐based analysis, grounded in fieldwork from the Yellow River Delta, which assesses the potential of native Suaeda salsa as an alternative for coastal protection. Our approach is distinctive in that it not only compares the flow velocity reduction effects of S. alterniflora and S. salsa, but also evaluates the efficacy of three distinct planting patterns in managing tidal dynamics. In this study, we present a significant finding: while both species can mitigate tidal flow velocity, S. alterniflora shows superior performance, which represents a novel point of discussion in the field of coastal wetland management. Furthermore, our research indicates that dense aggregated planting patterns are particularly effective during storm surges, offering a strategic insight for future ecological engineering. However, we recognize the potential limitations of our approach, including the need for continuous management to control the spread of S. alterniflora and the necessity for further research to optimize the restoration potential of S. salsa under varying tidal conditions. Our findings highlight the importance of a balanced approach, suggesting the retention of a portion of S. alterniflora while also emphasizing the strategic planting of S. salsa in high‐density configurations to enhance coastal resilience.

Vertical Distribution of Rocky Intertidal Organisms Shifts With Sea-Level Variability on the Northeast Pacific Coast.

Disentangling the effects of cyclical variability in environmental forcing and long-term climate change on natural communities is a major challenge for ecologists, managers, and policy makers across ecosystems. Here we examined whether the vertical distribution of rocky intertidal taxa has shifted with sea-level variability occurring at multiple temporal scales and/or long-term anthropogenic sea-level rise (SLR). Because of the distinct zonation characteristic of intertidal communities, any shift in tidal dynamics or average sea level is expected to have large impacts on community structure and function. We found that across the Northeast Pacific Coast (NPC), sea level exhibits cyclical seasonal variability, tidal amplitude exhibits ecologically significant variability coherent with the 18.6-year periodicity of lunar declination, and long-term sea-level rise is occurring. Intertidal taxa largely do not exhibit significant vertical distribution shifts coherent with short-term (monthly to annual) sea-level variability but do exhibit taxa-specific vertical distribution shifts coherent with cyclical changes in lunar declination and long-term SLR at decadal timescales. Finally, our results show that responses to cyclical celestial mechanics and SLR vary among taxa, primarily according to their vertical distribution. Long-term SLR is occurring on ecologically relevant scales, but the confounding effects of cyclical celestial mechanics make interpreting shifts in zonation or community structure challenging. Such cyclical dynamics alternatingly amplify and dampen long-term SLR impacts and may modify the impacts of other global change related stressors, such as extreme heat waves and swell events, on intertidal organisms living at the edge of their physiological tolerances. As a result, intertidal communities will likely experience cyclical periods of environmental stress and concomitant nonlinear shifts in structure and function as long-term climate change continues. Our results demonstrate that consistent, large-scale monitoring of marine ecosystems is critical for understanding natural variability in communities and documenting long-term change.

How do morphological characteristics affect tidal asymmetry in the Radial Sand Ridges?

While it is widely recognized that the Radial Sand Ridges (RSR) in the South Yellow Sea are predominantly shaped by tidal forces, there remains a limited understanding of how this distinctive morphological configuration—characterized by an interlaced channel-ridge system—can subsequently influence local tidal dynamics. This study examines the effects of morphological features on tidal asymmetry, taking into account seabed slope, relative depths between ridges and channels, and channel convergence. Three principal indices—namely tidal-duration-asymmetry (TDA), peak-current-asymmetry (PCA), and slack-water-asymmetry (SWA)—are employed to quantify various dimensions of tidal asymmetry. The findings indicate that SWA serves as the most morphology-sensitive indicator, whereas TDA exhibits minimal sensitivity to morphological changes. Furthermore, seabed steepness emerges as a critical factor influencing tidal asymmetry within the RSR; steeper slopes enhance intrinsic energy conversion processes, thereby inducing tidal asymmetries. Additional analysis reveals that streamwise advection accounts for an average of 88 % of total advection scale while controlling for spatial heterogeneity. Specifically, the average integral sum of advection terms along submerged sand ridges is 2.53 times greater than that along the deepest section of the tidal channel line—a significant contributor to spatial variability in SWA. With a positive seabed slope, the apex of the RSR acts as a source for overtides which interact with incoming astronomical tides, consequently generating tidal asymmetries. Moreover, this study illustrates varying dependencies of tidal asymmetry on bottom stress across channels and ridges, contributing to spatial variability in arc direction among RSRs. Ultimately, this research elucidates complex interactions between tidal flow and morphological characteristics within RSRs and provides insights into tide evolution in analogous ebb-shoal systems.

