Living in the front: Neomysis americana (Mysidacea) in the Río de la Plata estuary, Argentina-Uruguay

The Río de la Plata is one of the main estuarine systems of South America. It is characterized by a salt wedge regime, a well-developed bottom salinity front, and a maximum turbidity zone associated with it. We described, for the first time, the spatial distributional patterns of Neomysis americana, the most abundant mysid and the main food item for juvenile fishes in this estuary. We analyzed the link between mysid distribution and abundance and the bottom salinity gradient. A total of 242 plankton samples were taken from the Río de la Plata estuary in spring and fall between 1991 and 2001. Bottom salinity gradient was quantified from grids created on the basis of 348 oceanographic stations. The N. americana population was characterized by high abundances (up to 2500 ind. m⁻³), with juveniles, males, gravid and non-gravid females present in both spring and fall of different years. N. americana distribution followed the position of the bottom salinity front in different years and seasons. Pearson’s correlation analysis between mysid abundance and bottom salinity gradient confirmed the association of mysids with the bottom salinity front (maximum salinity gradient). No correlation was detected between mysid abundance and salinity per se or temperature (neither in spring nor in fall). We speculate that mysids concentrated at the front could take advantage of the high concentration of detrital material for feeding. The results of our work highlight the importance of the magnitude of salinity gradient for the ecological processes of a salt-wedge estuary like the Río de la Plata. The analysis of the spatial distribution of gradient values presented in this work also constitutes a useful tool to locate key ecological areas such as fronts.     

水肥耦合与旱地农业持续发展

水资源不足及人口增长给农业生产增加了很大压力,寻求开发旱农区相对丰富的土地资源成为满足需求增长的重要方法。文章讨论了旱地农业持续发展的限制因子、旱地水肥关系、水肥耦合效应机理以及水肥耦合对旱地农业持续发展的意义。     

Changes in hydrology and salinity accompanying a century of agricultural conversion in Argentina

Conversions of natural woodlands to agriculture can alter the hydrologic balance, aquifer recharge, and salinity of soils and groundwater in ways that influence productivity and sustainable land use. Using a land-use change chronosequence in semiarid woodlands of Argentina's Espinal province, we examined the distribution of moisture and solutes and estimated recharge rates on adjacent plots of native woodlands and rain-fed agriculture converted 6-90 years previously. Soil coring and geoelectrical profiling confirmed the presence of spatially extensive salt accumulations in dry woodlands and pervasive salt losses in areas converted to agriculture. A 1.1-km-long electrical resistivity transect traversing woodland, 70-year-old agriculture, and woodland, for instance, revealed a low-resistivity (high-salinity) horizon between approximately 3 m and 13 m depth in the woodlands that was virtually absent in the agricultural site because of leaching. Nine-meter-deep soil profiles indicated a 53% increase in soil water storage after 30 or more years of cultivation. Conservative groundwater-recharge estimates based on chloride tracer methods in agricultural plots ranged from approximately 12 to 45 mm/yr, a substantial increase from the <1 mm/yr recharge in dry woodlands. The onset of deep soil moisture drainage and increased recharge led to >95% loss of sulfate and chloride ions from the shallow vadose zone in most agriculture plots. These losses correspond to over 100 Mg of sulfate and chloride salts potentially released to the region's groundwater aquifers through time with each hectare of deforestation, including a capacity to increase groundwater salinity to >4000 mg/L from these ions alone. Similarities between our findings and those of the dryland salinity problems of deforested woodlands in Australia suggest an important warning about the potential ecohydrological risks brought by the current wave of deforestation in the Espinal and other regions of South America and the world.     

