Salinity stratification and vertical shear transport in an estuary

Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of Sao Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s−1 to 60 cm s−1 during flood tides and from 70 cm s−1 to 100 cm s−1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux.

An apparent lunar tidal cycle of phytoplankton blooming and community succession in the Gulf of Maine

The effect of stratification on the vertical structure of the tidal ellipse in the Changjiang River estuary, China

The increase of saltwater intrusion in recent years in the Pearl River Delta, has threatened the freshwater supply in the surrounding regions, especially the cities of Zhongshan, Zhuhai, Guangzhou in Guangdong Province and Macau. A numerical modeling system using nested grids was developed to simulate the salinity distribution in the Pearl River delta, and then to investigate the salt transport process and calculate the salt flux for each outlet in the Pearl River estuary. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the major tributaries in the Pearl River system. The model simulation results are in qualitative agreement with the available field data. The salt flux of the Pearl River delta during the spring tide in dry season is up to 19.5×106ton/ tidal period, while the salt flux during neap tide is only 5.1×106ton/ tidal period, 26.18% of that during the spring tide. The salt flux in Dahu and Guanchong stations are the highest among the stations of the eight outlets, indicating that Humen and Yamen are the most important entries for saltwater intrusion in the Pearl River delta. The most important reason is that the ratio of stream flow to tide flow is different for each outlet. The ratios at Humen and Yamen are the smallest among the eight outlets (<1 for each month), while the ratio at Modaomen is the biggest (>1 for each month), which leads to the lowest salt flux at Modaomen. Salinity distribution in different time periods shows that saltwater intrusion during the spring tide is much more serious than neap tide, and water in many cities during this time period will be unavailable for drinking, irrigation or for ecological purpose.

Saltwater intrusion has been serious in the Pearl River estuary in recent years. For better understanding and analysis of the saltwater movement to the estuary, the three-dimensional Finite-Volume Coastal Ocean Model (FVCOM) is made to simulate the salinity intrusion to the four western watercourses in the Pearl River estuary under three semilunar conditions. With the measured and simulated Root Mean Square Error (RMSE) and the mean absolute percentage error of water level and salinity at multiple sites, the results show that the numerical water levels, salinity and flow velocities are in agreement with the measured data. It is acceptable and feasible to apply the FVCOM to simulate the salt water intrusion in the western four watercourses of the Pearl River. With the numerical data, the time and spatial movement patterns of saltwater intrusion along the Modao watercourse are analyzed. The salinity contour reaches its peak generally during 3∼5 days before the spring tide. The salinity stratification is more obvious in the period of ebb tide than that in the rising tide whether in the spring or neap tides. Salt fluxes reflect changes of salt into the estuary, and the change rules are close to the rules of salinity intrusion.

Mangrove forest is one of the most carbon storing tropical forests. Thus, understanding the dynamics of greenhouse gas such as CO2 and CH4 are important to predict the future. CO2 is produced by plant respiration, dissolved in water and transported upwards with transpiration. Active respiration in mangrove roots occurs because salt from seawater must be excluded before water enters the xylem. Therefore, tidal fluctuation can affect the exchange of CO2 between tree surface and the atmosphere. CH4 is produced in anaerobic conditions in the sediment and is not readily soluble in water. However, it can diffuse through aerenchyma and pores in root and stem tissues, particularly during the day when the water potential becomes low and tissue air porosity increases. Because CH4 has a global warming potential that is 25 times more powerful than CO2 over 100 years, CH4 emissions have the potential to offset a part of the CO2 initially removed from the atmosphere by photosynthesis and buried as blue carbon in the sediment. Thus, both CO2 and CH4 fluxes should be considered in the carbon budget of mangrove ecosystems. The aim of this study was to determine how tidal and diurnal cycles affect CO2 and CH4 emissions from mangrove roots and stems, both physiologically and physically, for a more accurate understanding of gas exchange processes in mangrove forests.The research was conducted on Bruguiera gymnorrhiza and Rhizophora stylosa on the bank of the Miyara River in Ishigaki island. Chambers were placed at three heights along the stems (approximately 0.2, 1.0, and 1.5 m above the sediment) and on different root types to measure CO2 and CH4 fluxes during day and night and low and high tide in June, August and November 2024.CO2 efflux from the trunk showed no difference with height. In contrast, CH4 efflux was highest at the base of the trunks. At low tide, CH4 emissions from the roots were much higher than those from the trunk. Both CO2 and CH4 efflux from the trunk surface was lower at night than during the day, and there was no difference between high and low tide. Large variations in CH4 efflux was observed from the same position at different times. Continuous measurements are needed to better characterize these temporal variations. Characterizing the spatial distribution of roots is also a future challenge.