Tidal control on aerobic methane oxidation and mitigation of methane emissions from coastal mangrove sediments

Mangrove forests represent important sources of methane, partly thwarting their ecosystem function as an efficient atmospheric carbon dioxide sink. Many studies have focused on the spatial and temporal variability of methane emissions from mangrove ecosystems, yet little is known about the microbial and physical controls on the release of biogenic methane from tidally influenced mangrove sediments. Here, we show that aerobic methane oxidation is a key microbial process that effectively reduces methane emissions from mangrove sediments. We further demonstrate clear links between the tidal cycle and fluctuations in methane fluxes, with contrasting methane emission rates under different tidal amplitudes. Our data suggest that both the microbial methane oxidation activity and pressure-induced advective transport modulated methane fluxes in the mangrove sediments. Methane oxidation activity is limited by the availability of oxygen in the surface sediments, which in turn is controlled by tidal dynamics, further highlighting the interactive physico-biogeochemical controls on biological methane fluxes. Although we found some molecular evidence for anaerobic methanotrophs in the deeper sediments, anaerobic methane oxidation seems to play only a minor role in the mangrove sediments, with potential rates being two orders of magnitude lower than those of aerobic methane oxidation. Our findings confirmed the importance of surface sediments as biological barrier for methane. Specifically, when sediments were exposed to the air, methane consumption increased by ∼227%, and the methane flux was reduced by ∼62%, compared to inundated conditions. Our data demonstrate how tides can orchestrate the daily rhythm of methane consumption and production within mangrove sediments, thus explaining the temporal variability of methane emissions in the tidally influenced coastal mangrove systems.

Hydrodynamic Analysis of Sediment Deposition in Mangrove Wetlands of Zhanjiang Bay-Based on the Perspective of Sediment Grain Size

Grain size is an important parameter reflecting the characteristics of sedimentary dynamics, and mangrove wetland sediments record abundant sedimentary environmental information during transportation and deposition. To investigate the vertical characteristics of sediment grain size and sedimentary dynamic processes in the mangrove wetlands of Zhanjiang Bay, this study analyzed the grain size parameters, frequency distribution curves, and hydrodynamic characteristics of sediment cores collected from the mangrove wetlands in Nansan Island (A), the south of Techeng Island (B), Guanhai Promenade (C), Dongtoushan Island (D), Huguang Town (E), and the west of Donghai Island (F). The objective was to reveal the evolution of sedimentary dynamics in Zhanjiang Bay over the past century. The results showed that: (1) There were significant differences in the grain size characteristics among the sediment cores. Cores A and B had poor sorting and flat kurtosis, with sediment types of silty sand. Core C had a sinuous mean grain size and kurtosis curve, poor sorting, positive skewness, and a sediment type of silty sand. Core D had negative skewness, poor sorting, and moderate kurtosis, with a sediment type of sandy silt. Cores E and F had negative skewness, poor sorting, and large variations in kurtosis, with sediment types of sandy silt; (2) The grain size frequency distribution curves at depths of 25 cm, 50 cm, 75 cm, and 100 cm in each sediment core exhibited bimodal and unimodal characteristics, indicating the involvement of multiple sedimentary media in the depositional process. Cores A, B, C, and D had grain sizes concentrated on the sandy particle end, indicating strong sedimentary dynamics. Cores E and F had prominent silt-sized grains, indicating weaker sedimentary dynamics; (3) The grain size characteristics of the sediments were related to their terrestrial or island origins and the sediment supply from runoff. The depositional processes were deeply influenced by tidal dynamics, mangrove distribution, and human activities. The accelerated deposition rate in Zhanjiang Bay in recent years may affect navigation channels, and thus, attention should be paid to the ecological environment of wetlands within the bay.

