Tidal Stage Research Articles

Integrating time series Sentinel-2 images and tide height to mapping tidal flats in the Chinese mainland

As a unique and important ecosystem, tidal flats provide a variety of ecosystem functions and services. Mapping tidal flats is essential for the protection and management of coastal ecosystems. However, large-scale tidal flats mapping still faces challenges due to the tidal variation and spectral similarity between tidal flats and inland wetlands. Previous methods rely on the coastlines or maximum seawater extent to exclude inland areas, which is limited by its inability to effectively differentiate tidal flats from spectrally similar inland wetlands. To address these issues, we proposed a new tidal-flat mapping method by integrating Sentinel-2 time series imagery with tide height (TH) data from ground-based tide stations on Google Earth Engine. We first generated images at the lowest and highest tidal stages, established the statistical relationship between the Normalized Difference Water Index (NDWI) of each pixel and TH, and then concatenated them into a Random Forest classifier for further classification. The statistical relationship between NDWI and TH amplified the difference between tidal flats and inland wetlands, thus significantly reducing the influence of spectral similarity. This method could produce a high-precision tidal flats map with an overall accuracy of 97.30% in the coastal zone of the Chinese mainland. By quantitatively comparing with the previous tidal flat maps, we found that the strategies of tidal-level information simulation and inland area exclusion were the two main reasons producing the differences among the maps. The proposed method does not rely on space constraints to exclude inland wetlands and can capture more estuarine tidal flats, so it can be used as a reliable means to monitor the tidal flats in large-scale areas.

Seasonal dynamics of amino acids in the Southern Yellow Sea: Feedback on the mechanism of green tides caused by Ulva prolifera

The biogeochemical processes of amino acids in the Southern Yellow Sea (SYS) have become more dynamic under the influence of the world's largest-scale green tide. The potential relationship between amino acids and green tides has not been effectively assessed, despite its critical importance for exploring dissolved organic matter (DOM) cycling processes in marginal seas. In this study, three cruises were conducted to analyze the concentrations and compositions of total hydrolyzed amino acids (THAAs) in the SYS during the spring, summer, and autumn of 2019. The bioavailability potential of DOM was evaluated using the degradation index (DI) and THAA nitrogen normalized yield (THAA (%DON)) (DON as dissolved organic nitrogen). The variation dynamics of amino acid indicators during different stages of green tide were further explored. The results showed that the THAA concentrations and DOM bioavailability in the SYS were considerably influenced by biological processes. The THAA concentrations (0.96 ± 0.34 μmol L−1) exhibited the lowest mean values in the summer, while the DI values (0.106 ± 0.461) and mean THAA (%DON) values (18.20 ± 6.58 %) were the highest during this season. The distribution of amino acid indicators in the summer (the late-tide stage) was regulated by the green tide mechanism, and kept pace with the green tide floating region. In comparison with the waters in south of 35° N, the THAA concentrations and DI values experienced significant seasonal variations (p < 0.05) in north of 35° N, with the highest DI values (1.217) observed in the green tide aggregation area. This indicates the transformation of nutrient sources for Ulva prolifera in the late-tide stage and its impact on DOM bioavailability. Thus, as a potential feedback indicator of green tides, the study of amino acids is meaningful for understanding the occurrence of green tides and the source-sink pattern of organic nitrogen.

Wave-driven current and vortex patterns at an open beach: Insights from phase-resolving numerical computations and Lagrangian measurements

Wave-driven currents have a substantial impact on local circulation patterns in and across the surf zone, and are responsible for cross-shore and longshore exchange of mass and momentum over a broad range of spatial and temporal scales. Nearshore currents may drive sediment transport, lead to beach erosion, and also affect the spread of bacteria and other marine microorganisms, as well as the distribution of pollutants such as chemicals and microplastics. In addition, surf zone currents can cause hazardous conditions for beach-goers in the form of rip currents.It is known from previous work (Chen et al., 2003; Feddersen et al., 2011; Hally-Rosendahl and Feddersen, 2016) that Boussinesq-type models in combination with appropriate boundary conditions and wave breaking capabilities can function as powerful tools for the analysis of circulation patterns in the surf zone. In the present work, data from a recent field campaign reported on in Bjørnestad et al. (2021) are used to further validate the capability of Boussinesq systems to simulate nearshore dynamics.The numerical model is then used to study the influence of tidal elevation, peak direction and directional spread of the incoming wavefield on the quantity, extent, and circulatory magnitude of the nearshore circulation. In addition, fundamental features such as horizontal eddies are investigated, and comparisons are made to solid-body rotation and irrotational vortices.Overall, it is observed that local variations in the bathymetry across the surf zone are the controlling factor regarding the size of these circulations, and an increasing tidal level, which can be seen as a uniform offset to the bathymetry, favors the generation of larger vortex patterns. For a given tidal stage, the directional spread of the incoming wavefield has the most pronounced influence on the size and strength of the nearshore eddies while the peak direction has the strongest effect on the total number of circulations.

