Sulphate-reducing bacteria induce low-temperature Ca-dolomite and high Mg-calcite formation

In this study, we demonstrate that sulphate‐reducing bacteria induce anoxic low‐temperature Ca‐dolomite formation both in situ in Lagoa Vermelha and Brejo do Espinho, two neighbouring, dolomite‐precipitating hypersaline lagoons in Brazil, and in laboratory culture experiments. The metabolic activity of sulphate‐reducing bacteria facilitates dolomite formation under anoxic conditions, as demonstrated with experiments using dialysis bags. Overall changes in the chemical conditions of the medium exclusively, without the presence of bacteria, did not result in carbonate precipitation. Only pure cultures of metabolizing sulphate‐reducing bacteria induced Ca‐dolomite and high Mg‐calcite precipitates, indicating that the carbonate nucleation takes place in the locally changed microenvironment around the sulphate‐reducing bacterial cells. Not all pure strains, however, produced Ca‐dolomite under similar conditions, suggesting that the bacterial metabolism, activity and the rate of mineral precipitation have an influence on the type of carbonate formed.     

Desulfovibrio brasiliensis sp. nov., a moderate halophilic sulfate-reducing bacterium from Lagoa Vermelha (Brazil) mediating dolomite formation

A novel halotolerant sulfate-reducing bacterium, Desulfovibrio brasiliensis strain LVform1, was isolated from sediments of a dolomite-forming hypersaline coastal lagoon, Lagoa Vermelha, in the state of Rio de Janeiro, Brazil. The cells are vibrio-shaped and 0.30 to 0.45 m by 1.0 to 3.5 m in size. These bacteria mediate the precipitation of dolomite [CaMg(CO₃)₂] in culture experiments. The strain was identified as a member of the genus Desulfovibrio in the -subclass of the Proteobacteria on the basis of its 16S rRNA gene sequence, its physiological and morphological properties. Strain LVform1 is obligate sodium-dependent and grows at NaCl concentrations of up to 15%. The 16S rRNA sequence revealed that this strain is closely related to Desulfovibrio halophilus (96.2% similarity) and to Desulfovibrio oxyclinae (96.8% similarity), which were both isolated from Solar Lake, a hypersaline coastal lake in the Sinai, Egypt. Strain LVform1 is barotolerant, growing under pressures of up to 370 bar (37 MPa). We propose strain LVform1 to be the type strain of a novel species of the genus Desulfovibrio, Desulfovibrio brasiliensis (type strain LVform1 = DSMZ No. 15816 and JCM No. 12178). The GenBank/EMBL accession number for the 16S rDNA sequence of strain LVform1 is AJ544687.     

Carbonate Mineralogy Along a Biogeochemical Gradient in Recent Lacustrine Sediments of Gallocanta Lake (Spain)

Three sedimentary subenvironments, palustrine (GP), marginal lacustrine (GML) and central lacustrine (GCL), were compared regarding water chemistry and microbial activity in order to explain the differences in the carbonate mineralogical composition of the upper sediment layer in Gallocanta Lake, a shallow hypersaline environment in Northeastern Spain. Horizontal heterogeneity was considerable, salinity ranged from 5 to 116 (‰) for the GP and GCL subenvironments respectively. Sulfate, Mg ^{2 +} , and Ca ^{2 +} concentrations covaried among them and with salinity. The relative abundance of Mg-bearing carbonates, including high-Mg calcite, dolomite and hydrated Ca-magnesite, increased with the salinity. They were absent from the GP subenvironment, where only calcite precipitates, and maximum abundances were found in the GCL subenvironment (61%), where salinity, sulfate, and Mg ²⁺ concentrations were highest. Every subenvironment presented specific microecological characteristics. The microbial community of the GCL subenvironment lacked of oxygenic photosynthesis, while the microbial communities of GML and GP subenvironments were photosynthetically active. Vertical profiles of sulfide and pH at the water-sediment interface revealed clear differences between the GCL and GML subenvironments as well. Sulfide was detected below the oxic layer in the GCL subenvironment and increased with depth, but it was undetected in the GML subenvironment. The precipitation of Mg-bearing carbonates with different Mg:Ca proportions occurs at different stage along a biogeochemical gradient, where increasing salinity and sulfate content favour the anaerobic oxidation of organic carbon by dissimilatory sulfate reduction.     