Detangling past and modern zinc anthropogenic source contributions in an urbanized coastal river by combining elemental, isotope and speciation approaches

The accumulation of trace metals in the environmental compartments of coastal rivers is a global and complex environmental issue, requiring multiple tools to constrain the various anthropogenic sources and biogeochemical processes affecting the water quality of these environments. The Valao fluvio-estuarine system (Rio de Janeiro, Brazil) presents a challenging case of a coastal river contaminated by both modern and historical anthropogenic metal sources, located in the land and in the intra-estuary, continuously mixed by tidal cycles. This study employed a combination of spatial distribution analysis of trace metals including gadolinium (Gd), zinc (Zn) isotopic analyses, and X-ray absorption spectroscopy (XAS) to distinguish between these sources. The concentrations of metals in both dissolved (water samples) and surficial sediment compartments (Suspended Particulate Matter and sediment samples) display an overall enrichment trend from upstream to downstream. Multivariate statistical analysis allows to discriminate geogenic elements derived from watershed geology (Ti, K, and Mg) vs anthropogenic contaminants from urban runoff and domestic sewage discharges (Cu, Cr, Pb, Zn, and Gd); and legacy metal contaminants (Zn and Cd) remobilized from ancient metallurgical wastes and transported upstream in the estuary during tidal cycles. The anthropogenic Gd concentration in the dissolved compartment increases along the watercourse, highlighting continuous ongoing sewage discharge. Zinc solid speciation also indicates that Zn contribution from legacy metallurgy waste is primarily associated with sulfide-Zn and Zn-phyllosilicate in the outlet estuary, while in upstream sediments of fluvio-estuarine system, Zn is found bound to organic matter. Zinc isotope systematically reveals a progressive downstream shift to heavier isotope compositions. Upstream, the relatively pristine site and the urbanized section of the river exhibit a relatively uniform δ66/64Zn value (+0.20 ± 0.07 ‰) in suspended particulate matter (SPM) and surficial sediments. These results indicate that domestic sewage discharges contribute to Zn enrichment in sediments of the Valao fluvio-estuarine system but without modifying its isotope signature in sediments. The sediment of the downstream estuarine section shows a heavier δ66/64Zn value (+0.48 ± 0.08 ‰), indicating the strong influence of the intra-estuarine source identified as the historical metallurgic contamination. An integrated view of the geochemical tracers allows thus inferring that the untreated sewage and legacy metallurgical contamination are the primary sources of anthropogenic Zn contamination. It highlights the progressive mixing along the estuarine gradient under tidal dynamics. The influence of the former source continuously expands from the headland towards the estuary.

Estimating common whelk (Buccinum undatum) abundance using baited remote underwater video (BRUV)

Abstract Baited static gear fisheries targeting benthic invertebrates have expanded at a global scale. While improvements have been made in the monitoring and management of these fisheries, reliable survey methods for stock assessments of many key species are lacking. In this study we examine the viability of a baited remote underwater video (BRUV) system for obtaining abundance indices for a data-limited stock (common whelk, Buccinum undatum), and compare this method to catch per unit effort (CPUE) from commercial fishing. BRUVs capable of collecting 15 hours of timelapse footage were deployed on pot strings in two distinct whelk fishing sites, with replication over a spring-neap tidal cycle. Three potential BRUV metrics—the maximum (MaxN) and mean (MeanN) abundance and time of first arrival (T1)—were calculated for each deployment, and significant linear relationships were identified between MaxN, MeanN, and CPUE across sites. Temporal variability in BRUV indices driven by tidal dynamics was minimal, although results suggest high current speeds may reduce abundance estimates, and a potential method for predicting density on the seabed using arrival rates is also demonstrated. BRUVs are a valuable tool in developing stock assessment surveys for static gear fisheries.

