The influence of salinity on the kinetics of NH inf4 sup+ uptake in Spartina alterniflora

Summary The effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NH _inf4 ^sup+ uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent K_m but did not significantly affect V_max (mean V_max=4.23±1.97 mole·g^–1·h^–1). The apparent K_m increased in a nonlinear fashion from a mean of 2.66±1.10 mole/L at a salinity of 5 g/L to a mean of 17.56±4.10 mole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5–50 g/L was a competitive inhibition of NH _inf4 ^sup+ uptake in S. alterniflora. No significant NH _inf4 ^sup+ uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both V_max and K_m. Reduction of solution salinity from 35 to 5 g/L did not change V_max but reduced K_m by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in V_max of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NH _inf4 ^sup+ . For both experiments, increasing salinity led to an increase in the apparent K_m similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH _inf4 ^sup+ uptake kinetics.     

The effect of evapoconcentration on dissolved organic carbon concentration and quality in lakes of SW Greenland

1. Dissolved organic carbon (DOC) concentration was determined for a range of lakes of varying conductivity (30–4000 μS cm⁻¹) in the low Arctic of SW Greenland. DOC concentration range from <1 to >100 mg C L⁻¹, occasionally approaching 200 mg C L⁻¹ in meromictic, oligosaline lakes. DOC concentration was strongly related to [log₁₀] conductivity and total nitrogen. 2. Peak DOC concentrations (>80 mg L⁻¹) occur in lakes located approximately 50 km from the present ice sheet margin, a zone of low effective precipitation; evaporative concentration is the first‐order control on DOC concentration. Lakes at the coast and closer to the ice margin had lower DOC concentrations (<20 mg C L⁻¹). Local factors, notably the presence or absence of an outflow and catchment morphometry, resulted in considerable variability in concentration (20–100 mg C L⁻¹) within the area of maximum concentration around 51°W. 3. Despite their high DOC concentration, these lakes are essentially colourless. Dissolved organic matter (DOM) absorption (a₃₇₅) was low in most lakes (<10 m⁻¹) with maximum values (approximately 20 m⁻¹) occurring in one humic‐stained lake in the area. Absorption values corrected for DOC concentration () were very low (<0.6 m² g⁻¹ C) for all lakes apart from those at the coast, perhaps reflecting greater allochthonous inputs at these sites. 4. S, the spectral slope coefficient, ranged from 16 to 27 μm⁻¹ and was weakly correlated with DOC concentration. Both a₃₇₅ and S showed similar distribution patterns along the sampling gradient as did DOC, with maximum values at approximately 51°W. High and low S may indicate fresher, more rapidly flushed, systems with less degraded DOM or greater inputs from their catchments. 5. The lakes closer to the head of the fjord with higher conductivity, had low (<0.2 m² g⁻¹ C) and high S (>21 μm⁻¹) and this may reflect increasingly longer lake water residence times, greater DOM age and photochemical degradation.     

Distribution and regulation of urea in lakes of central North America

1. Urea accounts for ～50% of global nitrogen (N)‐based fertiliser; however, little is known of the factors regulating its distribution and abundance in freshwaters. Improved understanding of urea biogeochemistry is essential because its use as fertiliser is expected to double by 2050 and because pollution with urea can promote outbreaks of toxic cyanobacteria in phosphorus (P)‐rich lakes in regions with intensive agricultural or urban development. 2. Biweekly measurements of urea concentration and diverse limnological variables (water chemistry, hydrology, algae, zooplankton) were taken during two summers (2008, 2009) in a chain of seven productive lakes within a 52 000‐km² catchment in central Canada to quantify environmental and anthropogenic correlates of temporal and spatial patterns of urea occurrence. 3. Mean (±SD) urea concentrations varied between 29 ± 14 and 132 ± 65 μg N L^?1, generally increased from headwater to downstream sites and represented 10–50% of bioavailable N (as sum of , and urea). Principal components analysis demonstrated that urea concentrations were elevated in agriculturally impacted lakes with abundant dissolved organic and inorganic nutrients (N, P, C) and low O₂ concentrations, but were not correlated consistently with plankton abundance or community composition. Urea concentrations were more than twofold greater in lakes receiving N from cities than in agriculturally affected basins, despite low summer concentrations of urea in tertiary‐treated urban effluent (c. 50% of lake values). Multiple regression models evaluated using Akaike Information Criterion showed that mean water‐column O₂ concentration was the single best predictor of in situ urea concentrations (r² = 0.91, P = 0.002), but that urea concentrations were also correlated significantly with changes in longitudinal position and Secchi depth and with concentrations of , non‐urea dissolved organic N (DON) and dissolved inorganic carbon. 4. Additional seasonal surveys of up to 69 closed‐basin lakes within a 100 000‐km² region during 2004 and 2008 revealed that urea was abundant in 100% of measured sites and exhibited concentrations (81 ± 48 μg N L^?1) similar to those observed in lakes with surface drainage (58 ± 38 μg N L^?1). Further, non‐urea DON accounted for 50–99% of the total dissolved N pool in both open‐ and closed‐basin lakes. 5. When combined with an extensive literature review and previous mass‐budget analyses of the study lakes, these findings allowed the development of a first‐generation model of the mechanisms regulating urea content of P‐rich lakes of central North America. In this model, water‐column concentrations of urea are predicted to be regulated mainly by algal decomposition in anoxic environments (sediments, hypolimnion), followed by redistribution into surface waters. Consequently, anthropogenic activities can increase the urea content of lakes by stimulating primary production, sedimentation and deepwater anoxia and by increasing influx of undegraded urea from agricultural and urban sources.     