Trade-off between road avoidance and attraction by roadside salt pools in moose: An agent-based model to assess measures for reducing moose-vehicle collisions

Moose-vehicle collisions are a frequent traffic-safety issue, particularly in northern regions where moose are attracted to the near-road areas because they can consume sodium from de-icing salts that accumulate in pools at snowmelt. Moose that find salt pools near roads tend to remember their location and to re-visit them to get the sodium they need in their diet. This study investigated the trade-off between road avoidance and salt pool spatial memory in the movement behaviour of moose using an agent-based model to determine how the interplay of these two factors influences the frequency of road crossings in the Laurentides Wildlife Reserve (Québec, Canada). Mitigation measures studied were the removal of roadside salt pools and the construction of compensatory salt pools away from the road shoulder. A GPS telemetry program of moose in the study area was used to validate our model. The model moose with both road avoidance and salt pool spatial memory activated produced the best results when comparing to the real moose data. Results show that both road avoidance and salt pool spatial memory significantly affect moose road crossings, but that road avoidance explains most of the variance. Road avoidance tended to decrease the number of moose crossings, but this decrease was partly compensated by the spatial memory of salt pools which typically increased the likelihood that moose will cross the road. The trade-off between road avoidance and salt pool memory was largest when original salt pools were maintained. In simulations where road avoidance and salt pool memory were both turned off, the impact of mitigation measures on the number of road crossings was lowest. For the most realistic moose behavior, the management scenarios resulted in reductions in road crossings between 22% and 79%, and the best scenario is to completely remove roadside salt pools. If compensation salt pools are used, they should be located as far as possible from the roads (beyond 500 m) to have an impact on moose road crossings.     

Habitat as a mediator of mesopredator‐driven mammal extinction

A prevailing view in dryland systems is that mammals are constrained by the scarcity of fertile soils and primary productivity. An alternative view is that predation is a primary driver of mammal assemblages, especially in Australia, where 2 introduced mesopredators—feral cat (Felis catus) and red fox (Vulpes vulpes)—are responsible for severe declines of dryland mammals. We evaluated productivity and predation as drivers of native mammal assemblage structure in dryland Australia. We used new data from 90 sites to examine the divers of extant mammal species richness and reconstructed historic mammal assemblages to determine proportional loss of mammal species across broad habitat types (landform and vegetation communities). Predation was supported as a major driver of extant mammal richness, but its effect was strongly mediated by habitat. Areas that were rugged or had dense grass cover supported more mammal species than the more productive and topographically simple areas. Twelve species in the critical weight range (CWR) (35–5500 g) that is most vulnerable to mesopredator predation were extirpated from the continent's central region, and the severity of loss of species correlated negatively with ruggedness and positively with productivity. Based on previous studies, we expect that habitat mediates predation from red foxes and feral cats because it affects these species’ densities and foraging efficiency. Large areas of rugged terrain provided vital refuge for Australian dryland mammals, and we predict such areas will support the persistence of CWR species in the face of ongoing mammal declines elsewhere in Australia.     

Effects of salinity on multiplication and transmission of an intertidal trematode parasite

Salinity levels vary spatially in coastal areas, depending on proximity to freshwater sources, and may also be slowly decreasing as a result of anthropogenic climatic changes. The impact of salinity on host–parasite interactions is potentially a key regulator of transmission processes in intertidal areas, where trematodes are extremely common parasites of invertebrates and vertebrates. We investigated experimentally the effects of long-term exposure to decreased salinity levels on output of infective stages (cercariae) and their transmission success in the trematode Philophthalmus sp. This parasite uses the snail Zeacumantus subcarinatus as intermediate host, in which it asexually produces cercariae. After leaving the snail, cercariae encyst externally on hard substrates to await accidental ingestion by shorebirds, which serve as definitive hosts. We found that at reduced salinities (25 or 30 psu), the cercarial output of the parasite was lower, the time taken by cercariae to encyst was longer, fewer cercariae successfully encysted and encysted parasites had lower long-term survival than at normal seawater salinity (35 psu). The strong effect of salinity on the replication and transmission of this parasite suggests that there may be sources and sinks of transmission to birds along coastal areas, depending on local salinity conditions. Also, unless it evolves to adapt to changing conditions, the predicted reduction in salinity as a consequence of climate change may have negative impact on the parasite’s abundance.     