Interpretation of sulfur cycling in two catchments in the Black Forest (Germany) using stable sulfur and oxygen isotope data

The isotopic composition of SO ₄ ^2- in bulk precipitation, canopy throughfall, seepage water at three different soil depths, stream water, and groundwater was monitored in two forested catchments in the Black Forest (Germany) between November 1989 and February 1992. Isotope measurements on aqueous sulfate were complemented by ³⁴S-analyses on SO₂ in the air, total sulfur and inorganic sulfate in the soil, and bedrock sulfur, in order to identify sources and biogeochemical processes affecting S cycling in catchments with base poor, siliceous bedrock. Stable S isotope data indicated that atmospheric deposition and not mineral weathering is the major source of S in both catchments since ³⁴S-values for sulfate in the soil, in seepage water, and in stream water were generally found to be similar to the mean ³⁴S-values of precipitation SO ₄ ^2- (+2.1. However, ¹⁸O-values of seepage water SO ₄ ^2- at 30 cm and especially at 80 cm depth were depleted by several per mil with respect to those of the atmospheric deposition (+7.5 to +13.5. This indicates that in both catchments a considerable proportion of the seepage water SO ₄ ^2- is derived from mineralization of carbon-bonded soil S and must therefore have cycled through the organic soil S pool. ³⁴S-values for different S compounds in the solid soil were found to differ markedly depending on S fraction and soil depth. Since atmospheric S deposition with rather constant ³⁴S-values was identified as the dominant S source in both catchments, this is interpreted as a result ofin situ isotope fractionation rather than admixture of isotopically different S. The differences between the ³⁴S-values of seepage water and soil sulfate and those of organic soil S compounds are consistent with a model in which SO ₄ ^2- uptake by vegetation and soil microorganisms favours³⁴SO ₄ ^2- slightly, whereas during mineralization of organic soil S to aqueous SOSO ₄ ^2- ,³²S reacts preferentially. However, the data provide evidence for negligible isotope fractionation during physico-chemical S transformations such as adsorption/desorption in aerated forest soils.     

Effects of sulfate concentration in the overlying water on sulfate reduction and sulfur storage in lake sediments

We investigated the effects of sulfate concentration on sulfate reduction and net S storage in lake sediments using³⁴S as a tracer. The water overlying intact sediment cores from the hypolimnion of Mares Pond, MA, was replaced with two Na₂ ³⁴SO₄ solutions at either ambient (70 M) or elevated (260 M) sulfate concentrations. The ³⁴S of the added sulfate was 4974 . Over two months, the net sulfate reduction rate in the ambient sulfate treatment was zero, while the net rate for the high sulfate treatment was 140 moles/m²/d. The water overlying the cores was kept under oxic conditions and the sediment received no fresh carbon inputs, thus the net rate reported may underestimate the in situ rate. Gross sulfate reduction rates calculated by isotope dilution were approximately 350 moles/m²/d for both treatments. While the calculation of gross sulfate reduction rates in intact sediment cores can be complicated by differential diffusion of³⁴S and³²S, isotopic fractionation, and the possible formation of ester sulfates, we believe these effects to be small. The results suggest that sulfate reduction is not strongly sulfate-limited in Mares Pond. The difference in net sulfate reduction rates between treatments resulted from a decrease in sulfide oxidation and suggests the importance of reoxidation in controlling net S storage in lake sediments. In both treatments the CRS and organic S fractions were measurably labelled in³⁴S. Below the sediment surface, the CRS fraction was the more heavily labelled storage product for reduced sulfides.     

