Function Of Salinity Research Articles

Influence of salinity and mineralization on trace metal sorption to cyanobacteria in natural waters

The equilibrium distribution of Cd, Cu and Pb between the cyanobacterium Anabaena spp. and natural lake water as a function of salinity and degree of algal decomposition, was studied using batch experiments. Adsorption isotherms could be described with the Freundlich equation. Distribution coefficients ( K d) for Anabaena spp. were largest for Pb, followed by Cd and Cu. The variation in K d values for Cd was explained by a model accounting for metal complexation to the algal surface and complexation by dissolved ligands of variable concentration. The phytoplankton/water distribution of Cd in waters of different salinity appears to be regulated by the free Cd 2+ activity in solution. Mineralization of Anabaena spp. for 42 d resulted in a decrease of the solid to water ratio by a factor of 12, and an increase in K d for Cd (factor four), Cu (factor eight) and Pb (factor 10–20). The increase in K d is attributed to a higher affinity of the metals for the adsorbent. The use of constant K d values for mineralizing phytoplankton and detritus in trace metal fate models, may result in underestimations of bound fractions. The extent to which this fraction is underestimated, depends on the time course of the distribution coefficient and the solid to water ratio during the mineralization process.

Systematic salt effects in the automated determination of nutrients in seawater

The Bran + Luebbe TRAACS continuous flow analyser, commonly used for the determination of nutrients in seawater, primarily employs standard methods that have been modified and adapted for automated use. The analyser is fitted with self aligning spectrophotometric flow-cells with parallel optical ends of uniform thickness. As the light beam strikes both ends of the flow-cell perpendicularly, there is no refraction therefore no spurious absorbance caused by changes in the refractive index of the flow-cell contents. Investigation of methods for the determination of ammonia, nitrate, nitrite, orthophosphate, soluble reactive silica and urea, using TRAACS, confirmed that this was the case. Also, for the methods assessed, the absorbance signal was not biased by either sample turbidity or wetting agents. Chemical interference from salt was assessed by analysing nutrient spiked solutions, over the 0–35 ppt salinity range, relative to working calibration solutions prepared using demineralised water. A major salt effect which caused an apparent decrease in concentration, was detected over the analytical working ranges in the ammonia and nitrate methods. The salt effect results from a chemical interference which causes the production of reaction products to vary as a function of salinity. The salt effect is totally unrelated to refraction and produces a concentration error in the ammonia and nitrate methods up to 35 and 25% respectively. The degree of salt effect was shown to be dependent on both the salinity and true nutrient concentration of the sample. Over the salinity range 8.75–35.00 ppt, corrective equations were derived for ammonia and nitrate to express the true nutrient concentration in terms of the observed concentration and the salinity of the solution. Within the range 0–8.75 ppt no correction was required for ammonia wheras the nitrate showed a 3.5% increase in apparent concentration at 8.75 ppt. The orthophosphate, soluble reactive silica, urea and nitrite methods showed no significant salt effect as the coefficient of variation (CV) of the observed nutrient concentrations over the 0–35 ppt salinity range was within the accepted CV or analytical precision of the methods. The ammonia and nitrate corrective equations were applied in assessing the true nutrient content of “depleted” or low nutrient seawater, used in the preparation of working calibration solutions, after analysing relative to working calibration solutions prepared in demineralised water.