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the classical models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.In contrast, for lakes where the external loading was reduced, the measured internal loading explains most of the variation in the summer lake concentration. The external loading is of minor importance and the classical models cannot be applied. The internal loading measured before reduction of the external loading is not useful in predicting the concentration afterwards. Instead of the internal loading, the sediment composition can be used. The advantage of using sediment composition is that these variables are easier to determine and vary less in time. The most promising variable is the ratio between total P and total Fe in the sediment.Abbreviations: Q_s hydraulic loading (m y^-1) - hydraulic retention time (y) - L_ext external phosphorus loading (gP m^-2 y^-1) - L_int internal phosphorus loading (gP m^-2 y^-1) - P_lake phosphorus concentration in the lake (gP m^-3) - P_inlet phosphorus concentration in the inlet water (gP m^-3) - P_sed phosphorus content on the sediment (gP kg^-1 d.w.) - Fe_sed iron content of the sediment (gFe kg^-1 d.w.) - Y dependent variable multiple regression calculations - X1, X2 independent variables multiple regressions calculations - a, a1 constants - a2, b constants     

Spermidine application enhances tomato seedling tolerance to salinity-alkalinity stress by modifying chloroplast antioxidant systems

The purpose of this study was to elucidate whether exogenous spermidine (Spd) protection of tomato (Solanum lycopersicum L.) seedlings under salinity-alkalinity stress is associated with antioxidant enzymes in the chloroplast. The effects of exogenous Spd on antioxidant enzyme activity and antioxidant content in the chloroplast were evaluated in seedlings of salt-sensitive ecotype (Zhongza 9) grown in a 75 mM salinity-alkalinity solution, with or without 0.25 mM Spd foliar spraying. Results showed that salinity-alkalinity stress increased MDA content, superoxide anion O₂^?- generation rate, superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) activities and ratio of AsA/DHA and reduced contents of ascorbate (AsA), dehydroascorbate (DHA), AsA+DHA, glutathione (GSH), oxidized glutathione (GSSG), GSH+GSSG, dehydroascorbate reductase (DHAR) activity and ratio of GSH/GSSG in chloroplasts. The exogenous Spd application combined with salinity-alkalinity stress decreased the O₂^?- generation rate and MDA content compared to salinity-alkalinity stress alone. The exogenous Spd also increased AsA-GSH cycle components and increased all antioxidant enzyme activities in most cases. Therefore, exogenous Spd alleviates salinity-alkalinity stress damage using antioxidant enzymes and non-enzymatic systems in chloroplasts.     

