High Salinity Sites Research Articles

Mapping N2O production pathways in estuarine and coastal sediments through coupled 15N tracing and N2O isotopocules analysis

Estuarine and coastal zones are important sources of atmospheric nitrous oxide (N2O). Previous studies have explored the overarching roles of nitrification and denitrification in N2O production from these critical zones, yet the specific productions of various pathways within these processes remain elusive. Using dual 15N tracers (15NH4+ and 15NO3-) coupled with N2O isotopocules analysis, we quantified the contributions of multiple nitrification (both autotrophic and heterotrophic) and denitrification (bacterial, fungal, and chemical) pathways to N2O production in estuarine and coastal sediments. Furthermore, we leveraged microecology theory to dissect the biogeochemical mechanisms that govern the spatiotemporal dynamics of these pathways. Results showed that denitrification was the dominant process, accounting for over 70 % of N2O production. The remainder was attributed to autotrophic nitrification (5.80–23.92 %) and heterotrophic nitrification (1.07–7.10 %). Isotopocules analyses further showed that fungal denitrification was the primary source at freshwater sites (40.47–49.99 %), whereas bacterial denitrification prevailed at high-salinity sites (51.37–52.55 %). Moreover, these processes displayed contrasting spatial variability along the estuarine salinity gradient. Mechanistic investigation informed by microecology theory demonstrated that distinct assembly processes governed bacterial and fungal communities across this gradient, resulting in divergent ecological adaptation strategies of bacterial and fungal denitrifiers and, consequently, the contrasting contribution patterns of bacterial and fungal denitrification. Collectively, our work presents a comprehensive and refined picture of N2O sources and dynamics, offering essential insights for improving predictive models and formulating effective mitigation strategies targeting N2O emissions from estuarine and coastal zones.

Salinity increases under sea level rise strengthens the chemical protection of SOC in subtropical tidal marshes

The rise in sea levels due to global warming could significantly impact the soil organic carbon (SOC) pool in coastal tidal marshes by altering soil salinity and flooding conditions. However, the effects of these factors on SOC protection in coastal tidal marshes are not fully understood. In this study, we employed a space-for-time approach to investigate the variations in soil active carbon components and mineral-associated organic carbon under different salinity gradients (freshwater and brackish) and flooding frequencies (high and low tidal flats).The soil organic carbon (SOC) and easily oxidizable organic carbon (EOC) contents at the low-flooding frequency sites were higher than those at the high-flooding frequency sites. The dissolved organic carbon (DOC) content was higher at the high-salinity sites compared to the low-salinity sites, while the soil microbial biomass carbon (MBC) content was higher at the low-salinity sites than at the high-salinity sites. The EOC/SOC and DOC/SOC ratios were greater at the high-salinity sites than at the low-salinity sites, whereas the MBC/SOC ratios were higher at the low-salinity sites than at the high-salinity sites. Iron (Fe) and aluminum (Al) mineral-associated organic carbon [Fe(Al)-OC] and calcium-associated organic carbon (Ca-OC) contents were higher at the high-salinity sites compared to the low-salinity sites, and at the low-flooding frequency sites compared to the high-flooding frequency sites.Meanwhile, Fe(Al)-OC was the dominant fraction among mineral-associated organic carbon at all sites. The dominant phyla of bacterial community included Proteobacteria (49.31 %–66.36 %), Firmicutes (2.67 %–28.44 %), Chloroflexi (3.81 %–9.54 %), and Acidobacteria (4.28 %–7.02 %). In addition, Desulfobacca, a sulfate-reducing bacterium, promoted the formation of mineral-associated organic carbon.Random forest analysis showed that SOC and DOC were key factors in promoting mineral-associated organic carbon formation. Partial least squares path modeling (PLS-PM) indicated that sea level rise affects DOC content by altering soil physicochemical properties, promoting the formation of mineral-associated organic carbon.In summary, while soil organic carbon activity increases, the chemical association of minerals with organic carbon is becoming increasingly crucial for the protection of organic carbon under rising salinity conditions driven by sea level rise.

Comparative Transcriptomic Analyses Reveal Differences in the Responses of Diploid and Triploid Eastern Oysters to Environmental Stress.