Phosphate and calcium carbonate saturation in a stratified coastal lagoon

Seasonal variation of phosphate concentration and saturation index for calcite in water of a small stratified coastal lagoon have been studied. In surface waters, where salinity was low and pH high, the saturation index increased to values near 20, whereas in bottom water, with high salinity and low pH, they were usually lower. The ionic product for H₃PO₄ was strongly correlated with the ionic product of Ca(OH)₂ in surface and bottom waters, and with the ionic product of CaCO₃ in bottom, which suggested that chemical composition was mainly controlled by a calcium-phosphate solid phase.The low concentrations of phosphate in surface were due to chemical precipitation and organic sedimentation, whereas in bottom, calcium phosphate redissolved and organic matter was mineralized producing high concentrations of soluble phosphate (> 60 µmol l^–1).Decrease of calcium-bound phosphate in the upper layers of sediment was in agreement with a diminution of calcium-phosphate precipitation, probably due to a lower influence of seawater in the past.     

Temporal Changes in a Pink Feedlot Lagoon

Temporal changes in a feedlot drainage lagoon with a predominance of the phototrophic purple sulfur bacterium, Thiopedia rosea, were investigated during a 3-year period. The surface protein and bacteriochlorophyll concentrations, which indirectly measure T. rosea abundance, peaked annually during the fall months and coincided with the intensity of pink coloration. Surface bacteriochlorophyll concentration correlated with pH, alkalinity, and protein. The pH range was optimal for the survival of T. rosea. Surface sulfide concentration, which increased over the winter and early spring, reached low levels during the fall months. The most striking pattern to emerge was the marked increase in sulfate concentration that occurred each fall and winter. The protein peaks, which preceded the sulfate peaks, were indicative of the sulfate concentrations that would follow. During 1977 and 1978, the lagoon was essentially anaerobic and provided adequate growth conditions for T. rosea. Above-average precipitation during early 1979 raised the water level and altered the chemistry of the lagoon. Dissolved oxygen was higher during the final year, and, concurrently, concentrations of bacteriochlorophyll declined. Aeration of the lagoon resulted in a decrease in T. rosea.     

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quereus gambelii Nutt)

Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (D) and oxygen (¹⁸O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem D and ¹⁸O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.     

Sulfur isotopes in sulfate in the inputs and outputs of a Canadian Shield watershed

The isotopic composition of sulfur in sulfate from precipitation, stream inflows and outflow of the Lake 239 watershed at the Experimental Lakes Area, northwestern Ontario, was monitored for four years, 1982–1985. Rainfall had ³⁴S of 3–9and a three year mean value weighted by the mass of sulfate of 4.8 Sulfur in streams ranged from 5–15 and displayed a pronounced seasonal pattern in which ³⁴S showed significant negative correlations with both streamflows and sulfate concentrations. Outflow values were much more constant and averaged 6.4 over the three year period. Sulfate reduction or sorption favoring retention of the lighter isotope in the watershed was the probable cause of the isotopic variation of the sulfur isotopes in the streams. A sulfur isotope budget for the lake balanced within 1     

A recently landlocked brackish-water lagoon of Lake Tanganyika: physical and chemical characteristics,and spatio-temporal distribution of phytoplankton

Monthly measurements of physical and chemical characteristics were made at two localities in the eastern part of a recently landlocked lagoon of Lake Tanganyika. Variables analysed were: temperature, pH, conductivity, sodium, potassium, magnesium, calcium, carbonate, bicarbonate, chloride and sulphate. Large seasonal fluctuations of salinity were recorded (1.68–8.21 g l^–1). The seasonal water input controlled algal seasonality mainly through its effect on salinity and indirectly through its influence on nutrient concentration by dissolution and dilution of the excrements of the numerous cattle and other organic matter. Phytoplankton was mainly composed of Cyanophyta and Euglenophyta. Euglenophyta dominated during the dry periods with high salinity and probably very high nutrient levels, while Cyanophyta preferred moderate salt and nutrient concentrations during the rainy periods. The phytoplanktonic community was composed of a large number of perennial and a reduced quantity of annual organisms.A spatial study of the recently landlocked lagoon revealed an ascending salinity gradient, principally due to a sodium bicarbonate/carbonate enrichment, between locations near the lake and more inland situated stations. These facts point to a lake water supply and a salt concentration by evaporation in the swamps. Proportionally lower magnesium, calcium and potassium values were recorded at high salinities, due to chemical precipitation and biotic factors.A blue-green algal bloom was observed in the eastern water-body (salinity: 4.64 g l^–1); simultaneously an important development of diatoms dominated the western water-body (salinity: 2.18 g l^–1). No significant differences in morphometry, exposure, water temperature or nutrient levels (nitrate, nitrite, ammonia, orthophosphate) were observed. The relatively low salinity and high nutrient concentration in the western water-body probably favoured diatom development during the rainy season. The relatively higher salinity in the eastern water-body during the rainy season was probably responsible for the dominance of blue-green algae through its negative influence on silica concentration and notwithstanding the high inorganic nitrogen concentration.     