Hydrochemical fluctuations and crustacean community composition in an ephemeral saline lake (Sua Pan,Makgadikgadi Botswana)

Fluctuating hydrochemistry, as a result of extreme hydrological regimes, imposes major physiological constraints on the biota of ephemeral saline lakes. While the inverse relationship between salinity and zooplankton species richness is well-known across salinity gradients, few studies have documented closely the response of zooplankton to seasonal changes in salinity. Weekly sampling during two flood seasons at Sua Pan, an intermittent saline lake in central Botswana demonstrated the importance of spatial and temporal salinity gradients for crustacean community composition, associated with a decline in species richness, from 11 to three species. Conductivity ranged between 320 and 125,800 μS cm⁻¹ during seasonal flooding; changing from dominance by and , Ca²⁺ and Mg²⁺, at the beginning of the floods, to NaCl dominated waters as the lake dried out and salinities increased. pH estimates generally ranged between 8.6 and 10, with maximum values recorded during initial flooding. Crustaceans comprised mainly Branchinella spinosa, Moina belli, Lovenula africana and Limnocythere tudoranceai, all of which occurred across a wide range of salinities, while halotolerant freshwater species (Metadiaptomus transvaalensis, Leptestheria striatochonca and the ostracods Plesiocypridopsis aldabrae, Cypridopsis newtoni and a newly identified Potamocypris species) disappeared above conductivities of 1,500 μS cm⁻¹. A unique crustacean composition in southern Africa was attributed to Sua Pans’ rare chemical composition among southern African saline lakes; flood waters on Sua Pan contained a higher proportion of Na⁺ and , and less K⁺, Mg²⁺ and than over 80% of records from salt pans elsewhere in southern African. The freshwater species of crustaceans in Sua Pan were similar to those found in other southern Africa lakes, and these similarities decreased in lakes with higher pH and proportions of Na, and less SO₄ and Mg in their chemical composition. The predominant saline tolerant species on Sua Pan, however, showed a greater similarity to those in saline lakes in southern and East Africa with higher proportions of and, particularly, Mg²⁺ in their chemical composition. Handling editor: J. M. Melack     

Chinese and Mongolian saline lakes: a limnological overview

More than half of China's lakes are saline (viz. have salinities > 3 g L⁻¹). Most salt lakes are in northwestern China (Tibet, Qinghai, Sinkiang, Inner Mongolia). Most Mongolian salt lakes are in the west of that country. Tectonic movements have been of the greatest importance in lake origins, but aeolian activity and deflation have also played a role. Many salt lakes in Qinghai-Tibet lie at altitudes > 4 000 m.a.s.l.; Aiding Hu (Sinkiang) lies at −154 m.a.s.l. Again, many lakes are large in area and deep. Small, shallow lakes are also common. Dimictic thermal patterns prevail in deep lakes, polymictic patterns in shallow ones. The highest salinity recorded is 555 g L⁻¹. The salinity of Qinghai Lake, the largest Chinese salt lake, is 14 g L⁻¹, but mean lake salinity on the northern Tibetan plateau is about an order of magnitude greater. Lop Nor has a salinity of ∼ 5 g L⁻¹. Dominant ions are Na and Cl; Mg, Ca, SO₄ and HCO₃ + CO₃ are important in certain lakes. Most major ions originate by weathering and leaching from rocks. pH values are generally high (often > 9.0). There are no bird or fish species confined to salt lakes, though many are associated with lakes of low or moderate salinity. Artemia occurs widely inland and in coastal salt pans, but is the only major macroinvertebrate of highly saline lakes. In lakes of only low to moderate salinity, invertebrate communities comprise widespread halotolerant freshwater forms and halophiles, some regionally endemic. Submerged and emergent macrophytes occur in lakes of low salinity, but phytoplankton species are more halotolerant. Ctenocladus circinatus, a green alga, is known from a Tibetan salt lake with a salinity of 200 g L⁻¹. There is a dearth of basic limnological information on Chinese and Mongolian salt lakes. More work in particular is needed on a variety of geographically widespread lakes to (a) document seasonal physico-chemical events, and (b) compile comprehensive biological inventories of taxa present. Chinese salt lakes are significant sites for palaeoclimatic research, for conservation purposes, and for the resolution of several important biological questions (especially of an ecological and biogeographical sort). They also have important economic values. Unfortunately, the natural existence of many appears to be threatened by decreased inflows, largely the result of human impact on catchments.     

