Effects of pH on salinity tolerance of selected freshwater invertebrates

Salinization of freshwaters often co-occurs with other changes in the environment, including pH. We investigate the effect of pH on salinity tolerance in selected macroinvertebrates (Notalina fulva, Centroptilum sp. and Physa acuta—lethal effects only) and microinvertebrates (Paramecium caudatum and Hydra oligactis—lethal and sublethal effects). Despite seemingly plausible physiological arguments, no difference in salinity tolerance over 96-h period was detected between low (5 or 6 nominal) pH and circumneutral (7–8.2 nominal) pH. P. caudatum was more salt sensitive in pH 11 than in pH 5, 7, and 10 in terms of mortality, and in terms of a sublethal endpoint, number of individuals produced and survived over 72 h, more sensitive to salinity in pH 10 than in pH 5 and 7. No other effects of pH on salinity tolerance were detected. Acidification will likely have effects on freshwater organisms on its own, however, when combined with salinization (from saline waters approximating seawater) acidification level tested did not modify the direct effects of salinity on the sample of freshwater invertebrates tested from a range of taxonomic groups. Thus the risk of low (5 or 6) pH modifying the effect of salinity on freshwater invertebrates is not high. Logically, lower pH values might have modified the effect of salinity, but there is a limited scope for lower pH values that would keep the species studied alive. In contrast, alkaline pH may increase the effect of salinity in some freshwater invertebrates. It is possible that the effect of pH on salinity tolerance may, however, be increased in saline waters with low calcium concentrations.     

Acute and chronic effects of Cr(VI) on Hypsiboas pulchellus embryos and tadpoles

In the last few years there has been great concern about declines in the abundance of several species of amphibians around the world. Among amphibians, anurans have a biphasic life cycle, with aquatic tadpoles and generally terrestrial adults, and they have an extremely permeable skin, making them excellent indicators of the health of the environment. A number of different causes have been suggested for the global decline of anurans, the pollution of their habitat by chemical stressors being considered one of the major factors. Among chemical stressors, heavy metals are known for their high toxicity at very low concentrations. This study assessed short- (96 h, 'acute') and long-term (1272 h, 'chronic') exposure to Cr(VI) at lethal (3 to 90 mg 1(-1)) and sublethal concentrations (0.001 to 12 mg 1(-1)) on Hypsiboaspulchellus (previously called Hyla pulchella; see Faivovich et al. 2005) tadpoles (Fam. Hylidae) from central eastern Argentina. Fertilized eggs collected from a clean pond near La Plata (Buenos Aires Province) were used for acute and chronic toxicity testing. Assays were done under controlled laboratory conditions. Results of chronic exposure were used to assess the effect of factors such as toxicant concentration and age of organisms at the beginning of exposure on the response variables (growth, development and survival until metamorphosis). Results indicated a higher sensitivity to Cr(VI) of individuals first exposed as tadpoles than those first exposed as embryos during acute and chronic exposure. Exposure to the highest sublethal concentrations (6 to 12 mg 1(-1)) of the toxicant showed early inhibitory effects on growth of all treated organisms compensated at longer exposure periods with an increase in the growth rate compared to the control groups.     

中国林蛙生长发育盐效应的研究进展

本文对中国林蛙受精卵、蝌蚪和幼蛙等不同发育阶段生长发育的盐效应进行了综述。盐度对不同发育阶段的中国林蛙均有影响。中国林蛙受精卵孵化的最适盐浓度范围为0~5 g/L,孵化率超过80%;在2~3 g/L盐浓度下孵化的中国林蛙雌性个体超过90%,孵化后中国林蛙蝌蚪的存活率没有显著变化;中国林蛙蝌蚪和变态幼蛙的安全盐度上限分别为2 g/L和1.5 g/L。这些研究结果为中国林蛙野生资源的保护和人工养殖提供了一定的理论基础。     

Tolerance to Copper and to Salinity in Daphnia longispina: Implications within a Climate Change Scenario