Water and nitrogen dynamics in an arid woodland

Arid environments are characterized by spatial and temporal variation in water and nitrogen availability. differences in ¹⁵N and D of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil ¹⁵N. The spatial variation in soil ¹⁵N caused corresponding changes in plant ¹⁵N such that plant ¹⁵N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower ¹⁵N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus andArtemisia) relied on the episodic availability of water and nitrogen at the soil surface. ¹⁵N values did not vary through the year, while xylem pressure potentials and stem-water D values fluctuated with changes in soil moisture at the soil surface. In contrast,Chrysothamnus switched to a more stable water and nitrogen source during drought. ¹⁵N values ofChrysothamnus increased throughout the year, while xylem pressure potentials and stem-water D values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drougnt.     

Methanogenesis produces strong 13C enrichment in stromatolites of Lagoa Salgada,Brazil: a modern analogue for Palaeo‐/Neoproterozoic stromatolites?

Holocene stromatolites characterized by unusually positive inorganic δ¹³C_PDB values (i.e. up to +16‰) are present in Lagoa Salgada, a seasonally brackish to hypersaline lagoon near Rio de Janeiro (Brazil). Such positive values cannot be explained by phototrophic fixation of CO₂ alone, and they suggest that methanogenesis was a dominating process during the growth of the stromatolites. Indeed, up to 5 mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing δ¹³C values between ?15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, ¹³C‐depleted hopanoids diplopterol and 3β‐methylated C₃₂ 17β(H),21β(H)‐hopanoic acid (both ?40‰) are preserved in lagoon sediments and are most likely derived from aerobic methanotrophic bacteria thriving in the methane‐enriched water column. Loss of isotopically light methane through the water column would explain the residual ¹³C‐enriched pool of dissolved inorganic carbon from where the carbonate constituting the stromatolites precipitated. The predominance of methanogenic archaea in the lagoon is most likely a result of sulphate limitation, suppressing the activity of sulphate‐reducing bacteria under brackish conditions in a seasonally humid tropical environment. Indeed, sulphate‐reduction activity is very low in the modern sediments. In absence of an efficient carbonate‐inducing metabolic process, we propose that stromatolite formation in Lagoa Salgada was abiotically induced, while the ¹³C‐enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, ¹³C‐enriched stromatolitic deposits also appear in the geological record of prolonged periods in the Palaeo‐ and Neoproterozoic. Lagoa Salgada represents a possible modern analogue to conditions that may have been widespread in the Proterozoic, at times when low sulphate concentrations in sea water allowed methanogens to prevail over sulphate‐reducing bacteria.     

Estimating denitrification rates in estuarine sediments: A comparison of stoichiometric and acetylene based methods

We compared denitrification rates obtained using an adaptation of the acetylene block technique to rates estimated from benthic flux nutrient stoichiometry in the subtidal sediments of Tomales Bay, California (USA). By amending whole cores with acetylene and saturating nitrate concentrations, we obtained potential denitrification rates, which ranged between 4 and 30 mmol N m^–2 d^–1. We determined the apparent Michaelis constant (K_app) and the maximum potential rate (V_mp) of the denitrifying community and used these constants in a rectangular hyperbola to estimatein situ denitrification rates. Both the K_app and V_mp of the denitrifying community exhibited significant variation over both depth in the sediment column and time of sampling.Estimates ofin situ denitrification obtained using our kinetic-fix adaptation of the acetylene block ranged between 1.8 (March) and 9 (Sept.) mmol N m^–1 d^–1. Denitrification rates obtained using benthic flux stoichiometry ranged between 0.7 and 4.1 mmol N m^–2 d^–1. Average denitrification rates obtained using the kinetic-fix acetylene block approach exceeded those obtained from net benthic flux stoichiometry; however, these differences were not significant. We conclude that our kinetic-fix adaptation of the acetylene block technique provides realistic estimates of denitrification in sediments, even when pore water nitrate concentrations are low and nitrification and denitrification are closely coupled.     