Prokaryotic Community Structure Driven by Salinity and Ionic Concentrations in Plateau Lakes of the Tibetan Plateau

The prokaryotic community composition and diversity and the distribution patterns at various taxonomic levels across gradients of salinity and physiochemical properties in the surface waters of seven plateau lakes in the Qaidam Basin, Tibetan Plateau, were evaluated using Illumina MiSeq sequencing. These lakes included Lakes Keluke (salinity, <1 g/liter), Qing (salinity, 5.5 to 6.6 g/liter), Tuosu (salinity, 24 to 35 g/liter), Dasugan (salinity, 30 to 33 g/liter), Gahai (salinity, 92 to 96 g/liter), Xiaochaidan (salinity, 94 to 99 g/liter), and Gasikule (salinity, 317 to 344 g/liter). The communities were dominated by Bacteria in lakes with salinities of <100 g/liter and by Archaea in Lake Gasikule. The clades At12OctB3 and Salinibacter, previously reported only in hypersaline environments, were found in a hyposaline lake (salinity, 5.5 to 6.6 g/liter) at an abundance of ∼1.0%, indicating their ecological plasticity. Salinity and the concentrations of the chemical ions whose concentrations covary with salinity (Mg²⁺, K⁺, Cl⁻, Na⁺, SO₄²⁻, and Ca²⁺) were found to be the primary environmental factors that directly or indirectly determined the composition and diversity at the level of individual clades as well as entire prokaryotic communities. The distribution patterns of two phyla, five classes, five orders, five families, and three genera were well predicted by salinity. The variation of the prokaryotic community structure also significantly correlated with the dissolved oxygen concentration, pH, the total nitrogen concentration, and the PO₄³⁻ concentration. Such correlations varied depending on the taxonomic level, demonstrating the importance of comprehensive correlation analyses at various taxonomic levels in evaluating the effects of environmental variable factors on prokaryotic community structures. Our findings clarify the distribution patterns of the prokaryotic community composition in plateau lakes at the levels of individual clades as well as whole communities along gradients of salinity and ionic concentrations.     

Distribution and abundance of littoral benthic fauna in Canadian prairie saline lakes

The littoral benthos of 18 lakes in Alberta and Saskatchewan ranging in salinity from 3 to 126 (g1^–1 TDS) were investigated twice, in the spring and in the summer of 1986. Multiple Ekman dredge samples were taken at water depths of about 0.5, 1.0 and 2 metres in each transect. Two to three transects were used in each lake according to its estimated limnological diversity for a total of 114 stations. A total of 76 species was present varying from 29–31 species in the three lakes of lowest salinity (means of 3.1–5.55) to only 2 species in lakes exceeding 100. Species richness decreased rapidly in salinities greater than 15.Biomass maximum mean of 10.91 g m^–2 dry weight (maximum 63.0 g m^–2) occurred in culturally eutrophic Humboldt Lake (3.1) but one third as great in other low salinity lakes. However, biomass again increased to about 4.5 gm^–2 in two lakes of 15 As the salinity increased still further biomass declined steadily until a minimum of 0.0212 g m^–2 was recorded in most saline Aroma Lake (mean 119). Summer biomass (11 lakes) was greater than spring biomass (4 lakes) because some groups such as amphipods, corixids and ostracods became more abundant in summer. Wet weight biomass averaged 15.8 of dry weight biomass.Seasonality (spring or summer), sediment texture and organic matter content, water depth, pH, salinity (TDS) and the presence of aquatic plants ( plant cover) were considered in the matrix involving species dry weight biomass at each of 117 stations. TWINSPAN classification of the samples yielded a dendrogram with 18 indicator species. Successive dichotomies divided these indicator species into four main lake groups based on salinity, i.e., Group I: 3–10 (Gammarus, Glyptotendipes I, Chironomus cf. plumosus), Group II: 10–38%. (Hyalella, Enallagma,Bezzia), Group III: 38–63 (Hygrotus salinarius, Cricotopus ornatus), Group IV: >63 (Dolichopodidae, Ephydra hians). Each of these main groups was subdivided into smaller groups of lakes based on factors such as pH, seasonality (spring or summer species dominance), organic matter and plant cover. Depth of samples played no apparent role.     

