Low-salinity Zone Research Articles

Dynamics of trace metals with different size species in the Pearl River Estuary, Southern China

The Pearl River Estuary (PRE), the largest estuary in Southern China, regulates the fluxes of riverine trace metals into the South China Sea. However, the geochemical behavior of trace metals in this estuary is still ambiguous. In this study, we investigated the dynamics of trace metals in different phases (i.e., particulate, colloidal and truly dissolved) in the PRE in both wet and dry seasons characteristic of the region. Transformations of trace metals between particulate (>0.45 μm), colloidal (1 kDa to 0.45 μm) and truly dissolved (<1 kDa) phases were observed during the estuarine mixing. Colloidal metals (except for Pb) showed non-conservative ‘removal’ behavior in the low-salinity zone (S < 10‰), suggesting the coagulation of colloidal metals and subsequent deposition to bed sediments being an important sink of dissolved trace metals. By contrast, truly dissolved metals exhibited a mid-salinity maximum distribution (i.e., Mn, Ni, Cu and Cd) or little variation along the salinity gradient (i.e., Fe). The increase of truly dissolved metal concentration was accompanied by the decrease of particulate metal concentration, indicating that the desorption of suspended particles was an important source of dissolved Mn and Cd in the PRE. Metal released from the suspended particles increased in the dry season due to the high suspended particulate matter concentration. Dissolved Mn concentration in the bottom water in wet season was higher than that in dry season, implying that benthic Mn input increased as the bottom water became hypoxic. Abnormally high concentrations of particulate and dissolved Pb were observed at the lower PRE, implying the presence of a potential point source pollution. A flux model predicted that total dissolved Fe, Ni, Cu and Zn underwent net removal while dissolved Mn and Cd had net inputs during the estuarine mixing. This study raveled contrasting geochemical behaviors of trace metals with different size phases and the different sources and sinks of dissolved metals in the PRE.

The role of Mg2+ in inhibiting CaCO3 precipitation from seawater

The precipitation and dissolution processes of CaCO3 are essential to the marine carbon cycle and are relevant to climate research. Nevertheless, one of the nagging questions is why the precipitation of CaCO3 rarely occurs without the help of biological activity even in the surface seawater, where the calcite saturation state (Ω, expressed as the ratio of the calcium and carbonate ion concentration product to the stoichiometric solubility constant of calcite) is up to 5, but commonly takes place in hard-water lakes, springs and the low-salinity zone (practical salinity <5) of estuaries. Studies have found that Mg2+ plays a significant role in inhibiting all aspects of CaCO3 precipitation, namely crystal formation, solubility, and precipitation rate. However, whether the presence of Mg2+ in seawater is sufficient to inhibit CaCO3 precipitation has not been clearly stated, and the specific inhibitory role of Mg2+ in CaCO3 precipitation still requires clarification. In this study, we used four different seed materials to induce CaCO3 precipitation in artificial seawater of four Mg:Ca ratios and examined the crystal morphologies of the resulting precipitates using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). We found that Mg2+ inhibits the precipitation at the first stage of nucleation, which drastically increases the difficulty of calcite nucleation. At a seawater Mg:Ca ratio of 5.2, aragonite preferentially nucleates instead of calcite. Among the tested materials, only calcite and aragonite seeds exhibit marked catalysis of CaCO3 precipitation, indicating that not all the particles in natural seawater can catalyze CaCO3 precipitation. Our results demonstrate while the precipitation of CaCO3 is inhibited by Mg2+ at all stages, the inhibition at the nucleation stage is the main reason massive spontaneous CaCO3 precipitation does not occur under the surface seawater conditions, supporting the notion that the incorporation of foreign ions such as Mg2+ increases the surface energy and the solubility of the overgrowth or cluster, and thus inhibits CaCO3 precipitation.

