Temporal and Spatial Variability Scales of Salinity at a Large Microtidal Estuary

The Río de la Plata is a large fluvial–estuarine–sea system discharging into the southwestern Atlantic Ocean, which has relevant features such as high fluvial discharge, microtidal astronomical tidal scales, a relevant meteorological tide, and a strong atmospheric forcing effect, due to its large width. The objective of this study is to advance the understanding of the estuarine hydrodynamics and salt transport, as well as discussing the main characteristics of the spatiotemporal variability of the salinity field. To achieve this, the results of a 3D model of the Río de la Plata and its maritime front were used, simulating an extensive period of 10 years. In this study, the model was validated using vertical salinity profiles collected at different locations in the estuary. The temporal variability of the salinity stratification was characterised at different temporal scales: annual, monthly, and storm. At the same time, the influences of fluvial flow and winds were determined. The correlation analysis between fluvial flow and the salinity field showed that high annual fluvial flows generate an extension of the freshwater area, with larger longitudinal salinity gradients and a shift of the salinity front towards the ocean. The tendency at the monthly scale is not as clear as that observed at the annual scale. The results show that the effect of a storm coming from the northwest is quite similar to that of storms coming from the southwest, especially in the central and southern zones of the Río de la Plata, where mixing increases and stratification decreases, according to the intensity of the storm. The effect of south–southeasterly storms increases the mixing process and reduces stratification; the opposite effect was identified with respect to northeasterly storms, under the influence of which the stratified area increases. Synthesising the obtained results, a global zonification of the vertical salinity stratification for the Río de la Plata is proposed.

Investigation of the field of colored dissolved organic matter concentration and its relationship to the salinity field in Dnieper-Bug estuary

Based on the data of the expedition carried out in August 2012, the features of the structure of the salinity and fDOM concentration fields, as well as the correlation between these values ​​in the Dnieper-Bug estuary, are analyzed. It is shown that the salinity field in the Dnieper-Bug estuary is characterized by all structural features typical of coastal sea areas freshened by river runoff, while the fDOM concentration field has anomalous properties due to the presence of an anthropogenic component of this value. The presence of fDOM of anthropogenic origin is most significant in the western part of the Dnieper estuary and is related to the influence of the city and port of Ochakov. The maximum content of fDOM was recorded here, which caused an anomalous horizontal distribution of this substance concentration, increasing from the mouth of the Dnieper to the open Black Sea waters. The contaminated areas of the studied water areas, which are adjacent to settlements and occupy the predominant part of the water surface of the Dnieper-Bug estuary, are characterized by an increased fDOM content and intrusive feature of the vertical structure of this substance concentration field of. It is revealed that in the Dnieper-Bug estuary there is no inverse correlation between the salinity fields and fDOM concentration, which is typical for unpolluted coastal waters, freshened by river runoff. A similar anomaly is an indicator of the pollution of coastal waters with organic matter.

Сhanges of the Structure Indicators and the Salinity Field Average Value in the Sea of Azov

Purpose. The work is aimed at continuing the started in the previous papers investigations of structure of the Azov Sea salinity field based on the oceanographic survey data collected since 2000. Interest in studying this parameter is conditioned by its anomalous increase after 2006. Methods and Results. The data of 49 seasonal oceanographic surveys carried out in the Sea of Azov by the Azov-Black Sea Branch of "VNIRO" ("AzNIIRKH") from 2001 to 2016 permitted to calculate the following: the radii in the concentration region of the field spatial correlation function in the meridional and zonal directions for the surface and bottom layers (the characteristic of the field homogeneity); the ratio between these radii; the sea-average values of salinity field for the surface and bottom layers. The time graphical and the paired correlation analyses of the calculated indicators’ series were done. The average values of the meridional and zonal radii of the concentration region of the spatial correlation function (42.5 and 47.1 km) testify presence of two relatively isolated zones in the sea related to the water circulation. These zones are formed under the conditions of the eastern winds dominating in the region. The average values of the above-mentioned radii the near-bottom sea layer were approximately equal, whereas in the surface layer, the average zonal radii exceeded the meridional ones. In spring and summer, the meridional radius in the bottom layer surpassed the zonal one. Long-term variability shows that in the surface layer, the meridional radius values tend to increase, and in the bottom layer, the zonal radius ones – to decrease. These trends demonstrate a change in the nature of water exchange in the sea, namely from predominance of the zonal transport to that of the meridional one. Conclusions. Since 2006, the changes in the structure of the Azov Sea water salinity field (trends towards decrease of the zonal radii and increase of the meridional ones in the concentration region of the field spatial correlation function) resulted from decrease in the river water inflow and increase of water exchange with the Kerch Strait, and were accompanied by growth of average salinity. Water exchange with the Kerch Strait in the bottom layer was the most active in spring and summer. The anticipatory shift of the field structural characteristics by 1 and 2 years relative to its average values makes it possible to forecast them with a two-year advance time.