Factors Controlling Extremely Productive Heterotrophic Bacterial Communities in Shallow Soda Pools

Dilute soda lakes are among the worlds most productive environments and are usually dominated by dense blooms of cyanobacteria. Up to now, there has been little information available on heterotrophic bacterial abundance, production, and their controlling factors in these ecosystems. In the present study the main environmental factors responsible for the control of the heterotrophic bacterial community in five shallow soda pools in Eastern Austria were investigated during an annual cycle. Extremely high cyanobacterial numbers and heterotrophic bacterial numbers up to 307 × 10⁹ L^–1 and 268 × 10⁹ L^–1 were found, respectively. Bacterial secondary production rates up to 738 µg C L^–1 h^–1 and specific growth rates up to 1.65 h^–1 were recorded in summer and represent the highest reported values for natural aquatic ecosystems. The combination of dense phytoplankton blooms, high temperature, high turbidity, and nutrient concentration due to evaporation is supposed to enable the development of such extremely productive microbial populations. By principal component analysis containing the data set of all five investigated pools, two factors were extracted which explained 62.5% of the total variation of the systems. The first factor could be interpreted as a turbidity factor; the second was assigned to as concentration factor. From this it was deduced that bacterial and cyanobacterial abundance were mainly controlled by wind-induced sediment resuspension and turbidity stabilized by the high pH and salinity and less by evaporative concentration of salinity and dissolved organic carbon. Bacterial production was clustered with temperature in factor 3, showing that bacterial growth was mainly controlled by temperature. The concept of describing the turbid water columns of the shallow soda pools as fluid sediment is discussed.     

Salinity and dissolved substances in the Vaal River at Balkfontein,South Africa

Seasonal variation and interrelationships between environmental variables and salinity in the Vaal River at Balkfontein were investigated between 1985 and 1989. The salinity levels were high (min. 108, max. 1032, average 512 mg 1^-1), and indications are that salinisation of the river water is continuing at a rate of 15 mg 1^t-1 a^-1. The concentration of major ions (mg 1^-1 or meq 1^-1) in the river water occurred in the proportions of cations, i.e. Ca²⁺ Na⁺ > Mg²⁺ > K⁺ and of anions, i.e. SO₄ ^2- > HCO₃ ^- > Cl^-. The high S0₄ concentration (average 212 mg 1^-1) indicated SO₄ pollution at Balkfontein. High Ca concentrations (average 59 mg 1^-1) evidently interfered with the solubility of phosphate (PO₄) which could play an important role in the availability of PO₄ to phytoplankton in the system. Measured electrical conductivity (EC) values can be used to estimate salinity (S, in mg 1^-1) in the Vaal River, i.e. e. S = 7.13 * EC - 27.8 (from regression analysis) or S = 6.72 * EC (from TDS:EC ratio). Salinity in the river tends to be higher during the dry season, i.e. winter months. All the dissolved ions (except Si) showed decreased concentrations with increased run-off. The main importance of the high salinity in the Vaal River is evidently its influence on turbidity and the possible clarification of the water-column, which results in a deeper euphotic zone and thus more favourable conditions for photosynthesis followed by a biomass build-up of phytoplankton.     

Diurnal variation in physical-chemical properties and primary production in the interconnected marine,mangrove and freshwater biotopes of Kakinada coast,Andhra Pradesh,India

Diurnal variation in hydrological variables and dissolved inorganic nutrients such as PO _inf4 ^sup3– -P, N O _inf2 ^sup– -N, NO _inf3 ^sup– -N and NH _inf4 ^sup+ -N were studied in three interconnected biotopes including freshwater, marine and mangrove brackish water of the Kakinada coastal zone, Andhra Pradesh. Samples were collected at intervals of 3 hours, for a period of 24 hours. In the marine environment salinity varied from 26 to 32 whereas in the mangrove waters it fluctuated from 12 to 20 and in both biotopes salinity showed bimodal type of oscillation. Dissolved oxygen content was high in the mangrove waters during day time but decreased rapidly during the night hours. In the marine environment PO_f4 ^p3–-P concentration varied from 0.345 to 1.195 g at l^–1, NO _inf3 ^sup– -N from 1.03 to 6.62 g at l^–1 and NO _inf2 ^sup– -N from 0.086 to 0.506 g at l^–1. The highest and the lowest concentrations of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N, NO _inf2 ^sup– -N recorded in the mangrove waters were 0.790 and 0.325 g at l^–1, 7.10 and 1.60 g at l^–1 and 0.278 and 0.060 g at l^–1, respectively. The concentration of PO _inf4 ^sup3– -P, NO _inf3 ^sup– -N and NO _inf2 ^sup– -N were high in the freshwater canal, the maximum and minimum values being 1.110 and 0.730 g at l^–1, 26.40 and 9.98 g at l^–1 and 0.520 and 0.252 g at l^–1 respectively. The concentration of ammonia was relatively high in the mangrove water. Gross and net primary production in the mangrove water was 4 times higher than in the marine biotope. There was no export of dissolved nutrients from the mangrove environment to the adjacent marine waters.     