Spatio-temporal dynamics of net primary productivity and the economic value of Spartina alterniflora in the coastal regions of China

This study employs an improved Carnegie–Ames–Stanford Approach (CASA) model to calculate the Net Primary Productivity (NPP) of Spartina alterniflora (SA) and various other land use/land cover types (LULC) across coastal China over multiple years. The research aims to provide significant theoretical and practical insights into carbon sink research in coastal zones, sustainable development, and resource management. Key findings include identifying the first εmax value of 2.219 g C/MJ for SA, addressing a critical data gap in CASA modeling research on invasive plants. SA's NPP exhibited higher values in Shanghai and Zhejiang due to factors such as genetic diversity, invasion duration, and tidal dynamics. In contrast, other LULC exhibited higher NPP values in southern and inland regions, characterized by greater vegetation cover and favorable growing conditions. In 2020, SA and other LULC sequestered 16.352 kt C and 0.821*106 kt C, respectively. From 2000 to 2020, the average annual NPP and total carbon storage of SA and other LULC increased significantly, primarily driven by Shanghai and deciduous needleleaf forests, respectively. Seasonal NPP trends followed summer> spring> autumn> winter, influenced by climate conditions and plant life activities. Economic assessments in 2020 estimated SA's carbon storage value at RMB0.409 billion (Market Value method) or RMB5.562 billion (Carbon Tax method), with RMB2.054 billion attributed to oxygen release values, underscoring its economic and ecological potential. Among other LULC, evergreen broadleaf forests showed the highest carbon storage value (RMB183.463 billion). The study emphasizes the critical role of all LULC in carbon storage and oxygen release, advocating for targeted conservation and land management strategies. It suggests that managing SA should balance stringent control in high-risk areas, lenient measures in low-risk areas, eradication of scattered populations, and maximizing ecological benefits in retention areas, with continuous monitoring and adaptive management strategies to balance conservation and development efforts.

Evaluation of water quality fluctuation in the tidal reach under the impact of on shore wastewater discharges based on MIKE 21 model in dongguan, China

The text discusses the study of water quality simulation in the lower reaches of the Dongjiang River basin—tidal river section. Utilizing MIKE 21 software, an integrated 2D hydrodynamic-water quality coupled mathematical model was constructed for the lower reaches and estuary area of the Dongjiang River. The model simulated the hydrodynamic characteristics near the wastewater outlet and the impact of pollutant discharge on the water quality in the study area. The simulation results indicate: (1) During wet and dry seasons, the overall flow pattern remains consistent, with the ebb current velocity slightly greater than the flood tide current velocity beside the outlet. The flow velocity is generally greater during wet season compared to dry season. Interestingly, the flow direction during flood and ebb tides is completely opposite and tides have a great impact on the maximum flow velocity in Xialu Connecting channel. (2) Under normal discharge conditions, the increment of COD, NH3-N, and TP in most areas of Zhongtang Waterway, Daoyun Waterway, and Hengchong Waterway is less than 5 mg/L, 0.1 mg/L, and 0.02 mg/L, respectively, indicating a minor impact on water quality; While in the event of an accidental discharge, the concentration increment is in the range of 5–30 mg/L, 0.1–0.25 mg/L, and 0.02–0.05 mg/L, respectively. COD causes the most significant water quality changes and will severely impact the water environment of the Dongjiang River basin. Furthermore, pollutant dispersion is more extensive during dry seasons due to reduced river flow and weaker dilution capacity. (3) Overall, the results underscore the critical importance of accounting for both tidal and seasonal dynamics in simulation of estuarine water quality.