Сhanges of the Structure Indicators and the Salinity Field Average Value in the Sea of Azov

Purpose. The work is aimed at continuing the started in the previous papers investigations of structure of the Azov Sea salinity field based on the oceanographic survey data collected since 2000. Interest in studying this parameter is conditioned by its anomalous increase after 2006. Methods and Results. The data of 49 seasonal oceanographic surveys carried out in the Sea of Azov by the Azov-Black Sea Branch of "VNIRO" ("AzNIIRKH") from 2001 to 2016 permitted to calculate the following: the radii in the concentration region of the field spatial correlation function in the meridional and zonal directions for the surface and bottom layers (the characteristic of the field homogeneity); the ratio between these radii; the sea-average values of salinity field for the surface and bottom layers. The time graphical and the paired correlation analyses of the calculated indicators’ series were done. The average values of the meridional and zonal radii of the concentration region of the spatial correlation function (42.5 and 47.1 km) testify presence of two relatively isolated zones in the sea related to the water circulation. These zones are formed under the conditions of the eastern winds dominating in the region. The average values of the above-mentioned radii in the near-bottom sea layer were approximately equal, whereas in the surface layer, the average zonal radii exceeded the meridional ones. In spring and summer, the meridional radius in the bottom layer surpassed the zonal one. Long-term variability shows that in the surface layer, the meridional radius values tend to increase, and in the bottom layer, the zonal radius ones – to decrease. These trends demonstrate a change in the nature of water exchange in the sea, namely from predominance of the zonal transport to that of the meridional one. Conclusions. Since 2006, the changes in the structure of the Azov Sea water salinity field (trends towards decrease of the zonal radii and increase of the meridional ones in the concentration region of the field spatial correlation function) resulted from decrease in the river water inflow and increase of water exchange with the Kerch Strait, and were accompanied by growth of average salinity. Water exchange with the Kerch Strait in the bottom layer was the most active in spring and summer. The anticipatory shift of the field structural characteristics by 1 and 2 years relative to its average values makes it possible to forecast them with a two-year advance time.

River Freshwater Contribution in Operational Ocean Models along the European Atlantic Façade: Impact of a New River Discharge Forcing Data on the CMEMS IBI Regional Model Solution

River freshwater contribution in the European Atlantic margin and its influence on the sea salinity field are analyzed. The impacts of using a new river discharge database as part of the freshwater forcing in a regional ocean model are assessed. Ocean model scenarios, based on the CMEMS (Copernicus Marine Environment Monitoring Service) operational IBI-MFC (Iberia Biscay Ireland Monitoring Forecasting Centre) model set-up, are run to test different (observed, modeled and climatological) river and coastal freshwater forcing configurations throughout 2018. The modelled salinity fields are validated, using as a reference all known available in-situ observational data sources. The IBI model application is proven to adequately simulate the regional salinity, and the scenarios showcase the effects of varying imposed river outflows. Some model improvement is achieved using the new forcing (i.e., better capture of salinity variability and more realistic simulation of baroclinic frontal structures linked to coastal and river freshwater buoyancy plumes). Major impacts are identified in areas with bigger river discharges (i.e., the French shelf or the northwestern Iberian coast). Instead, the Portuguese shelf or the Gulf of Cadiz are less impacted by changes in the imposed river inflows, and other dynamical factors in these areas play a major role in the configuration of the regional salinity.

Seawater Salinity Estimating Module Based on the Sound Velocity Measurements

Purpose. Reliability of knowledge about the ocean dynamics and climate variability is largely limited for lack of systematic in situ observations of the sea surface layer salinity, which is one of the basic hydrological parameters determining circulation and stratification of the water masses. The study is aimed at developing an autonomous device for long-term monitoring of salinity in the seawater upper layer. Methods and Results. One of the most effective tools for in situ observations of the ocean upper layer is the global network of surface drifting buoys – drifters. At present, the network consists of more than 1500 buoys, but only a few of them provide sea surface salinity observations within the framework of a limited number of pilot experiments. In the drifters, salinity is calculated by the traditional method using the results of the electrical conductivity and temperature measurements. There are a few problems related both to the principle of determining salinity by this method and to providing long-term stable running of conductivity sensors under the conditions of pollution and biological fouling. A drifter equipped with the module for the sound velocity and temperature measurements used for calculating salinity by an alternative method just aboard the drifter, was developed in Marine Hydrophysical Institute, Russian Academy of Sciences. The sound velocity and temperature module includes a specially designed time-of-flight sound velocity sensor with the fixed base and a quartz temperature sensor. In course of two years, numerous laboratory and in situ tests of several prototypes of the sound velocity and temperature module were performed. The laboratory tests showed that the repeatability limits for the results of the sound velocity measurements in the distilled water were 0.02 m/s. According to the data of the long-term in situ tests performed at intensive biological fouling, the error of salinity estimation resulted from of the sound velocity and temperature measurements were within 0.05 ‰. This result permits to expect that the sound velocity and temperature module parameters will remain stable in real conditions of long-term autonomous operation. Conclusions. The obtained results make it possible to recommend application of the drifters equipped with the modules for the sound velocity and temperature measurements as an effective tool for regular operational monitoring of the salinity field of the upper sea layer.