Factors relating to annual planktonic primary production in five small oligotrophic lakes in Terra Nova National Park,Newfoundland

Annual planktonic primary production in five small oligotrophic lakes in Newfoundland, as measured at maximum depth (P-area) ranged from 5.6 to 12.0 g C/m²/year. Production per unit surface area representative of the whole lake basin (P-area ) was considerably lower in the shallower lakes (3.3 to 6.4 g C/m²/yr), while production expressed per unit volume representative of the whole lake basin (P-vol ) was the higher in the shallower lakes (0.6 to 4.1 g C/m³/yr). Production efficiency varied from lake to lake and throughout the year, but it remained fairly stable in a shorter term. Production efficiency and insolation was used to estimate annual production. Lakes with higher summer concentration of total phosphorus had higher rates of carbon assimilation.     

Diatom–environmental relationships and nutrient transfer functions from contrasting shallow and deep limestone lakes in Ontario, Canada

Diatom assemblages were analysed in the surface sediments of 44 alkaline lakes in south-western Ontario, Canada, and combined with a pre-existing 58 south-eastern Ontario lake set: (1) to determine if shallow, polymictic Ontario lakes contain different diatom assemblages from deeper, dimictic lakes, and if so, which environmental variables most influence assemblages; (2) to improve the existing transfer functions; (3) to construct and compare transfer functions separately for dimictic, deep lakes and for polymictic, shallow lakes. Polymictic and dimictic lakes covered a similar nutrient range (spring total phosphorus (TP)=4–54 g/l, spring total nitrogen (TN)=200–927 g/l; n=101) and spring pH levels (7.6–9; n=101). However, polymictic lakes were shallower (median mean depth = 2.9 m vs. 7.3 m in dimictic lakes). Benthic diatoms (average 60% relative abundance) dominated the polymictic lakes, whereas planktonic diatoms (average 60%) dominated dimictic lakes. A Canonical Correspondence Analyses with forward selection (p < 0.05, 999 Monte Carlo permutations) identified TP, alkalinity, watershed to volume ratios and lake depth as the most important measured environmental variables influencing diatom distribution in both polymictic and dimictic lakes. Additionally, pH was identified as an important variable in polymictic lakes, whereas TN was also forward selected in the dimictic lakes. Adding more lakes to the original southern Ontario calibration set improved the TN transfer function (r² _jack=0.42, root mean squared error of prediction (RMSEP)_jack=0.11 [log g TN/l]), although there was a high systematic error in the revised model (r² _residual = 0.48). However, the strongest TP model was derived from the polymictic lakes (r² _boot =0.44, RMSEP_boot=0.20 [log g TP/l]), which was the smallest lake set (n=30) with the lowest number of diatom species. The stronger TP model from the polymictic lakes may be partly due to the relatively low macrophyte cover in our polymictic lakes, which may lead to stronger benthic–pelagic coupling than in lakes with large macrophyte populations. Additionally, our study suggests that the Chrysophyceae cyst:diatom frustule ratio may be useful for indicating trends in TP levels of 35 g/l in alkaline lakes that are dimictic, but is not necessarily indicative of trophic state changes in shallow, polymictic lakes. Our study demonstrates that it may be important to construct separate diatom-based nutrient transfer functions for polymictic and dimictic lakes.     

Development of diatom-based salinity models for paleoclimatic research from lakes in British Columbia (Canada)