Thermodynamic studies of the carbonate system in seawater

Laboratory measurements of the components of the carbonate system (pH, fCO 2, TCO 2 and TA) were made on Gulf Stream seawater and Certified Reference Material (CRM) as a function of salinity (33–37), temperature (0–40°C) and the ratio TA/ TCO 2 (1.02 to 1.25). The pH (±0.002) was determined by spectrophotometry; the fCO 2 (±2 μatm) with an infrared detector; the TCO 2 (±2μol kg −1) by coulometry; and the TA (±2μmol kg −1) by potentiometric titrations. The results were used to examine the internal consistency of the thermodynamics of the carbonate system by calculation of various parameters (e.g., TA) from two input parameters (e.g., pH- TC0 2). The measurements on CRMs at 25°C were found to be consistent with the thermodynamic constants of Goyet and Poisson (1989) and Roy et al. (1993) to °0.005 in pH, °3 μmol kg −1 in TCO 2 and ±3 μmol kg −1 in TA. The measurements on Gulf Stream seawater from 0 to 40°C were also found to be consistent in pH to ±0.006, in TCO 2 to ±4μmol kg −1 and in TA to ±4μmol kg −1 when these constants were used. The pH and fCO 2 were measured on seawater with a know TA and TCO 2 from 5 to 35°C. The results give slopes of (d In fCO 2 dt) × 10 2 = 4.26 ± 0.03 and (dpH/d t) × 10 2 = 1.65 ± 0.01, in agreement with the values calculated from the thermodynamic constants of Roy et al. (1993) and Goyet and Poisson (1989) over the same temperature range. The measured fugacities were internally consistent to ±7 μatm with the values calculated with these same constants and with pH- TA or pH- TCO 2 as input parameters. All of these laboratory measurements indicate that the carbonic acid dissociation constants of Roy et al. (1993) and Goyet and Poisson (1989) give the best description of the thermodynamics of the CO 2 system in seawater over temperature from 0 to 40°C, salinity from 33 to 37 and the ratio TA/ TCO 2 from 1.02 to 1.25. By using the p K 1 determined from the equations of Roy et al. (1993), we have used our measurements to determine p K 2 and fitted the results by the equation (σ = 0.0048): p K 2 = 70.8632 − 1386.5890/ T− 10.0561ln( T) + 0.0573 ( S − 35) − [15.4502 T]( S − 35) which is valid for S = 33–37 and T = 273.15–331.15 K.

The uptake of cadmium by the midge larvae Chironomus riparius as a function of salinity

The effect of salinity on the uptake of cadmium by the freshwater midge larvae Chironomus riparius was studied. Fourth instar larvae were exposed to a salinity range up to 10 ppt, by diluting chemically defined seawater with chemically defined river water. The influence of prior acclimation at different salinities on cadmium uptake rate was examined for three salinities of acclimation. To discriminate between the effect of the free cadmium ion concentration, the free cadmium ion activity and the calcium ion concentration, we tested solutions of different composition. Within each acclimation group, cadmium uptake decreased with increasing salinity in all exposure solutions. At a constant salinity of exposure, cadmium uptake increased with increasing salinity of acclimation. Changes in the free ion activity explained most of the observed variation (52%). The cadmium uptake was also influenced by the salinity of acclimation. Changes in calcium concentration had only a minor effect on cadmium uptake. The integration of the different effects explained almost 80% of the total variation in cadmium uptake by midge larvae. The remaining variation could not be explained and was attributed to natural variation in cadmium uptake among midge larvae.

Modelling the effect of salinity on radium desorption from sediments

The desorption of the four naturally occurring radium isotopes 223Ra, 224Ra, 226Ra, and 228Ra from estuarine sediments is investigated. These isotopes are created within sediments by the radioactive decay of insoluble thorium parents. Due to competition from other ions for the occupation of adsorption sites on the sediment grains, radium desorption is a function of salinity. A model is developed to describe radium desorption as it is affected by salinity, grain size, and sediment concentration. It is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters. One of these parameters is the concentration of radium available for exchange on the sediment; the other depends on major ion and radium exchange coefficients. Desorption experiments performed on sediments from the bed of the Bega River, Australia, are used to validate the model and to evaluate the parameters needed for model application.

