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This paper presents a systematic review of the research available on salinity optic fiber sensors (OFSs) for seawater based on the refractive index (RI) measurement principle for the actual measurement demand of seawater salinity in marine environmental monitoring, the definition of seawater salinity and the correspondence between the seawater RI and salinity. To further investigate the progress of in situ measurements of absolute salinity by OFSs, the sensing mechanisms, research progress and measurement performance indices of various existing fiber optic salinity sensors are summarized. According to the Thermodynamic Equation of Seawater-2010 (TEOS-10), absolute salinity is recommended for sensor calibration and measurement. Comprehensive domestic and international research progress shows that fiber-optic RI sensors are ideal for real-time, in situ measurement of the absolute salinity of seawater and have excellent potential for application in long-term in situ measurements in the deep ocean. Finally, based on marine environmental monitoring applications, a development plan and the technical requirements of salinity OFSs are proposed to provide references for researchers engaged in related industries.

As a new seawater standard, the International Thermodynamic Equation of Seawater-2010 (TEOS-10) has made it possible to accurately calculate salinity changes caused by small changes in the relative proportions of inorganic dissolved constituents in the open ocean and thus to accurately calculate the thermodynamic properties of seawater. However, offshore and semi-enclosed seas are subject to the dual influence of geo-biochemical processes and terrestrial inputs, and the spatial and temporal variations in the relative composition of their seawater are more complex, resulting in very few studies of salinity change due to changes in the relative composition in these regions, and the applicability of the TEOS-10 in these seas needs to be further evaluated. The Bohai Sea is a typical semi-enclosed sea that has accumulated a large amount of physical oceanography and marine chemistry measurements, making it an ideal area to study the effects of changes in the relative composition of seawater on salinity. Based on repeated measurements of Section B in the Bohai Sea from 1985 to 2020, complemented by quasi-synchronous large-scale measurements of the Bohai Sea from 2006 to 2007, this study found that [ HCO 3 − ] and [ Ca 2 + ] ions anomalies dominate the relative composition anomalies of dissolved substances in this sea relative to the Standard Sea Water (SSW). These additional components could increase the absolute salinity S A by up to 0.1 g·kg -1 and the practical salinity S P by 0~0.04 according to the TEOS-10 algorithm and the mathematical model Pa08. Based on the specific environmental characteristics of the Bohai Sea, the sources, spatial and temporal variations, and influencing factors of the relative compositional anomalies are analyzed, and the empirical formulas of the absolute salinity anomaly δ S A and practical salinity anomaly δ S P for the Bohai Sea are further fitted, with the uncertainty of 0.007~0.021 g·kg -1 for δ S A and 0.003~0.008 for δ S P . The above results can not only improve the accuracy of the thermodynamic properties of the Bohai Seawater, but also provide the compatibility level of the long-term salinity changes in the Bohai Sea and its adjacent waters.

Li, Y.; Wang, J., and Wang, S., 2020. Absolute salinity measurement based on microfiber coaxial Mach-Zehnder interferometer. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 194-201. Coconut Creek (Florida), ISSN 0749-0208. Seawater salinity is a key parameter in the study of ocean dynamics, accurate measurement of seawater salinity is crucial. However, current seawater salinity is a measure of practical salinity, which limits the accuracy of salinity measurements. The proposed absolute salinity compensates for the flaws in the practical salinity that does not accurately reflect the true salinity of seawater. Microfiber sensors enable highly sensitive measurements of absolute salinity in seawater. In this paper, a new method based on microfiber coaxial Mach-Zehnder interferometer (coaxial MZI) is proposed for seawater absolute salinity measurement. The absolute salinity sensitivity of microfiber coaxial MZI is calculated by finite element program, and the theoretical results show that the sensitivity increases with increasing wavelength and increases with the decrease of waist diameter. Thirteen microfiber coaxial MZIs with different diameter were fabricated and used for the measurement of absolute salinity. Experimental results show that the highest sensitivity is 2.38nm/ ‰, and the variations rules of sensor sensitivity are consistent with theoretical calculations. In addition, in order to promote the practical application of microfiber devices in the marine field, coaxial MZI is encapsulated. The salinity response time, time stability and vibration influence of the sensor were tested by experiments. Encapsulated coaxial MZI is expected to realize the in-site measurement of the absolute salinity of seawater.