Considering IPPC climate change scenarios, it is pertinent to predict situations where coastal ecosystems already impacted with chemical contamination became exposed to an additional stressor under a future scenario of seawater intrusion. Accordingly, the present study aimed at evaluating if a negative association between tolerance to a metal and to saltwater exists among genotypes of a freshwater organism. For this, five clonal lineages of the cladoceran Daphnia longispina O.F. Müller, exhibiting a differential tolerance to lethal levels of copper, were selected. Each clonal lineage was exposed to lethal and sublethal concentrations of sodium chloride (assumed as a protective surrogate to evaluate the toxicity of increased salinity to freshwater organisms). Mortality, time to release the first brood and total number of neonates per female were monitored and the somatic growth rate and intrinsic rate of natural increase were computed for each clonal lineage. Data here obtained were compared with their lethal responses to copper and significant negative correlations were found. These results suggest that genetically eroded populations of D. longispina, due to copper or salinity, may be particularly susceptible to a later exposure to the other contaminant supporting the multiple stressors differential tolerance.     

不同生态型芦苇种群对盐胁迫的生长和光合特性

土壤盐渍化是影响我国土壤利用效率的主要因素之一,芦苇是改良土壤盐渍化的良好实验材料,但芦苇有着多种的生态型,比较各生态型芦苇的耐盐差异成为亟待解决的问题。通过设置淡水(0.00%)与加盐(质量浓度2.00%)处理控制实验,测量芦苇的生长指标和光合指标,比较河口型芦苇与内陆型芦苇耐盐性,寻找合适生态型的芦苇作为改良土壤盐渍化的生物材料。在实验中,与淡水条件相比,加盐(2.00%)处理条件下,河口型芦苇和内陆型芦苇的株高(height)、蒸腾速率(E)均显著性下降,但是两种生态型的芦苇的水分利用效率(WUE)明显提高;河口型的芦苇相对生长速率(RGR)和气孔导度(Gs)都明显高于内陆型芦苇。在淡水环境中,河口型芦苇的相对生长速率(RGR)和净光合速率(A)都显著性地高于内陆型芦苇。结果表明两种生态型的芦苇在进化过程中存在一定程度上的分化,盐胁迫会抑制两种芦苇的生长,两种生态型芦苇的相对生长速率和气孔导度在盐胁迫下出现明显地差异,表明两种生态型的芦苇对盐度的响应机制有所差异。相比于内陆型芦苇,河口型芦苇有着更强的耐盐性,内陆型及河口型芦苇的表型性状差异主要是由于其原生境的差异所决定的。     

Episodic salinization and freshwater salinization syndrome mobilize base cations,carbon, and nutrients to streams across urban regions

Urbanized watersheds in colder climates experience episodic salinization due to anthropogenic salt inputs and runoff from impervious surfaces. Episodic salinization can be manifested as a ‘pulse’ in concentrations and fluxes of salt ions lasting from hours to days after snowstorms in response to road salting. Episodic salinization contributes to freshwater salinization syndrome, characterized by cascading mobilization of chemicals and shifting acid–base status. We conducted laboratory experiments and analyzed high-frequency sensor data to investigate the water quality impacts of freshwater salinization syndrome and episodic salinization across 12 watersheds draining two major metropolitan regions along the U.S. East Coast. Sediments from 12 watersheds spanning land use gradients across two metropolitan regions, Baltimore, Maryland and Washington DC, were incubated across a range of replicated salinity treatments (0–10 g/L sodium chloride). There were statistically significant linear increasing trends in calcium and potassium concentrations with experimental salinization across all 12 sites and in magnesium concentrations at 11 of 12 sites (p?<?0.05), with mean rates of increase of 1.92?±?0.31 mg-Ca per g-NaCl, 2.80?±?0.67 mg–K per g-NaCl, and 1.11?±?0.19 mg-Mg per g-NaCl, respectively. Similarly, there were statistically significant increasing linear trends in total dissolved nitrogen (TDN) concentrations with experimental salinization at 9 of the 12 sites, with a mean rate of increase of 0.07?±?0.01 mg-N per g-NaCl. There were statistically significant increasing linear trends in soluble reactive phosphorus (SRP) concentrations with experimental salinization at 7 of the 12 sites (p?<?0.05), with a mean rate of increase of 2.34?±?0.66 µg-P per g-NaCl. The response of dissolved inorganic carbon (DIC) and organic carbon (DOC) concentrations to experimental salinization varied between sites, and dissolved silica did not show any significant response. High-frequency sensors near the experimental sites showed statistically significant positive linear relationships between nitrate concentrations, specific conductance, and chloride concentrations similar to relationships observed in laboratory incubations. Our results suggested that episodic salinization and freshwater salinization syndrome can mobilize base cations and nutrients to streams through accelerated ion exchange and stimulate different biogeochemical processes by shifting pH ranges and ionic strength. The growing impacts of freshwater salinization syndrome and episodic salinization on nutrient mobilization, shifting acid–base status, and augmenting eutrophication warrant serious consideration in water quality management.     