Tidal Impacts on Zooplankton Dynamics in a Major Ocean-Lagoon Channel: Insights from a 25-Hour Intensive Survey in the Cotonou Channel, Benin

This study investigates the effects of tidal cycles on the zooplankton community within the Cotonou Channel, an important waterway connecting the large Nokoué Lagoon to the Atlantic Ocean in Benin. From the determination of zooplankton composition from 25-hour samples collected in July 2020, alpha diversity indices and abundance were assessed, while relationships between biotic and abiotic parameters were analyzed through Pearson correlation, analysis of variance, and principal component analysis. A total of 66 zooplankton taxa were identified, with rotifers exhibiting the highest species richness (35 taxa), while copepods dominated in abundance (71%). Zooplankton abundance varied significantly, ranging from 2 to 95 ind L−1 depending on the tidal phase. A negative correlation was found between species richness (r = −0.51, p < 0.01) and increasing salinity (3–37), indicating that higher salinity reduced diversity (r = 0.06, p > 0.05). Resilient species like Synchaeta bicornis persisted despite salinity changes. The tidal cycle structurally altered the zooplankton community, with abundance and diversity peaking at different phases, notably higher at high tide (15 ind L−1.) These initial findings underscore the complex interactions between tidal dynamics and estuarine biodiversity, suggesting the need for further research across different tidal and seasonal conditions to inform effective management and conservation efforts.

Internal-tide vertical structure and steric sea surface height signature south of New Caledonia revealed by glider observations

Abstract. In this study, we exploit autonomous underwater glider data to infer internal-tide dynamics south of New Caledonia, an internal-tide-generation hot spot in the southwestern tropical Pacific. By fitting a sinusoidal function to vertical displacements at each depth using a least-squares method, we simultaneously estimate diurnal and semidiurnal tides. Our analysis reveals regions of enhanced tidal activity, strongly dominated by the semidiurnal tide. To validate our findings, we compare the glider observations to a regional numerical simulation that includes tidal forcing. This comparison assesses the simulation's realism in representing tidal dynamics and evaluates the glider's ability to infer internal-tide signals and their signature in sea surface height (SSH). The glider observations and a pseudo glider, simulated using hourly numerical model output with identical sampling, exhibit similar amplitude and phase characteristics along the glider track. Existing discrepancies are in large part explained by tidal incoherence induced by eddy–internal-tide interactions. We infer the semidiurnal internal-tide signature in steric SSH by the integration of vertical displacements. Within the upper 1000 m, the pseudo glider captures roughly 78 % of the steric SSH total variance explained by the full water column signal. This value increases to over 90 % when projecting the pseudo glider's vertical displacements onto climatological baroclinic modes and extrapolating to full depth. Notably, the steric SSH from glider observations aligns closely with empirical estimates derived from satellite altimetry, highlighting the internal tide's predominant coherent nature during the glider's sampling.

Climate change impacts on a sedimentary coast—a regional synthesis from genes to ecosystems

Climate change effects on coastal ecosystems vary on large spatial scales, but can also be highly site dependent at the regional level. The Wadden Sea in the south-eastern North Sea is warming faster than many other temperate coastal areas, with surface seawater temperature increasing by almost 2 °C over the last 60 years, nearly double the global ocean mean increase. Climate warming is accompanied by rising sea levels, which have increased by approximately 2 mm yr−1 over the last 120 years. For this sedimentary coast, the predicted acceleration of sea-level rise will have profound effects on tidal dynamics and bathymetry in the area. This paper synthesises studies of the effects of ocean warming and sea level rise in the northern Wadden Sea, largely based on research conducted at the Wadden Sea Station Sylt of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. An increasing rate of sea level rise above a critical threshold will lead to coastal erosion and changes in sediment composition, and may cause the transition from a tidal to lagoon-like environment as tidal flats submerge. This involves changes to coastal morphology, and the decline of important habitats such as muddy tidal flats, salt marshes and seagrass meadows, as well as their ecological services (e.g. carbon sequestration). Ocean warming affects plankton dynamics and phenology, as well as benthic community structure by hampering cold-adapted but facilitating warm-adapted species. The latter consist mostly of introduced non-native species originating from warmer coasts, with some epibenthic species acting as ecosystem engineers that create novel habitats on the tidal flats. Warming also changes interactions between species by decoupling existing predator–prey dynamics, as well as forming new interactions in which mass mortalities caused by parasites and pathogens can play an understudied but essential role. However, Wadden Sea organisms can adapt to changing abiotic and biotic parameters via genetic adaptation and phenotypic plasticity, which can also be inherited across generations (transgenerational plasticity), enabling faster plastic responses to future conditions. Important research advances have been made using next-generation molecular tools (-omics), mesocosm experiments simulating future climate scenarios, modelling approaches (ecological network analysis), and internet-based technologies for data collection and archiving. By synthesising these climate change impacts on multiple levels of physical and biological organisation in the northern Wadden Sea, we reveal knowledge gaps that need to be addressed by future investigations and comparative studies in other regions in order to implement management, mitigation and restoration strategies to preserve the uniqueness of this ecosystem of global importance.