Reduction of Fe(III) with sulfite in natural waters

The Fe(III) in marine aerosols and rainwaters can be reduced to Fe(II) by photochemical processes and by reactions with sulfite. In this paper, measurements of the rates of reduction of nanomolar levels of Fe(III) with sulfite (without O2) have been determined in NaCl and seawater solutions as a function of temperature (0° to 40°C), pH (2 to 6.8), ionic strength (I = 0.1 to 6 M), and composition (Na+, Mg2+, Ca2+, F−, Cl−, Br−, HCO3−, SO42−). The overall rate constant (k, M−1 min−1) for the reaction, products, is given by d[Fe(III)]/dt = −k[Fe(III)] [S(IV)]. The reaction was found to be first order with respect to Fe(III) and S(IV). The rate constants as a function of pH increased from a pH = 2 to 4 and decreased at higher pH. The effect of temperature and ionic strength on the rates could be represented by log k = log k0 + AI0.5/(1 + I0.5), where A = −1.1 in NaCl and −2.2 in seawater and log k0 = 25.39 − 6,323/T. The energy of activation was found by 121±6 kJ mol−1. The measured rates in seawater as a function of salinity were lower than the rates in NaCl at the same ionic strength. Measurements in NaCl solutions with added sea‐salt ions (Mg2+, Ca2+, F−, Br−, and SO42−) at pH = 3.5 indicate that the formation of inert FeF2+ may be responsible for the lower rates. The effect of changes in the composition on the rates was interpreted by examining the speciation of Fe(III) and S(IV). This analysis indicates that the rate‐determining steps from a pH of 2.5 to 4.0 are FeOH2+ + HSO3− ↔ HOFeSO3H + and · and at pH of 4 to 6, the reactions Fe(OH)2+ + HSO3− ↔ (HO)2FeSO3H and · become important. The changes in the concentration of FeOH2+ and HSO3− as a function of pH and composition can account for most of the changes in the rates. These kinetic studies indicate that the rates of reduction of Fe(III) with S(IV) in acidic water droplets at natural levels of S(IV) may be an important source of Fe(II).

Speed of sound in seawater as a function of salinity, temperature, and pressure

In view of the adoption of the International Temperature Scale of 1990 (ITS-90) which defines the International Celsius Temperatures t90, the dependence on temperature of the speed of sound in seawater is reexamined. The coefficients for two seawater sound-speed equations published previously, one by UNESCO (1983) which is also known as the Chen and Millero equation of 1977, and one by DelGrosso (1974) are revised for application with the new temperature scale ITS-90. It is found that the resulting change in seawater sound speed is within 0.024 and 0.017 m/s for the UNESCO and the DelGrosso equations, respectively. These changes are comparable to the numerical correction values suggested by Dushaw et al. (1993).

Thermodynamics of the carbon dioxide system in the oceans

In the next ten years, a number of studies on the carbonate system are planned as part of the JGOFS/WOCE programs. The carbon dioxide system will be studied by measuring at least two of the controlling parameters; pH, total alkalinity (TA), total inorganic CO 2 (TCO 2), and the fugacity of CO 2 ( f CO2). The other parameters can be calculated using thermodynamic relations. In the present paper the thermodynamic equations necessary to characterize the CO 2 system in the oceans as a function of salinity and temperature are given. This includes equations for the dissociation of carbonic acid, boric acid, phosphoric acid, silicic acid, water, hydrogen sulfide, and ammonia in seawater as a function of temperature (0 to 45°C) and salinity (0 to 45). The equations are of the form ln K i = A + B T + C ln T , where A, B, and C are functions of salinity. Equations are also given for calculating the effect of temperature and salinity on the fugacity and pH of seawater using the carbonic acid constants of Roy et al. (1993a).

The osmotic significance of the heteroside floridoside in the mangrove alga Catenella nipae (Rhodophyta: Gigartinales) in Eastern Australia

The effect of salinity on the intracellular floridoside concentration in the mangrove alga Catenella nipae (Rhodophyta: Gigartinales) was investigated using 13 C nuclear magnetic resonance spectroscopy ( 13 C-NMR), and gasliquid and high pressure liquid chromatography (GLC, HPLC). The floridoside content increased linearly with increasing salinity, and hence is strongly involved in osmotic acclimation in this species. In addition, during 13 C-NMR analysis isethionic acid was identified as a new metabolite in C. nipae . The concentration of this compound also varied as a function of salinity. The steady-state contents of floridoside were also recorded in various geographically isolated field populations of C. nipae from mangroves along New South Wales coastline of Australia. All isolates contained very high floridoside levels. From these data, together with the physiological capability to accumulate floridoside under hypersaline conditions for osmotic acclimation, it is interpreted that floridoside is essential for survival in the extreme mangrove habitat.