Salt transport during a dry season in the Modaomen Estuary, Pearl River Delta, China

The transport of microplastics (MPs) from rivers to the ocean remains poorly constrained, highlighting the need for quantitative evaluation of the MPs budget in estuarine environments. This study quantitatively evaluated MPs transport in an estuary using field observations and numerical simulations, focusing on vertical distribution, mass flux, and salt-wedge processes. MPs in estuarine environments were found to accumulate vertically and be re-transported by tidal forcing, resulting in seaward export over a single tidal cycle.Field observations were conducted in the estuarine reach of the Oita River during one tidal cycle, with five sampling campaigns from high tide through low tide to the subsequent high tide. Water samples were collected at 50 cm vertical intervals from the surface to near the riverbed, and flow velocities were measured simultaneously.The results showed that vertical MPs mass fluxes consistently followed a depth-dependent pattern of bottom > mid-depth > surface during both neap and spring tides, indicating persistent bottom-dominated transport. Spatiotemporal integration of MPs mass fluxes yielded a net seaward transport of +1.22 × 10³ mg/m over one tidal cycle, quantitatively demonstrating MPs export from land to the coastal ocean. This net export resulted from dominant land-to-sea transport during ebb tide, driven by upstream MPs fluxes and resuspension from the bottom layer, exceeding landward transport during flood tide.Numerical simulations reproduced MPs accumulation near the halocline formed during ebb tide and subsequent seaward transport by bottom currents, consistent with the observed positive net flux. These results demonstrate that estuarine MPs transport is asymmetrically controlled by tidal flow and salinity stratification, with the halocline playing a key role in MPs accumulation and seaward export.

Interpreting grain-size trends associated with bedload transport on the intertidal flats at Dafeng, central Jiangsu coast

Understading the hydrodynamic behavior and suspended particulated matter (SPM) transport are of great importance in port regions such as Vitória Harbor, which is located at Vitória Bay, Vitória - ES, Brazil. Vitória Bay is an estuary that has not been systematically assessed through a temporal analysis in order to identify its hydrodynamics characteristics and SPM exchange. This study aims to investigate salt and suspended particulate matter flux at the estuarine mouth of Vitória Bay by understanding the temporal variation of salinity, temperature and tidal currents within the water column and at the channel crosssection. Results showed that the estuarine mouth tended to present partial stratification periods during neap tides and little stratification in spring tides. The circulation pattern was mainly influenced by the tide, with little influence from river discharge. With regard to the SPM, the mouth of the estuary tended to show low concentrations, with the highest values occurring during the dry season. A close relationship between momentary discharge, SPM and salt fluxes was observed. Despite all the data was collected at the mouth of the estuary, the system showed an importation trend of salt in all cycles and SPM importation for three of the four studied tidal cycles. Thus, Vitoria Bay is not exporting SPM to the adjacent inner shelf.

The results of a numerical study of Delaware Bay using the Regional Ocean Modeling System (ROMS) are presented. The simulations are run over a range of steady river inputs and used M2 and S2 tidal components to capture the spring–neap variability. Results provide a description of the spatial and temporal structure of the estuarine exchange flow and the salinity field, as well the along-channel salt flux in the estuary. The along-channel salt flux is decomposed into an advective term associated with the river flow, a steady shear dispersion Fe associated with the estuarine exchange flow, and a tidal oscillatory salt flux Ft. Time series of Fe and Ft show that both are larger during neap tide than during spring. This time variability of Ft, which is contrary to existing scalings, is caused by the lateral flows that bring velocity and salinity out of quadrature and the stronger stratification during neap tide, which causes Ft to be enhanced relative to spring tide. A fit for the salt intrusion length L with river discharge Q for a number of isohalines is performed. The functional dependences of L with Q are significantly weaker than Q−1/3 scaling. It is concluded that the response of the salt field with river discharge is due to the dependence of Fe and Ft with Q and the relative importance of Ft to the total upstream salt flux: as river discharge increases, Fe becomes the dominant mechanism. Once Fe dominates, the salt field stiffens because of a reduction of the vertical eddy viscosity with increasing Q.