Irreversibility of a bad start: early exposure to osmotic stress limits growth and adaptive developmental plasticity

Harsh environments experienced early in development have immediate effects and potentially long-lasting consequences throughout ontogeny. We examined how salinity fluctuations affected survival, growth and development of Fejervarya limnocharis tadpoles. Specifically, we tested whether initial salinity effects on growth and rates of development were reversible and whether they affected the tadpoles’ ability to adaptively accelerate development in response to deteriorating conditions later in development. Tadpoles were initially assigned to either low or high salinity, and then some were switched between salinity levels upon reaching either Gosner stage 30 (early switch) or 38 (late switch). All tadpoles initially experiencing low salinity survived whereas those initially experiencing high salinity had poor survival, even if switched to low salinity. Growth and developmental rates of tadpoles initially assigned to high salinity did not increase after osmotic stress release. Initial low salinity conditions allowed tadpoles to attain a fast pace of development even if exposed to high salinity afterwards. Tadpoles experiencing high salinity only late in development metamorphosed faster and at a smaller size, indicating an adaptive acceleration of development to avoid osmotic stress. Nonetheless, early exposure to high salinity precluded adaptive acceleration of development, always causing delayed metamorphosis relative to those in initially low salinity. Our results thus show that stressful environments experienced early in development can critically impact life history traits, having long-lasting or irreversible effects, and restricting their ability to produce adaptive plastic responses.     

Influence of density and salinity on larval development of salt‐adapted and salt‐naïve frog populations

Environmental change and habitat fragmentation will affect population densities for many species. For those species that have locally adapted to persist in changed or stressful habitats, it is uncertain how density dependence will affect adaptive responses. Anurans (frogs and toads) are typically freshwater organisms, but some coastal populations of green treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands. Tadpoles from coastal populations metamorphose sooner and demonstrate faster growth rates than inland populations when reared solitarily. Although saltwater exposure has adaptively reduced the duration of the larval period for coastal populations, increases in densities during larval development typically increase time to metamorphosis and reduce rates of growth and survival. We test how combined stressors of density and salinity affect larval development between salt‐adapted (“coastal”) and nonsalt‐adapted (“inland”) populations by measuring various developmental and metamorphic phenotypes. We found that increased tadpole density strongly affected coastal and inland tadpole populations similarly. In high‐density treatments, both coastal and inland populations had reduced growth rates, greater exponential decay of growth, a smaller size at metamorphosis, took longer to reach metamorphosis, and had lower survivorship at metamorphosis. Salinity only exaggerated the effects of density on the time to reach metamorphosis and exponential decay of growth. Location of origin affected length at metamorphosis, with coastal tadpoles metamorphosing slightly longer than inland tadpoles across densities and salinities. These findings confirm that density has a strong and central influence on larval development even across divergent populations and habitat types and may mitigate the expression (and therefore detection) of locally adapted phenotypes.     

Etudes expérimentalles sur la sensibilité du phytoplancton d'eau douce aux variations de salinité dans le fjord du Saguenay (Estuaire du St-Laurent), Canada Experimental Studies on the Susceptibility of Freshwater Phytoplankton to Variation of Salinity in the Fjord of Saguenay (St. Lawrence Estuary), Canada

The Saguenay Fjord is characterized by very high run-offs. The annual flow varies between 1.2 and 4.2 × 10³ m³ . s⁻¹. During the summer the surface salinity shows a sharp horizontal gradient (from 1 0/00 to 20 0/00). The surface phytoplankton, mainly freshwater and even some euryhaline species is submitted to salinity stress. Continuous culture experiments were conducted to study the susceptibility of this freshwater phytoplankton under different salinity conditions. Some pennate diatoms (dominant group) are found as relatively tolerant to salinity. Microflagellates have been found to be resistant to salinity up to 26 0/00. Chlorophyll concentrations and rates of primary production of those organisms are as high as those of the control.     