The influence of temperature and salinity on<Emphasis Type="Italic"> Acartia</Emphasis> (Copepoda: Calanoida) nauplii survival

Adult Acartia congeners, A. bifilosa, A. clausi, A. discaudata and A. tonsa, have distinct seasonal and spatial distribution patterns in Southampton Water (UK), reflecting patterns of temperature and salinity, respectively. The effect of these factors on other life stages, hatch success and naupliar survival was investigated by exposing the congeners to a range of salinity (15.5–33.3) and of temperature (5–20°C). A. clausi is known to prefer more saline waters, and showed highest hatch success at 33.3 salinity. A. tonsa is most tolerant to dilution, and at 15.5 salinity it had the highest hatch success of all the congeners. Hatch success in both A. bifilosa and A. discaudata was similar over the range of salinities investigated, confirming that they are intermediate species in terms of spatial distribution. The nauplii of all species survived well at the higher salinities and best at 33.3, which allows for differential transport of the poorly swimming nauplii to the mouth of the estuary until size and swimming ability increase, after which they can then return to regions of preferred salinity. The summer species, A. clausi and A. tonsa showed higher hatch success at 20°C, whereas A. discaudata, which is present in the water column all year round, showed no significant temperature-related differences in hatch success. A. bifilosa, which diapauses over summer, showed significantly higher hatch success at 10°C than at 20°C. The physiological relationship between temperature and development time was clear; naupliar survival of all species was highest at 20°C and all congeners reached the first copepodite stage (CI) significantly faster at 20°C. However, no consistent pattern was seen for salinity. It would appear that the adult Acartidae in Southampton Water remain in regions of their preferred salinity and lay eggs there which hatch well. However, because the nauplii are not good swimmers, they are swept towards the mouth of the estuary and into areas of higher salinity, where they remain and develop into more advanced stages before moving back up the estuary to take up their adult distribution pattern.Communicated by J.P. Thorpe, Port Erin     

Effects of salinity on the net uptake of zinc by the common mussel Mytilus edulis

The net uptake of zinc by the common mussel Mytilus edulis (L.) has been investigated under different natural and artificial salinity stresses. The effects of stable and fluctuating salinities on the uptake of zinc by the mussel are discussed in terms of three possible modes of action. Under certain highly-stressful conditions, salinity may affect the uptake of zinc by the mussel. This factor should be considered when the mussel is used as an indicator of environmental pollution by zinc in estuarine areas, or spurious conclusions may result.     

The effects of salt exclusion during ice formation on circulation in lakes

Laboratory experiments have been performed to investigate the effects of salt exclusion on the behaviour of lakes with salinities up to 8 g L^?1. At these salinities the freezing temperature is less than the temperature of maximum density and, unlike sea-ice, a reverse temperature stratification forms beneath the ice that can support at least some of the excluded salt. Temperature time series at four depths showed that salt exclusion drives cascades of localised overturning, while the persistence of reverse temperature stratification indicated that mixing was not complete. While our array of temperature sensors had insufficient spatial resolution to provide full details of the flow, we hypothesize that: at salinities of 1 and 2 g L^?1  salt is released relatively uniformly and forms a layer of elevated salinity immediately below the ice, which supports double-diffusive salt-fingering; and at salinities of 4 and 8 g L^?1, salt plumes penetrate the reverse stratification. After the ice melted, a relatively fresh surface layer formed above a more saline layer, sufficient to suppress spring turnover. Our measurements compare favourably with field observations from lakes, and highlight the importance of salt exclusion on biogeochemical processes in lakes.     

Influence of salinity on eggs,sperm and larvae of low-vertebral herring reproducing in the coastal waters of the Soviet Union

Clupea harengus pallasi deposit their eggs in the coastal zone, which is the most dynamic part of the sea in respect to its regime. Salimity is one of the most variable factors on the spawning grounds. Observations were made in the seas of the European North and Far East, and in experiments where salinities varied from 0 to 70. Study of the influence of salinity on the development of sea herring eggs is of interest for several reasons. Firstly, salinity fluctuation patterns differ in different parts of a given habitat. In each area inhabited, salinity boundaries exist within which eggs can develop normally. Secondly, the levels of limiting salinities on spawning grounds differ noticeably in different seas. Hence, attention was paid to population-specific differences in the responses of eggs to salinity. Finally, detailed knowledge on the responses of sexual cells to low-salinity conditions helps to elucidate the ecological situation in areas with reduced salinity, and the reproductive potential of the population considered. Sexual cells of numerous populations of Pacific herring are capable of fertilization over a wide range of salinities. Fertilizability in low salinities and, partially, also in high salinities, reveals a population specificity. The responses of the eggs ofC. harengus pallasi to reduced salinities differ from those ofC. harengus harengus. Osmotic resistance of eggs to low salinities is considerably higher in the first-mentioned subspecies. Under salinity stress, eggs reveal individual differences which are not seen under optimum conditions of salinity. In all populations studied, responses to salinity change during embryology.     

