Estuary Mouth Research Articles

Tidally Forced Saltwater Intrusions might Impact the Quality of Drinking Water, the Valdivia River (40° S), Chile Estuary Case.

The Valdivia River estuary (VRE) located in south-central Chile is known as one of the largest estuarine ecosystems on the Pacific coast. This research aims to determine the intra-tidal and sub-tidal variability of saline intrusions into the VRE between November 2017 and March 2019 derived from salinity sensors located along the VRE. Complementary hydrographic measurements were conducted during flood and ebb conditions of the spring and neap tides for each of the four seasons of the year along the central axis of the VRE. The results of the salinity time series showed that saline intrusions (values greater than 0.5 Practical Salinity Units) occurred ~20 km from the estuary mouth, when the total flow of the Cruces and Calle-Calle rivers (main tributaries of the estuary) was low, around 280–300 m3 s−1. During the same period, the best co-variability was observed between the saline intrusions and the mixed-semidiurnal tide and the fortnightly and monthly periods of the tide. Regression analyses indicated that salinity intrusion length (L) is best correlated to discharge (D) with a fractional power model L α D−1/2.64 (R2 = 0.88). The decreasing discharge trend, found between 2008–2019, implies that saline water intrusions would negatively impact the Valdivia’s main drinking water intake during the low rainfall season under future climate conditions.

Evaluation of along-channel sediment flux gradients in an anthropocene estuary with an estuarine dam

While estuarine dams can develop freshwater resources and block the salt intrusion, they can result in increased sediment deposition in the estuary. The mechanism of increased sediment deposition in an estuary with an estuary dam is not well understood. To fill this knowledge gap, 7 ADCP measurements of flow and suspended sediment concentration (SSC) were collected along-channel in an estuary with an estuarine dam over a neap-spring cycle. Flow and SSC were used to calculate the sediment flux and sediment flux gradients, which in turn were decomposed into mean-flow fluxes (which occur on timescales longer than a tidal cycle) and correlation fluxes (which occur on the tidal timescale). Then the fluxes were compared with the external forcing to determine the relative importance of tides, dam discharge, and waves in driving the fluxes. The results indicated that the cumulative sediment fluxes at all stations were directed landward. The along-channel sediment flux gradient was negative, which indicated deposition along the channel. The landward mean-flow fluxes were dominant in the deep portion of the channel near the estuary mouth, whereas landward correlation fluxes were dominant in the shallow portion of the channel near the estuarine dam. The tides were the dominant forcing driving the sediment fluxes throughout the estuary. Waves and dam discharge were locally important forcing however. In particular, the dam discharge resulted in seaward mean-flow fluxes for 7 km seaward of the estuarine dam. This distance corresponded approximately to a tidal excursion seaward from the dam. It was the distance a freshwater mass discharged during an ebb tide could propagate before the currents returned landward during the following flood tide. Overall, the along-channel sediment flux gradient in an estuary with an estuarine dam was different from a similar estuary without an estuarine dam because the mean-flow fluxes in the inner estuary are forced by episodic dam discharge which was limited in seaward extent by the tidal excursion. This imposed a limitation for the seaward mean-flow fluxes which counteract the landward mean-flow and correlation fluxes due to the tides. With restricted seaward flushing, the tides could dominate, which could in turn result in increased depositional rates for an estuary with an estuarine dam.

Carbon dioxide dynamics in a tropical estuary over seasonal and rain-event time scales

Tropical estuaries may release large amounts of carbon dioxide to the atmosphere, but remain understudied relative to temperate systems. Here, we investigate hydrological drivers of pCO2 along ~22 km of an aquatic continuum from the mountain river to the coastal ocean, including highly modified aquaculture and urban zones (Perancak Estuary, Ijo Gading River, Bali, Indonesia). We report seasonal spatial surveys (n = 8) and stationary time series observations (n = 4 days) during a rain event. Overall, pCO2 ranged from 330 μatm to 12,126 μatm, with the lowest values observed near the estuary mouth and in the river upstream of the urban zone, and the highest values in the upper estuary where radon (222Rn, a natural groundwater tracer) revealed elevated groundwater discharge. Average atmospheric CO2 fluxes in the upper estuary (107.3 mmol m−2 d−1) were ~5-fold greater than in the lower estuary (19.1 mmol m−2 d−1), while the river was a minor atmospheric CO2 sink (−0.2 mmol m−2 d−1). Overall, the estuary was a source of CO2 to the atmosphere with area-weighted emissions of ~67 mmol m−2 d−1. Seasonal spatial surveys revealed strong correlations between antecedent rainfall and pCO2 and 222Rn in the estuary, implying pCO2 responds to delayed groundwater discharge. A detailed 4-day time series covering an episodic flooding event revealed sharp transitions in CO2 drivers, including dilution by rainfall followed by inputs of CO2-enriched groundwater. Groundwater discharge, as traced by radon, explained most of pCO2 variability in dry and wet conditions. Overall, we highlight the importance of seasonal and episodic rainfall and how delayed groundwater seepage may drive CO2 distribution in a highly modified tropical estuary.

Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.

Numerical study of sediment transport time scales in an ebb-dominated waterway

Understanding the transport time scales for sediment in an estuary is important to study the retention of pollutants, such as heavy metals, that adsorb to sediment. In this study, a three-dimensional hydrodynamic and sediment transport model was developed to investigate the transport time scales of sediment in the Modaomen Waterway (MW), China. The results show that approximately 100 and 240 days are required for fine-grained (settling velocity, ωs = 0.2 mm/s) and coarse-grained sediment (ωs = 0.5 mm/s) to transport from the headwater to the estuary mouth, respectively. Unlike dissolved materials, sediment undergoes intermittent settling and resuspension, which are key processes that determine the sediment age. The sediment stays longer at the seabed than suspended in the water column. The sediment has more potential to be eroded from the seabed in the freshwater region because of the strong bottom shear stress. These eroded sediments can be rapidly diffused to the upper water column and advected seaward. In contrast, sediment tends to deposit within the turbidity maximum zone due to low bottom shear stress and weak turbulent mixing. Downstream of the turbidity maximum, turbulent mixing is further suppressed by density stratification and the eroded sediment is rarely diffused to the upper water column. As a result, sediment travels more rapidly in the freshwater region than in the brackish zone. The sediment in the upper water column is mostly advected from upstream, and the difference in the sediment ages near the surface and at the bottom can reach 40 days for the coarse-grained sediment. The sediment age has an effect on water quality because a long sediment retention time facilitates the accumulation of pollutants in the MW.

Ten Principles to Determine Environmental Flow Requirements for Temporarily Closed Estuaries

Temporarily closed estuaries require seasonal opening to tidal flows to maintain normalecological processes. Each estuary has specific environmental flow (EFlow) requirements basedon the relationship between freshwater inflow, coastal dynamics, rate of sandbar formation,and the open/closed state of the mouth. Key abiotic processes and ecosystem services linkedto mouth state were highlighted. We reviewed completed EFlow requirement studies for temporarilyclosed estuaries in South Africa and found that the formulation of these requirements shouldconsider the timing and magnitude of flows in relation to the morphology of an estuary, its mouthstructure, catchment size, and climate. We identified ten key principles that could be adapted tosimilar systems in equivalent climatic settings. Principle 1 recognizes that each estuary is unique interms of its EFlow requirements because size, scale, and sensitivity of core elements to freshwaterinflow are specific for each system; EFlows cannot be extrapolated from one estuary to another.Principle 2 highlights the importance of baseflows in keeping an estuary mouth open because a smallreduction in flow can cause the mouth to close and alter essential ecological processes. Principle 3outlines the role of floods in resetting natural processes by flushing out large volumes of sedimentand establishing the equilibrium between erosion and sedimentation. Principle 4 emphasizes the needfor open mouth conditions to allow regular tidal flushing that maintains water quality throughreducing retention times and preventing the onset of eutrophic conditions. Principle 5 advisesartificial breaching to be practiced with caution because execution at low water levels encouragessedimentation that reduces the scouring eect of flushing. Principle 6 holds that elevated inflowvolumes from wastewater treatment works or agricultural return flows can increase the frequency ofmouth opening and cause ecological instability. Principle 7 states that water released from dams tosupply the environmental flow cannot mimic the natural flow regime. Principle 8 specifies the needfor short- and long-term data to increase the confidence levels of EFlow assessments, with data tobe collected during the open and closed mouth states. Principle 9 advocates the implementation ofa monitoring program to track the achievement of EFlow objectives as part of a strategic adaptivemanagement cycle. Finally, Principle 10 recommends the adoption of a holistic catchment-to-coastmanagement approach underpinned by collaboration with regulatory authorities and stakeholdersacross a range of sectors. These principles can be used to guide the formulation and managementof EFlows, an essential strategy that links the maintenance of estuarine ecological integrity withsocial well-being.