Triploid oysters are commonly used as the basis for production in the aquaculture of eastern oysters along the USA East and Gulf of Mexico coasts. While they are valued for their rapid growth, incidents of triploid mortality during summer months have been well documented in eastern oysters, especially at low salinity sites. We compared global transcriptomic responses of diploid and triploid oysters bred from the same three maternal source populations at two different hatcheries and outplanted to a high (annual mean salinity = 19.4 ± 6.7) and low (annual mean salinity = 9.3 ± 5.0) salinity site. Oysters were sampled for gene expression at the onset of a mortality event in the summer of 2021 to identify triploid-specific gene expression patterns associated with low salinity sites, which ultimately experienced greater triploid mortality. We also examined chromosome-specific gene expression to test for instances of aneuploidy in experimental triploid oyster lines, another possible contributor to elevated mortality in triploids. We observed a strong effect of hatchery conditions (cohort) on triploid-specific mortality (field data) and a strong interactive effect of hatchery, ploidy, and outplant site on gene expression. At the low salinity site where triploid oysters experienced high mortality, we observed downregulation of transcripts related to calcium signaling, ciliary activity, and cell cycle checkpoints in triploids relative to diploids. These transcripts suggest dampening of the salinity stress response and problems during cell division as key cellular processes associated with elevated mortality risk in triploid oysters. No instances of aneuploidy were detected in our triploid oyster lines. Our results suggest that triploid oysters may be fundamentally less tolerant of rapid decreases in salinity, indicating that oyster farmers may need to limit the use of triploid oysters to sites with more stable salinity conditions.

Planktonic ciliate communities along an environmental gradient in the Nile Delta (Damietta region, Egypt)

The spatial patterns of planktonic ciliate communities were studied from May to June 2019 in the Nile Delta’s Damietta region, southeastern Mediterranean. The ciliate communities were sampled from twenty-five sites of five stressed domains with spatial gradients of environmental status. A total of 32 ciliate taxa with six dominant species were identified, comprising 21 tintinnids and 11 aloricate ciliates. The abundance and richness of each ciliate group varied geographically and were most strongly influenced by salinity variations; tintinnid ciliates attained high abundance and richness at high salinity sites in the harbour and coastal region and decreased within the estuary upstream. Aloricate ciliates were poorly represented at most sites but were a substantial proportion of upstream estuarine sites. Multivariate/univariate analyses demonstrated that spatial patterns of the ciliate communities were significantly correlated with environmental variables, especially salinity, chlorophyll-a, and nutrients, either alone or in combination with one another. These results indicate that the ciliates can be useful bioindicators in stressed environments while also allowing the detection of impacts on short time scales by rapidly responding to environmental variations.

Extremophile hypolithic communities in the Vestfold Hills, East Antarctica

Abstract The Vestfold Hills are a 400 km2, isolated ice-free oasis in eastern Antarctica featuring large areas with translucent quartz rocks that provide habitat for hypolithic microbial communities underneath. We used high-throughput DNA sequencing of 16S and 18S ribosomal RNA amplicons to characterize bacterial and eukaryotic hypolithic communities across the Vestfold Hills. We found high-level, local heterogeneity in community structure consistent with limited dispersal between hypoliths. Hypolithic communities were dominated by heterotrophic Bacteroidetes (mean bacterial relative read abundance: 56%) as well as Cyanobacteria (35%), with the eukaryote component often dominated by Chlorophyta (43%). Small but significant proportions of the variation in microbial community composition and function were explained by soil salinity (5–7%) and water availability (8–11%), with distinct taxa associated with different salinities and water availabilities. Furthermore, many inferred bacterial metabolic pathways were enriched in hypolithic communities from either dry or high-salinity sites. Vestfold Hills hypolithic habitats are likely to be local refuges for bacterial and eukaryotic diversity. Gradients in soil salinity and water availability across the Vestfold Hills, in addition to the number and diversity of lake types and fjords as potential source populations, may contribute to the observed variation in the extremophile, hypolithic microbial community composition.

Salt Water Exposure Exacerbates the Negative Response of Phragmites australis Haplotypes to Sea-Level Rise.