Seedling Diversity Considerably Changes Near Localities in Three Salinity Zones of Sundarbans Mangrove Forest, Bangladesh

The status of natural seedlings near localities in the Sundarbans Mangrove Forest was assessed through a stratified random sampling method to observe seedling composition and diversity, importance value index, family importance value, and species evenness. A total of 63 sample plots of 3,990 m2 area were surveyed in three natural and anthropogenic influenced salinity zones of Sundarbans. A total of 16,166 seedlings of 15 species under 12 families were found. Family Euphorbiaceae showed the highest (59.59%) family relative density and Rhizophoraceae presented the highest (20%) family relative diversity Index (FRDI). The maximum family importance value (FIV) showed by Euphorbiaceae (66.26) and the maximum importance value index (IVI) of species has been observed in Excoecaria agallocha (114.74). Excoecaria agallocha showed the highest relative density (59.6%) and relative abundance (39.87%) where the highest relative frequency (15.27%) was observed by Heritiera fomes. The mean stem density and species diversity index in the whole survey area was 2701 stem ha-1 and 0.0009 correspondingly. The Shannon-Wienner's diversity index was 1.52 where the maximum Shannon-Wienner's diversity index was 2.708. The Simpson's diversity index and Dominance of Simpson index were 0.38 and 0.62 with Simpson's reciprocal index 2.632. The Species evenness index, Menhinick’s, and Margalef's indices were 0.561, 0.118, and 1.445 respectively. The Species diversity index of the three salinity zones were 0.0017, 0.0029, and 0.0035 respectively. The Shannon-Wienner's diversity index of Low Salinity Zone (LSZ), Moderate Salinity Zone (MSZ), and Strong Salinity Zone (SSZ) were 0.887, 1.369, and 1.845 correspondingly where LSZ (0.632) showed the highest Simpson's diversity index follow ed by MSZ (0.394) and SSZ (0.21). The Species evenness index for LSZ, MSZ, and SSZ were 0.346, 0.505, and 0.742 where Menhinick’s Index were 0.148, 0.210, and 0.207 respectively. The analysis showed poor diversity indices and the area was dominated by few species with few families. The status is also reduced with increasing salinity.

Unraveling bacterial community structure and function and their links with natural salinity gradient in the Yellow River Delta

Salinization can change the soil environment and affect microbial processes. In this study, soil samples were collected from Zone A (Phragmites australis wetlands), Zone B (P. australis and Suaeda salsa wetlands), and Zone C (Spartina alterniflora wetlands) in the Yellow River Delta. The microbial community and functional potential along the natural salinity gradient were investigated. Total nitrogen, ammonia nitrogen, and soil organic matter presented a downward trend, and salinity first increased and then decreased from Zone A to Zone C. Nitrospira and norank_f_Nitrosomonadaceae were widely distributed throughout the zones. Denitrifying bacteria Alcanivorax, Marinobacterter, and Marinobacterium were abundant in Zone B and preferred high salinity levels. However, denitrifying bacteria Azoarcus, Flavobacterium, and Pseudomonas were mainly distributed in low-salinity Zones A and C, suggesting their high sensitivity to salinity. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Aeromonas and Geobacter dominated Zone C, whereas Caldithrix performed DNRA in Zone B. Interestingly, DNRA with organic matter as the electron donor (C-DNRA) occurred in Zone A; DNRA coupled with sulfide oxidation (S-DNRA) was dominant in Zone B; and C-DNRA and DNRA with divalent iron as electron donor and S-DNRA occurred simultaneously in Zone C. Salinity was the key factor distinguishing low and high salinity zones, and total nitrogen and total phosphorus had important effects at the phylum and genus levels. The abundance of genes encoding cell growth and death was relatively stable, indicating that the microbial community had good environmental adaptability. The genes related to the biodegradation of xenobiotics and the metabolism of terpenoids and polyketides were abundant in Zone B, revealing high metabolic potential for exogenous refractory substances. The microorganisms under low-salinity Zones A and C were more sensitive to environmental changes than those under Zone B. These results suggest that salinity plays important roles in microbial processes and shapes specific functional zones in coastal wetlands.