Effects of adult salinity acclimation on larval survival and early development of Strongylocentrotus droebachiensis and Strongylocentrotus pallidus (Echinodermata: Echinoidea)

Larval survival and development rates of Strongylocentrotus droebachiensis and Strongylocentrotus pallidus were determined as a function of salinity in two experiments by (i) directly transferring fertilized eggs obtained from adults acclimated to sea water at a salinity of 30‰ to cultures containing seawater at salinities of 30, 27.5, 25, 22.5, 20, 17.5, 15, 12.5, and 10‰ at 10 °C; and (ii) acclimation of adult sea urchins to the salinity–temperature conditions described above for 2, 3, and 4 weeks prior to spawning. Subsequent development occurred under these acclimation conditions. Development rates and percent survival of larvae prior to metamorphosis varied directly with salinity. Survival of S. pallidus plutei to metamorphosis decreased at salinities below 30‰. Strongylocentrotus droebachiensis plutei survived to metamorphosis at 20‰ and above. Lactic acid concentrations in the coelomic cavity fluid of adult S. droebachiensis and S. pallidus acclimated to low salinity were significantly higher than initial controls at 30‰. Cell volumes of fertilized eggs of both species exhibited osmotic swelling when exposed to lowered salinity. LC50 values (‰), development rates, and percent survival to metamorphosis indicate that acclimation of adult urchins to lower salinity prior to spawning and fertilization does not enhance development or survival of embryos of these two species exposed to low salinity. Furthermore, our results show that S. pallidus larvae are stenohaline when compared with larvae of other echinoderm species.

On the electrokinetics of shear stress behavior in fluid‐mud suspensions

The variability of rheological properties in fluid‐mud suspensions is studied as a function of salinity and sediment concentration. It was found that the steady state shear stress increases exponentially with increasing sediment concentration and increases logarithmically by increasing the salinity of the suspension. An analytic model predicting shear stress as a function of electroviscous properties is developed. The model shows that the effect of salts in the suspension is to decrease the zeta potential by compressing the electric double layer and, thereby, elevating the shear stresses acting on the shearing planes at each particle‐fluid interface in the suspension. These stresses increase with the increase in salt content. The model incorporates the classical double‐layer theories of Gouy‐Chapman and the Helmholtz‐Smoluchowski theory for electrokinetics of charged particles. The model shows good correlation with experimental data at low sediment concentrations where the basic assumptions of the Gouy‐Chapman formulation are satisfied. At progressively higher sediment concentrations, there are increasingly larger departures between the model predictions and the experimental data due to violations of these basic assumptions. The advantages and limitations of the model are discussed.

Microalgae for use in tropical aquaculture II: Effect of salinity on growth, gross chemical composition and fatty acid composition of three species of marine microalgae

The influence of salinity on the growth, gross chemical composition and fatty acid composition of three species of marine microalgae,Isochrysis sp.,Nannochloropsis oculata andNitzschia (frustulum), was investigated. There was no significant change in growth rate ofIsochrysis sp. andN. (frustulum) over the experimental range of salinity (10–35 ppt), whileN. oculata had a significantly slower growth rate only at 35 ppt. The ash content of all three species increased with increasing salinity. Two species,Isochrysis sp. andN. oculata, showed significant linear increases in total lipid content with increasing salinity over the range 10 to 35 ppt.N. (frustulum) showed significant linear decrease in total lipids, with the highest percentage at low salinity within the range 10–15 ppt. Variation in salinity had only a slight effect on the total protein, the soluble carbohydrate and chlorophylla content of all species. All species responded to change in salinity by modifying their cellular fatty acid compositions. Significant positive correlations were observed between increase in salinity and increase in the percentage ofcis-9-hexadecenoic acid [16:1 (n-7)] over the entire experimental range inN. (frustulum) and between 25–35 ppt inN. oculata. There were curved relationships between salinity and percentage of hexadecanoic acid [16:0] inN. oculata andN. (frustulum), with maxima within the range 25–30 ppt for both species. A curved relationship was found between salinity and percentage of eicosapentaenoic acid [20–5(n-3)], forN. (frustulum), with lowest percentages of the fatty acid within the range 25–30 ppt. There was no consistent pattern in the percentages of other major fatty acids as functions of salinity. The Northern Territory isolateN. (frustulum) was unusual in having a substantial increase in total fatty acids with decreasing salinity (85 mg g−1 dry wt at 10 ppt compared with 33 mg g−1 at 35 ppt). The optimum salinities for the production of maximum amount of lipids and the essential fatty acids 20:5(n-3) and/or 22:6(n-3) were as follows:

Growth and osmoregulation of Boekelovia hooglandii in relation to salinity

The growth and osmoregulation of Boekelovia hooglandii Nicolai et Baas Becking (Chrysophyceae) was investigated as a function of salinity. This chromophyte microflagellate was found to be euryhaline and also to require sodium for growth. The optimum sodium chloride concentration for growth was 0.2–0.4 M, and growth was severely inhibited in media with sodium chloride concentrations above 1 M. Using gas chromatography – mass spectroscopy analysis of cell extracts, it was established that the alga contains D-mannitol, myo-inositol, as well as the inositol derivative, cyclohexanetetrol. The content of cyclohexanetetrol and mannitol increased with increased salinity, while the content of inositol remained almost constant. Potassium and magnesium were the major intracellular cations. However, the content of cations and amino acids showed only minor increases with salinity over a wide salinity range. The contents of cyclohexanetetrol and mannitol increased rapidly when cells were subjected to hypertonic shocks. It was concluded that B. hooglandii utilizes cyclohexanetetrol and mannitol as osmoregulatory substances. Key words: Boekelovia hooglandii, chromophyte, cyclohexanetetrol, euryhaline, D-mannitol, myo-inositol, osmoregulation, salinity.

Environmental variability as a factor controlling spatial patterns in distribution and species diversity of zooplankton in the St. Lawrence Estuary

Field studies on a variety of organisms have suggested that environmental variability plays a major role in determining spatial patterns in distribution and species diversity of estuanne organisms due to the effect of abiotic fluctuations on the physiology of animals. However, there is no study examining the effect of environmental variability on zooplankton distribution and diversity in estuaries. As vertical m~gration is obligatory behavior for the retention of planktonic animals at intermediate positions in the St. Lawrence Estuary (Canada), vertical stratification of the water column is considered the major source of environmental variability for zooplankton in this system. To evaluate the importance of this source of variability as a factor controlling the distribution and diversity of zooplankton in the estuary, we examined the relative contribution of each of the environmental factors of salinity, temperature, turbidity and vertical stratification in explaining spatial patterns of summer zooplankton distribution and diversity. Multivariate analyses revealed the presence of a longitudinal succession of seasonally stable species assemblages (tidal freshwater, true-estuarine and euryhalinemarine assemblages) whose spatial distribution was mainly a function of salinity and vertical stratification. Turbidity and temperature played a minor role in explaining spatial distribution. For all sampling periods, the limit between the true-estuarine and euryhaline-marine assemblages, the lowest number of population centers and the lowest zooplankton abundance all corresponded to the most vertically stratified waters. In contrast, population centers of all species were concentrated in the most abiotically stable parts of the estuary. We conclude that environmental variability is a major factor determining zooplankton distribution and diversity in the estuary. The spatio-temporal stability of the species assemblages and the seasonal variabihty in the abundance of some species also suggest that trophic interactions may play an important role in the regulation of zooplankton populations in the estuary.

