Multivariate analysis of heavy metal and nutrient concentrations in sediments of Taihu Lake, China*

Taihu Lake is one of the largest freshwater lakes in China. The Lake is very shallow with a mean depth of 1.9 m and an area of 2428 km². Nutrient concentrations (Org-C, Tot-N and Tot-P) and heavy metal concentrations (Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sr, Zn, etc.) in the lake's surface sediments were sampled at 13 locations. This was done to determine if industrialized areas along the lake's coastline were impacting the nutrient and heavy metal distribution of the lake's surface sediments. Principal Component Analysis (PCA) was used to assess the degree of contamination and spatial distribution of heavy metals and nutrients in different areas of Taihu Lake.A distinctive spatial distribution of heavy metals and nutrients was observed. Sediments from a large embayment of Taihu Lake called Lake Wulihu had significantly higher nutrient concentrations (Org-C, 2.05–3.83%; Tot-N, 0.28–0.54%; Tot-P, 0.10–0.33%) than any other area of Taihu Lake. These high nutrient levels were associated with the input of untreated domestic sewage from the large (circa one million people) City of Wuxi, which discharges its effluents into the Liangxi River. As a result, Lake Wulihu is the most eutrophic area of Taihu Lake. The nearby Meiliang Bay suffered from the worst heavy metal contamination in Taihu Lake (e.g. As, 64.0; Ag, 4.2; Cd, 0.93; Co, 14.2; Cr, 155.0; Cu, 144.0; Hg, 0.25; Ni, 79.8; Pb, 143.0 and Zn, 471 mg kg^–1). These high heavy metal concentrations were ascribed to the discharge of untreated and partially treated industrial waste water from Changzhou and Wujin via the Zhihugang River. Surface sediment samples from the east basin of Taihu Lake were characterized by high Org-C (1.0–2.3%) and Tot-N (0.18–0.37%) and low Tot-P (0.048–0.056%) concentrations. It is likely that macrophytes removal accounts for a major reduction of phosphorus in the sediments of the east basin of Taihu Lake.     

Binding of phosphate in sediment accumulation areas of the eastern Gulf of Finland,Baltic Sea

The relationships between P and components binding P were studied by analysing the concentrations of N, P, Fe, Mn, Ca and Al in sediments and pore water along the estuarine transect of the River Neva in August 1995. The high sediment organic matter concentration resulted in low surface redox potential and high pore-water o-P concentration, whereas the abundance of amphipods resulted in high surface redox potentials and low pore-water o-P concentration. However, despite the variation in sediment organic matter and the abundance of amphipods, very reduced conditions and slightly variable concentrations of Tot-P (0.7–1.1 mg g^–1 DW) were observed in the 10–15 cm sediment depth along the estuarine gradient, indicating that the pools of mobile P were largely depleted within the depth of 0–15 cm. Multiple regression analysis demonstrated that organic matter and Tot-Fe concentration of the sediment were closely related to the variation in Tot-P concentration of the sediments (r ² = 0.817, n=32). In addition, the high total Fe:P ratio suggested that there is enough Fe to bind P in sediments along the estuarine gradient. However, low Fe_diss concentrations in the pore water of reduced sediment (redox-potential <–50 mV) indicated efficient precipitation of FeS (FeS and FeS₂), incapable to efficiently bind P. Consequently, the low Fe_diss:o-P ratio (< 1) recorded in pore water in late summer implied that Fe³⁺ oxides formed by diffusing Fe_diss in the oxic zone of the sediments were insufficient to bind the diffusing o-P completely. The measured high o-P concentrations in the near-bottom water are consistent with this conclusion. However, there was enough Fe_diss in pore water to form Fe³⁺ oxides to bind upwards diffusing P in the oxic sediment layer of the innermost Neva estuary and the areas bioturbated by abundant amphipods.     

The response of a freshwater wetland to long-term ’low level‘ nutrient loads: nutrients and water budget

Boney Marsh is a small constructed freshwater wetland located along thefloodplain of the Kissimmee River in south Florida, USA. River water, withaverage Tot-P concentrations of 0.052 mg l⁻¹, Tot-N of 1.70 mgl⁻¹, and Cl⁻ of 15.95 mg l⁻¹, wasdiverted through the marsh to quantify mass retention and fate.Comprehensive mass balance budgets for Tot-P, Tot-N, and Cl⁻were developed based on input (inflow, precipitation) and output (outflow,evapotranspiration, seepage). Cl⁻, as well asNa⁺, budgets indicated that groundwater accounted forapproximately 7% of the total water budget. Annual mass loadings toBoney Marsh were 0.5, 15.7, and 147.9 g m⁻²year⁻¹ for Tot-P, Tot-N, and Cl⁻, respectively.Mean annual nutrient removal was estimated at 72% for Tot-P and34% for Tot-N, and P-assimilation capacity remained high andunchanged for the period of record. The subtropical marsh system accumulatedTot-P at a much higher rate than Tot-N, with averaged net sedimentationrates of 20.4 and 8.3 year⁻¹, respectively. Boney Marsh netsedimentation coefficients were higher than lakes with similar depths. TheN:P mass ratio in the wetland water column increased during the period ofrecord, and was primarily due to a high P-sedimentation rate and a decliningN-sedimentation rate. The authors use the editor‘s suggestion forabbreviations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reconstruction of historical productivity using visible-near-infrared (VNIR) reflectance properties from boreal and saline lake sediments