Relationship between water salinity and vegetation distribution in the Tiaozini reclamation area

To investigate the water and salt distribution, and explore the relationship between vegetation circles and soil water salinity conditions in the Tiaozini reclamation area of Dongtai, Jiangsu, a measurement and sampling campaign was conducted in a reclaimed zone used for wetland restoration in September 2019. Electrical resistivity tomography (2-D and 3-D) was conducted near the plant circles. Precipitation, surface water, soil profiles, and shallow groundwater samples were collected and analyzed for water and salt parameters as well as for D and 18O isotopic compositions. The results show that low resistivities and high salinity zones exist underneath the plant circles within 3 m of depth, and soil water salinity increases with the plant type, changing from the center of the circle to the outer area. Soil water salinity is an essential factor for the circular distribution of different plant types. Spatial variation of surface soil water salinity results from both precipitation infiltration and salt accumulation caused by evaporation. Soil salinity affects the distribution pattern of different plant types, while plants can also influence the salinity distribution by affecting the infiltration and evaporation processes.

Dissolved neodymium and hafnium isotopes and rare earth elements in the Congo River Plume: Tracing and quantifying continental inputs into the southeast Atlantic

The Congo River is the second largest river by discharge in the world and a major source of element inputs into the South Atlantic Ocean. Yet, the element fluxes and transport mechanisms across and beyond its estuary and their impacts on the marine distribution and cycling of many major and trace elements are not well understood. We present the first combined dissolved neodymium (Nd) and hafnium (Hf) isotope and rare earth element (REE) concentration distributions following the Congo River plume along its flow path off the West African coast and along a connected offshore latitudinal section at 3°S. The Congo River freshwater itself is characterized by extraordinarily high Nd and Hf concentrations of up to 4000 pmol/kg and 54 pmol/kg, and by Nd (εNd) and Hf (εHf) isotope compositions that range between −15.6 and −16.4 and between 0.35 and −1.4, respectively. Our near- and offshore data indicate that at salinities above 23 conservative mixing of Congo-derived Nd and Hf concentrations and isotopic signatures with ambient surface seawater occurs for at least 1000 km to the northwest of the river mouth. This demonstrates a large spatial extent of the influence of the Congo plume on trace metal distributions in the eastern south Atlantic surface waters. A comparison between dissolved Nd and Hf fluxes from the Congo River and the shelf zone estimated based on radium isotope compositions indicate that release from Congo-derived particulate phases likely balances strong estuarine REE and Hf removal in the low salinity zone. The combined riverine and shelf zone flux for Nd is almost twice as high as that estimated for the Amazon River, despite that the Amazon discharge is about five times higher than that of the Congo River. Even the offshore Nd flux estimated for the 3 °S transect based on radium isotope compositions still corresponds to ∼40% of the Congo-shelf-zone flux and reaches 150 ± 50 Mg/year for Nd. Moreover, intermediate waters below the plume are strongly affected by exchange with particulate inputs from the Congo River given that Nd isotope signatures are inconsistent with values expected from large-scale water mass mixing and instead support unradiogenic Nd release either from sinking or deposited Congo-derived detrital material. Deep and bottom water isotopic signatures are also slightly affected by interaction with particles and benthic Nd release.

Variation of Yellow River Runoff and Its Influence on Salinity in Laizhou Bay

Salinity in estuaries has an important influence on sediment deposition, delta formation, and fishery economy. The change of runoff in Yellow River (YR), which is the largest river along the Bohai Sea, has an important effect on the salinity of the sea, particularly in Laizhou Bay. In this study, measured runoff data in Lijin hydrological station from 1950 to 2018 are adopted to examine the change of YR runoff into the sea. The study concludes that the runoff of YR is distributed unevenly throughout the year and the amount of runoff mainly concentrates during the flood season. Results show a decreasing trend at a significant level of alpha=0.95, with a range of -6.17 x 10(9) m(3) yr(-1). Three groups of runoff are set by using the Finite-Volume Coastal Ocean Model numerical model to analyze the change of salinity in Laizhou Bay. These modeled effects were compared with water level observations made by Bayuquan, Tanggu, and Penglai gauge stations and salinity surveys conducted by Longkou gauge station. Model results and observations of salinity distributions and coastal water level showed good agreement. We can conclude that the distribution of salinity in Laizhou Bay is high in the east and low in the west. Furthermore, it responded quickly to the change of the runoff from YR. Under conditions of high river discharge, the low-salinity zone increased in horizontal direction and reached the bottom in vertical direction.

