Multidecadal Sea-level Rise in the Southeast Indian Ocean: The Role of Ocean Salinity Change

Regional sea-level rise in the Southeast Indian Ocean (SEIO) exerts growing threats to the surrounding Australian and Indonesian coasts, but the mechanisms of sea-level rise have not been firmly established. By analyzing observational datasets and model results, this study investigates multidecadal steric sea-level (SSL) rise of the SEIO since the mid-20th century, underscoring a significant role of ocean salinity change. The average SSL rising rate from 1960 through 2018 was 7.4±2.4 mm decade−1, and contributions of the halosteric and thermosteric components were ~42% and ~58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400-1000 m depths. Further insights are gained through the decomposition of temperature and salinity changes into the Heaving (vertical displacements of isopycnal surfaces) and Spicing (density-compensated temperature and salinity change) modes. The subsurface freshening trend since 1960 is mainly attributed to the Spicing mode, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. Also noteworthy is a dramatic acceleration of SSL rise (20.3±7.0 mm decade−1) since ~1990, which was predominantly induced by the thermosteric component (16.3±5.5 mm decade−1) associated with the Heaving mode. Enhanced Ekman downwelling by surface winds and radiation forcing linked to global greenhouse-gas warming mutually caused the depression of isopycnal surfaces, leading to the accelerated SSL rise through thermosteric effect. This study highlights the complexity of regional sea-level rise in a rapid-changing climate, in which the role of ocean salinity is vital and time-varying.

Temporal Changes in Physiological Responses of Bay Scallop: Performance of Antioxidant Mechanism in Argopecten irradians in Response to Sudden Changes in Habitat Salinity

Changes to habitat salinity may induce oxidative stress in aquatic organisms. The effect of salinity on the antioxidant function of bay scallops was investigated at 55, 70, 85 and 120% of seawater salinity (SW), with 100% SW as the control. The scallops were sampled 0, 6, 12, 24, 48 and 72 h after the salinity change to measure superoxide dismutase (SOD), catalase (CAT), hydrogen peroxide (H2O2), and lipid peroxidation (LPO) levels, as well as apoptosis in the digestive diverticula and/or hemolymph. The SOD immunohistochemistry and apoptotic response were assessed at 55% and 120% SW at 12 h. Antioxidant expressions at 55% and 70% SW peaked at 24 h or 48 h, and then decreased. At 120% SW, they increased with exposure time. The H2O2 and LPO levels at each SW increased significantly with time. A comet assay also revealed that changes in salinity increased the rate of nuclear DNA damage in all the salinity groups. Thus, variations in salinity result in significant physiological responses in bay scallops. A change in habitat salinity of 15% or more produces oxidative stress that cannot be resolved by the body’s antioxidant mechanism, suggesting that excessive generation of reactive oxygen species can lead to cell death.

Hemocyte Responses of the Oyster Crassostrea hongkongensis Exposed to Diel-Cycling Hypoxia and Salinity Change

Marine hypoxia caused by nutrient enrichment in coastal waters has become a global problem for decades, especially diel-cycling hypoxia that occurs frequently in the summer season. On the contrary, sudden rainstorms, and freshwater discharge make salinity in estuarine and coastal ecosystems variable, which often occurs with hypoxia. We found mass mortality of the Hong Kong oyster Crassostrea hongkongensis in the field where hypoxia and salinity fluctuation co-occur in the summer season during the past several years. To investigate the effects of diel-cycling hypoxia and salinity changes on the hemocyte immune function of C. hongkongensis, oysters were exposed to a combined effect of two dissolved oxygen (DO) concentrations (24 h normal oxygen 6 mg/L, 12 h normal oxygen 6 mg/L, and 12 h hypoxia 2 mg/L) and three salinities (10, 25, and 35‰) for 14 days. Subsequently, all treatments were restored to constant normal oxygen (6 mg/L) and salinity under 25‰ for 3 days to study the recovery of hemocyte immune function from the combined stress. Hemocyte parameters were analyzed by flow cytometry, including hemocyte mortality (HM), total hemocyte count (THC), phagocytosis (PHA), esterase (EST) activity, reactive oxygen species (ROS), lysosomal content (LYSO), and mitochondrial number (MN). The experimental results showed that diel-cycling hypoxia and salinity changes have obvious interactive effects on various immune parameters. In detail, diel-cycling hypoxia and decreases in salinity led to increased HM, and low salinity caused heavier impacts. In addition, low salinity, and diel-cycling hypoxia also led to decreases in LYSO, EST, and THC, while the decrease of PHA only occurs in the early stage. On the contrary, ROS production increased significantly under low salinity and hypoxic conditions. After 3-day recovery, THC, PHA, EST, LYSO, and MN were basically restored to normal, while HM and ROS were still significantly affected by diel-cycling hypoxia and salinity change, indicating that the combined stress of diel-cycling hypoxia and salinity changes had latent effects on the immune function of C. hongkongensis. Our results highlight that diel-cycling hypoxia and salinity change may impair the health and survival of the Hong Kong oyster C. hongkongensis and may be the key factors for the mass mortality of this oyster in the field.