Fish and fisheries of the Muni lagoon in Ghana, West Africa

Muni is a closed lagoon that opens to the sea when the water level is high, especially during the rainy season. During the dry period, the water level is very low and the salinity of the lagoon water increases sometimes exceeding that of seawater. Only three finfish species and 8 shellfish species (live animals and empty shells) were found in the lagoon and surrounding wetlands during the study which was conducted in March–June 1994. The blackchin tilapia Sarotherodon melanotheron (Cichlidae) is the dominant species. The castnet is the most important fishing gear used in the lagoon followed by various finfish and shellfish traps. The fishing effort in the lagoon is high and, like catch rates, it was found to be inversely related to the volume of water in the lagoon and the fishing situation in the sea. The estimated exploitation level (E = 0.65) indicates that S. melanotheron in the Muni lagoon could be considered as over-exploited. From the gonadosomatic index, S. melanotheron appears to spawn continuously in the Muni lagoon between the months of March and June. The von Bertalanffy growth parameters were estimated for S. melanotheron as L = 12.5 cm standard length (SL), K = 0.70 yr^–1 and t₀ = 0.01 yr. The longevity of the fish is estimated as ca. 4.2 yr. Taboos and cultural practices contribute to the management and conservation of fisheries in the lagoon. However, additional formal management is required to ensure sustainability of the fisheries in the lagoon. In addition, there is a need to manage the lagoons physical and chemical environment including planting of mangroves around the lagoon in order to restore its mangrove cover. The poverty problem in the fishing community needs to be addressed to divert labour from the lagoon.     

Holocene palaeoenvironment in a former coastal lagoon of the arid south eastern Iberian Peninsula: salinization effects on δ<Superscript>15</Superscript>N

The palaeoenvironment of a former coastal lagoon in the south eastern Iberian Peninsula (San Rafael, Almeria, Spain) were inferred from one core analyzed for particulate organic matter content (POM) together with its C/N, δ¹³C, δ¹⁵N to depict the biogeochemical record from the Late Glacial to the Holocene. The results, complemented by previously reported pollen assemblages, indicate the appearance of a freshwater lagoon at 7300 b.p. (uncalibrated ¹⁴C age), its salinization at 6200 b.p. and its disappearance at 4400 b.p. The period of existence of the lagoon coincided with a period of wetter conditions as inferred from terrestrial vegetation. The lagoon’s salinization was not related to a decrease in precipitation but to a stronger maritime influence since there were no parallel changes in terrestrial vegetation. Salinization caused an increase in δ¹³C, associated with a higher relative presence of C₄ plants, and an increase in δ¹⁵N, due to a decrease in plant N demand. The late period of the lagoon, from about 5100 to 4400 b.p., shows a progressive drying and salinization not detected in isotopes but reflected in a decrease in POM, and in the pollen records. Increases in δ¹⁵N were related to increases in salinity within the lagoon, and are indicative of a more open N cycle, because the absence of changes in terrestrial vegetation rules out changes in the catchment area as the cause for changes in δ¹⁵N.     