The response of coastal wetlands to sea-level rise (SLR) largely depends on the tolerance of individual plant species to inundation stress and, in brackish and freshwater wetlands, exposure to higher salinities. Phragmites australis is a cosmopolitan wetland reed that grows in saline to freshwater marshes. P. australis has many genetically distinct haplotypes, some of which are invasive and the focus of considerable research and management. However, the relative response of P. australis haplotypes to SLR is not well known, despite the importance of predicting future distribution changes and understanding its role in marsh response and resilience to SLR. Here, we use a marsh organ experiment to test how factors associated with sea level rise-inundation and seawater exposure-affect the porewater chemistry and growth response of three P. australis haplotypes along the northern Gulf of Mexico coast. We planted three P. australis lineages (Delta, European, and Gulf) into marsh organs at five different elevations in channels at two locations, representing a low (Mississippi River Birdsfoot delta; 0-13 ppt) and high exposure to salinity (Mermentau basin; 6-18 ppt) for two growing seasons. Haplotypes responded differently to flooding and site conditions; the Delta haplotype was more resilient to high salinity, while the Gulf type was less susceptible to flood stress in the freshwater site. Survivorship across haplotypes after two growing seasons was 42% lower at the brackish site than at the freshwater site, associated with high salinity and sulfide concentrations. Flooding greater than 19% of the time led to lower survival across both sites linked to high concentrations of acetic acid in the porewater. Increased flood duration was negatively correlated with live aboveground biomass in the high-salinity site (χ2 = 10.37, p = 0.001), while no such relationship was detected in the low-salinity site, indicating that flood tolerance is greater under freshwater conditions. These results show that the vulnerability of all haplotypes of P. australis to rising sea levels depends on exposure to saline water and that a combination of flooding and salinity may help control invasive haplotypes.

Spatial and Temporal Variability in Water Quality and Phytoplankton Community Composition in Charleston Harbor

The Charleston Harbor estuary is a dynamic ecosystem draining three rivers that surround the rapidly urbanizing greater Charleston area. Projected climate change impacts include elevated sea surface temperature and local changes in water quality that will likely alter biogeochemical cycling as well as phytoplankton abundance and community composition. Partnering with the local non-profit organization Charleston Waterkeeper, surface water samples were collected from late April through October 2021 at 20 sites in the Charleston Harbor estuary system. Water quality parameters measured included sea surface temperature (SST), salinity, pH, dissolved inorganic carbon, chromophoric dissolved organic matter (CDOM), phytoplankton pigments and nitrate and phosphate concentrations. Relatively high (6—16 μM) nitrate values were observed throughout the year and the nitrate to phosphate (N:P) ratios were consistently elevated (50—200) relative to the Redfield ratio of 16. Analysis of variance indicated that the delivery of nitrate, phosphate, and CDOM into the estuary came from upriver of the Charleston Harbor. For instance, CDOM was significantly impacted by tidal stage (p < 0.05) and was negatively correlated with salinity at low salinity sites, but no correlation was observed at high salinity sites. Seasonal patterns in phytoplankton community abundance and composition were driven by changes in SST, as overall phytoplankton biomass increased with a community shift from diatoms during colder months to flagellated cells such as prasinophytes during the warmer summer months. Phytoplankton diversity was greatest during the early summer and lowest in October. This study provides a reference baseline for water quality parameters and phytoplankton community composition in an estuarine ecosystem that is changing rapidly due to dredging and climate change processes.

Using Unoccupied Aerial Systems (UASs) to Determine the Distribution Patterns of Tamanend’s Bottlenose Dolphins (Tursiops erebennus) across Varying Salinities in Charleston, South Carolina

The Charleston Estuarine System Stock (CESS) of Tamanend’s bottlenose dolphins (Tursiops erebennus) exhibit long-term site fidelity to the Charleston Harbor, and the Ashley, Cooper, and Wando Rivers in Charleston, South Carolina, USA. In the Cooper River, dolphins have been irregularly sighted in upper regions where salinity levels are below what is considered preferred dolphin habitat. We conducted unoccupied aerial system (UAS) surveys in high-salinity (>15 parts per thousand) and low-salinity (<15 parts per thousand) regions (n = 8 sites) of the Cooper River and surrounding waters to assess dolphin distribution in terms presence/absence, detection rate, abundance, and density. We also assessed the influence of ecological factors (salinity, water temperature, season, and prey availability) on dolphin distribution. Dolphins were detected at five sites, with higher salinity and water temperature being significant predictors of presence and abundance. Dolphins were detected year-round across high-salinity sites, and were infrequently detected in low-salinity sites during months with warmer water temperatures. The results from this study contribute to the overall understanding of dolphin distribution across various habitats within the Charleston Estuary System and the potential drivers for their movement into low-salinity waters.