As one of few who have been involved in the equation of state of seawater over the last 40 years, I was invited to review some of the history behind its early development and also the more recent thermodynamic equation of state. The article first reviews early (late 1800s) work by Knudsen and others in defining the concept of salinity. This summary leads into the development of the practical salinity scale. Our studies at the University of Miami Rosenstiel School, along with the work of Alain Poisson's group at Laboratoire de Physique et Chimie, Université Pierre et Marie Curie, and that of Alvin Bradshaw and Karl Schleicher at Woods Hole Oceanographic Institution, were instrumental in deriving the 1980 equation of state (EOS-80) that has been used for 30 years. The fundamental work of Ranier Feistel at Leibniz Institute for Baltic Sea Research led to the development of a Gibbs free energy function that is the backbone of the new thermodynamic equation of state (TEOS-10). It can be used to determine all of the thermodynamic properties of seawater. The salinity input to the TEOS-10 Gibbs function requires knowledge of the absolute salinity of seawater (SA), which is based upon the reference salinity of seawater (SR). The reference salinity is our best estimate of the absolute salinity of the seawater that was used to develop the practical salinity scale (SP), the equation of state, and the other thermodynamic properties of seawater. Reference salinity is related to practical salinity by SR= SP(35.16504/35.000) g kg⁻¹ and absolute salinity is related to reference salinity by SA= SR+ δSA, where δSAis due to the added solutes in seawater in deep waters resulting from the dissolution of CaCO₃(s) and SiO₂(s), CO₂, and nutrients like NO₃ and PO₄ from the oxidation of plant material. The δSAvalues due to the added solutes are estimated from the differences between the measured densities of seawater samples compared with the densities calculated from the TEOS-10 equation of state (Δρ) at the same reference salinity, temperature, and pressure, using δSA= Δρ/0.75179 g kg⁻¹. The values of δSAin the ocean can be estimated for waters at given longitude, latitude, and depth using correlations of δSAand the concentration of Si(OH)₄ in the waters. The SAvalues can then be used to calculate all the thermodynamic properties of seawater in the major oceans using the new TEOS-10. It will be very useful to modelers examining the entropy and enthalpy of seawater.

Accurate and traceable measurements are required to understand ocean processes, to address pressing societal challenges, such as climate change and to sustainably manage marine resources. Although scientific and engineering research has resulted in advanced methods to measure Essential Ocean Variables (EOVs) there is a need for cross comparison of the techniques and traceability to recognized standards. Metrological laboratories are experienced in accredited methods and assessment of methodology. An EU INFRAIA-02-2020: Integrating Activities for Starting Communities project MINKE (Metrology for Integrated marine maNagement and Knowledge-transfer nEtwork https://minke.eu ) brings European marine science and metrology Research Infrastructures together to identify synergies and create an innovative approach to Quality Assurance of oceanographic data. Quality depends both on the accuracy (that can be provided through the metrology component) and the completeness of the data sets. The collaboration between different Marine Research Infrastructures (RIs) places a fundamental role on assuring the completeness of the datasets, particularly at global scales. The MINKE project encourages enhancement through collaboration of national metrology laboratories and the oceanographic community. Metrological assessment of the accuracy and uncertainties within multidisciplinary ocean observations will provide data that are key to delivering policy information. Objectives across all the RIs are to facilitate ocean observation and build wider synergies. MINKE will investigate these synergies, then introduce metrology to the core of various EOV measurements. Currently the marine RIs cover laboratory and field operations, from the surface seafloor, coastal waters to deep sea, fixed ocean stations to ship and autonomous vehicle operations to ships of opportunity, and flux stations focusing on carbonate system variables. The nexus of these operations is the focal point for coordinated improvement of ocean observing methods. Measurement intercomparisons, traceability and uncertainty assessments should be at the core of the scientific observations. Specifically, MINKE will work with RIs and Metrology Institutes to improve the quality of dissolved oxygen, carbonate system, chlorophyll-fluorescence, ocean sound and current meter measurements, through access to metrology laboratories, Transnational Access and intercomparison studies across existing marine consortia and RIs. MINKE will also promote the development of absolute salinity observation, and improvements in marine litter measurements.