Estimation of denitrification potential in a karst aquifer using the <Superscript>15</Superscript>N and <Superscript>18</Superscript>O isotopes of NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>

A confined aquifer in the Malm Karst of the Franconian Alb, South Germany was investigated in order to understand the role of the vadose zone in denitrifiaction processes. The concentrations of chemical tracers Sr²⁺ and Cl^– and concentrations of stable isotope ¹⁸O were measured in spring water and precipitation during storm events. Based on these measurements a conceptual model for runoff was constructed. The results indicate that pre-event water, already stored in the system at the beginning of the event, flows downslope on vertical and lateral preferential flow paths. Chemical tracers used in a mixing model for hydrograph separation have shown that the pre-event water contribution is up to 30%. Applying this information to a conceptual runoff generation model, the values of ¹⁵N and ¹⁸O in nitrate could be calculated. Field observations showed the occurence of significant microbial denitrification processes above the soil/bedrock interface before nitrate percolates through to the deeper horizon of the vadose zone. The source of nitrate could be determined and denitrification processes were calculated. Assuming that the nitrate reduction follows a Rayleigh process one could approximate a nitrate input concentration of about 170 mg/l and a residual nitrate concentration of only about 15%. The results of the chemical and isotopic tracers postulate fertilizers as nitrate source with some influence of atmospheric nitrate. The combined application of hydrograph separation and determination of isotope values in ¹⁵N and ¹⁸O of nitrate lead to an improved understanding of microbial processes (nitrification, denitrification) in dynamic systems.     

Stable isotopic signature of El Niño-Southern Oscillation events in eastern tropical Pacific reef corals

In this study we measured stable oxygen and carbon isotope ratios in the skeletons of massive reef-building corals (Porites lobata and Pavona gigantea) at four widespread locations in the tropical eastern Pacific, each with a unique marine climate. Annual variation in sea surface temperature (SST) varied from 5–7°C at upwelling sites (Gulf of Panamá and Galápagos Islands) to 2–3°C where upwelling was absent [Caño Isand (Costa Rica) and Gulf of Chiriquí (Panamá)]. Annual range in salinity was high in the gulfs of Panamá and Chiriquí (up to 15) and low at Cãno Island and Galápagos (2–3). We characterize the isotopic signatures of recent (15–40 y long records) El Niño/Southern Oscillation (ENSO) events, particularly the very severe 1982–1983 event. Subannual sampling at 1–2 month resolution reveals that the ¹⁸O signal at Caño Island records strong to very strong ENSOs. In the Gulf of Chiriquí, this signal is governed mainly (80%) by salinity and thus is a weak recorder of ENSO events: only the 1951–1952, 1957–1958 and 1972–1973 events appear as significant ¹⁸O anomalies over the period 1940–1984. In the Gulf of Panamá, high variation in both SST and salinity contribute to the ¹⁸O signal. ENSO events at this site are poorly recorded, probably due to ameliorating effects of cool upwelled water during the early stage of the ENSO event. The ¹⁸O record in Galápagos, however, shows a strong correlation with SST and accurately records all but the most severe 1982–1983 ENSO event. Thus, ENSOs are most clearly recorded at sites where salinity variation is minimal. At Caño only strong ENSOs are recorded while mild to strong events appear in the Galápagos record. Nowhere did the ¹⁸O signal accurately record the full range of temperatures that occurred during the 1982–1983 ENSO; however, a stress band on the coral skeleton was evident at all sites. By comparing the ¹⁸O records and skeletal features across sites it may be possible to identify the occurrence of strong to very severe ENSOs prior to instrumental records. The relationship between ¹⁸O and ¹³C was examined and found to be significantly positive and in phase (i.e., either depleted or enriched at the same time) at three of the four sites studied. Weak correlations at some sites may be explained by high variability in water column clarity leading to depleted ¹³C when waters are cool and surface isolation high.     

Biological,chemical and hydrological characters of coastal lagoons of Ghana,West Africa

Two types of lagoon which occur along the West Coast of Africa described as open and closed, were studied using oceanographic methods. The parameters were temperature, salinity, dissolved oxygen and productivity. It was observed that rainfall, evaporation and the tides have a combined effect on the variations in the temperature and the salinity. The opening of the lagoons into the sea affects the level of the productivity and the rate at which salinity increases with time.Minimum temperatures at the heads of the lagoons were found to be lower than those near the ocean, while the maximum temperatures were higher at the heads. During the dry period, due to continuous evaporation of water, the salinity in the closed lagoon usually rises steeply, while those in the open lagoon rise gently.The levels of dissolved oxygen concentration in the open lagoon are higher than those in the closed lagoon. This is the result of the fact that phytoplankton count in the open lagoon is higher than that in the closed lagoon. There is also a greater diversity of phytoplankton and more live phytoplankton cells per cubic centimetre of water in the open lagoon than in the closed lagoon.As a result of all this, the main organic productivity in the open lagoon is higher than those in the closed lagoon.     