Intertidal spring dynamics and coastal nutrient loading revealed through geophysics, drone surveys, and in‐situ monitoring

AbstractCoastal eutrophication poses an increasing risk to ecosystem health due to enhanced nutrient loading to the global coastline. Submarine groundwater discharge (SGD) represents a significant pathway for nitrate‐nitrogen (NO3‐N) transport to the coast, but diffusive SGD transport is difficult to monitor directly, given the low flux rates and expansive discharge areas. In contrast, focused SGD from intertidal springs can potentially be sampled and directly gauged, providing unique insight into SGD and associated contaminant transport. Basin Head is a coastal lagoon in Prince Edward Island, Canada that is a federally protected ecosystem. Nitrate‐nitrogen is conveyed from agricultural fields in the contributing watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater‐dominated tributaries. We used several field methods to characterize groundwater discharge, nutrient loading, and in‐channel mixing associated with intertidal springs. The tributaries and intertidal springs were gauged and sampled to estimate a representative summer nitrate load to the lagoon. Our analysis revealed that NO3‐N export to the lagoon through tributaries and springs throughout summer 2023 was on average 401 kg N/month, with the combined spring loading comparable in magnitude to the combined tributary loading. We collected thermal infrared and visual imagery using drone surveys and found spatial overlap between cold‐water plumes from the spring discharge and macroalgae blooms, indicating the local thermal and ecosystem impacts of the focused SGD. We also mapped the electrical resistivity (salinity) distribution in the water column around one large spring with electromagnetic geophysics at different tidal stages to reveal the three‐dimensional spring plume dynamics. Results showed that the fresher spring water floated above the saline lagoon water with the brackish plume oriented in the direction of the tidal current. Collectively, our multi‐pronged field investigations help elucidate the hydrologic, thermal, and nutrient dynamics of intertidal springs and the cascading ecosystem impacts.

Fecal Bacteria Contamination of Floodwaters and a Coastal Waterway From Tidally-Driven Stormwater Network Inundation.

Inundation of coastal stormwater networks by tides is widespread due to sea-level rise (SLR). The water quality risks posed by tidal water rising up through stormwater infrastructure (pipes and catch basins), out onto roadways, and back out to receiving water bodies is poorly understood but may be substantial given that stormwater networks are a known source of fecal contamination. In this study, we (a) documented temporal variation in concentrations of Enterococcus spp. (ENT), the fecal indicator bacteria standard for marine waters, in a coastal waterway over a 2-month period and more intensively during two perigean spring tide periods, (b) measured ENT concentrations in roadway floodwaters during tidal floods, and (c) explained variation in ENT concentrations as a function of tidal inundation, antecedent rainfall, and stormwater infrastructure using a pipe network inundation model and robust linear mixed effect models. We find that ENT concentrations in the receiving waterway vary as a function of tidal stage and antecedent rainfall, but alsosite-specificcharacteristics of the stormwater network that drains to the waterway. Tidal variables significantly explain measured ENT variance in the waterway, however, runoff drove higher ENT concentrations in the receiving waterway. Samples of floodwaters on roadways during both perigean spring tide events were limited, but all samples exceeded the threshold for safe public use of recreational waters. These results indicate that inundation of stormwater networks by tides could pose public health hazards in receiving water bodies and on roadways, which will likely be exacerbated in the future due to continued SLR.

Spatial and Temporal Variability in Water Quality and Phytoplankton Community Composition in Charleston Harbor