Mangroves: obligate or facultative halophytes? A review

Salinity plays significant roles in regulating the growth and distribution of mangroves, and the salt tolerance mechanisms of mangroves have been the focus of research for several decades. There are contradictory views regarding the relationship between mangroves and salt: (1) Mangroves are facultative halophytes, i.e. freshwater is a physiological requirement and salt water is an ecological requirement for mangroves because they are capable of growing in freshwater. The former prevents excess respiratory losses while the latter prevents invasion and competition from non-halophytes. (2) Mangroves are obligate halophytes, i.e. salt is necessary for their growth. Mangroves cannot survive in freshwater permanently and salt water is a physiological requirement. Up to now, mangroves are usually considered as facultative halophytes. In this review, we provided five lines of evidence to evaluate these two contradictory views: (1) the results of laboratory culture experiments and field investigations; (2) the viviparous nature of mangroves; (3) the salt accumulation of mangroves under freshwater or low salinity; (4) the effect of salinity on the photosynthetic rate and in vitro enzyme activities, and (5) the effects of salinity fluctuation on mangrove growth and physiology. Contrary to widely accepted view, our evaluations of the aforementioned evidence suggest that mangroves are obligate halophytes. Mangroves can grow in freshwater for a limited time by drawing upon the nutrients and salt reserves in their hypocotyls while prolonged culture in freshwater is fatal to them. Mangroves have the ability to absorb Na⁺ and Cl⁻ rapidly and preferentially under low-salinity conditions. Not all of the enzymes in mangroves are sensitive to salt. In fact, the activities of some enzymes are even stimulated by low or moderate salinity. Plants grown under constant salinity in a laboratory setting are unlikely to behave in the same way as those in their natural habitat with fluctuating salinity. Thus, studies on the effects of freshwater or low salinity and salinity fluctuation on mangroves, as well as the physiological mechanisms that allow maintenance of function under fluctuating salinity conditions should be strengthened in future research.     

A tale of two pesticides: how common insecticides affect aquatic communities

1. Recent ecotoxicology studies show that pesticide exposure can alter community composition, structure and function. Generally, community responses to pesticides are driven by trait‐ and density‐mediated indirect effects resulting from sublethal and lethal effects of pesticide exposure on vulnerable taxa. These effects depend upon the concentration of the pesticide and the frequency of exposure. 2. While more research is needed to understand community‐level responses to pesticide exposure, testing the effects of multitudes of registered chemicals on ecologically relevant communities is overwhelming. Recent reviews suggest that contaminants with similar modes of action should produce comparable community‐level responses because they have similar direct effects and, as a result, similar indirect effects; this hypothesis remains largely untested. 3. We subjected pond communities [containing zooplankton, phytoplankton, periphyton and leopard frog tadpoles (Rana pipiens)] to several applications (single applications of medium or high concentrations or weekly applications of a lower concentration) of two acetylcholine esterase inhibiting insecticides, malathion and carbaryl that have comparable toxicity for aquatic organisms. 4. We found that both insecticides cause comparable trophic cascades that affect zooplankton and phytoplankton abundances; however, their effects on amphibians diverged, especially when exposed to higher concentrations of insecticides. Malathion caused a trophic cascade beginning with a decline in cladocerans followed by increases in phytoplankton. At a medium concentration, this cascade also caused a subsequent decrease in periphyton. Carbaryl caused a similar trophic cascade with the highest application, a weak trophic cascade with the medium application and no cascade with smallest application. Malathion directly reduced tadpole survival at all concentrations. Survivors in the two higher treatments were larger at metamorphosis while survivors in the lowest treatments were smaller and developed slowly. In contrast, carbaryl was not directly toxic to tadpoles, but indirectly reduced survival because slow growth and development prevented some tadpoles from metamorphosing before the mesocosms dried at medium and low applications. 5. These results suggest that these common pesticides, which share the same mode of action, have similar effects on zooplankton and algae, but differences in the strength and timing of their effects on tadpoles reduce the generality of responses at higher trophic levels. Overall, general predictive models of contaminant effects could be improved by incorporating the relative timing of direct and indirect effects of exposure.     