Population Dynamics of a Single-Stage Sulfidogenic Bioreactor Treating Synthetic Zinc-Containing Waste Streams

Waste streams from industrial processes such as metal smelting or mining contain high concentrations of sulfate and metals with low pH. Dissimilatory sulfate reduction carried out by sulfate-reducing bacteria (SRB) at low pH can combine sulfate reduction with metal-sulfide precipitation and thus open possibilities for selective metal recovery. This study investigates the microbial diversity and population changes of a single-stage sulfidogenic gas-lift bioreactor treating synthetic zinc-rich waste water at pH 5.5 by denaturing gradient gel electrophoresis of 16S rRNA gene fragments and quantitative polymerase chain reaction. The results indicate the presence of a diverse range of phylogenetic groups with the predominant microbial populations belonging to the Desulfovibrionaceae from δ-Proteobacteria. Desulfovibrio desulfuricans-like populations were the most abundant among the SRB during the three stable phases of varying sulfide and zinc concentrations and increased from 13% to 54% of the total bacterial populations over time. The second largest group was Desulfovibrio marrakechensis-like SRB that increased from 1% to about 10% with decreasing sulfide concentrations. Desulfovibrio aminophilus-like populations were the only SRB to decrease in numbers with decreasing sulfide concentrations. However, their population was <1% of the total bacterial population in the reactor at all analyzed time points. The number of dissimilatory sulfate reductase (DsrA) gene copies per number of SRB cells decreased from 3.5 to 2 DsrA copies when the sulfide concentration was reduced, suggesting that the cells' sulfate-reducing capacity was also lowered. This study has identified the species present in a single-stage sulfidogenic bioreactor treating zinc-rich wastewater at low pH and provides insights into the microbial ecology of this biotechnological process.     

Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2

The stable isotopic composition of soil water is controlled by precipitation inputs, antecedent conditions, and evaporative losses. Because transpiration does not fractionate soil water isotopes, the relative proportions of evaporation and transpiration can be estimated using a simple isotopic mass balance approach. At our site in the shortgrass steppe in semi-arid northeastern Colorado, ¹⁸O values of soil water were almost always more enriched than those of precipitation inputs, owing to evaporative losses. The proportion of water lost by evaporation (E/ET) during the growing season ranged from nil to about 40% (to >90% in the dormant season), and was related to the timing of precipitation inputs. The sum of transpiration plus evaporation losses estimated by isotopic mass balance were similar to actual evapotranspiration measured from a nearby Bowen ratio system. We also investigated the evapotranspiration response of this mixed C₃/C₄ grassland to doubled atmospheric [CO₂] using Open-Top Chambers (OTC). Elevated atmospheric [CO₂] led to increased soil-water conservation via reduced stomatal conductance, despite greater biomass growth. We used a non-invasive method to measure the ¹⁸O of soil CO₂ as a proxy for soil water, after establishing a strong relationship between ¹⁸O of soil CO₂ from non-chambered control (NC) plots and ¹⁸O of soil–water from an adjacent area of native grassland. Soil–CO₂ ¹⁸O values showed significant treatment effects, particularly during a dry summer: values in ambient chambers (AC) were more enriched than in NC and elevated chamber (EC) plots. During the dry growing season of 2000, transpiration from the EC treatment was higher than from AC and lower than from NC treatments, but during 2001, transpiration was similar on all three treatments. Slightly higher evaporation rates from AC than either EC or NC treatments in 2000 may have resulted from increased convection across the soil surface from the OTC blowers, combined with lower biomass and litter cover on the AC treatment. Transpiration-use efficiency, or the amount of above-ground biomass produced per mm water transpired, was always greatest on EC and lowest on NC treatments.