Plasticity in branch water relations and stem hydraulic vulnerability enhances hydraulic safety in mangroves growing along a salinity gradient.

Coping with water stress depends on maintaining cellular function and hydraulic conductance. Yet measurements of vulnerability to drought and salinity do not often focus on capacitance in branch organs that buffer hydraulic function during water stress. The relationships between branch water relations, stem hydraulic vulnerability and stem anatomy were investigated in two co-occurring mangroves Aegiceras corniculatum and Rhizophora stylosa growing at low and high salinity. The dynamics of branch water release acted to conserve water content in the stem at the expense of the foliage during extended drying. Hydraulic redistribution from the foliage to the stem increased stem relative water content by up to 21%. The water potentials at which 12% and 50% loss of stem hydraulic conductivity occurred decreased by ~1.7 MPa in both species between low and high salinity sites. These coordinated tissue adjustments increased hydraulic safety despite declining turgor safety margins at higher salinity sites. Our results highlight the complex interplay of plasticity in organ-level water relations with hydraulic vulnerability in the maintenance of stem hydraulic function in mangroves distributed along salinity gradients. These results emphasise the importance of combining water relations and hydraulic vulnerability parameters to understand vulnerability to water stress across the whole plant.

DNA metabarcoding reveals evidence of inter- and intra-guild predation by Scylla paramamosain in a marine ecosystem

IntroductionCrustacean predators exhibit diverse trophic interactions across various habitats in aquatic ecosystems. To assess their ecological roles, it is essential to understand their complete dietary spectrum. Recently, DNA-based techniques such as DNA metabarcoding and high-throughput sequencing have provided insights into trophic networks and their response to environmental drivers.MethodsIn this study, we used these methods to investigate the impact of salinity gradients on the trophic networks of mud crabs (Scylla paramamosain) in marine ecosystems.ResultsOur results revealed that mud crabs prey on a wide range of taxa, including crabs (other than the host species), fish, shrimps, and other prey factions. Salinity gradients had a significant influence on the specific prey items, such as high predation on Metapenaeus, and Trigonopterus species in high salinity sites. Notably, crabs exhibited a greater infestation of Portunion parasites in environments with high salinity levels, in contrast to environments with lower salinity levels. Ordination analysis showed that mud crabs adapt their diet according to environmental constraints.DiscussionFurthermore, the results indicate that mud crabs showed opportunistic predation behavior as they were observed preying on other Portunidae species sharing the same trophic guild. Overall, these findings demonstrate the dietary plasticity and associated parasite infestation of an important crustacean predator, and provide evidence that environmental changes may affect incidences of the associated parasites as well as the specific predator-prey interactions indirectly through fluctuations in marine ecosystems.

Deterministic processes dominate archaeal community assembly from the Pearl River to the northern South China Sea.

Archaea play a significant role in the biogeochemical cycling of nutrients in estuaries. However, comprehensive researches about their assembly processes remain notably insufficient. In this study, we systematically examined archaeal community dynamics distinguished between low-salinity and high-salinity groups in water and surface sediments over a 600-kilometer range from the upper Pearl River (PR) to the northern South China Sea (NSCS). Neutral community model analysis together with null model analysis showed that their C-score values were greater than 2, suggesting that deterministic processes could dominate the assembly of those planktonic or benthic archaeal communities at both the low-salinity and high-salinity sites. And deterministic processes contributed more in the low-salinity than high-salinity environments from the PR to the NSCS. Furthermore, through the co-occurrence network analysis, we found that the archaeal communities in the low-salinity groups possessed closer interactions and higher proportions of negative interactions than those in the high-salinity groups, which might be due to the larger environmental heterogeneities reflected by the nutrient concentrations of those low-salinity samples. Collectively, our work systematically investigated the composition and co-occurrence networks of archaeal communities in water as well as sediments from the PR to the NSCS, yielding new insights into the estuary's archaeal community assembly mechanisms.