Evolutionary rescue and adaptation to abrupt environmental change depends upon the history of stress

Whether evolution will be rapid enough to rescue declining populations will depend upon population size, the supply of genetic variation, the degree of maladaptation and the historical direction of selection. We examined whether the level of environmental stress experienced by a population prior to abrupt environmental change affects the probability of evolutionary rescue (ER). Hundreds of populations of two species of yeast, Saccharomyces cerevisiae and Saccharomyces paradoxus were exposed to a range of sublethal concentrations of salt for approximately a hundred generations before transfer to a concentration of salt lethal to the ancestor (150 g l^–1 NaCl). The fitness of surviving populations of both species was a quadratic function of yield: fitness was greatest for large populations that had been selected on low salt concentrations (less than 20 g l⁻¹ NaCl) and small populations that had adapted to high salt (more than 80 g l⁻¹ NaCl). However, differences occurred between species in the probability of ER. The frequency of ER was positively correlated with salt concentration for S. cerevisiae, but negatively correlated with salt concentration in S. paradoxus. These results not only demonstrate that past environmental conditions can determine the probability of ER after abrupt environmental change, but also suggest that there may even be differences between closely related species that are worth further exploration.     

Eco-physiological characteristics of alfalfa seedlings in response to various mixed salt-alkaline stresses

Soil salinization and alkalization frequently co-occur in nature, but little is known about the mixed effects of salt-alkaline stresses on plants. An experiment with mixed salts （NaCI, Na2SO4, NaHCO3 and Na2CO3） and 30 salt-alkaline combinations （salinity 24-120 mmollL and pH 7.03-10.32） treating Medicago sativa seedlings was conducted. The results demonstrated that salinity and alkalinity significantly affected total biomass and biomass components of seedlings. There were interactive effects of salt composition and concentration on biomass （P 〈 0.001）. The interactions between salinity and alkalinity stresses led to changes in the root activity along the salinity gradient （P 〈 0.001）. The effects of alkalinity on seedling survival rate were more significant than those of salinity, and the seedlings demonstrated some physiological responses （leaf electrolyte leakage rate and proline content） in order to adapt to mixed salt-alkaline stresses. It was concluded that the mixed salt-alkaline stresses, which differ from either salt or alkali stress, emphasize the significant interaction between salt concentration （salinity） and salt component （alkalinity）. Further, the effects of the interaction between high alkalinity and salinity are more severe than those of either salt or alkali stress, and such a cooperative interaction results in more sensitive responses of ecological and physiological characteristics in plants.     

Context-dependent effects of ranaviral infection on northern leopard frog life history traits

Pathogens have important effects on host life-history traits, but the magnitude of these effects is often strongly context-dependent. The outcome of an interaction between a host and an infectious agent is often associated with the level of stress experienced by the host. Ranavirus causes disease and mortality in amphibian populations in various locations around the world, but most known cases of ranaviral infection have occurred in North America and the United Kingdom. While Ranavirus virulence has been investigated, the outcome of Ranavirus infection has seldom been related to the host environment. In a factorial experiment, we exposed Northern leopard frog (Lithobates pipiens, formerly Rana pipiens) tadpoles to different concentrations of Ranavirus and investigated the effect of host density on certain life-history traits, namely survival, growth rate, developmental stage and number of days from virus exposure to death. Our results suggest a prominent role of density in driving the direction of the interaction between L. pipiens tadpoles and Ranavirus. We showed that increasing animal holding density is detrimental for host fitness as mortality rate is higher, day of death earlier, development longer and growth rate significantly lower in high-density tanks. We observed a linear increase of detrimental effects when Ranavirus doses increased in low-density conditions, with control tadpoles having a significantly higher overall relative fitness. However, this pattern was no longer observed in high-density conditions, where the effects of increasing Ranavirus dose were limited. Infected and control animals fitness were consequently similar. We speculate that the host may eventually diverts the energy required for a metabolic/immune response triggered by the infection (i.e., direct costs of the infection) to better cope with the increase in environmental "stress" associated with high density (i.e., indirect benefits of the infection). Our results illustrate how the net fitness of organisms may be shaped by ecological context and emphasize the necessity of examining the direct/indirect costs and benefits balance to fully understand host-pathogen interactions.     

