Long-term trends of salinity in coastal wetlands: Effects of climate, extreme weather events, and sea water level

Numerical simulation of thermal effects on seawater intrusion management by coastal cutoff walls

Seawater intrusion into coastal aquifers poses significant challenges for groundwater management, particularly under fluctuating thermal conditions. This study employs a two-dimensional numerical model to explore the combined effects of temperature gradients and geological stratification on seawater intrusion and the performance of coastal cutoff walls. The results show that temperature gradients between seawater and groundwater substantially affect intrusion patterns, with the wedge extension under variable temperature gradients is 54 times greater compared to the scenario with a constant temperature gradient. Colder seawater exacerbates intrusion, increasing the wedge by 18.7 %, while warmer seawater mitigates it, reducing the wedge by 14.5 %. The performance of cutoff walls is temperature-dependent, with efficiency decreasing as temperature differences widen. In heterogeneous aquifers, cutoff walls can reduce the saltwater wedge by up to 66.73 % under the influence of colder seawater intrusion. In such aquifers, the extent of the saltwater wedge is not solely determined by the thickness of the highly permeable layer, but rather by the proportion of its thickness at the base of the cutoff walls. This study highlights the importance of incorporating thermal and stratigraphic factors in the design of mitigation measures, emphasizing the need for site-specific strategies to address the challenges posed by changing thermal conditions and aquifer heterogeneity.

Amino Acid Substitutions in the Na + /K + -ATPase May Contribute to Salinity Tolerance in Insects

Environmental salinity levels vary naturally across terrestrial ecosystems but can be heightened locally by rising sea levels and desertification as well as human activities such as road salt application and agriculture. Since salt is essential for many physiological processes in insects, rising environmental sodium concentrations may drive behavioral changes, where insects select environments and food sources with suitable sodium levels, or evolutionary changes in constitutive or plastic physiological mechanisms to process salt, potentially altering ecological dynamics and species interactions. Numerous hematophagous (blood feeding) insects are known to breed in relatively saline environments, while others such as the yellow-fever mosquito Aedes aeqypti are expanding their range to coastal regions. Among phytophagous (plant feeding) insects, grasshoppers can be important herbivores in arid and coastal salt-affected regions, whereas the monarch butterfly ( Danaus plexippus ) appears to perform relatively well on milkweed host plants growing in roadsides influenced by salt runoff. Several of these insects share a common trait: amino acid substitutions in the first extracellular loop of the Na + /K + -ATPase (NKA), a highly conserved sodium pump crucial for maintaining ion balance. For the monarch these substitutions confer resistance to toxic cardenolides from milkweeds, but it is unclear whether NKA substitutions may influence salt tolerance. Here, we investigate whether the NKA substitutions found in these insects may contribute to salt tolerance using gene-edited Drosophila melanogaster mutant strains as models. We show that flies with the Q111L and A119S substitutions alone or in combination (found in grasshoppers and several hematophagous insects) exhibited greater salt tolerance, whereas flies carrying the combination of substitutions found in the monarch (Q111V, A119S, and N122H) did not. Our results suggest that the monarch may rely on alternate mechanisms for salt tolerance and that its NKA substitutions are important primarily for cardenolide resistance. However, substitutions Q111L and A119S may be important for salt tolerance in a variety of insects. Uncovering mechanisms of salt tolerance enhances our understanding of species distributions, ecological interactions, and evolutionary physiology in response to changing environmental salinity levels.

Salinity levels, trends and drivers of surface water salinization across China's river basins

The salinization of freshwater resources constitutes an increasingly global challenge, exacerbated by climate change and human activities. Despite its growing significance, comprehensive assessments of salinity dynamics and the roles of natural and anthropogenic factors remain scarce. This study investigates surface water salinity levels and their long-term (2003-2022) and seasonal trends across 1356 sampling sites in ten major Chinese river basins. Our results reveal that >20 % of the sites exhibit relatively high long-term average salinity levels compared to the irrigation water threshold, primary in arid or semi-arid regions experiencing intensified human activities. Nearly 20 % of low-salinity sites exhibit significant trends towards increased salinity, primarily in humid region. Southern basins, such as the Pearl River and Yangtze River, generally have relatively low salinity but demonstrate upward trends, whereas northern basins, like the Yellow River and Huai River, experience moderate to high salinity levels with more rapid increases. Winter salinity levels and their rate of increase surpass those of other seasons. Anthropogenic drivers, particularly population density and agricultural water use, emerge as key contributors to rising salinity, in conjunction with hydroclimatic variables. Furthermore, seasonal salinity trends underscore the critical role of agricultural water use during summer and autumn months. These findings emphasize the necessity to address the compounded pressures of climate variability and human activities, which are increasingly threatening surface water quality through rising salinity and extreme weather events.