Accurate and traceable measurements are required to understand ocean processes, to address pressing societal challenges, such as climate change and to sustainably manage marine resources. Although scientific and engineering research has resulted in advanced methods to measure Essential Ocean Variables (EOVs) there is a need for cross comparison of the techniques and traceability to recognized standards. Metrological laboratories are experienced in accredited methods and assessment of methodology. An EU INFRAIA-02-2020: Integrating Activities for Starting Communities project MINKE (Metrology for Integrated marine maNagement and Knowledge-transfer nEtwork https://minke.eu ) brings European marine science and metrology Research Infrastructures together to identify synergies and create an innovative approach to Quality Assurance of oceanographic data. Quality depends both on the accuracy (that can be provided through the metrology component) and the completeness of the data sets. The collaboration between different Marine Research Infrastructures (RIs) places a fundamental role on assuring the completeness of the datasets, particularly at global scales. The MINKE project encourages enhancement through collaboration of national metrology laboratories and the oceanographic community. Metrological assessment of the accuracy and uncertainties within multidisciplinary ocean observations will provide data that are key to delivering policy information. Objectives across all the RIs are to facilitate ocean observation and build wider synergies. MINKE will investigate these synergies, then introduce metrology to the core of various EOV measurements. Currently the marine RIs cover laboratory and field operations, from the surface seafloor, coastal waters to deep sea, fixed ocean stations to ship and autonomous vehicle operations to ships of opportunity, and flux stations focusing on carbonate system variables. The nexus of these operations is the focal point for coordinated improvement of ocean observing methods. Measurement intercomparisons, traceability and uncertainty assessments should be at the core of the scientific observations. Specifically, MINKE will work with RIs and Metrology Institutes to improve the quality of dissolved oxygen, carbonate system, chlorophyll-fluorescence, ocean sound and current meter measurements, through access to metrology laboratories, Transnational Access and intercomparison studies across existing marine consortia and RIs. MINKE will also promote the development of absolute salinity observation, and improvements in marine litter measurements.

The thermodynamic properties of seawater have recently been redefined as the International Thermodynamic Equation of Seawater—2010 (TEOS-10 for short), and here we summarize the changes to oceanographic practices that are needed to take advantage of this new international standard. A key feature of TEOS-10 is that the thermodynamic quantities are functions of a new salinity variable, Absolute Salinity, which incorporates the effects of spatial differences in seawater composition. TEOS-10 also treats the “heat content” of seawater in a more consistent and natural fashion through the introduction of a new temperature variable, Conservative Temperature, which replaces potential temperature. Since TEOS-10 includes fundamental equations of state also for ice and for humid air, thermodynamically consistent and complete relationships now exist between all the thermodynamic properties of fresh water, seawater, ice and humid air.