Salinity of incubation media influences embryonic development of a freshwater turtle

Variations in water potential have marked effects on aspects of embryological development in reptiles. Therefore variation in the salinity of the incubation environment is likely to have significant consequences on the early life stage. The combination of an extended incubation period, coupled with the real threat of soil salinisation within their range makes Chelodina expansa an ideal model to assess the influence of salinity on turtle embryology. We quantified the influence of salt on the development of C. expansa hatchlings in four substrate treatments varying in salinity. Embryos incubated in higher salinities had 39 % less survival than those incubated in substrates with freshwater. Hatchlings that emerged from eggs in saline treatments were smaller with higher concentrations of plasma sodium, chloride, urea, and potassium. The physiological effects of salinity mirror those of turtles incubated in drier media with low water potential. Salinisation of river banks has the potential to reduce hatching success and fitness of nesting reptiles.     

Survival and behavioral characteristics of amphidromous goby larvae of Sicyopterus japonicus (Tanaka, 1909) during their downstream migration

To understand the ecology and environmental tolerances of newly hatched larvae of the amphidromous fish Sicyopterus japonicus during their downstream migration, the salinity tolerance of eggs, 0-15 day old larvae, and adults, and the temperature tolerance, specific gravity and phototaxis of hatched larvae were examined. Tolerances of adults were measured as survival after a 24 h challenge in freshwater (FW), brackish water (1/3 SW) and seawater (SW). The survival rate of adult S. japonicus was 100% in FW and 1/3 SW, while none survived in SW. Hatching success of eggs (30 eggs each) was significantly higher in FW (mean: 73%) and 1/3 SW (73%) than in SW (19%). Tolerance of newly hatched larvae to salinity and temperature was investigated in different combinations of salinities (FW, 1/3 SW and SW) and temperatures (18, 23 and 28 °C). Larval survival was significantly different in each salinity and temperature. Survival rate was significantly higher in 1/3 SW than in FW and higher in SW than in FW at 23 °C and 28 °C. At the latter part of the experiment, there was no survival in FW and at 28 °C. Survival was higher in lower temperatures, but larval development did not occur in FW. Specific gravity of newly hatched larvae was 1.036 at 28 °C and 1.034 at 23 °C. When exposed to a light source on one side of an aquarium, larval distribution was not affected. Our results indicated larval S. japonicus are more adapted to brackish water and seawater than freshwater, while the adults and eggs are more adapted to freshwater and brackish water than seawater. This is consistent with their amphidromous life history with growth and spawning occurring in freshwater and the larval stage utilizing marine habitats.     

Evolutionary mismatch along salinity gradients in a Neotropical water strider

The evolution of local adaptation is crucial for the in situ persistence of populations in changing environments. However, selection along broad environmental gradients could render local adaptation difficult, and might even result in maladaptation. We address this issue by quantifying fitness trade‐offs (via common garden experiments) along a salinity gradient in two populations of the Neotropical water strider Telmatometra withei—a species found in both fresh (FW) and brackish (BW) water environments across Panama. We found evidence for local adaptation in the FW population in its home FW environment. However, the BW population showed only partial adaptation to the BW environment, with a high magnitude of maladaptation along naturally occurring salinity gradients. Indeed, its overall fitness was ~60% lower than that of the ancestral FW population in its home environment, highlighting the role of phenotypic plasticity, rather than local adaptation, in high salinity environments. This suggests that populations seemingly persisting in high salinity environments might in fact be maladapted, following drastic changes in salinity. Thus, variable selection imposed by salinization could result in evolutionary mismatch, where the fitness of a population is displaced from its optimal environment. Understanding the fitness consequences of persisting in fluctuating salinity environments is crucial to predict the persistence of populations facing increasing salinization. It will also help develop evolutionarily informed management strategies in the context of global change.     