Temporal streamflow reduction and impact on the salt dynamics of the São Francisco River Estuary and adjacent coastal zone (NE/Brazil)

During the past decades, the São Francisco River basin encountered substantial changes due to intensive human activities such as river regularization and damming, and more recently, climate change. As a result, a decrease of streamflow in the lower São Francisco River sector has received considerable attention due to its drastic geomorphological and hydrodynamic changes. This study presents a detailed investigation on the changes of streamflow and its effect on spatiotemporal migration of estuarine plume along the São Francisco coastal zone from 1994 to 2017 based on salinity and water temperature extracted from HYCOM numerical modeling.The progressive saltwater intrusion from in situ observation for 2005 and 2017, is analyzed. The results show two distinct periods with substantial inter-annual variability of streamflow during the period from 1994 to 2011 and accentuated decreasing trends from the second semester of 2012 onwards, and discontinuous periodicities from later 2012 due to the extreme hydrological events of droughts. The spatiotemporal variability in salinity and water temperature on the coastal zone revealed a severe retraction of the estuarine plume due to the low streamflow rates, which started after the second semester of 2012, and the presence of Tropical Surface Waters (TSW, S > 35.9 g/kg) closer to the river mouth, shifting the previous estuarine mixing zone from the coastal region to inward areas of the estuary. The continuous penetration of saltwater towards inner areas of the São Francisco estuary (∼10 km upstream from estuary mouth) is attributed to the decline of water flow supply from the river upper reaches due to the extreme drought events and water conservation measures. These changes may lead to potential ecological and human problems due to persistent high salinity above the limit for human consumption (S > 0.5 g/kg) from the estuary mouth to the limits of local cities along the river.

Transcriptomic Analysis of Gill and Kidney from Asian Seabass (Lates calcarifer) Acclimated to Different Salinities Reveals Pathways Involved with Euryhalinity

Asian seabass (or commonly known as barramundi), Lates calcarifer, is a bony euryhaline teleost from the Family Latidae, inhabiting nearshore, estuarine, and marine connected freshwaters throughout the tropical Indo-West Pacific region. The species is catadromous, whereby adults spawn in salinities between 28 and 34 ppt at the mouth of estuaries, with resultant juveniles usually moving into brackish and freshwater systems to mature, before returning to the sea to spawn again as adults. The species lives in both marine and freshwater habitats and can move quickly between the two; thus, the species’ ability to tolerate changes in salinity makes it a good candidate for studying the salinity acclimation response in teleosts. In this study, the transcriptome of two major osmoregulatory organs (gills and kidneys) of young juvenile Asian seabass reared in freshwater and seawater were compared. The euryhaline nature of Asian seabass was found to be highly pliable and the moldability of the trait was further confirmed by histological analyses of gills and kidneys. Differences in major expression pathways were observed, with differentially expressed genes including those related to osmoregulation, tissue/organ morphogenesis, and cell volume regulation as central to the osmo-adaptive response. Additionally, genes coding for mucins were upregulated specifically under saline conditions, whereas several genes important for growth and development, as well as circadian entrainment were specifically enriched in fish reared in freshwater. Routing of the circadian rhythm mediated by salinity changes could be the initial step in salinity acclimation and possibly migration in euryhaline fish species such as the Asian seabass.