Diatoms were identified and enumerated from the surface sediments of 65 lakes located on the Cariboo and Chilcotin Plateaux (British Columbia, Canada). These lakes span a large gradient in lakewater ionic concentration (fresh through hypersaline) and composition, as well as other physical/chemical variables. Almost all of the study lakes had higher salinities in the late-summer than in the spring. The lakes with spring salinities >8 g l^–1 showed the largest seasonal increases in salinity. Ionic composition was similar in the spring and late-summer for most lakes. Both ionic concentration (i.e. salinity) and composition were important environmental variables that could account for the different diatom floras in the lakes. Diatom assemblages characteristic of carbonate-dominated and sulfate-dominated waters were identified. Other variables such as water depth and phosphorus concentration were also important.The majority (87%) of diatom taxa had estimated salinity optima < 3 g l^–1 Halophilic diatom taxa had broader tolerances to salinity when compared to the fresh water taxa, however taxa with narrow and broad tolerances could be identified across the salinity gradient. Species diversity was weakly but significantly correlated to lakewater salinity (r ² = 0.18 to 0.3, P < 0.05).Salinity inference models were developed based on the relationship between the diatom assemblages and the spring, late-summer and average salinity. The correlations between the measured and diatominferred salinity, based on the spring (r = 0.95), late-summer (r = 0.94) and average (r = 0.95) salinity data, are high because there was an extremely strong correlation (r = 0.98) between the log transformed spring and late-summer measured salinities. These salinity reconstruction models provide a tool that can be used to infer past climatic changes as part of paleolimnological studies from appropriate closed-basin lakes in British Columbia.     

Temporal and spatial distribution of the baetid Afroptilum sudafricanum in the sediment surface of a tropical stream

Shallowness and tropicality primarily relate to physical aspects of environmental regulation. These concern water input and output, with correlates associated with water level and salinity; energy balance and heat distribution, with correlates of temperature and density stratification; and largely wind-driven water movements, with consequences in chemical and biological distributions. Shallowness in a water-column affects the quantitative relationship between many stock quantities and flux rates per unit surface area. Evaporative loss of water is one familiar example; sensitivity to surface energy exchanges provides others. Somewhat different are processes that depend upon transmission with depth. Here light penetration, convective penetration and wind-generated turbulence with flow-depth relations are illustrative. Tropicality further influences by climatic factors, especially of solar radiation and rainfall. Energy balance tends to yield year-long elevated water temperature at all depths, but at a level dependent upon altitude. The magnitude and seasonal periodicity of water input is widely dependent upon the inter-tropical convergence zone in atmospheric circulation. Over the arid and semi-arid tropics, the lakes may lie in closed drainage basins and be influenced by evaporative concentration with salinization. Shallow lakes of tropical Africa are used to illustrate these varied environmental constraints and some of their biological consequences.     

Phenanthrene mineralization along a natural salinity gradient in an Urban Estuary,Boston Harbor,Massachusetts

The effect of varying salinity on phenanthrene and glutamate mineralization was examined in sediments along a natural salinity gradient in an urban tidal river. Mineralization was measured by trapping¹⁴CO₂ from sediment slurries dosed with trace levels of [¹⁴C]phenanthrene or [¹⁴C]glutamate. Sediments from three sites representing three salinity regimes (0, 15, and 30%.) were mixed with filtered column water from each site. Ambient phenanthrene concentrations were also determined to calculate phenanthrene mineralization rates. Rates of phenanthrene mineralization related significantly to increasing salinity along the transect as determined by linear regression analysis. Rates ranged from 1 ng/hour/g dry sediment at the freshwater site to > 16 ng/hour/g dry sediment at the 30 salinity site. Glutamate mineralization also increased from the freshwater to the marine site; however, the relationship to salinity was not statistically significant.To examine the effect of salinity on mineralizing activities, individual sediments were mixed with filtered water of the other two sites. Slurries were also made with artificial seawater composed of 0, 15, or 30 g NaCl/ liter to substitute for overlying water. Rates of phenanthrene mineralization in the 0 ambient salinity sediments were not affected by higher salinity waters. Activities in the 15 and 30 ambient salinity sediments, however, were significantly inhibited by incubation with 0 salinity water. The inhibition, in large part, appears to be due to the decreased NaCl concentration of the water phase. Glutamate mineralization was affected in a similar manner, but not as dramatically as phenanthrene mineralization. The results suggest that phenanthrene degraders in low salinity estuarine sediments subject to salt water intrusion are tolerant to a wide range of salinities but phenanthrene degradation in brackish waters is mainly a function of obligate marine microorganisms.     