The Charleston Harbor estuary is a dynamic ecosystem draining three rivers that surround the rapidly urbanizing greater Charleston area. Projected climate change impacts include elevated sea surface temperature and local changes in water quality that will likely alter biogeochemical cycling as well as phytoplankton abundance and community composition. Partnering with the local non-profit organization Charleston Waterkeeper, surface water samples were collected from late April through October 2021 at 20 sites in the Charleston Harbor estuary system. Water quality parameters measured included sea surface temperature (SST), salinity, pH, dissolved inorganic carbon, chromophoric dissolved organic matter (CDOM), phytoplankton pigments and nitrate and phosphate concentrations. Relatively high (6—16 μM) nitrate values were observed throughout the year and the nitrate to phosphate (N:P) ratios were consistently elevated (50—200) relative to the Redfield ratio of 16. Analysis of variance indicated that the delivery of nitrate, phosphate, and CDOM into the estuary came from upriver of the Charleston Harbor. For instance, CDOM was significantly impacted by tidal stage (p &lt; 0.05) and was negatively correlated with salinity at low salinity sites, but no correlation was observed at high salinity sites. Seasonal patterns in phytoplankton community abundance and composition were driven by changes in SST, as overall phytoplankton biomass increased with a community shift from diatoms during colder months to flagellated cells such as prasinophytes during the warmer summer months. Phytoplankton diversity was greatest during the early summer and lowest in October. This study provides a reference baseline for water quality parameters and phytoplankton community composition in an estuarine ecosystem that is changing rapidly due to dredging and climate change processes.

The two salinity peaks mode of marine salt supply to coastal underground brine during a single tidal cycle

Seawater salt is constantly supplied from the marine environment to coastal underground brine deposits, meaning that brine has the potential for continuous extraction. There is currently a lack of information about the processes that drive the fluxes of seawater salt to underground brine deposits in tidal-driven brine mining areas. We chose the Yangkou salt field on the southern coast of Laizhou Bay, a brine mining area, as our study site. We monitored the spatial and temporal distribution of the underground brine reserve and the changes in water level and salinity in the mining area and adjacent tidal flats using electrical resistivity tomography and hydrogeological measurements. We monitored cross-sections along two survey lines and observed that the underground brine reserve receives a stable supply of seawater salt, and calculated that the rate of influx into the brine body in the mining area near the boundary of the precipitation funnel was 0.226−0.232 t/h. We calculated that a total salt flux of approximately 5.50 t enters the underground brine body every day through a 150 m long shoreline and a 1322.3 m2 window, which is sufficient to sustain the daily extraction of one brine well. During tidal cycles, there are two peaks in the salinity of the water supplied to the underground brine reserve, which means that the brine supply is from at least two high-salinity salt sources in different tidal stages. The first salinity peak occurs during the initial stage of the rising tide after seawater inundates the tidal flat. At this time, seawater, which is a solution and carries a large amount of evaporated salt, is transported into the brine layer through highly permeable areas or biological channels and replenishes the brine in the mining area. The second salinity peak occurs during the early stage of the falling tide. Influenced by hysteresis-driven tidal pumping, high-salinity brine from the lower intertidal zone is rapidly transported into the mining area, thereby increasing the salinity of the underground brine.

Observations of beach cusp morphodynamics on a composite beach

Substantial volumes of sediment are moved on coarse grained beaches through the formation and destruction of beach cusps, yet the processes governing cusp behaviour remain poorly understood. Here, we combine continuous 2D LiDAR scan data with daily UAV topographic surveys and RGB imagery to provide detailed morphological measurements of beach cusps on a composite beach with the aim to investigate the role of sediment size and tidal stage on cusp behaviour. Rapid changes in cusp height and prominence were observed when storm-eroded gravels were readily transported over relatively impermeable sandy substratum, showing the high-order control that mixed matrix sediments have on this process. Post storm, gravels armoured the swash zone overlaying the sand and were rapidly reworked into horns, with the incipient bays simultaneously experiencing erosion. We provide some of the first field observations to show divergent behaviours occurring simultaneously along the beach as cusps adjust to the new dominant hydrodynamic regime, with some areas experiencing a shortening of cusp wavelength through new horn growth, whilst others contemporaneously experienced wavelength growth through the removal of intermediate horns. Cusp formation was generally more pronounced on the falling tide, but some evidence indicates horns can also be consolidated and grow in relief at some stages of individual rising tides. This research demonstrates how feedback between sediments, morphology and hydrodynamics are key to determining cusp form, with these insights only made possible through the novel combination of LiDAR, UAV, and camera methods to provide sufficient temporal and spatial resolution to resolve key processes.