Trade‐offs between salinity preference and antipredator behaviour in the euryhaline sailfin molly Poecilia latipinna

Salinity preference and responses to predatory chemical cues were examined both separately and simultaneously in freshwater (FW) and saltwater (SW)‐acclimated sailfin mollies Poecilia latipinna, a euryhaline species. It was hypothesized that P. latipinna would prefer FW over SW, move away from chemical cues from a crayfish predator, and favour predator avoidance over osmoregulation when presented with both demands. Both FW and SW‐acclimated P. latipinna preferred FW and actively avoided predator cues. When presented with FW plus predator cues v. SW with no cues, P. latipinna were more often found in FW plus predator cues. These results raise questions pertaining to the potential osmoregulatory stress of salinity transitions in euryhaline fishes relative to the potential fitness benefits and whether euryhalinity is utilized for predator avoidance. This study sheds light on the potential benefits and consequences of being salt tolerant or intolerant and complicates the understanding of the selection pressures that have favoured the different osmoregulatory mechanisms among fishes.     

Short- and Long-Term Effects of Salinity on Leaf Growth in Rice (Oryza sativa L.)

Addition of 50 mM NaCl to Oryza sativa L. had little effectupon the time of leaf initiation, but leaf mortality prior tothe normal phase of senescence was increased and the onset ofsenescence was advanced. There was no significant effect uponthe day-to-day pattern of growth, nor upon the ultimate length,of leaves that were developing at the time of, or shortly after,salinization with 50 mM NaCI. Leaves that developed after prolongedexposure of the plants to salinity were shorter. Addition ofNaCl, KC₁ or mannitol to the root medium brought about a cessationof leaf elongation within one minute. Growth at a reduced raterestarted abruptly after a lag period that depended upon theexternal concentration. Elongation rate recovered its originalvalue within 24 h after exposure to 50 mM NaCl, though not athigher concentrations. Addition of NaCl at concentrations upto 100 mM elicited no short-term effects upon photosyntheticgas exchange. Na uptake contributed to osmotic adjustment ofthe growing zone. When plants were rapidly exposed to 50 mMNaCl, no change in turgor pressure was detectable in the growingzone with the resolution of the miniature pressure probe used(about 70 kPa). It is concluded that the initial growth reductionin rice caused by salinization is due to a limitation of watersupply. A clear distinction is made between the initial effectsof low salinity which are recoverable and the long-term effectswhich result from the accumulation of salt within expanded leaves. Key words: Leaf elongation, gas exchange, photosynthesis, water relations     

A salty landscape of fear: responses of fish and zooplankton to freshwater salinization and predatory stress

Predator–prey relationships are altered by anthropogenic contaminants. Road salt is a widespread contaminant among freshwater ecosystems, yet a relatively understudied subject in community ecology. Unknown is whether road salt salinization interacts with predatory stress to influence the growth, behavior, or reproduction of freshwater organisms. Using rainbow trout (Oncorhynchus mykiss) and zooplankton (Daphnia pulex), we exposed them to variable levels of road salt (NaCl) crossed with the presence or absence of alarm cues or kairomones. Alarm cue reduced trout activity and aggression and increased shoaling behavior. Road salt reduced trout growth in the high compared to moderate salt concentration, but neither concentration was different from the control. There was no interaction between alarm cues and salt for trout. Road salt and predatory stress had an additive effect on Daphnia abundance. Predatory stress decreased Daphnia abundance by 11%. Compared to the control, salt decreased Daphnia abundance by 40% in 860 mg Cl^?/L and 79% in 1300 mg Cl^?/L, and by the final day abundance was reduced by 85% in 1300 mg Cl^?/L. Road salt and predatory stress had an interactive effect on Daphnia reproduction. Predatory stress in control water and moderate salt levels (230 mg Cl^?/L) increased sexual reproduction of Daphnia, but these responses disappeared at high salt concentrations. Thus, road salt could limit reproductive adaptations to natural and anthropogenic stressors in Daphnia. Our results indicate road salt salinization could alter zooplankton population dynamics directly and by interacting with predatory stress, which might affect energy flow through freshwater food webs.