Soil organic matter and salinity as critical factors affecting the bacterial community and function of Phragmites australis dominated riparian and coastal wetlands

Soil salinization poses a great threat to the natural ecosystem and interferes with the structure and function of the biological community, resulting in different vegetation distributions. However, little attention is paid to the changes in microbial community in different wetland types with the same vegetation. In this study, the Yellow River Delta was used as a model because of its typical and extensive distribution of Phragmites australis-dominated saltwater and freshwater wetlands. We investigated the differences in the structure and function of bacterial communities, as well as their relationships with soil properties in coastal (Zone A) and riparian (Zone B) wetlands. Results showed higher salinity and pH in Zone A than Zone B (p < 0.05), whereas TN (p < 0.05) and SOM were lower than those in Zone B. Significant differences existed in microbial community composition between Zones A and B. The nitrifying-bacteria Nitrospira and norank_f_Nitrosomonadaceae had high abundance in Zones A and B. Alcanivorax, Halomonas, and Marinobacter were extensively distributed in Zone A, whereas Flavobacterium and Pseudomonas were dominant in Zone B, indicating the diversity characteristics of denitrifying bacteria. Conversely, methane-oxidizing bacteria Methylophaga were significantly higher in Zone A than in Zone B (p < 0.05), indicating that high salinity was conducive to aerobic methane oxidation and that the genetic diversity at strain level endowed it with a certain denitrification potential. Salinity and SOM played important roles in shaping microbial community at phylum and genus levels. The gene abundances related to xenobiotics metabolism and repair were high in Zone A, whereas the genes encoding energy metabolism and signal transduction were relatively high in Zone B. Denitrification was more favored for the low-salinity Zone B, whereas methane oxidation was enriched in the high-salinity Zone A. Therefore, our study emphasized the importance of an in-depth understanding of the microbial-community structure and function in Phragmites australis-dominated saltwater and freshwater wetlands.

Establishing an ecological forest system of salt-tolerant plants in heavily saline wasteland using the drip-irrigation reclamation method

Salinization in the low-lying edges of the Yinchuan plain, in northwest China, is a threat to productive farmland. An experiment, adopting “drip irrigation + high ridge + salt tolerant plant” reclaiming pattern, was conducted to develop a method of forest construction. The drip irrigation was scheduled by soil matric potentials (SMPs) monitored by tensiometers at both 20 cm and 50 cm depths exactly under drip emitters. There were five treatments marked as S1–S5, with their corresponding SMPs controlled higher than − 5, − 10, − 15, − 20 and − 25 kPa. Two salt-tolerant plants, the Elaeagnus angustifolia and the Astragalus adsurgens, were planted on ridges for reclamation. Results showed that low salinity zones formed quickly under drip emitters and the average electrical conductivity of saturated soil paste extracts (ECe) within 60 cm depth increased as SMP thresholds decreased. The 3-year average ECe values were 1.8, 4.6, 6.0, 7.1 and 7.7 dS m−1 for S1–S5, respectively, reduced by 93.4%, 83.1%, 77.9%, 73.9% and 71.7% compared with initial values. Soil salt was progressively leached and reached to an equilibrium state when SMP was kept above − 10 kPa within one growing season. Two growing seasons were required to reach a similar equilibrium for SMP threshold between − 15 and − 25 kPa. The soil pH increased 0.25, 0.11, 0.09, 0.05 and 0.07, and the soil sodium adsorption ratios declined by 59.3%, 36.2%, 37.8%, 44.9% and 42.7% in S1–S5, respectively, indicating a dealkalizing soil environment in all treatments. Plant growth indicators decreased as SMP thresholds declined and a threshold of − 20 kPa was important to maintain plant survival rates and growths. Therefore, keeping SMP higher than –10 kPa in the first two years was important to ensure desalinization and that higher than –20 kPa in the third year was essential to maintain salt balance and plant growth. A method consisting of agronomic practices and a four-stage drip-irrigation schedule based on the key SMPs is recommended for constructing a forest system.