Diversity of Carbon Monoxide-Oxidizing Bacteria in Five Lakes on the Qinghai-Tibet Plateau,China

Carbon monoxide-oxidizing (COX) bacteria play an important role in controlling the flux of carbon monoxide among natural reservoirs, and thus studying their diversity in natural environments is of great significance to understanding the carbon cycle. In this study, the COX bacterial diversity was investigated in five lakes (Erhai Lake, Gahai Lake1, Gahai Lake2, Xiaochaidan Lake, Lake Chaka) on the Qinghai-Tibet Plateau and its correlation with environmental variables of the lakes was explored. Phylogenetic analyses showed that the CO-oxidizers were dominated by Proteobacteria and Actinobacteria in the Qinghai-Tibet Plateau lakes, and their relative abundance varied with salinity: in the freshwater Erhai Lake, the COX bacteria in the water were dominated by the Betaproteobacteria, in contrast to the Actinobacteridae dominance in the sediment; in the saline and hypersaline lakes of Gahai Lake1, Gahai Lake2 and Xiaochaidan Lake, alphaproteobacterial COX bacteria were dominant in the water, whereas Actinobacteridae and alphaproteobacterial COX bacteria were dominant in the sediment. In the hypersaline Lake Chaka, an unknown COX bacterial clade and alphaproteobacterial COX bacteria were dominant in the water and sediment, respectively. Statistical analyses showed that salinity, pH, and major ions (e.g., K⁺, Na⁺, Ca²⁺, Mg²⁺, SO₄ ^2-, and Cl^-) were important factors affecting the COX bacterial community compositions in the investigated lakes. Overall, our results provided insights into the COX bacterial diversity in Qinghai-Tibetan lakes.     

Does the impact of nutrients on the biological structure and function of brackish and freshwater lakes differ?

The effects of nutrients on the biological structure of brackish and freshwater lakes were compared. Quantitative analysis of late summer fish, zooplankton, mysid and macrophyte populations was undertaken in 20–36 shallow brackish lakes of various trophic states and the findings compared with a similar analysis of shallow freshwater lakes based on either sampling (fish) or existing data (zooplankton, mysids and macrophytes). Special emphasis was placed on differences in pelagic top-down control. Whereas the fish biomass (CPUE, multiple mesh-size gill nets) rose with increasing P-concentration in freshwater lakes, that of brackish lakes was markedly reduced at P-concentrations above ca. 0.4 mg P l^-1 and there was a concomitant shift to exclusive dominance by the small sticklebacks (Gasterosteus aculeatus and Pungitius pungitius); as a result, fish density remained relatively high. Mysids (Neomysis integer) were found at a salinity greater than 0.5 and increased substantially with increasing P-concentration, reaching levels as high as 13 ind. l^-1. This is in contrast to the carnivorous zooplankton of freshwater lakes, which are most abundant at intermediate P levels. The efficient algal controller, Daphnia was only found at a salinity below 2 and N. integer in lakes with a salinity above 0.5. Above 2 the filter-feeding zooplankton were usually dominated by the less efficient algal controllers Eurytemora and Acartia. In contrast to freshwater lakes, no shift to a clearwater state was found in eutrophic brackish lakes when submerged macrophytes became abundant. We conclude that predation pressure on zooplankton is higher and algal grazing capacity lower in brackish eutrophic-hypertrophic lakes than in comparable freshwater lakes, and that the differences in trophic structure of brackish and freshwater lakes have major implications for the measures available to reduce the recovery period following a reduction in nutrient loading. From the point of view of top-down control, the salinity threshold dividing freshwater and brackish lakes is much lower than the conventionally defined 5.     

Response of Aerobic Anoxygenic Phototrophic Bacterial Diversity to Environment Conditions in Saline Lakes and Daotang River on the Tibetan Plateau,NW China

Variations of the pufM gene [encoding the M subunit of the photosynthetic reaction center in aerobic anoxygenic phototrophic (AAnP) bacteria] diversity in response to environmental changes were investigated in waters of six aqueous regimes (including Daotang River and five saline/hypersaline lakes on the Tibetan Plateau) representing a full salinity gradient from freshwater to NaCl-saturation. AAnP bacterial community structures responded to salinity change: Gamma-like AAnP community was predominant in freshwater Daotang River (0.01% salinity). AAnP community structure shifted from Loktanella-like sequences of the Alphaproteobacteria in saline Qinghai Lake to Roseobacter-like sequences in hypersaline lakes (Gahai, Xiaochaidan and Charhan). In addition to salinity, other environmental variables (e.g. N and P availability, TOC and/or DOC, and HCO^? ₃/CO^2? ₃ ions) were also important in affecting the pufM gene diversity in hypersaline lakes. These data have important implications for our understanding of the response of AAnP bacterial community to environmental variables in high-altitude aquatic ecosystems.