Impact of Nax genes for Na+ exclusion from leaves on bread wheat yield on saline soils

AbstractWheat production in many countries is threatened by climate change and rising sea levels causing increases in salt‐water intrusion in low‐lying coastal areas. Large areas of the coastal zone of Bangladesh remain fallow during the dry season primarily due to salinity. It is estimated that 0.86 million hectares of land currently under fallow in the dry season would be suitable for wheat production using varieties with enhanced salt tolerance. This study investigated the impact of salt tolerance Nax genes (Na+ exclusion) on the grain yield of bread wheat grown on saline soils in southern Bangladesh. Nax1 and Nax2 genes were separately crossed into two Bangladeshi bread wheat varieties, BARI Gom 25 and BARI Gom 26, through conventional crossing with marker‐assisted selection. The key outcome was strong evidence that both Nax genes have the capacity to lower leaf Na+ concentration in locally adapted bread wheat and consequently deliver improved yields in challenging field environments in southern Bangladesh with moderate to high salinity. The genetic backgrounds of the adapted varieties played a significant role in the expression of these genes. There was a differential response in the phenotypic expression of Nax genes to reduce leaf Na+ levels between the two local varieties selected as recurrent parents and the resultant variation in yield. The average reduction in leaf Na+ concentrations from the third backcrossed lines in the BARI Gom 25 background was about twice that of lines in the BARI Gom 26 background. These same lines typically yielded between 10% and 20% higher than BARI Gom 25 on moderate to high salinity sites, whereas the backcrossed lines in the BARI Gom 26 background yielded similarly to their recurrent parent at these same sites. The contrast in the differential phenotypic expression of the Nax genes evident in the two recurrent parents represented in this study highlights the importance of crossing salt tolerance Nax genes into a range of locally adapted high‐yielding genetic backgrounds to identify the greatest potential for improvement in salt tolerance.

Reproductive Phenology of the Eastern Oyster, Crassostrea virginica (Gmelin, 1791), Along a Temperate Estuarine Salinity Gradient

Low salinity can negatively affect reproduction in estuarine bivalves. The spatial and temporal extents of these effects are important to inform models of population dynamics, environmental risk assessments, restoration efforts, and predictions of climate change effects. A hypothesis of delayed gametogenesis for oysters at low salinity sites was tested relative to their higher salinity counterparts in downstream experimental cages. In 2018, the timing of gametogenesis and spawning was observed June–August for 2-year-old oysters from three distinct ancestries (native, hatchery, aquaculture), outplanted at age 1 month along the salinity gradient (3–30 psu) of a temperate estuary. A second season of data was collected in 2019 from a 3-year-old aquaculture line and mixed-age native adult oysters dredged and transplanted 1 year prior. Dermo was tested in 2019 and prevalence was 1.3% (n = 240). Gametogenesis and spawning were retarded for all ancestries at low salinity relative to higher salinity sites during July and August. The reverse pattern was found in June, with low salinity sites having more advanced gonad index than at a high salinity site. This difference in average gonad index was 2.65 vs 1.46, respectively, for the native line and 2.62 vs 2.08 for aquaculture. Low salinity seemed to not only induce earlier gametogenesis in June, but also extended the reproductive season relative to higher salinity sites. Among oyster ancestries, the aquaculture line stood out as having 30–48% lower gametogenic synchrony within sites, but only in 2018. Because the native oysters used in this study have been restricted to low salinity conditions for many generations, demonstration of their reproductive plasticity across salinities is notable and broadens the range of potential future restoration strategies.

Differential responses of soil nutrients to edaphic properties and microbial attributes following reclamation of abandoned salinized farmland

Salinization is a global threat to the sustainability of farmland in arid areas. However, the mechanisms by which soil physicochemical and microbial properties affect soil nutrients in salinized farmlands are not completely understood. Here, the direct and indirect responses of edaphic properties (bulk density [BD], saturated hydraulic conductivity [SC], field capacity [FC], alkalinity[pH], microbial biomass carbon [MBC], and microbial biomass nitrogen [MBN]) and soil microbial properties (denitrifying genes [nirS], nitrogen-fixing genes [nifH], catalase [CAT] and urease [UR] activities, and species richness [Chao1]) to the total soil N content (TN), soil organic carbon (SOC), alkali hydrolyzable N (AN), potassium (K+), and calcium (Ca2+) ions at low-salinity (2–4 dS m−1), mid-salinity (4–8 dS m−1), high-salinity (8–15 dS m−1) sites, and non-salinity (Control, 0–2 dS m−1) farmland in arid areas of northwest China were examined. Soil UR activity (range: 703.5–956.6 U UR L-1 ) and nifH, AN, SOC, and MBN content were higher at the low-salinity than at the medium- and high-salinity sites. Furthermore, FC was indirectly affected by the soil nutrients due to their effects on nifH and CAT activities. The relative contribution of soil TN, SOC, and AN content on the direct responses of the microbial attributes (52–70%) was higher than that of the soil physicochemical properties (27–45%) in different reclamation types. The reclamation process on abandoned salinized farmland promoted the activity of microorganisms, further improving the soil's physical properties and nutrient status. This study has found that improving microbial metabolic activity in combined with reclamation measures will be crucial for future salinity management.