Salinity drives growth dynamics of the mangrove tree Sonneratia apetala Buch. -Ham. in the Sundarbans, Bangladesh

Mangroves throughout the world are threatened by environmental changes apart from anthropogenic disturbances. Many of these changes may inhibit the growth and survival of mangrove species. To understand and predict the effects of global change on mangrove forests, it is necessary to obtain insights on the growth dynamics of mangroves in relation to environmental factors. This study was conducted on Sonneratia apetala, a mangrove species which grows under a range of salinity conditions across the Sundarbans in Bangladesh. We studied trees growing under respectively high, medium, and low salinity conditions based on the influence of freshwater discharge. First, the periodicity of radial growth across the year was detected by applying cambial analyses. Based on tree-ring analyses, we calculated the growth response of S. apetala to monthly variation in precipitation and temperature as well as river discharge, as a proxy for salinity. We found the cambium of S. apetala being active during the monsoon and post-monsoon period whereas it was dormant in the pre-monsoon. This periodicity in radial growth leads to the formation of distinct annual rings with ring boundaries being marked by radially flattened fibres. S. apetala trees growing under low salinity conditions generally show higher growth rates indicating the positive impact of river discharge, i.e. freshwater input on mangrove growth. Wet and warm conditions during the monsoon period positively affected S. apetala growth, especially in the low salinity zone. Our results show that salinity is the primary driver of growth dynamics of S. apetala in the Sundarbans. A gradual or seasonal increase in salinity, e.g. as a consequence of sea-level rise may therefore importantly alter the growth of this species, possibly leading to changes in mangrove forest dynamics and zonation.

Deciphering the genetic diversity of Avicennia officinalis L. across the salinity gradient in the Sundarbans mangrove forest of Bangladesh

Mangroves adaptive plasticity in the changing environmental conditions is of vital importance for conservation management. Genetic diversity of mangrove brings about adaptive plasticity, enabling a species to cope with different habitat conditions. The Sundarbans of Bangladesh is the largest coastal wetland and mangrove forest with diversified habitat conditions that support a wide diversity of flora and fauna. Avicennia officinalis L. is the pioneer species in mangrove succession in the low-salinity, medium-salinity and high-salinity zones of the Sundarbans. Adopting RAPD-PCR analysis, the genetic diversity of A. officinalis was studied to explore the ecotypes of this species in the Sundarbans. The genetic distances of A. officinalis between the low-salinity and medium-salinity zones (0.50) and between the low-salinity and high-salinity zones (0.52) are significantly (p < 0.05) higher than that between the medium-salinity and high-salinity zones (0.09). The expected heterozygosity of A. officinalis of medium-salinity (0.54 ± 0.14) and high-salinity zones (0.55 ± 0.10) are higher than that of low-salinity zone (0.37 ± 0.12). The genetic diversity of A. officinalis of medium-salinity (0.5417 ± 0.3167) and high-salinity (0.5458 ± 0.3189) zones is significantly (p < 0.05) higher than that of low-salinity zone (0.3750 ± 0.2313). This genetic diversity of A. officinalis bears significant ecological consequences. A. officinalis growing in the low-salinity zone is the low salt-adapted ecotype while that growing in the medium-salinity and high-salinity zones is the high salt-adapted ecotype in the Sundarbans. These two ecotypes of A. officinalis are of both in-situ and ex-situ conservation importance for the Sundarbans and coastal plantations of Bangladesh to face the climate change induced salinity regime changes in the future.

Prospects of using drip irrigation for ecological conservation and reclaiming highly saline soils at the edge of Yinchuan Plain

Reclaiming saline soils at the edge of Yinchuan Plain is of paramount importance for ecological conservation. Therefore, efficiency of drip irrigation method was evaluated in a field experiment. The experiment using drip irrigation as reclamation method was carried out that included five soil matric potentials (SMPs) controlled at −5, −10, −15, −20 and −25 kPa (treatments S1–S5, respectively) directly under drip emitters at 20 cm depth. Measures including vegetation collocation, high-raised ridge and three-stage drip irrigation scheduling were adopted. Results showed that SMP thresholds significantly affected the spatial distributions of both electrical conductivity (ECe) and sodium adsorption ratio (SARe) based on extracts of saturated paste. The low salinity and SARe zones either kept enlarging or remained stable when SMP thresholds were above −20 kPa. However, for treatment S5(−25 kPa), these gradually shrank with time. By the end of the second year, the average ECe values in treatments S1–S5 were 2.05, 2.59, 3.36, 3.05 and 3.9 dS m−1, respectively, indicating that ECe increased linearly as SMP threshold decreased. The SARe showed the same trend, with corresponding values of 9.56, 10.48, 12.26, 12.82 and 15.23 (mmol L−1)0.5. The pH of the saturated soil extract (pHe) in all treatments increased from 7.64 to around 8.00 and was not significantly affected by treatments. Moreover, the two-year survival rates of the dominant trees were >87.5% when SMP threshold was above −20 kPa. The growth indexes of plants reduced with the decrease in SMP thresholds and statistical analysis suggested that −20 to −17 kPa was an important SMP range, below which the forage yield of the herbaceous legume Astragalus adsurgens dropped sharply. This study suggested the SMP threshold of −20 kPa was important for drip irrigation scheduling and the drip irrigation, combined with ridging plantation, was efficient for both vegetation reclamation and reclaiming saline soils in these saline areas with shallow ground water.