Chlorophylls preserved in lake sediments have been used as a proxy to infer past trophic status. Recently, it has been demonstrated that visible-near-infrared (VNIR) reflectance spectroscopy can provide a rapid and non-destructive estimation of fossil chlorophylls from alpine lake sediments. The present study explores, (a) the applicability of VNIR reflectance spectroscopy to reconstructing historical productivity from boreal and saline lakes, and (b) the ability of an inference model combining all lake types to reconstruct historical chlorophyll concentrations from lake sediments. Results revealed that regardless of the lake type, a common sediment spectral feature of a reflectance trough centered near 675 nm, was observed. Additionally, the amplitude of reflectance in the VNIR region differs within and among lakes depending on their trophic states. The inferred concentration of total chlorophylls and derivatives from sediment spectral properties reflected a recent nutrient enrichment in most of the study lakes. Predicted chlorophyll concentration, when plotted against high-pressure liquid chromatography (HPLC) measured concentration combining all lake types, was found to be statically significant (r ² = 0.80, P < 0.01). Collectively, results from this study indicate that regardless of the lake type, a common chlorophyll absorption feature near 675 nm can be detected, which is associated with contrasting limnological settings and, therefore, can be used as a viable tool to reconstruct paleoproductivity. A similar approach can be implemented for rapid and non-destructive detection of historical lake water quality in a wide range of lake sediments.     

Nutrient balances and phytoplankton dynamics in two agriculturally loaded shallow lakes

The impact of agriculture was estimated on two shallow, eutrophic lakes, Lake Kotojärvi and Lake Villikkalanjärvi in southern Finland. The main emphasis was on phosphorus and nitrogen budgets and on the phytoplankton dynamics. Special attention was paid to internal P loading and blue-green algal blooms. The mean Tot-P load from agricultural land was 1.2 kg ha^-1 a^-1 in both basins and Tot-N loads were 19 kg ha^-1 a^-1 in L. Villikkalanjärvi and 12 kg ha^-1 a^-1 in L. Kotojärvi. The Tot-P input to L. Kotojärvi was on an average 0.62 g m^-2 a^-1 (per lake surface area), and the Tot-N input 9.1 g m^-2 a^-1. The corresponding inputs to L. Villikkalanjärvi were 3.1 and 57 g m^-2 a^-1, respectively. The annual variation followed the runoff volumes. About half of the Tot-P and one third of the Tot-N load was retained in L. Kotojärvi. In L. Villikkalanjärvi the retention was only 24% for Tot-P and 19% for Tot-N. The difference was very probably due to a longer theoretical retention time in L. Kotojärvi. In L. Villikkalanjärvi the mean concentration of Tot-P was 120 µg 1^-1 and that of Tot-N 1700 µg 1^-1 and the corresponding figures in L. Kotojärvi 67 and 990 µg 1^-1, respectively. The mean chlorophyll a concentration was, however, higher in L. Kotojärvi (26 µg 1^-1) than in L. Villikkalanjärvi (20 µg 1^-1). This was probably due to an internal P load in L. Kotojärvi: in 1988 the internal load of dissolved P was estimated to be as much as twofold the external load. In L. Villikkalanjärvi the internal dissolved P load was only up to 50% of the external input. In L. Kotojärvi the high internal P load coupled with a low DIN:DIP ratio resulted in a strong blue-green algal bloom in the summer of 1988. In L. Villikkalanjärvi blue-green algae were observed only in small amounts. Even in August 1990, when the DIN:DIP ratio was low enough to favor the occurrence of blue-green algae, they contributed only up to 10–15% of the total phytoplankton biomass.     

Sedimentary phosphorus fractions and bioavailability as influenced by repeated sediment resuspension

Concentrations of phosphorus (P) fractions and changes in their bioavailability in the sediments as influenced by repeated resuspension were determined by sequential fractionation in laboratory experiments. The water and sediment samples used were taken from the campus canal. Sequential fractionation indicated that the concentrations of the iron bound P (BD–P) were predominant, consisting of over 50% of total P (Tot-P) in the sediments that did and did not undergo resuspension. BD–P mobility was reduced due to resuspension resulting from the decline of the proportion ratio of non-occluded Fe–P and occluded Fe–P from 0.53 to 0.29. Therefore, under sediment resuspension conditions, using the sum of loosely sorbed P (NH₄Cl–P), BD–P, aluminium bound P (Al–P), and organic-P (NaOH–nrP) to estimate bio-available P (BAP) might be problematic. However, BAP could be accurately estimated by the sum of NH₄Cl–P, % BD–P (bio-available, non-occluded Fe–P), and NaOH–nrP. By this estimation, the amount of BAP in the sediments as influenced by repeated resuspension decreased by about 10% of Tot-P, compared with the initial state (raw sediments). The results suggest that repeated resuspension could accelerate the transformation of P from mobile fractions to refractory fractions, which can be attributed to the increase of occluded Fe–P, Al–P, and calcium bound P (HCl–P).     