Relative salinity tolerance of warm season turfgrass species

Fresh water, coupled with soil salinization in many areas has resulted in an increased need forscreening of salt tolerant turf grasses. Relative salinity tolerance of eightwarm season turfgrass species were examined in this study in sand culture. Grasses were grown in a glasshouse, irrigated with either distilled water or saline sea water adjusted to 24, 48 or 72 dSm-1. Salt tolerances of the grasses were assessed on the basis of their shoot and root growth, leaf firing and turf quality. Regression analysis indicated that Zoysiajaponica (Japanese lawn grass) (JG), Stenotaphrum secundatum (St. Augustine) (SA), Cynodon dactylon (satiri) (BS), Zoysia teneuifolia (Korean grass) (KG), Digitaria didactyla (Serangoon grass) (SG), Cynodon dactylon (Tifdwarf) (TD), Paspalum notatum (Bahia grass) (BG) and Axonopus compressus(Pearl blue) (PB) suffered a 50% shoot growth reduction at 36.0, 31.8, 30.9, 28.4, 26.4, 25.7, 20.0 and 18.6 dSm1 of salinity, respectively and a root growth reduction at44.9, 43.7, 33.4, 31.0, 29.5 27.5, 21.5 and 21.4 dSm- of salinity, respectively. Leaf firing and turf quality of the selected species, as a whole, were also found to be affected harmoniously with the change in root and shoot growth. On the basis of the experimental results the selected species were ranked for salinity tolerance as JG>SA>BS>KG>SG >TD>BG>PB.     

Blue mussels, Mytilus edulis, at the edge of the range: population structure, growth and biomass along a salinity gradient in the north-eastern Baltic Sea

The blue mussel, Mytilus edulis, is the most conspicuous animal species in the northern Baltic rocky sublittoral. In the studied area the species lives at the margin of its salinity tolerance. Although dwarfed by the low-salinity conditions, blue mussels in the northern Baltic are very abundant and have a decisive role in the benthic and pelagic ecosystems. We studied abundance, size distribution, biomass and growth rate of blue mussels along a 270 km salinity gradient in the northern Baltic Sea. Samples (n=317, 1998-1999) from moderately exposed and exposed rocky shores at seven study areas were collected in the southern Archipelago Sea in the west and into the central Gulf of Finland in the east, where the species is becoming increasingly rare. The results show a marked decline in mean mussel size and biomass from the saline west to the less saline east. The growth rate also decreased with lower ambient salinity. However, abundance of small mussels was considerably higher in the central and eastern parts of the study area. Vertically, the highest biomass was recorded at intermediate depths (5 and 8 m), being lower at both shallower (3 m) and deeper bottoms (12 m). It is concluded that salinity is the foremost factor determining size structure and growth rate among populations within the area. The results suggest that predation further influences the population structure of blue mussels living at the edge of their range in the central Gulf of Finland ultimately set by their salinity tolerance.     

Estimating soil salinity in different landscapes of the Yellow River Delta through Landsat OLI/TIRS and ETM+ Data

Soil salinization has increasingly become a serious issue in coastal zone due to global climate changes and human disturbances. Assessment of soil salinity, especially at the landscape scale, is critical to coastal management and restoration. Two data from OLI/TIRS and ETM+ sensors of Landsat satellite were used to compare their ability to invert the spatial pattern of soil salinity in both farmland and salt marsh landscapes in the Yellow River Delta, China, respectively. The results showed that the in situ electrical conductivity (EC _a ) of soil, representing soil salinity, were closely related with spectral parameters and salinity indices calculated by the remote sensing data. The results of multiple regression models have showed that nearly all the spectral parameters and salinity indices calculated by OLI/TRIS data were more sensitive to soil salinity than those by ETM+ data. Therefore, the models based on OLI/TIRS data are superior to those on ETM+ data in estimating the spatial pattern of soil salinity in farmland and salt marsh landscapes. Our results were very helpful to evaluate the levels of soil salinization in the Yellow River Delta.     