Impacts of secondary and quarter-diurnal tidal species on backwater hydrodynamics in tidal rivers

As river flow debouches into the sea, it is affected by the tidal fluctuation at the estuary mouth, which results in a backwater zone, where the rising residual water level (averaged over a lunar day) in the landward direction is changing periodically. This is known as backwater hydrodynamics, especially the variation of the residual water levels that are controlled by the interplay between tide and river flows, while these hydrodynamics follow the traditional stage-river discharge relation in the upstream river-dominated region. However, the tidal asymmetry contributions to the residual water level that are caused by secondary tidal species (e.g., diurnal tide D1), overtide generation (e.g., the quarter-diurnal tide D4) and their interplay with river flow are poorly understood. In this study, we aim to understand the impacts of secondary and quarter-diurnal tidal species on the increase in residual water level in tidal rivers. To quantify the contributions made by different components, we decomposed the numerically computed subtidal friction into different components representing the contributions made by the river flow alone, the tide-river interaction, and tidal asymmetry due to the nonlinear interaction between different tidal species. The results show that the contribution by tidal asymmetry to the residual water level is minor, while the contributions by river flow and tide-river interaction are dominant, accounting for more than 90% and 80% at the upstream and downstream end of the estuary, respectively. The contribution made by river flow increases, while the tide-river interaction decreases in the landward direction. The semidiurnal tidal species (D2) predominantly control the residual water level induced by the tide-river interaction along the Yangtze estuary, which accounts for 70%, while the contributions made by the diurnal and quarter-diurnal tidal species are minor and comparable in magnitude in the seaward reach where tide-river interactions are important. The method proposed in this study to quantify the contributions of river flow and different tidal species to the residual water level will enhance our understanding of riverine and tidal impacts on estuarine backwater hydrodynamics and can be used to guide effective and sustainable water management in the Yangtze estuary and other tidal rivers with substantial river flow.

A tidally dependent plume bulge at the Pearl River Estuary mouth

Both observations and numerical simulations have proven the existence of a plume bulge at the trumpet-like Pearl River Estuary (PRE). The formation dynamic of the PRE plume bulge during the southeasterly wind period is investigated with numerical model simulations based on the Regional Ocean Modelling System (ROMS). The results indicate that the formation and evolution of the PRE plume bulge is strongly modulated by the tidal cycle. The bulge is sustained by the strong jet flow along the longitudinal Lantau Channel, which is induced by the ebb current, and transports freshwater offshore. Its size is significantly modulated by spring-neap cycle, being large during spring tide and very small during neap tide. Besides, the existence of abundant freshwater in the estuary is a precondition for the formation of bulge. The ebb jet flow could be strengthened by lateral buoyancy forcing, which is induced by the accumulated freshwater in the estuary, thus guarantees the offshore extension of bulge. The input momentum flux by the river runoff, however, makes no significant contribution. The work indicates a new mechanism of plume bulge formation at a wide estuary and evaluates the impact of different forcings.

Water Balance Standardization Approach for Reconstructing Runoff Using GPS at the Basin Upstream

While in-situ estuarine discharge has been correlated and reconstructed well with localized remotely-sensed data and hydraulic variables since the 1990s, its correlation and reconstruction using averaged GPS-inferred water storage from satellite gravimetry (i.e., GRACE) at the basin upstream based on the water balance standardization (WBS) approach remains unexplored. This study aims to illustrate the WBS approach for reconstructing monthly estuarine discharge (in the form of runoff (R)) at Mekong River Delta, by correlating the averaged GPS-inferred water storage from GRACE of the upstream Mekong Basin with the in-situ R at the Mekong River Delta estuary. The resulting R based on GPS-inferred water storage is comparable to that inferred from GRACE, regardless of in-situ stations within Mekong River Delta being used for the R reconstruction. The resulting R from the WBS approach with GPS water storage converted by GRACE mascon solution attains the lowest normalized root-mean-square error of 0.066, and the highest Pearson correlation coefficient of 0.974 and Nash-Sutcliffe efficiency of 0.950. Regardless of using either GPS-inferred or GRACE-inferred water storage, the WBS approach shows an increase of 1–4% in accuracy when compared to those reconstructed from remotely-sensed water balance variables. An external assessment also exhibits similar accuracies when examining the R estimated at another station location. By comparing the reconstructed and estimated Rs between the entrance and the estuary mouth, a relative error of 1–4% is found, which accounts for the remaining effect of tidal backwater on the estimated R. Additional errors might be caused by the accumulated errors from the proposed approach, the unknown signals in the remotely-sensed water balance variables, and the variable time shift across different years between the Mekong Basin at the upstream and the estuary at the downstream.