Effects of geomorphic-induced turbulence on horizontal mixing in the coastal lagoon Xiaohai in China

Geomorphic conditions and the resulting turbulence are significant factors that influence coastal lagoonal environments. In this study, a shallow-water hydrodynamic transport model was employed to investigate turbulence related to geomorphic features in a large shallow lagoon (Xiaohai Lagoon) in China and to explore the effects these features have on horizontal mixing and the dissipation of tidal energy within the lagoon. At various tidal stages, the vorticity distribution revealed the presence of large-scale turbulent structures associated with the local geomorphic features found in the downstream part of Xiaohai Lagoon, which enhanced flow shear in the affected area and governed local mixing. Increased tidal range enhanced the horizontal mixing flux in the upstream part of the lagoon during spring-neap cycles. In contrast, the mixing flux in the downstream region declined during spring tides and increased during neap tides due to phase differences between tidal flow processes and tracer concentration. In addition, tidal energy dissipation was most concentrated in the vicinity of the local geomorphic features. The rate of dissipation from the inlet to the upstream end of the deep groove accounted for 99.6% of the total in Xiaohai Lagoon, resulting in a loss of 72% of the tidal range in the upstream region, which substantially limited the upstream tidal prism and water mixing. This study suggests that turbulent mixing and energy dissipation associated with local geomorphic features are important mechanisms governing water exchange and environmental heterogeneity in lagoons and should be considered when formulating management and optimization strategies for coastal lagoons.

Temporal and Spatial Variations in Subterranean Estuary Geochemical Gradients and Nutrient Cycling Rates: Impacts on Groundwater Nutrient Export to Estuaries

AbstractSubterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p &lt; 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were &lt;8 mmoles N m−2 d−1. The total groundwater discharge rate was 38 ± 11 L m−2 d−1; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m−2 d−1 for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to &gt;100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for &lt;10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics.

Design Scour Levels for Dune Revetments and Seawalls

A new method to determine design scour levels for dune revetments and seawalls is presented. The method is based on the thesis that toe scour is a function of the nearshore incident wave energy and was calibrated with results from several moveable bed model studies covering a large range of scales. The method determines scour levels to a still water level datum defined at the incident wave-breaking location. Therefore, the method incorporates many more parameters than those that pertain to existing methods including tidal stage, storm surge, wave setup, wave period, and seabed slope. However, application of the method is not difficult with a methodology being presented. The sensitivity of the method to variations in the values for the input parameters is examined and the limits of their applicability are presented. Recommendations for further research are given.

Virome reveals effect of Ulva prolifera green tide on the structural and functional profiles of virus communities in coastal environments

Viruses are widely distributed in marine environments, where they influence the transformation of matter and energy by modulating host metabolism. Driven by eutrophication, green tides are a rising concern in Chinese coastal areas, and are a serious ecological disaster that negatively affects coastal ecosystems and disrupts biogeochemical cycles. Although the composition of bacterial communities in green algae has been investigated, the diversity and roles of viruses in green algal blooms are largely unexplored. Therefore, the diversity, abundance, lifestyle, and metabolic potential of viruses in a natural bloom in Qingdao coastal area were investigated at three different stages (pre-bloom, during-bloom, and post-bloom) by metagenomics analysis. The dsDNA viruses, Siphoviridae, Myoviridae, Podoviridae, and Phycodnaviridae, were found to dominate the viral community. The viral dynamics exhibited distinct temporal patterns across different stages. The composition of the viral community varied during the bloom, especially in populations with low abundance. The lytic cycle was most predominant, and the abundance of lytic viruses increased slightly in the post-bloom stage. The diversity and richness of the viral communities varied distinctly during the green tide, and the post-bloom stage favored viral diversity and richness. The total organic carbon, dissolved oxygen, NO3−, NO2−, PO43−, chlorophyll–a contents, and temperature variably co-influenced the viral communities. The primary hosts included bacteria, algae, and other microplankton. Network analysis revealed the closer links between the viral communities as the bloom progressed. Functional prediction revealed that the viruses possibly influenced the biodegradation of microbial hydrocarbons and carbon by metabolic augmentation via auxiliary metabolic genes. The composition, structure, metabolic potential, and interaction taxonomy of the viromes differed significantly across the different stages of the green tide. The study demonstrated that the ecological event shaped the viral communities during algal bloom, and the viral communities played a significant role in phycospheric microecology.