滨海地区湿地重建的生态工程范例

在天津泰达区盐田上通过淡水浸泡、消浪墙构建、土壤更换、水生植物引种等措施，重建了湿地生态系统的结构。结果表明，人工湿地在初期阶段是成功的。     

Significance of salinity and silicon levels for growth of a formerly estuarine eelgrass (Zostera marina) population (Lake Grevelingen, The Netherlands)

Since the early 1980s, the eelgrass, Zostera marina L., population in the saline Lake Gevelingen, The Netherlands, is rapidly declining. An earlier study, in which long-term data on eelgrass coverage in this former estuary were correlated with several environmental variables, showed only one significant correlation: coverage was positively related to water column silicon levels. In addition, a negative correlation with salinity was observed, but this was not significant. In the present study, the effect of silicon and the effect of salinity on the development of Z. marina were investigated experimentally. Enhancement of dissolved silicon concentrations in the water did not stimulate Z. marina above-ground production or an increase in final above- and below-ground biomass. The highly significant correlation between eelgrass coverage and water column silicon levels, thus, remains to be explained. The results of the growth experiments did, however, demonstrate a clear effect of salinity on Z. marina growth. Plants cultured at 22 psu showed a higher production of shoots and leaves, resulting in more above-ground biomass, than plants grown at 32 psu. In addition, below-ground biomass was also higher at 22 psu. Measurements of chlorophyll a fluorescence, performed with a PAM-fluorometer, indicated a reduction of photosynthesis in the high-salinity treatments. Thus, low salinity stimulates development of Z. marina from Lake Grevelingen. Eelgrass from such a historically estuarine area may be more sensitive to high salinities than other, more marine populations. Recovery of the autochthonous eelgrass population is expected to be favoured when the estuarine conditions of the seagrass area are re-established, or when restoration programmes are carried out with allochthonous ecotypes that are less sensitive to high salinities. Received: 23 June 1998 / Accepted: 19 November 1998     

The effect of water movement,temperature and salinity on abundance and reproductive patterns of<Emphasis Type="Italic"> Macrocystis</Emphasis> spp. (Phaeophyta) at different latitudes in Chile

This study describes the density variation and phenology of Macrocystis integrifolia and M. pyrifera populations from northern and southern Chile, respectively. Samples of both species were taken in wave-exposed and wave-protected areas. In addition, spore production, germination and early growth rate of sporophytes of each population was studied at monthly intervals under three temperature and salinity regimes. Results indicate that M. integrifolia from northern Chile presents perennial plants with a mean density of three individuals per 0.25 m² throughout the year and that it reproduces mainly during spring and winter. Although, M. pyrifera in exposed areas of southern Chile also have a perennial-type life strategy, they are able to reproduce all year round. In contrast, M. pyrifera populations in protected areas of southern Chile show a clear annual cycle, with high recruitment during late winter and fertile sporophytes in summer and autumn, although the populations become completely decimated thereafter. The effect of temperature and salinity on M. integrifolia shows that it is independent of water movement, but requires low temperatures and high salinities for the release of zoospores, germination and early sporophyte growth. This pattern differs from that of M. pyrifera in southern Chile, which has a broader tolerance range for salinity and temperature than does M. integrifolia. However, in southern Chile wave-protected populations showed higher spore release and germination at 15°C and 18°C, whereas sporophyte growth responded better at the lowest temperature tested (8°C). In general, these results are contrary to those expected, since a seasonal reproductive pattern was observed in M. integrifolia inhabiting a less seasonally variable environment. In exposed sites of southern Chile, plants showed greater tolerance and continuous reproduction throughout the year, despite the greater environmental variability. Finally, population dynamics of protected kelps in southern Chile shows an annual pattern, which is contrary to the expected perennial strategy shown by exposed populations.Communicated by P.W. Sammarco, Chauvin     