Asymmetric responses of spatial variation of different communities to a salinity gradient in coastal wetlands

Various ecological communities are susceptible to the salinity gradients in coastal wetlands. Remane diagram has well described the macrozoobenthos diversity pattern along salinity gradients. Yet, further research is still needed, that is, the changes in diversity and biomass of other communities (e.g. plants, fish) along salinity gradients, and whether these changes are consistent or different among different communities. In this study, using China's Yellow River Delta wetland as a case study, we analyzed the variation of the community composition, species richness, and biomass of plant, macrozoobenthos, and fish communities along a salinity gradient from <0.5 to 30 ppt. We found that plant community composition exhibited more distinct variation along the salinity gradient than macrozoobenthos, with the least distinction for fish. Plant species richness decreased greatly along the gradient, whereas macrozoobenthos richness first decreased and then increased with increasing salinity, with the low richness occurring at a salinity of 0.9–12.3 ppt. Fish had the highest richness at a salinity of 14.8–16.0 ppt. The sum of plant, macrozoobenthos, and fish species and macrozoobenthos richness were both similar to the Remane diagram. Plants had higher biomass in low-salinity zones than in high-salinity zones, except for high biomass at a salinity of 14.8–16.0 ppt, whereas macrozoobenthos and fish showed the opposite trend. Principal-coordinate analysis showed an obvious dissimilarity map based on the composition, richness, and biomass of the plant, macrozoobenthos, and fish communities. Overall, the effects of salinity gradient differed among different communities. These findings demonstrate the asymmetric responses of different communities to salinity gradients, and have practical implications for maintaining a salinity gradient in coastal wetlands.

Reclamation of Coastal Saline Wasteland Using Drip Irrigation and Embedded Subsurface Pipes

A 3‐yr field experiment investigated the impact of drip irrigation and embedded subsurface drain pipes on salt leaching and Chinese rose (Rosa chinensis Jacq.) growth in a severely degraded saline soil located in north China. The experiment consisted of five soil matric potential (SMP) treatments where the soil matric potential at 0.2 m below the emitter was −5, −10, −15, −20 and −25 kPa in 2014 and 2015 and then decreased to −15, −20, −25, −30 and −35 kPa in 2016. With time, the size of the low salinity zone increased and the soil extract electrical conductivity (ECe) in surface 40 cm decreased from 27.32 to &lt;1.5 dS m−1. Soil water content increased with increasing SMP threshold values in the surface soil profile, but there were no significant differences among treatments in the deep layer. Soil pH values increased and then decreased for all treatments and obvious alkalization occurred, especially in the root zone. High survival rates of Chinese rose with values &gt;96% were obtained in 2014 for all treatments and these decreased with time, especially in the lower SMP treatments in 2016. An SMP higher than −5 kPa in the first year, −10 kPa in the second year and −20 kPa in the third year, combined with an irrigation water event of 10 mm, can be used for rose drip‐irrigation scheduling, which can maintain survival rates of &gt;85% on reclamation of coastal saline sandy loam soil with embedded subsurface pipes.Core Ideas Chinese roses were grown on coastal saline soils under drip irrigation and embedded subsurface pipe. Five soil matric potential thresholds were set for drip irrigation. The drip irrigation scheduling for rose grown in coastal saline soils was determined.