Salty surprises: Developmental and behavioral responses to environmental salinity reveal higher tolerance of inland rather than coastal Bufo spinosus tadpoles

Salinization is predicted to intensify due to climate change, impacting biodiversity and ecosystem functioning. Amphibians, particularly embryos and larvae, are highly susceptible to environmental salinity. Yet, local adaptation may cause differing vulnerabilities between coastal and inland populations. In this study, we investigated the physiological, behavioural, and life-history responses to environmental salinity (0, 2 and 4 g l-1) of embryos and larvae of a widespread amphibian species (spined toad, Bufo spinosus) from salt-exposed (coastal) and salt-free (inland) populations. Moderate salinity (4 g l-1) altered embryonic and larval development in both populations, causing increased malformations, decreased body size and survival, and altered behavior, but did not affect telomere length or oxidative status. Individuals exposed to low salinity (2 g l-1) performed better across most traits. However, moderate salinity had stronger negative effects on coastal individuals, indicating a lack of local adaptation and overall lower performance compared to their inland counterparts. These findings suggest that increasing salinity will have varied impacts on organisms depending on their population origins and developmental stages.

Precipitation changes alter plant dominant species and functional groups by changing soil salinity in a coastal salt marsh

Specific mechanisms of precipitation change due to global climate variability on plant communities in coastal salt marsh ecosystems remain unknown. Hence, a field manipulative precipitation experiment was established in 2014 and 5 years of field surveys of vegetation from 2017 to 2021 to explore the effects of precipitation changes on plant community composition. The results showed that changes in plant community composition were driven by dominant species, and that the dominance of key species changed significantly with precipitation gradient and time, and that these changes ultimately altered plant community traits (i.e., community density, height, and species richness). Community height increased but community density decreased with more precipitation averaged five years. Furthermore, changes in precipitation altered dominant species composition and functional groups mainly by influencing soil salinity. Salinity stress caused by decreased precipitation shifted species composition from a dominance of taller perennials and grasses to dwarf annuals and forbs, while the species richness decreased. Conversely, soil desalination caused by increased precipitation increased species richness, especially increasing in the dominance of grasses and perennials. Specifically, Apocynaceae became dominance from rare while Amaranthaceae decreased in response to increased precipitation, but Poaceae was always in a position of dominance. Meanwhile, the dominance of grasses and perennials has the cumulative effect of years and their proportion increased under the increased 60% of ambient precipitation throughout the years. However, the annual forb Suaeda glauca was gradually losing its dominance or even becoming extinct over years. Our study highlights that the differences in plant salinity tolerance are key to the effects of precipitation changes on plant communities in coastal salt marsh. These findings aim to provide a theoretical basis for predicting vegetation dynamics and developing ecological management strategies to adapt to future precipitation changes.

Variations of salinity during reproduction and development affect ontogenetic trajectories in a coastal amphibian

Although coastal ecosystems are naturally submitted to temporal variations of salinity, salinization has been increasing over time threatening coastal biodiversity. Species that exploit such habitats can thus be exposed to brackish water at different life stages. However, the impacts of variations of salinity on wildlife remain poorly understood. This is particularly true for coastal amphibians, due to the strong dependency of early life stages (embryos and larvae) on aquatic environments. In order to investigate the effect of salinity during egg laying and embryonic and larval development of coastal amphibians, we used a full-factorial design to expose reproductive adults, eggs, and larvae of coastal spined toads (Bufo spinosus) to fresh (0 g.l-1) or brackish water (4 g.l-1). At egg laying, we evaluated parental investment in reproduction. During embryonic and larval development, we assessed effects on survival, development, and growth. We highlighted strong effects of environmental salinity on reproduction (reduced egg laying time, marginally reduced egg size, and reduced investment in reproduction). Responses to salinity were highly dependent on the developmental stages of exposure (stronger effects when individuals were exposed during embryonic development). These effects carried over when exposure occurred at egg laying or during embryonic development, highlighting the importance of the environmental conditions during early life on ontogenetic trajectories. We also highlighted partial compensation when individuals were transferred back to freshwater. Whether the magnitude of these responses can allow coastal biodiversity to overcome the observed detrimental effects of salinization remain to be assessed.