Elemental composition ofMedicago sativa under saline conditions. Relation to biological nitrogen fixation

The salinity tolerance of aRhizobium meliloti strain, used as inoculum, was established by growing the strain for seven consecutive generations in a broth containing 0–1.2% NaCl. Identical generation times and viable cell numbers were observed. Furthermore, the nodulation, plant yield and elemental composition ofM. sativa grown on agar slopes responded identically to all inocula, irrespective of the levels of NaCl with which they were grown. The effect of salinity on the ability ofM. sativa to grow and fix nitrogen was tested on agar slopes containing 0–1.2 % NaCl. At 0.0, 0.2 and 0.4 % NaCl the induced fixation was identical as indicated by the constant values of nodulation and plant yield. However, a significant reduction at 0.8 and almost a total suppression at 1.2 % NaCl occurred. Commensurate was the effect of inoculation on the elemental composition ofM. sativa as a function of salinity at the agar medium. The concentration of Mo, Mn, Sr, Cu and Zn, is clearly affected by fixation while salinity has no effect. Their concentration in the inoculated plants is significantly lower compared to the uninoculated at 0–0.4 % NaCl levels, when significant fixation occurred. In contrast, at 0.8 and 1.2 % NaCl their concentration in inoculated and uninoculated plants tends to overlap. On the other hand, the concentration of K, Rb, Br and Cl is affected mainly by salinity. Finally, Ni is affected by neither salinity nor nitrogen fixation.

Particle-solution behaviour of plutonium in an estuarine environment, Esk Estuary, UK

The particle-solution ( K d ) relationships of Pu(III,IV) in the Esk Estuary are investigated, using new experimental data together with a synthesis of our earlier work. Adsorption of 236Pu(IV) by a suspension of intertidal mud (40 mg L −1) from the Esk Estuary was determined after 1 h, as a function of salinity and pH, in a series of controlled laboratory experiments. Desorption of 239, 240Pu(III,IV) from this environmentally contaminated sediment was determined concurrently. The short term (i.e., within a tidal cycle) non-conservative behaviour of Pu(III,IV) in both the laboratory experiments and the estuary appears to be dominated by a rapid, reversible surface complexation reaction, although only a small fraction (<5%) of the 239,240Pu(III,IV) activity of the environmentally contaminated sediment is able to participate in the reaction. The existence of this exchangeable or labile fraction can explain the apparent dependence of Pu(III,IV) K d on sediment concentration. Thus, a single K d value is able to describe both the observed adsorption and desorption behaviour and the effect of sediment concentration, when desorption is expressed with respect to the labile 239,240Pu(III,IV) fraction. For example, in R. Esk water, the adsorption of 236Pu(III,IV) and the desorption of 236,240Pu(III,IV), over a two order of magnitude concentration range of unwashed Esk Estuary sediment, can be accounted for by a single K d of ∼3 × 10 3 L kg −1. The K d for the labile Pu(III,IV) fraction varies mainly as a function of salinity. pH is not an important factor over the pH range 4–9, although H + ion exchange does occur, probably through a similar surface complexation reaction. The kinetic and equilibrium characteristics of the Pu (III,IV) sorption behaviour conform to the pattern observed by Jannasch et al. (1988) and other workers for a range of trace elements.

Biological criteria, environmental health and estuarine macrobenthic community structure

Biological criteria for defining water quality and the presence of acceptable levels of benthic resources are evaluated for estuarine macrobenthic communities of the lower Chesapeake Bay, USA. Models of expected community values as a function of salinity are presented for community biomass, numbers of individuals, species richness, percent biomass of deep-dwelling species, percent biomass of equilibrium species, and percent biomass of opportunistic species. The models presented may serve as, or be used to develop, biological criteria for estuaries. Tidal freshwater and oligohaline regions had the highest variability in model parameters due to patchily distributed, large-sized bivalve species. In the absence of data from pristine habitats, the models were developed from a 5 year data set (1985–1989) for stations considered to be minimally impacted. The models produced were used to evaluate benthic communities of two regions of the Chesapeake Bay—one exposed to summer low dissolved oxygen events (hypoxia/anoxia) and the other characterized by sediments contaminated with heavy metals and polynuclear aromatic hydrocarbons. Stations exposed to stress from either low dissolved oxygen events or contaminated sediments were characterized by 1. reduced community biomass, 2. reduced species richness, 3. less biomass consisting of deep-dwelling species and equilibrium species and 4. more biomass consisting of opportunistic species. Some unstressed habitats can be highly dominated by shallow-dwelling long-lived species, thus dominance of deep-dwelling species in biomass must be used with caution as a biological criterion. The number of individuals per m 2 was highly variable for some stressed stations and this parameter is probably of limited value as a biological criterion characterizing the quality of estuarine habitats. No single method or analysis is likely to produce stress classifications without unacceptable misclassifications. Ecological stress, from any source, is best measured by multiple methods or analyses with different assumptions. The consistency of classification between different approaches would provide the robustness necessary to judge the reliability of a stress classification.