Adaptation of Some Quinoa Genotypes (Chenopodium quinoa Willd.), Grown in a Saharan Climate in Algeria.

Agriculture in southern Algeria faces several challenges that hinder its development, including drought, high temperatures and the excessive salinity of soil and groundwater. The introduction of crops resistant to these factors is one of the solutions chosen to address these abiotic constraints. This research aimed to evaluate the behavior of quinoa (Chenopodium Quinoa Willd.) grown in the Ouargla region of southeastern Algeria. Five varieties of quinoa (Santa maria, Giza1, Amarilla Sacaca, Blanca de Junin and Kancolla) were tested at two sites that differed in terms of soil salinity (9.95 mS/cm and 0.85 mS/cm) during 2019 and 2020. A complete random block experimental design with four repetitions was used for the agronomic tests. Our results clearly show that higher grain yields were obtained at the high salinity site (site 1) compared to the low salinity site (site 2). However, plant height, grain yield per plant and harvest index differed between varieties and sites. In contrast, stem diameter was not greatly affected by salinity. The varieties that seem to be best adapted to the growing conditions of the Ouargla region are, in descending order: Santa Maria, Giza1, Amarilla Sacaca and Blanca de Junin. When testing quinoa in new environments, it is critical to adapt the cropping cycle of varieties to avoid very high temperatures. The choice to switch to winter cultivation instead of spring cultivation can be an essential criterion for success. The biogeographical approach conducted in this research opens up new perspectives for the adaptation and cultivation of quinoa outside its region of origin to satisfy the food security of the people of North Africa.

Gross nitrogen transformations in intertidal sediments of the Yangtze Estuary: Distribution patterns and environmental controls

Nitrogen (N) transformations in estuarine and coastal ecosystems play a crucial role in the global biogeochemical cycles. However, simultaneous estimation of gross N transformations and the underlying drivers across estuarine and intertidal wetlands remain poorly understood. Here, the spatial changes in gross N transformation rates and related functional gene abundances were explored in varying habitats of the intertidal zone in the Yangtze Estuary via an optimized 15N tracing model and quantitative PCR. The results indicate that the gross N transformation rates changed remarkably in estuarine and intertidal wetlands, with comparatively higher gross N transformation rates at the upper freshwater and low-salinity sites relative to high-salinity sites. Meanwhile, the functional gene abundances (amoB, UreC, hzo, nirS and nrfA) exhibited a similar spatial distribution pattern to corresponding N transformation processes. Sedimentary total organic carbon (TOC), nitrite (NO2–), ferrous iron (Fe(II)), and microbial functional gene abundances jointly mediated the gross N transformations. This study carried out the first simultaneous estimation of the gross N transformation pattern and driving factors in estuarine and intertidal ecosystems based on the optimized 15N tracing model, providing a better understanding of internal mechanisms on the N cycling and strategy for estuarine and coastal N management.

Combined effect of salinity and temperature on copepod reproduction and oxidative stress in brackish-water environment

Climate-induced warming and increased river inflows are forcing the Baltic Sea to radical changes in the near future; organisms living in this brackish-water ecosystem are already experiencing osmotic stress, which, together with thermal stress, may have severe consequences on the ecosystem. The aim of this work was to study the combined effect of decreasing salinity and increasing temperature on reproductive success and oxidative stress in zooplankton by using a calanoid copepod Acartia sp. as a model organism. The field study was conducted during summer 2020 in the western Gulf of Finland, using three sampling sites with naturally differing salinity levels. Additionally, the copepods from these sites were experimentally exposed to ambient or 3°C elevated temperature for 72 h. The copepods derived from the deepest and the most saline sampling site suffered less oxidative damage and exhibited relatively high reproduction, while the temperature treatment itself had little effect. On the other hand, the field-based monitoring data showed otherwise; temperature increased lipid peroxidation, glutathione-s-transferase activity, or both in all three sampling sites. Meanwhile, egg production rate was negatively associated with temperature in the area with the lowest salinity. Moreover, egg production rate decreased from June to September along with increasing temperatures in the mid-salinity sampling site, while similar change occurred also in the highest-salinity site between August and September. The combined effect of salinity and sampling date on reproduction indicates the importance of even subtle salinity changes on copepods. Moreover, the data suggest that the unusually strong heatwave was responsible for increased oxidative stress during the sampling season and possibly forced a trade-off between antioxidant activity and reproductive effort.