Surface flow modulation by river discharges in a tropical micro-tidal estuary

The typical bidirectional pattern of subtidal flow observed at the mouths of estuaries is produced by inflows during high-tide (flood) and outflows during low-tide (ebb) periods. This flow pattern can be modulated in the presence of a strong river discharge, as observed for the Alvarado Estuary (western Gulf of Mexico) during the rainy season. Herein, six transect repetitions were obtained using a towed Acoustic Doppler Current Profiler (ADCP) at an average speed of approximately 2 m/s during a diurnal tidal cycle from September 20 to 21, 2009. The water column during this period was strongly stratified at the mouth of the estuary due to the amount of discharge from the Papaloapan River. Furthermore, the temperatures within the estuary were directly related to the distance from the Papaloapan River. Based on salinity data obtained at four locations along the estuary, the Alvarado Estuary could be considered for the first time as a transitional system from oligohaline to mesohaline (salinities between 5 and 18). The currents and backscatter intensities of the ADCP revealed concentrated areas of suspended particles in the deepest area of the estuary during flood tide periods. During ebb tide periods, estuarine water primarily exited through its mouth (∼90%), followed by the artificial channel (∼10%) located in the northernmost area of the estuary. The residence time during this season was 3.99 days. Finally, the typical estuarine flow pattern observed during flood and ebb tide periods was only depicted near the bottom of the mouth of the estuary. However, the near-surface flow was always toward the ocean (outflow), regardless of the tidal stage.

Shorebird Monitoring Using Spatially Explicit Occupancy and Abundance

Loss of habitat and human disturbance are major factors in the worldwide decline of shorebird populations, including that of the threatened migratory piping plover (Charadrius melodus). From 2013 to 2018, we conducted land-based surveys of the shorebird community every other week during the peak piping plover season (September to March). We assessed the ability of a thin plate spline occupancy model to identify hotspot locations on Whiskey Island, Louisiana, for the piping plover and four additional shorebird species (Wilson’s plover (Charadrius wilsonia), snowy plover (Charadrius nivosus), American oystercatcher (Haematopus palliatus), and red knot (Calidris canutus)). By fitting single-species occupancy models with geographic thin plate spline parameters, hotspot priority regions for conserving piping plovers and the multispecies shorebird assemblage were identified on the island. The occupancy environmental covariate, distance to the coastline, was weakly fitting, where the spatially explicit models were heavily dependent on the spatial spline parameter for distribution estimation. Additionally, the detectability parameters for Julian date and tide stage affected model estimations, resulting in seemingly inflated estimates compared to assuming perfect detection. The models predicted species distributions, biodiversity, high-use habitats for conservation, and multispecies conservation areas using a thin-plate spline for spatially explicit estimation without significant landscape variables, demonstrating the applicability of this modeling approach for defining areas on a landscape that are more heavily used by a species or multiple species.

How do ambient conditions and management actions affect manatee movements and habitat use?

AbstractKings Bay in northwest Florida, USA, is an important winter home of the largest aggregation of Florida manatee (Trichechus manatus latirostris), and the only location in the United States where visitors legally swim and interact with manatees. In addition to ambient conditions, visitors to the area and management actions have the potential to influence manatee behaviors. We tracked 32 manatees with satellite‐linked global position system (GPS) telemetry tags in Kings Bay from 2006 to 2018. Also, personnel at Crystal River National Wildlife Refuge collected manatee counts at Three Sisters Springs from 2014 to 2017. Our objectives were to document the use of springs and other habitat components in Kings Bay relative to ambient water temperature, time of day, tide stage, and management actions within Three Sisters Springs and other manatee sanctuaries in the bay. Manatees that we tagged in Kings Bay spent a median 87% of the winter within the local study area, compared to 47% for animals that we tagged in the Florida Panhandle. Manatees showed preferences for King and Magnolia springs basins at low tide, indicating that they function as tidal refuges, when other locations may be less accessible; we also recorded within spring basin movements. Magnolia Spring Basin showed a significant diel pattern overall, and within basins, King Spring, Tarpon Hole, and Three Sisters Springs were used more during the night than during the day. All areas outside Kings Bay were used more at warmer ambient gulf water temperatures, while all of the springs basins were used more when gulf temperature was colder, especially Three Sisters Springs Basin. Management of Three Sisters Springs had an influence on manatee use during times of cold ambient gulf water temperatures; manatees used the springs more when the spring was closed to visitors, versus when the springs were completely open or when the spring lobes alone were closed. Manatee decisions were consistent with avoiding human interactions.