The fate of river-borne contaminants in the marine environment: Characterising Regions of Freshwater Influence (ROFIs) and estuary plumes using idealised models and satellite images

A combination of idealised modelling and satellite imagery has been used to assess the dispersal of estuarine plume water and suspended material throughout the ecologically significant marine area off the west coast of the central North Island, New Zealand. The idealised modelling was used to elucidate the relative importance of oceanographic and meteorological conditions in controlling the horizontal structure of the estuary plumes, and then compared to plumes visible in satellite imagery and environmental monitoring data. Under low to average river flows the plumes can be categorised as either non-rotational or rotational. Rotational plumes are directed southwards under light (<5 m/s SW or <10 m/s E) winds and northwards under stronger (>5 m/s SW) winds. Non-rotational plumes remain close to the estuary mouth. The type, orientation and extent of the plumes have implications for estuarine flushing and for the dispersal of land-derived contaminants into the marine environment.

Tidal Patterns and Sediment Dynamics in a Hypertidal Estuary Influenced by a Tidal Power Station

Rtimi, R.; Sottolichio, A. and Tassi, P., 2020. Tidal patterns and sediment dynamics in a hypertidal estuary influenced by a tidal power station. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1520–1524. Coconut Creek (Florida), ISSN 0749-0208.The Rance estuary is a relatively small low-inflow steep-sided ria, located in the Brittany coast of northern France, with a maximum spring tidal range of 13.5 m at the mouth. Taking advantage of this hypertidal regime, the first and largest operational tidal power station in the world was built at the estuary mouth and is in operation since the 1960s. Despite a well-known effect of the plant on the damping of estuarine water levels, little attention has been given to the propagation of the tidal wave along the estuary. Moreover, net siltation has been reported by several observations, but there is no specific knowledge on the role of the plant on sedimentation. In this study, tidal wave patterns and sediment dynamics are analyzed in this particular man-engineered system. A numerical model, based on a two-dimensional depth-averaged approach is implemented to predict the tide propagation and tidal currents along the estuary. Numerical results are analyzed and discussed to highlight the role of the plant on the transformation of the tidal wave in the estuary and its possible consequences on sediment dynamics and morphological evolution.

Study on the Transport of Terrestrial Dissolved Substances in the Pearl River Estuary Using Passive Tracers

Highly populated river deltas are experiencing marine environment degradation resulting from the tremendous input of terrestrial dissolved substances (TeDS). The Pearl River Delta is one of the deltas with degradation of the water quality and ecological condition. The Pearl River Estuary (PRE) was investigated to reveal the fate and transport timescales of TeDS in order to provide guidance on water resource management and pollutant transport prediction. By using passive tracers in a calibrated 3D numerical model, the TeDS transports from five different outlet groups were investigated systematically. The TeDS transport time was computed by using the concept of water age, which is a measure of the time that has elapsed since the tracer was transported from the upstream boundary to the downstream concerned area. The tracer impacted area was defined by the area with tracer concentrations &gt; 0.2 (arbitrary unit). The domains that were impacted by the tracer coming from each outlet group were identified separately. In the wet season, the impacted area was larger than in other seasons. The most prominent variations appeared in the Jiaomen–Hengmen–Hongqili (JHH) and Modaomen (MD) outlets. The hydrodynamic conditions controlled the offshore spreading of the TeDS. Assuming the TeDS were conservative, it took approximately 10–20 days for the TeDS to be transported from the head water to the entrance of the outlet. For the TeDS coming from the head water of the Humen outlet, it took approximately 40 (80) days for the TeDS to be transported out of the mouth of the Lingding Bay during the wet (dry) season. For the case of the TeDS coming from the head water of the JHH outlets, it took approximately 20 (40) days for the TeDS to be transported out of the Lingding Bay during the wet (dry) season. For the MD, Jiti and Yamen–Hutiao outlets, it usually took approximately 10 days for the TeDS to be transported from the head water to the inner shelf. The correlation coefficient between the river flow and tracer concentrations was 0.78, and between the river flow and transport time it was −0.70 at a station in the lower Lingding Bay. At the estuary mouth, the impacts of other forcing fields got stronger.