Seawater Salinity Estimating Module Based on the Sound Velocity Measurements

Purpose. Reliability of knowledge about the ocean dynamics and climate variability is largely limited for lack of systematic in situ observations of the sea surface layer salinity, which is one of the basic hydrological parameters determining circulation and stratification of the water masses. The study is aimed at developing an autonomous device for long-term monitoring of salinity in the seawater upper layer. Methods and Results. One of the most effective tools for in situ observations of the ocean upper layer is the global network of surface drifting buoys – drifters. At present, the network consists of more than 1500 buoys, but only a few of them provide sea surface salinity observations within the framework of a limited number of pilot experiments. In the drifters, salinity is calculated by the traditional method using the results of the electrical conductivity and temperature measurements. There are a few problems related both to the principle of determining salinity by this method and to providing longterm stable running of conductivity sensors under the conditions of pollution and biological fouling. A drifter equipped with the module for the sound velocity and temperature measurements used for calculating salinity by an alternative method just aboard the drifter, was developed in Marine Hydrophysical Institute, Russian Academy of Sciences. The sound velocity and temperature module includes a specially designed time-of-flight sound velocity sensor with the fixed base and a quartz temperature sensor. In course of two years, numerous laboratory and in situ tests of several prototypes of the sound velocity and temperature module were performed. The laboratory tests showed that the repeatability limits for the results of the sound velocity measurements in the distilled water were ± 0.02 m/s. According to the data of the long-term in situ tests performed at intensive biological fouling, the error of salinity estimation resulted from of the sound velocity and temperature measurements were within ± 0.05 ‰. This result permits to expect that the sound velocity and temperature module parameters will remain stable in real conditions of long-term autonomous operation. Conclusions. The obtained results make it possible to recommend application of the drifters equipped with the modules for the sound velocity and temperature measurements as an effective tool for regular operational monitoring of the salinity field of the upper sea layer

Investigation of Spatial and Temporal Salinity Distribution in a River Dominated Delta through Idealized Numerical Modelling

The world’s river deltas are increasingly vulnerable due to pressures from human activities and environmental change. In deltaic regions, the distribution of salinity controls the resourcing of fresh water for agriculture, aquaculture and human consumption; it also regulates the functioning of critical natural habitats. Despite numerous insightful studies, there are still significant uncertainties on the spatio-temporal patterns of salinity across deltaic systems. In particular, there is a need for a better understanding of the salinity distribution across deltas’ channels and for simple predictive relationships linking salinity to deltas’ characteristics and environmental conditions. We address this gap through idealized three-dimensional modelling of a typical river-dominated delta configuration and by investigating the relationship between salinity, river discharge and channels’ bifurcation order. Model results are then compared with real data from the Mississippi River Delta. Results demonstrate the existence of simple one-dimensional and analytical relationships describing the salinity field in a delta. Salinity and river discharge are exponentially and negatively correlated. The Strahler-Horton method for stream labelling of the delta channels was implemented. It was discovered that salinity increases with decreasing stream order. These useful relationships between salinity and deltas’ bulk features and geometry might be applied to real case scenarios to support the investigation of deltas vulnerability to environmental change and the management of deltaic ecosystems.

Influence of Salinity Gradient Changes on Phytoplankton Growth Caused by Sluice Construction in Yongjiang River Estuary Area

Though the number of sluices and dams in coastal areas has increased rapidly in recent years, the influence of their construction on phytoplankton in estuary areas is hardly known. This paper aims to provide a reference for quantitative research on the ecological influence of sluice construction and give ecological justifications for the setting of environmental standards in the estuary areas. The survey data gained at the lower reach of the Yongjiang River and its estuarine areas in June 2015 were used in MIKE21 software (Danish Hydraulic Institute (DHI), Denmark)) for establishing a two-dimensional numerical model to simulate the salinity field distribution after sluice construction. Based on the simulation results, the salinity gradient changes caused by the construction were analyzed. The one-dimensional Gaussian model was applied to calculated the phytoplankton’s ecological threshold interval over the salinity changes, which helped predict the influence of salinity changes on phytoplankton cell density. The study shows that salinity in the Yongjiang estuary increases obviously, beyond the phytoplankton ecological threshold, after sluice construction without water discharge. Salinity will become a restriction factor to phytoplankton growth after sluice construction in the study area, which may cause a sharp decrease of certain phytoplankton species.