Photosynthetic performance,oxidative damage and antioxidants in <Emphasis Type="Italic">Cylindrotheca closterium</Emphasis> in response to high irradiance,UVB radiation and salinity

Cylindrotheca closterium is a common marine diatom living in intertidal environments where it can be present both in the water column and on sediments, depending on the tidal regime. In the present work this diatom was employed to investigate the responses to desiccation and to increase in PAR and UVB intensity, as occurs during emersion. Under these circumstances, the production of active oxygen species (AOS) may be enhanced resulting in an oxidative stress. Stress responses in this species were measured by exposing it to normal (30) and double salinity (60), supplying light of low or high intensity for 12 h, in the latter case either without or with moderate dose rates of UVB. Pulse amplitude modulated fluorometry was used to measure Chl a autofluorescence (F ₀), an index of photosynthetic efficiency of PSII (F _v/F _m) and the relative electron transfer rate (rETR). The oxidative stress was evaluated by analysing GSH pools and SOD activity. It was observed that at double salinity and under low light, intracellular pools of reduced glutathione (GSH) were higher than under the two conditions of high light without and with UVB at both salinities. The antioxidative defence activity of superoxide dismutase (SOD) was far higher under hypersaline conditions. The oxidative damage was evaluated as protein and lipid damage. The results showed that it expressed itself mainly through protein peroxidation: at normal salinity relative protein carbonyl content was (a) twice as high as in cells grown at double salinity, and (b) three times as high under UVB. Total unsaturated lipid contents doubled under hypersalinity conditions. The lipid peroxidation marker malondialdehyde showed the strongest response to low light and UVB at salinity value of 60. Lipid peroxide content was significantly higher at salinity of 60 compared to normal salinity and was the highest under low light and high light with UVB. The simulated emersion condition of the diatom seems to lead to the establishment of a balance between damage and repair, expressed mainly as (a) oxidative protein damage at normal salinity, in particular due to UV radiation, (b) sufficient protection by SOD activity mainly under hypersaline conditions.     

Effect of environmental flows on deep,anoxic pools

A series of field surveys were carried out on two permanent pools of the upper Glenelg River in SW Victoria, Australia. One was representative of the wider and deeper pools while the other was representative of the more-narrow and shallower pools. Both pools showed a typical seasonal cycle of warm, brackish, oxygen-poor, summer conditions and cool, oxygen-rich, low-salinity, winter conditions. The summer salinity increases were larger than expected, suggesting possible saline groundwater inflow from unidentified springs. Both pools contained anoxic water in their deeper sections but this was permanent only in the deeper pool. A simple model of the flushing rate of such anoxic pools subject to flows, such as environmental flow releases, was developed, based on an energy balance between the potential energy required to lift the anoxic layer and the kinetic energy derived from the river flow. The results were tested against and in agreement with the field measurements. The model also suggests that the anoxic layers are resilient to all but the largest environmental flows.     

Different salinity tolerance mechanisms in Atlantic and Chesapeake Bay conspecific oysters: glycine betaine and amino acid pool variations

Crassostrea virginica (Gmelin) collected in 1989 from several sites within the Chesapeake Bay have narrower salinity tolerances than conspecific oysters collected in 1989 from several Atlantic coast sites (Georgia to Cape Cod). The basis of this physiological difference appears to be the biochemical mechanisms that control cellular osmolality following salinity stress. When adapted to the same salinity, the amino acid pools of both gill and adductor muscles of Atlantic oysters are larger than those of Bay oysters and different in composition. The Atlantic oyster tissues rely primarily on taurine for salinity tolerance, while the Bay oyster tissues have relatively less taurine, depending instead upon alanine, glycine and proline to adapt to high salinity. In addition, Atlantic oyster gill and adductor have 10 to 25 times the glycine betaine concentrations of these tissues from Bay oysters, depending upor the salinity of acclimation. The betaine concentration varies with salinity in Atlantic oysters, but does not change in Bay oysters. The results suggest that these biochemical differences are the basis of the narrower salinity tolerance in Bay oysters. The biochemical differences may reflect genetic differences between Bay and Atlantic oysters.