Tidal dynamic response to riverbed evolution in the Yangtze River Estuary

Since 1958, there have been significant changes in the Yangtze River estuary. Due to extensive reclamation and construction of ports and channels, the water area has drastically decreased, resulting in corresponding changes in hydrodynamics and riverbeds at the mouth of the river. According to the analysis of measured topographic data and Delft3D-FLOW model for seven typical historical periods since 1958 at the Yangtze River Estuary, this study investigates the characteristics of riverbed evolution and tidal flow dynamics. From 1958 to 2019, driven by strong human activities, the total area of the Yangtze River Estuary decreased from 2084 km2 to 1403 km2, with a decrease of 32.7%, while the total volume of the corresponding river channel changed slightly and remained stable. Compared with 1958, the volume of the Yangtze River Estuary in 2019 only increased by 345 million m3, with an increase of about 4.1%. The tidal dynamic change of the Yangtze Estuary is closely related to the riverbed evolution of each reach, which not only shapes the estuary landform, but also is affected by the riverbed evolution. Tidal level, tidal range and water area change are closely related. With the decrease of water area in the Yangtze River Estuary, tidal range tends to increase. Tidal prism change is closely related to channel volume. In the past 60 years, the tidal volume at the mouth of the Yangtze River has decreased by 8%. The research findings will provide technical support for enhancing flood control and tide resistance measures at the Yangtze River Estuary, as well as formulating comprehensive management plans for estuaries, contributing to the protection and sustainable development of the Yangtze River Estuary.

Asymmetric Tidal Dynamics in the Macro-tidal Hangzhou Bay, China

Asymmetric Tidal Dynamics in the Macro-tidal Hangzhou Bay, China

Circulation in a tropical embayment with two inlets and complex geometry: The Itamaracá-Itapessoca system, Brazil

Along the northeast coast of Brazil, numerous estuaries with intricate morphologies, characterized by multiple channels, islands, and intertidal areas, are found. These estuaries undergo significant seasonal variations, experiencing pronounced wet and dry periods. While some of these systems have small drainage basins with minimal freshwater inflow and sediment production, they boast extensive intertidal mangrove areas. This study focuses on assessing the hydrodynamics and residual circulation of the Itamaracá-Itapessoca Estuarine System (IIES) to elucidate the impact of residual circulation on estuary-shelf exchange dynamics. The IIES comprises two main islands and two inlets surrounded by extensive intertidal areas. Field observations collected during the wet season were used to calibrate and validate a numerical model. Two model scenarios were examined: a 'dry' scenario, considering hydrodynamics driven solely by interactions between morphology and tides, and a 'wet' scenario, incorporating freshwater inflow. Key findings include: (a) the inclusion of freshwater inflow minimally alters system dynamics; (b) tidal dynamics are amplified within the system, resulting in a null zone between the inlets; and (c) residual circulation circulates around the islands, with net importation through the larger inlet and net exportation through the smaller one. The intensity of exchange is heightened under wet conditions.