Growing pollution hazards in large watersheds, including Egypt’s Nile River, necessitate precise and effective monitoring of water quality indices. Although considerable efforts have been made to model individual parameters such as pH, Total Dissolved Solids (TDS), Electrical Conductivity (EC) and sodium (Na ), the emphasis on isolated forecasts has often overlooked their interdependencies, thereby limiting estimation accuracy and practical applicability. Conventional machine learning methodologies further compound this issue, as they frequently lack adaptability and physical interpretability, resulting in inefficiencies for sustainable water management. To address these limitations, this study develops a physics-informed neural network (PINN) framework integrated with optimization boosting techniques to jointly predict pH, TDS (mg/L), EC ( S/cm), and Na (mg/L) under three critical management strategies: industrial discharge regulation, salinity management and irrigation planning. The approach incorporates prior hydrodynamic and chemical knowledge to guide input categorization, employs adaptive weighting mechanisms to dynamically adjust feature relevance, and introduces a deep interaction module to capture intricate physicochemical couplings. A physics-constrained loss function ensures consistency with ecological processes. Field analysis revealed that while pH values (6.01–6.87) consistently met FAO standards, TDS, EC, and Na frequently exceeded permissible thresholds, particularly during the dry season. Model evaluation showed that PINN outperformed conventional optimizers, achieving values of 0.945–0.999 and RMSE between 0.012–0.088. Cross-validation further confirmed its robustness, yielding consistently low RMSE and near-unity across folds. Interpretability analysis highlighted irrigation intensity, salinity loads, and industrial effluents as the dominant drivers of water quality dynamics. Overall, the proposed PINN not only advances predictive accuracy and computational efficiency but also provides a practical framework for supporting regulatory decision-making, optimizing salinity control, and guiding sustainable irrigation planning. Ultimately, this approach offers a viable pathway for mitigating contamination risks and ensuring the long-term sustainability of the Nile River ecosystem.

Salt stress is one of the most serious abiotic factors that inhibit plant growth. Dunaliella salina has been recognized as a model organism for stress response research due to its high capacity to tolerate extreme salt stress. A proteomic approach based on isobaric tags for relative and absolute quantitation (iTRAQ) was used to analyze the proteome of D. salina during early response to salt stress and identify the differentially abundant proteins (DAPs). A total of 141 DAPs were identified in salt-treated samples, including 75 upregulated and 66 downregulated DAPs after 3 and 24 h of salt stress. DAPs were annotated and classified into gene ontology functional groups. The Kyoto Encyclopedia of Genes and Genomes pathway analysis linked DAPs to tricarboxylic acid cycle, photosynthesis and oxidative phosphorylation. Using search tool for the retrieval of interacting genes (STRING) software, regulatory protein–protein interaction (PPI) networks of the DAPs containing 33 and 52 nodes were built at each time point, which showed that photosynthesis and ATP synthesis were crucial for the modulation of early salinity-responsive pathways. The corresponding transcript levels of five DAPs were quantified by quantitative real-time polymerase chain reaction (qRT-PCR). These results presented an overview of the systematic molecular response to salt stress. This study revealed a complex regulatory mechanism of early salt tolerance in D. salina and potentially contributes to developing strategies to improve stress resilience.

Conductivity–temperature–depth (CTD) transects across continental slope of the Eurasian Basin and the St. Anna Trough performed during NABOS (Nansen and Amundsen Basins Observing System) project in 2002–2015 and a transect from the 1996 Polarstern expedition are used to describe the temperature and salinity characteristics and volume flow rates (volume transports) of the current carrying the Atlantic water (AW) in the Arctic Ocean. The variability of the AW on its pathway along the slope of the Eurasian Basin is investigated. A dynamic Fram Strait branch of the Atlantic water (FSBW) is identified in all transects, including two transects in the Makarov Basin (along 159∘ E), while the cold waters on the eastern transects along 126, 142, and 159∘ E, which can be associated with the influence of the Barents Sea branch of the Atlantic water (BSBW), were observed in the depth range below 800 m and had a negligible effect on the spatial structure of isopycnic surfaces. The geostrophic volume transport of AW decreases farther away from the areas of the AW inflow to the Eurasian Basin, decreasing by 1 order of magnitude in the Makarov Basin at 159∘ E, implying that the major part of the AW entering the Arctic Ocean circulates cyclonically within the Nansen and Amundsen basins. There is an absolute maximum of θmax (AW core temperature) in 2006–2008 time series and a maximum in 2013, but only at 103∘ E. Salinity S(θmax) (AW core salinity) time series display a trend of an increase in AW salinity over time, which can be referred to as an AW salinization in the early 2000s. The maxima of θmax and S(θmax) in 2006 and 2013 are accompanied by the volume transport maxima. The time average geostrophic volume transports of AW are 0.5 Sv in the longitude range 31–92∘ E, 0.8 Sv in the St. Anna Trough, and 1.1 Sv in the longitude range 94–107∘ E.