Bacterial composition reflects fine-scale salinity changes while phylogenetic diversity exhibits a strong salt divide

Climate change induced salinization events are predicted to intensify and lead to increased salt stress in freshwater aquatic ecosystems. As a consequence, formerly distinct abiotic conditions and associated biotic communities merge, and the emergence, loss, and persistence of microbial taxa modify the types and rates of ecosystem processes. This study examined how bacterial taxonomic and phylogenetic diversity and ecosystem function respond to acute salinization events where freshwater and estuarine communities and environments coalesce. We hypothesize that if the salinity change outpaces microbial adaptation or saline microbial populations are not yet established in formerly freshwater conditions, then we predict diminished carbon cycling rates, decreased microbial diversity, and altered the composition of microbial communities compared to historically freshwater communities. We used an experimental mesocosm approach to determine how salinity and the merging of distinct communities influenced resultant bacterial community structure and function. Each mesocosm represented different salinities (0, 5, 9, 13 psu). Two dispersal treatments, representing aquatic communities sourced from brackish 13 psu ponds and a mix of 13 psu and freshwater ponds, were added to all salinity levels and replicated four times. Results revealed that salinity, but not dispersal, decreased bacterial taxonomic and phylogenetic diversity. Carbon mineralization rates were highest in freshwater conditions and associated with low relative abundance indicator taxa. Acute salinity changes, such as localized flooding due to storm surge, will more negatively affect freshwater aquatic communities compared to chronic exposure to salinization where the communities have had time to adapt or turnover resulting in recovered biogeochemical functions.

Tilapia Farming in Bangladesh: Adaptation to Climate Change

In Bangladesh, aquaculture is critically important in terms of providing food and nutrition, sustainable livelihoods, income, and export earnings. Nevertheless, aquaculture in Bangladesh has faced recent concerns due to climate change. Aquaculture is vulnerable to a combination of climatic factors, such as global warming, rainfall variation, flood, drought, temperature fluctuation, and salinity change. Considering the vulnerability of fish production to the impacts of climate change, tilapia farming is one of the possible strategies for adaptation to climate change. The positive culture attributes of tilapia are their tolerance to low water levels and poor water quality with rainfall variation, temperature fluctuation, and salinity change. In fact, tilapia farming is possible in a wide range of water environments, including freshwater, brackish water, and saltwater conditions. We suggest that appropriate tilapia culture strategies with institutional support and collaboration with key stakeholders are needed for adaptation to environmental change.

Decomposing oceanic temperature and salinity change using ocean carbon change

As the planet warms due to the accumulation of anthropogenic CO2 in the atmosphere, the global ocean uptake of heat can largely be described as a linear function of anthropogenic CO2 uptake. This relates the oceans mitigation of atmospheric warming and carbon sequestration, as well as its increasing heat content. Patterns of ocean salinity also change as the earth system warms due to hydrological cycle intensification and perturbations to air-sea freshwater fluxes. Local temperature and salinity change in the ocean may result from perturbed air-sea fluxes of heat and freshwater (excess temperature, salinity), or from variability resulting from reorganisation of the preindustrial temperature and salinity fields (redistributed temperature, salinity), which are largely due to circulation changes. Here, we present a novel method in which, by tracking the redistribution of preindustrial carbon, we may estimate the redistribution of temperature and salinity using only local spatial information. We demonstrate this technique by estimating the redistribution of heat and salinity in the NEMO OGCM coupled to the MEDUSA-2 Biogeochemistry model under a RCP8.5 scenario over 1860–2099. We find on the longest timescales, the patterns of excess heat and salinity storage are dominated by increases in excess heat and salinity in the Atlantic, and that excess salinity is generally negative in other basins, compensating for strong atmospheric transport of excess salinity to the Atlantic. We also find significant redistribution of heat away from the North Atlantic, and of salinity to the South Atlantic, consistent with AMOC slowdown. Temperature change at depth is accounted for predominately by redistributed, rather than excess heat, but the opposite is true for salinity, where the excess component accounts for the majority of changes at depth. Though by the end of the simulation excess heat is the largest contribution to density change and steric sea level rise, the storage of excess salinity greatly reduces variability in excess density, particularly in the Atlantic. Here, redistribution of the preindustrial heat and salinity fields also produce generally opposing changes in sea level, though patterns are less clear elsewhere. As expected, the regional strength of excess heat and salinity signal grows through the model run. In addition, the regional strength of the redistributed temperature and salinity signals also grow, indicating increasing circulation variability or systematic circulation change on at least the time scale of the model run.