Climate trends in the East Antilles Islands

AbstractThis paper analyzes climate trends in satellite‐blended model reanalyses for the east Antilles islands of Guadeloupe, Dominica and Martinique (14–17°N, 62.5–60.5°W). Trends are studied for the dry winter (JFM) and wet summer (JAS) seasons, for parameters including: aerosol profile, tropospheric NO2 concentration, rainfall and hydrology, outgoing longwave radiation, potential evaporation, sea level, air and sea temperature and salinity in the upper ocean. Climate change emerges in the form of 2–4 and 6–8‐year surges in near‐surface temperature and mid‐tropospheric subsidence. While near‐surface specific humidity exhibits a rising trend especially in summer, the mid‐troposphere is drying. Coastal zones show little trend in rainfall, but cloud forests on the volcanic slopes are getting wetter. Faster rising trends are noted for sea surface height, and for sea temperatures in low salinity zones that affect tropical cyclone intensification. This work quantifies the spatial and temporal nature of climate variability in the east Antilles islands of Guadeloupe, Dominica and Martinique and advocates for increasing integration of mesoscale analysis into studies on local socio‐economic vulnerability.

Modeling the Influence of Outflow and Community Structure on an Endangered Fish Population in the Upper San Francisco Estuary

The aim of this community modeling study was to evaluate potential mechanisms by which freshwater outflow in the upper San Francisco Estuary, CA, controls the fall habitat and abundance of subadult delta smelt Hypomesus transpacificus and its community. Through analyses of the community matrix, community stability and the direction of change of community variables were qualitatively and quantitatively modeled under four outflow–input scenarios. Three subsystems were modeled in the low salinity zone (1–6 psu), each overlapping the location corresponding to the distance from the mouth of the estuary to upstream positions where the near-bottom 2 psu isohaline (X2) is at 74, 81, and 85 km (corresponding to high-, mid-, and low-outflows). Results suggested communities were qualitatively stable at each X2 position, but simulations showed the percent of stable models decreased from low- to high-X2 positions. Under all outflow–input scenarios, the predicted qualitative population responses of delta smelt were: (1) consistently positive for the low X2 position, and (2) uncertain under both mid- and high-X2 positions. Qualitative predictions were generally consistent with quantitative simulations and with the relations between relative abundance of delta smelt and X2. Thus, high outflow seems beneficial to subadult delta smelt when X2 reaches 74 km during fall.

European smelt Osmerus eperlanus in the eastern Gulf of Finland, Baltic Sea: Stock status and fishery.

In the eastern Gulf of Finland, European smelt Osmerus eperlanus occurs as an anadromous ecological form that spawns in coastal, low-salinity zones and in several rivers entering the gulf, most importantly in the River Neva. Osmerus eperlanus is a key commercial fish species for the population of St. Petersburg, and the city's fish symbol. However, the state of the smelt stock has considerably deteriorated over the past few decades. Monitoring shows that annual catches during 1965-1992 averaged 2274 t, compared to 292 t in 2002-2017, although some improvement is inidicated by a gradual increase in catches in recent years (e.g., to 595 t in 2017). This paper addresses long-term and recent changes in the fisheries for smelt in the eastern Gulf of Finland, including both commercial and recreational fisheries, and the range of fishing gears used. The commercial smelt fishery is mainly conducted during the spawning period (April-May) on migrating fish, using trap nets of various designs and beach seines. There has been an increase in recreational fisheries for smelt; in years when ice cover on the gulf during winter (December-April) is steady, recreational fishers from St. Petersburg, using hook and line, may catch quantities that are comparable to those of commercial catches. Several factors may account for the stock dynamics and associated changes in catches of smelt: these include a decrease in prey availability in the gulf; loss and degradation of spawning and nursery habitats, partly associated with large-scale hydroengineering operations in Neva Bay; and illegal, unreported and unregulated (IUU) fishing which became widespread in post-Soviet decades. In spite of a gradual increase in smelt abundance in the most recent 15 years, a failure to accurately monitor and manage the extent of IUU and recreational fishing for smelt, to assess the status of the stock, or to forecast catches, increases the risk of overfishing of the spawning stock.