High variability of Blue Carbon storage in seagrass meadows at the estuary scale

Seagrass meadows are considered important natural carbon sinks due to their capacity to store organic carbon (Corg) in sediments. However, the spatial heterogeneity of carbon storage in seagrass sediments needs to be better understood to improve accuracy of Blue Carbon assessments, particularly when strong gradients are present. We performed an intensive coring study within a sub-tropical estuary to assess the spatial variability in sedimentary Corg associated with seagrasses, and to identify the key factors promoting this variability. We found a strong spatial pattern within the estuary, from 52.16 mg Corg cm−3 in seagrass meadows in the upper parts, declining to 1.06 mg Corg cm−3 in seagrass meadows at the estuary mouth, despite a general gradient of increasing seagrass cover and seagrass habitat extent in the opposite direction. The sedimentary Corg underneath seagrass meadows came principally from allochthonous (non-seagrass) sources (~70–90 %), while the contribution of seagrasses was low (~10–30 %) throughout the entire estuary. Our results showed that Corg stored in sediments of seagrass meadows can be highly variable within an estuary, attributed largely to accumulation of fine sediments and inputs of allochthonous sources. Local features and the existence of spatial gradients must be considered in Blue Carbon estimates in coastal ecosystems.

Mangrove colonisation of the Mlalazi Estuary, South Africa: a response to artificial breaching

The Mlalazi Estuary has 40 ha of mangroves, but before the 1930s there were none. The purpose of this study is to understand why this change occurred, and how the understanding thus gained can provide ecological information that will help understand the dynamics of similar estuaries in the region. The approach was to accumulate evidence gathered from historical records, published literature and scientific investigations. In the past 6 000 years, the Mlalazi Estuary evolved from an open bay to a permanently open estuary and then to its current state of a temporarily closed estuary. There is evidence that the mangroves that occurred in the system were lost when the estuary mouth started closing for extended periods. Since 1896, Mlalazi has been artificially breached whenever closure resulted in the backing up of water. This mouth-breaching regime resulted in physical conditions suitable for mangrove growth. Mangrove propagules would have dispersed from adjacent estuaries; taking more than 30 years for Avicennia marina and Bruguiera gymnorrhiza to colonise, and approximately 100 years for Rhizophora mucronata. Once established the mangrove population expanded and there is evidence that available habitat is becoming limiting. Any alteration to the current mouth management strategy is likely to affect the mangroves and must not be implemented without a full understanding of its consequences. The Mlalazi Estuary is a unique estuary in South Africa in that mouth manipulation has led to the colonization of mangroves not previously current in the system.

Modeling Mud and Sand Transfers Between a Macrotidal Estuary and the Continental Shelf: Influence of the Sediment Transport Parameterization

AbstractCoastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants transported by sediments from continental areas to the sea is crucial for marine resources protection. However, the complexity of estuarine dynamics makes it difficult to quantify sediment fluxes from field measurements alone and requires numerical modeling. Thus, using a realistic 3‐D hydrodynamic and sediment transport model, this study aims at evaluating the influence of model empirical parameters on sediment fluxes and estimating uncertainties on mud and sand transfers at a macrotidal estuary mouth. A sensitivity analysis, considering changes in sediment transport parameters, revealed that the system is sensitive not only to settling and erosion parameterizations but also to the spin‐up period and the sediment sliding process. Both estuarine circulation and tidal pumping induce a residual up‐estuary transport, which is balanced by seaward export during spring tides. Although more fine sediments are exported within the surface turbid plume during high river discharge, the net mud transport is directed up‐estuary due to increased baroclinic circulation. Besides, model results highlighted a strong seasonal variability in sediment fluxes with a short and high import during high river flow and a long and weak export during low river flow. Uncertainties associated with the simulated fluxes were about 93% for mud and 51% for noncohesive classes, based on the best performing parameter sets for surface suspended sediment concentrations. These results can be reliably extrapolated to similar macrotidal estuarine systems.