Low shifts in salinity determined assembly processes and network stability of microeukaryotic plankton communities in a subtropical urban reservoir

Background Freshwater salinization may result in significant changes of microbial community composition and diversity, with implications for ecosystem processes and function. Earlier research has revealed the importance of large shifts in salinity on microbial physiology and ecology, whereas studies on the effects of smaller or narrower shifts in salinity on the microeukaryotic community in inland waters are scarce. Our aim was to unveil community assembly mechanisms and the stability of microeukaryotic plankton networks at low shifts in salinity. Results Here, we analyzed a high-resolution time series of plankton data from an urban reservoir in subtropical China over 13 consecutive months following one periodic salinity change ranging from 0 to 6.1‰. We found that (1) salinity increase altered the community composition and led to a significant decrease of plankton diversity, (2) salinity change influenced microeukaryotic plankton community assembly primarily by regulating the deterministic-stochastic balance, with deterministic processes becoming more important with increased salinity, and (3) core plankton subnetwork robustness was higher at low-salinity levels, while the satellite subnetworks had greater robustness at the medium-/high-salinity levels. Our results suggest that the influence of salinity, rather than successional time, is an important driving force for shaping microeukaryotic plankton community dynamics. Conclusions Our findings demonstrate that at low salinities, even small increases in salinity are sufficient to exert a selective pressure to reduce the microeukaryotic plankton diversity and alter community assembly mechanism and network stability. Our results provide new insights into plankton ecology of inland urban waters and the impacts of salinity change in the assembly of microbiotas and network architecture.

Influence of Salinity on the Survival and Growth of Juvenile Coregonus Ussuriensis Berg

To explore the suitable salinity range of Coregonus ussuriensis Berg, we investigated the effect of induced salinity change in captivity on C. ussuriensis with an initial body weight of 35 ± 1.5 g. After 30 days of salinity acclimation, the survival, growth performance, blood biochemical profiles, antioxidative capacity, and tissue structure of juveniles under four salinity conditions (8‰, 16‰, 24‰, and 32‰) were investigated. Our results revealed that serum penetration, blood glucose, and serum Na+, Cl−, and Mg2+ gradually increased with increasing salinity until 32‰ salinity, when a significant difference was observed, whereas the K+ concentration showed a downward trend. The tissue sections showed that under high salinity (32‰), the liver and gill tissues of the fish were severely damaged and the vacuolation was serious. The levels of superoxide dismutase, glutathione peroxidase, and serum cortisol gradually increased with increasing salinity. A gene expression analysis showed that the increase in salinity induced higher expression of stress-, growth-, and inflammation-related genes (HSP70, Gh and Igf-1, and IL-1β, respectively). The downregulation of stress-related gene expression at 32‰ salinity may indicate that this level of salinity exceeded the regulatory capacity of C. ussuriensis. We concluded that C. ussuriensis may survive in an estuary under 0–24‰ salinity. Our findings provide insights into the physiological adaptation of C. ussuriensis to salinity change. These results could improve our knowledge of the stress response and resilience of estuarine fish to hyposalinity and hypersalinity stress.

Global-scale patterns of observed sea surface salinity intensified since the 1870s

Sea surface salinity patterns have intensified between the mid-20th century and present day, with saline areas becoming saltier and fresher areas fresher. This change has been linked to a human-induced strengthening of the global hydrological cycle as global mean surface temperatures rose. Here we analyse salinity observations from the round-the-world voyages of HMS Challenger and SMS Gazelle in the 1870s, early in the industrial era, to reconstruct surface salinity changes since that decade. We find that the amplification of the salinity change pattern between the 1870s and the 1950s was at a rate that was 54 ± 10% lower than the post-1950s rate. The acceleration in salinity pattern amplification over almost 150 years implies that the hydrological cycle would have similarly accelerated over this period.

Global-scale surface salinity change since the 1870s.Implications for the global hydrological cycle

<p>Based on the first ever combined analysis of observations from the round-the-world voyages of HMS Challenger and SMS Gazelle in the 1870s, early in the industrial era, this paper shows that the amplification of the global surface salinity signal (saline areas becoming saltier and fresh areas fresher) has increased by 63±5% since the 1950s compared to the period 1870s to 1950s. Other analyses of regional salinity change between the mid-20<sup>th</sup> century and present day have linked this amplification to anthropogenically-driven strengthening of the global hydrological cycle in line with increasing global temperatures. Our results show that the rate of change has indeed accelerated but more closely in line with changes in sea surface temperature than with surface air temperature over almost 150 years. This is the first global-scale analysis of salinities from these two expeditions in the 1870s and the first observational evidence of changes in the global hydrological cycle since the late 19<sup>th</sup> century.</p>