Exchange of Water and Nutrients Between the Skagerrak and the Kattegat

Simultaneous CTD and current measurements have been made repeatedly at several stations in a cross-section between the Danish island Læsø and the Swedish west coast. The cross-section is situated in the northern Kattegat which forms the outer part of the Baltic estuary. Parallel nutrient and oxygen measurements were made in order to obtain a suitable set of background data for ecological modelling of the Kattegat. Here, this dataset is used to calculate fluxes of water, nutrients and oxygen, as functions of salinity. A division is made between the biologically passive winter period, November to February (when the inorganic nutrients components are nearly conservative) and a biologically active 'summer' period from March to October. A striking feature is the much stronger inflow of high salinity deep water during winter, approximately 64 000 m 3 s -1, compared to only 28 000 m 3 s -1 during summer. The inflow is concentrated to salinities higher than 33 PSU. The surface water outflow occurs in the interval 23 to 27 PSU. While the inflow of oceanic deep water to the Kattegat occurs on the deeper eastern side of Læsø, the outflow of surface water from the Baltic seems to be dominated by the flux west of Læsø.

Trace metals in suspended matter from the Rhine/Meuse estuary

The contents of trace metals in suspended matter of the river Rhine have decreased during the last few decades but still exceed background contents. Contamination factors for suspended matter in the river Rhine at Lobith are found to be: Cd (7), Cu (5), Zn (9), Pb (3.5), Ni (2.5) and Cr (3). In the Rhine/Meuse estuary geochemical trace-metal behaviour is influenced by industrial discharges. Budget calculations show that the additional enrichment due to industry is a few percent (0 to 3%) for Cr, Cu, Hg, Ni, Pb and Zn, but is much higher for As (11.5%) and Cd (48.6%). Suspended-matter samples collected in the estuary along a salinity gradient were analysed for major elements and trace metals. The results show a decrease in the suspended-matter contents of Cd, Co, Cu, Cr, Ni, Pb and Zn as a function of salinity. The Sr content shows an increase with salinity. By using Cr as a conservative tracer, the differences in trace-metal contents between freshwater and seawater can be attributed to the physical mixing of relatively contaminated Rhine/Meuse suspended matter with relatively uncontaminated North Sea suspended matter. The results show an enrichment in Mn, Co, Ni and Pb in the samples collected in May 1988 in comparison to the samples collected in March 1989. This may be due to the enhanced oxidation of reduced Mn at higher temperatures. As a result of the formation of Mn-oxyhydroxides the trace metals Ni, Co and Pb are scavenged from the solution.

Effect of salinity on the critical nitrogen concentration of Spartina alterniflora Loisel

Nitrogen was withheld from the salt marsh grass Spartina alterniflora Loisel., in order to determine the effect of salinity (sea salts) on critical tissue nitrogen concentrations (defined here as the minimum tissue concentration required to sustain biomass accumulation). The critical nitrogen concentration per kilogram dry weight of above-ground tissue increased non-linearly from a mean of 8.2 g kg −1 at 5 g l −1 and 20 g l −1 salinity to 13.6 g kg −1 and 22.9 g kg −1 at salinities of 40 g l −1 and 50 g l −1, respectively. Below-ground tissue nitrogen concentrations averaged 62% of the above-ground values irrespective of salinity treatment. These results suggest that the critical nitrogen concentration is a function of salinity and indicate that the internal nitrogen supply required in support of growth increases with salinity. Above-ground tissue nitrogen concentrations reported in the literature and the relationship between salinity and critical nitrogen concentration observed in this study were used to evaluate the nitrogen status of S. alterniflora over a wide range of geographical locations. Comparisons suggest that both short and tall forms of S. alterniflora are nitrogen limited in the majority of marshes along the Gulf and Atlantic Coasts of the US.