The Elbe is one of the longest European rivers and features a large, turbid and well-mixed estuary, which runs through the inner city of Hamburg. The Elbe has been closely monitored using classical catch techniques in the past. Here we tested a COI-based eDNA approach for assessing the biodiversity within the Elbe. We sampled three stations in the Elbe, included low and high tide events, as well as two adjoining lakes to compare the recovered faunas. To analyze the data, we employed two different pipelines: the automated mBRAVE pipeline utilizing the BOLD database and one including NCBI BLAST. The number of OTUs with species or higher-level identifications were similar between both approaches with 352 OTUs and 355 OTUs for BLAST and mBRAVE, respectively, however, BLAST searches recovered another 942 unidentified metazoan OTUs. Many taxa were well represented; however, fish species were poorly represented, especially in the Elbe estuary samples. This could be a result of the universal COI primers, which also yielded high read numbers for non-metazoan OTUs, and small-bodies taxa like Rotifera, which might have been sampled together with the eDNA. Our results show a strong tidal influence on the recovered taxa. During low tide, downstream stations resembled sites further upstream, but the former showed a very different OTU composition during high tide and early tide. Such differences might be due to varying impacts of upstream-originating eDNA during tide cycles. Such factors need to be considered when routinely employing eDNA for monitoring programs.

High-resolution current velocity and sus- pended sediment concentration (SSC) data were collected by using an Acoustic Doppler Current Profiler (ADCP) at two anchor stations and a cross-section in the South Channel of the Changjiang River mouth during meso and neap tides on Nov. 16, 2003. In addition, tidal cycle (13-hour) observation at two stations was carried out with traditional methods during the spring tide. Results indi- cated that resuspension occurred not only at the flood and ebb maximum, but also in the early phase of ebb in the meso and neap tide. When tidal current transited from high to ebb phase, current speed accelerated. Subse- quently, fine-grained sediment with low critical threshold was resuspended and increased concentration. The river mouth area remained in siltation in the meso and neap tidal phase during the observation season, with calculated resuspension flux in the order of magnitude of 10 24 - 10 27 kg?m 22 /s. Suspended sediment transport in the South Channel was dominated by freshwater discharge, but the Storks drift, vertical circulation and vertical shear effect due to tidal oscillation also played an important role in resuspension and associated sediment transport. In contrast, resuspension sediment flux in the spring tide was larger than that in meso and neap tide, especially at the ebb maximum and flood maximum. The present study revealed that intensive resuspension corresponded well with the larger current velocity during winter. In addition, the 'tidal pumping' effect and tidal gravity circulation were also vital for forming the turbidity maximum in the Changjiang River estuary.

The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach

The spatial and vertical structure of the water currents and its relationship with the tidal cycles were studied using current meters in the Gulf of Nicoya. In the upper gulf, the vertical marine current differences increase as the depth increases. The water column at the station near Chira Island (upper gulf) shows the smaller changes in currents and in temperature. The flow at the station between San Lucas Island and Puntarenas (middle gulf) is the most stratified in this region. Currents with magnitudes over 100 cm/seg were measured during spring tides. Changes in the lags of the surface and bottom tidal flows were measured on the order of 100 minutes. In general, in this upper region the flows are toward the head of the gulf when the tide is flooding and toward the mouth when the tide is ebbing. In the lower gulf the circulation is more complex. Along an axis between Tárcoles and Negritos Islands, changes of velocity vectors are identified between surface and bottom. The current rotates in a different way in the water column in this region and their patterns cannot be explained only by the tidal cycles predicted for Puntarenas. These results demonstrate that the spatial and vertical variation of the marine currents of the Gulf of Nicoya is not only related to the thermohaline structure, but also to the tidal cycles and tide ranges that take place in this estuary.