Dynamics of upstream saltwater intrusion driven by tidal river in coastal aquifers

For the coastal aquifers, recent research have shown that the tidal has a significant effect on saltwater intrusion in the near-shore aquifer. However, it is currently unclear how the tidal river contributes to the groundwater flow and salinity distribution in the upstream aquifer of the estuary. This study examined the effects of a tidal river on the dynamic characteristics of groundwater flow and salt transport in a tidal river-coastal aquifer system using field monitoring data and numerical simulations. It was found that changes in tidal-river level led to the reversal of groundwater flow. For a tidal cycle, the maximum area of seawater intrusion is about 41.16 km2 at the end of the high tide stage. Then the area gradually decreased to 39.02 km2 at the end of the low tide stage. More than 2 km2 area variation can be observed in a tidal cycle. Compared to the low tide stage, the area of SWI increased by 5 % at high tide stage. The SWI region was also spreading landward from the tidal river. In addition, we quantified the water exchange and salt flux between the tidal river and aquifer. When the tidal fell below the level of the riverbed, the water exchange rate was stabilized at about −1.6 m/h. The negative value indicated that the river was recharged by the groundwater. With the increasing of tidal water level, the water exchange rate gradually changes from negative to positive and reached the maximum value of 3.2 m/h at the beginning of the falling tide stage. The presence of a physical river dam can amplify the difference in water level between high and low tides, thereby enhancing the influence of a tidal river on water exchange and salt flux. The findings lay the foundation for gaining a comprehensive understanding of the tidal river on groundwater flow and salt transport in upstream aquifers.

Do urban rivers in the amazon coast trap macroplastic?

Amazon Rivers are important sources of macroplastic that enter the oceans. The estimated macroplastic transport is still inaccurate as hydrodynamics are not considered and data are not collected in situ. The present study shows the first quantification of floating macroplastics at different temporal scales, and an annual transport estimate through urban rivers in the Amazon: the Acará and Guamá Rivers, which discharge into the Guajará Bay. We conducted visual observations of macroplastics (>2.5 cm) in different river discharges and tidal stages, and we measured current intensity and direction in the three rivers. We quantified 3481 floating macroplastics, which varied according to tidal cycle and seasonality. Although connected to and affected by the same tidal regime, the urban estuarine system had an import rate of 12 ton.y−1 through the Guamá River and an export rate of 21.7 ton.y−1 of macroplastics through the Guajará Bay, affected by local hydrodynamics.

Observations of River Plume Mixing in the Surf Zone

Abstract We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions. Significance Statement River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.

Faunal engineering stimulates landscape-scale accretion in southeastern US salt marshes

The fate of coastal ecosystems depends on their ability to keep pace with sea-level rise—yet projections of accretion widely ignore effects of engineering fauna. Here, we quantify effects of the mussel, Geukensia demissa, on southeastern US saltmarsh accretion. Multi-season and -tidal stage surveys, in combination with field experiments, reveal that deposition is 2.8-10.7-times greater on mussel aggregations than any other marsh location. Our Delft-3D-BIVALVES model further predicts that mussels drive substantial changes to both the magnitude (±<0.1 cm·yr−1) and spatial patterning of accretion at marsh domain scales. We explore the validity of model predictions with a multi-year creekshed mussel manipulation of >200,000 mussels and find that this faunal engineer drives far greater changes to relative marsh accretion rates than predicted (±>0.4 cm·yr−1). Thus, we highlight an urgent need for empirical, experimental, and modeling work to resolve the importance of faunal engineers in directly and indirectly modifying the persistence of coastal ecosystems globally.

Vessel Wake Impact Forces on Marsh Scarps

Priestas, A.M.; Styles, R., and Bain, R., 2023. Vessel wake impact forces on marsh scarps. Journal of Coastal Research, 39(2), 207–220. Charlotte (North Carolina), ISSN 0749-0208. A common morphological feature of high-energy coastal plain salt marshes is an erosive scarp separating the marsh platform from the abutting tidal flat. A noted deficiency in the understanding of scarp erosion is a lack of direct wave impact measurements to better constrain the physical processes driving material loss. To measure wave impact forces, load cells were embedded into a scarp face along the Intracoastal Waterway in northern Florida, which is frequented by shallow-draft vessels. The largest impact forces occurred when the mean water surface elevation was just below the midpoint of the scarp and decayed as the mean depth either increased or decreased. Early wave breaking around low tide reduced the direct impact on the scarp, whereas wave reflection around high tide reduced wave breaking and the associated maximum impact force. The sensitivity of impact force to tidal stage suggests that wave energetics (breaking vs. reflection) and scarp morphology (ramp vs. near-vertical surface) combine to produce a force pattern that favors maximum impact around midtide. Regression analysis indicated that the impact pressures were positively correlated with wave power and dynamic pressure, which are primary metrics used to model shoreline erosion. This study argues that the critical threshold for erosion may be refined by evaluating the relationship between impact pressures and marsh resistance, since wave exposure alone does not necessarily initiate erosion.