Tidal velocities on the Mid-Atlantic Bight continental shelf using high-frequency radar

Surface current velocity from high-frequency radar data are used to study tidal currents in the Mid-Atlantic Bight, which represent an average 32% of the total kinetic energy of the surface circulation. Average annual and seasonal tidal current characteristics and the associated variance of the semi-major axis are computed. The M2 constituent dominates the tide, representing a maximum of 72.26% and an average of 51.63% of the total studied tidal signal. Its semi-major axis reaches a maximum of 10 cm s???1 around mid-shelf and its phase sweeps from north to south. Local maxima of the same order of magnitude are found near the mouths of estuaries (Long Island Sound, Hudson River, Delaware Bay, Chesapeake Bay) and an absolute maximum of 50 cm s???1 is found in Nantucket Shoals. These regions of localized elevated tidal amplitude are associated with high interannual and intra-annual variability due to seasonal stratification, internal waves, and sand waves. Seasonal tidal changes are large in some of these regions; thus, should not be neglected in making tidal predictions. Lastly, the authors position this paper as an updated alternative to the Moody tidal atlas as the source for tidal current values in the Mid-Atlantic Bight.

Short-term variations of platinum concentrations in contrasting coastal environments: The role of primary producers

Short-term variations of Pt concentrations and primary production indicators were compared in three contrasting coastal sites during spring bloom: (i) the Gironde Estuary mouth (SW France), (ii) the semi-enclosed Arcachon Bay (SW France), and (iii) the urbanized Genoa Harbor (NW Italy). At each site, surface seawater sampling and physical-chemical measurements were combined to study diel cycles (over 25 h) of dissolved Pt concentrations in seawater (PtD) and master variables reflecting primary production activity (chlorophyll-a, phaeopigments, and particulate organic carbon, POC concentrations). Plankton nets were used in all sites, providing for the first time plankton Pt concentrations (PtPK) over a whole diel cycle (Gironde Estuary mouth) and spot sampling (Arcachon Bay and Genoa Harbor) in the coastal zone. Bivalves (wild oysters or mussels), reflecting organisms at higher trophic levels, were also collected at all sites. The POC/Chl-a ratios in the collected particulate material suggested high contribution of phytoplankton to the particulate matter in the productive Gironde Estuary mouth. At this site, phytoplankton activity partly controlled Pt cycling and particle/dissolved Pt partitioning during daytime. During the night, zooplankton grazing may release Pt into the dissolved phase. These processes are partly masked by external factors such as tide or local Pt sources, especially in more confined and/or urbanized coastal water bodies such as the Arcachon Bay and the Genoa Harbor. Platinum levels in plankton and bivalves from these contrasting sites along the Atlantic and Mediterranean coasts tended to reflect the general Pt levels in seawater. These results clearly suggest that (i) Pt contamination of coastal waters and marine organisms has become a common feature in urbanized sites and (ii) Pt transfer to the marine food chain starts at the basic level of primary producers. By concentrating Pt (Bioconcentration Factor: BCF ~ 104), phytoplankton may serve as a biomonitor to assess Pt contamination in coastal environments.

Sea Level, Tidal, and River Flow Trends in the Lower Columbia River Estuary, 1853–present

AbstractFew tidal records are available pre‐1900 for the Pacific Ocean. We improve data coverage by recovering historical tabulations and digitizing analog tide rolls from Astoria, Oregon, for 1853–1876. Nearly 13,500 overlapping images of tides from 1855–1870 were digitized at a 6 min resolution using a line‐finding algorithm. Available hourly and high/low tabulations were also digitized, as were nearby hourly records from 1933 to 1943. Uncertainty was assessed by evaluating manual staff measurements, historical documents, and leveling surveys. Results suggest that uncertainty in mean sea level varies from ±0.07 m (early 1850s) to ±0.03 m (1867–1876) and is driven primarily by datum and benchmark uncertainty, rather than measurement precision, data reduction procedures, or hydrodynamic changes. We also corrected an up‐to 0.05 m error in the 1925–1960 tidal datum at Astoria. Harmonic analysis shows that major tidal constituents increased by up to 7% between 1855 and 2018. Mean tidal range increased by 0.1 m (5%), with more change occurring in July (0.17 m larger) than winter (0.07 m larger). By contrast, sea level increased most in winter and least in spring/summer. Tidally based estimates of river discharge suggest that these observations are caused by a ~50% reduction in peak spring discharge and a 30–60% increase in winter discharge. No evidence of altered upwelling is found. Overall, Astoria relative sea level (RSL) increased by 0.06 m ± 0.04 m since the 1858–1876 epoch or, after accounting for vertical land motion, 0.11 ± 0.09 m. Consistent with GNSS measurements, RSL has dropped near the estuary mouth since 1905, indicating a strong tectonic influence.