The low-salinity zone in the San Francisco Estuary as a proxy for delta smelt habitat: A case study in the misuse of surrogates in conservation planning

Abstract The use of surrogates in conservation planning for at-risk species is both a necessary and a fraught practice. Here we assess the use of the position of the low-salinity zone in California’s Sacramento-San Joaquin Delta as a surrogate for the extent and quality of habitat available to the imperiled delta smelt. The U.S. Fish and Wildlife Service issued a biological opinion and incidental take statement under the Endangered Species Act analyzing the impacts of ongoing operation of two large water infrastructure projects on the delta smelt. The Service’s analysis and the conservation actions it imposed are based on the assumption that the low-salinity zone can serve as a “surrogate indicator” for delta smelt habitat. We demonstrate that available scientific information on the species countermands use of the low-salinity zone to represent delta smelt habitat in conservation planning for the species, and that the U.S. Fish and Wildlife Service erred by assuming the existence of a surrogate relationship absent validation using the best available scientific information. Notably, large expanses within the low-salinity zone are unoccupied by delta smelt and the species consistently occurs outside of it. This case serves to remind scientists of the dysfunction between a consensus in the scientific literature that calls for analytical validation prior to the use of species surrogates and habitat proxies, and the commonplace practice of using surrogates and proxies based on surmise and assertion.

Modeling the Potential of Submarine Groundwater Discharge to Facilitate Growth of Vibrio cholerae Bacteria

Submarine groundwater discharge (SGD), the discharge of terrestrial groundwater to the ocean, can govern the coastal benthic environment. Bacteria such as Vibrio cholerae inhabit coastal waters and sediments, whose growth can be influenced by SGD. In particular, salinity changes introduced by SGD could have a positive effect on the abundance but also virulence of non-halophilic V. cholera bacteria dwelling in coastal waters and shallow marine sediments. Here we assess potential effects of SGD on the environmental properties that favor V. cholerae in a numerical modeling study representing multiple scenarios. Approaching natural systems, simulation results reveal a high sensitivity of non-halophilic Vibrio cholerae growth to SGD and its primary driving factors. This dependency leads to highest growth potential at high groundwater inflow and low hydraulic conductivity of the aquifer as well as for steep sea-side boundary slopes. Besides its minor impact on the extent of SGD in our model, dispersion is a crucial limiting factor for V. cholerae habitat. We conclude that there is a close connection between the driving factors of SGD and low salinity zones along a coastal slope, and recommend taking these into consideration for evaluating local V. cholerae outbreaks.

Restoring oysters to urban estuaries: Redefining habitat quality for eastern oyster performance near New York City.

Restoring and conserving coastal resilience faces increasing challenges under current climate change predictions. Oyster restoration, in particular, faces threats from alterations in precipitation, warming water temperatures, and urbanization of coastlines that dramatically change salinity patterns, foster the proliferation and spread disease, and disrupt habitat connectivity, respectively. New York City (NYC) coastal waters, once home to a booming oyster fishery for eastern oysters (Crassostrea virginica), are now nearly devoid of live oyster reefs. Oyster restoration in urban estuaries is motivated by the synergistic ecosystem benefits this native keystone species can deliver. Recent surveys have documented substantial remnant populations of adult oysters in the upper low salinity zone of the Hudson/Raritan Estuary (HRE) near Tarrytown, NY. This study assessed fitness-related performance across the HRE salinity gradient to evaluate habitat suitability on an estuarine scale. Oysters were hatchery-produced from wild, moderate-salinity broodstock, then outplanted for measurement of growth, survival, reproduction and disease prevalence over two years. Survival was generally higher in the lower salinity river sites and in the higher salinity Jamaica Bay sites relative to mesohaline NYC harbor sites. Growth rate was highest in Jamaica Bay and had high variation among other sites. Surprisingly, the highest proportion of individuals with sex-differentiated gametes and the highest average gonad maturation index was found at a low salinity site. Consistent with the advanced gametogenesis measured in experimental animals at low salinity, annual wild recruitment was documented near the low salinity remnant population in each of five monitored years. These results suggest that the remnant HRE oyster population is a robust, self-sustaining population that can be leveraged to support restoration of subpopulations in other parts of the estuary, but further research is required to determine if the mesohaline and near-ocean reaches of the HRE can support the full oyster life cycle.