Invader at the edge - Genomic origins and physiological differences of round gobies across a steep urban salinity gradient.

Species invasions are a global problem of increasing concern, especially in highly connected aquatic environments. Despite this, salinity conditions can pose physiological barriers to their spread, and understanding them is important for management. In Scandinavia's largest cargo port, the invasive round goby (Neogobius melanostomus) is established across a steep salinity gradient. We used 12,937 SNPs to identify the genetic origin and diversity of three sites along the salinity gradient and round goby from western, central and northern Baltic Sea, as well as north European rivers. Fish from two sites from the extreme ends of the gradient were also acclimated to freshwater and seawater, and tested for respiratory and osmoregulatory physiology. Fish from the high-salinity environment in the outer port showed higher genetic diversity, and closer relatedness to the other regions, compared to fish from lower salinity upstream the river. Fish from the high-salinity site also had higher maximum metabolic rate, fewer blood cells and lower blood Ca2+. Despite these genotypic and phenotypic differences, salinity acclimation affected fish from both sites in the same way: seawater increased the blood osmolality and Na+ levels, and freshwater increased the levels of the stress hormone cortisol. Our results show genotypic and phenotypic differences over short spatial scales across this steep salinity gradient. These patterns of the physiologically robust round goby are likely driven by multiple introductions into the high-salinity site, and a process of sorting, likely based on behaviour or selection, along the gradient. This euryhaline fish risks spreading from this area, and seascape genomics and phenotypic characterization can inform management strategies even within an area as small as a coastal harbour inlet.

The seasonal response of in situ denitrification and DNRA rates to increasing nitrate availability

Climate change may dramatically increase nitrate (NO3−) production and transport from watersheds to the coastal area due to short-term heavy precipitation events. However, the response of the coastal filter to such pulse NO3− delivery is poorly studied. We measured NO3− reduction rates via denitrification and DNRA in intact sediments collected from the Goro Lagoon (Po River Delta, Italy). We contrasted 2 seasons (spring and summer) and 2 sites (with lower and higher than average lagoon salinity). We performed 15NO3− additions along a concentration series (50–2000 μM) far exceeding the annual NO3− variations (20–150 μM) to quantify nitrate reduction saturation thresholds. In both seasons and sites denitrification largely outcompeted DNRA and increased along the NO3− gradient, following Michaelis-Menten kinetics. At the site characterized by low salinity, maximum potential denitrification rates decreased from spring to summer, following the decrease in the abundance of burrowing macrofauna. DNRA followed the same seasonal trend but was not affected by NO3− addition. At the site characterized by high salinity, denitrification rates slightly increased and DNRA decreased from spring to summer, despite the establishment of chemically reduced conditions and the increase in pore water free sulfides concentration. The Goro Lagoon sediments displayed elevated buffering capacity against NO3− pulse additions. Such buffer capacity was mainly supported by denitrification through spring bioturbation, decreased summer pathlength of NO3− to the anoxic zone, and unknown mechanisms preventing sulfide toxicity and the dominance of DNRA at the high salinity site.

Exploring the interplay of biotic interactions and salinity stress in freshwater invertebrate assemblages: reply to Chessman (2022)

The most parsimonious interpretation of our experimental findings (Bray 2019; Kefford 2022) is that the effect of experimental increases in salinity on stream macroinvertebrate communities, was altered by organisms from a high-salinity site. This interpretation is consistent with long-established and widely accepted ecological and ecotoxicological theory. The suggestion (Chessman 2022) that the initial community was confounded, that it converged within the first week of the experiment and then diverged again, is neither parsimonious nor explicable by theory. All experiments are imperfect, yet we maintain that our experiment is informative, and we encourage other researchers to generate new data to inform on the degree to which abiotic and biotic effects of salinity, and other abiotic stressors, affect ecological communities.