Bacterial Diversity and Geochemical Profiles in Sediments from Eutrophic Azorean Lakes

In the Azores, the advanced trophic state of the lakes requires a fast intervention to achieve the good ecological status prescribed by the Water Framework Directive. Despite the considerable effort made to describe the phytoplankton growing on the water column, the lack of information regarding the microbial processes in sediments is still high. Thus, for the successful implementation of internal management actions, the present work explored the relationships between geochemical profiles and dominant members of the bacterial community in sediments from eutrophic Azorean lakes. Lake Azul geochemical profiles were quite homogeneous for all parameters, while in lake Furnas the total iron profile presented a peak below the aerobic layer. For lake Verde, the concentrations of all studied parameters (20 ± 2% loss-on-ignition; 2.10 ± 0.08 mg g^?1 total phosphorus; 1.31 ± 0.50 mg g^?1 total nitrogen; 8.06 ± 0.13 mg g^?1 total iron) in the uppermost sediment layer were approximately two times higher than the ones in sediments from other lakes, decreasing with sediment depth. The higher amounts of phosphorus and organic matter in lake Verde suggested a higher internal contribution of phosphorus to eutrophication. The dominant members of the sediment bacterial community, investigated by denaturing gradient gel electrophoresis, were mostly affiliated to Proteobacteria phylum (Alpha-, Delta-, and Gamma-subclasses), group Bacteroidetes/Chlorobi and phylum Chloroflexi. The Cyanobacteria phylum was solely detected in sediments from lake Verde and lake Furnas that presented the highest amounts of nitrogen and phosphorus both in the water column and sediments, while the other phyla were detected in sediments from the three studied lakes. In conclusion, management measurers to achieve the good ecological status until 2015 should be distinct for the different lakes taking into account the relative magnitude of the nutrient sources and the bacterial diversity in sediments.     

Long-term zooplankton composition data reveal impacts of invasions on community composition in the Waikato lakes,New Zealand

Data collected on zooplankton community composition over longer time periods (>?10 years) are rare. We examined among-lake spatial and temporal trends of zooplankton communities from a monitoring programme undertaken in the Waikato region, New Zealand. A total of 39 lakes were sampled over a period of 12 years, between 2007 and 2019, with varying degrees of temporal effort. We focussed particularly on eight lakes, considered here as ‘long-term lakes’, where samples were collected with greater regularity (including 5 with 12 years of data). Among lakes, suspended sediment concentrations and indicators of lake trophic state were inferred to be important in determining the zooplankton distributions; as this region is dominated by shallow lakes, the relative importance of suspended sediments was high. Among the long-term lakes, the greatest dissimilarities in zooplankton community composition among years were in Lake Waahi, where the Australian Boeckella symmetrica was first detected in 2012. That is, the greatest temporal changes to zooplankton composition during the study period were due to the invasion by non-indigenous species, rather than changes in trophic state or other environmental variables; non-native species commonly dominated the individual counts of species through much of 2014 and 2015, with most samples since 2016 being again dominated by native species. Following this lake, the largest and shallowest lakes in the dataset—Whangape and Waikare—exhibited the greatest variability in community composition among years.

     

Resuspension in a shallow eutrophic lake

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arresø, Denmark (41 km², mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2–8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l^–1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours.A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988.These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m^–2 d^–1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l^–1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arresø is reduced to 0.5 m.Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.     

Sublittoral and profundal Oligochaeta fauna of the Lake Constance (Bodensee-Obersee)

Fourteen tubificid species and one lumbriculid make up the oligochaete population of the northern half of Lake Constance, a meso- to mesoeutrophic lake. Tubifex tubifex makes up 72% of the total abundance, the clean water species Stylodrilus heringianus only 0.1%. No correlation exists between total worm abundance and organic carbon content of the sediment, only 4% of the organic carbon being used by the worms. A highly significant correlation between particulate organic matter loading and its organic carbon content from 10 tributaries and total oligochaete abundance is demonstrated. The ecological index I_PA is defined and used to assess the biologically relevant trophic status of the lake bed. This integrates information on the percent representation of three groups of worm species classified on atrophic basis and total abundance of the worms. Despite control of the increase in phosphorus loadings progressive eutrophication of sediments was observed after an 8 year interval.     

Nitrogen transformations at the sediment–water interface across redox gradients in the Laurentian Great Lakes

The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO₃ ^?) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment–water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH₄ ⁺ and N₂ out of the sediment decreased, and sediments shifted from a net sink to a net source of NO₃ ^?. Denitrification of NO₃ ^? from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO₃ ^? through denitrification, likely as a result of the thick oxygenated sediment layer.     

Eutrophication as a driver of microbial community structure in lake sediments

Lake sediments are globally important carbon sinks. Although the fate of organic carbon in lake sediments depends significantly on microorganisms, only few studies have investigated controls on lake sedimentary microbial communities. Here we investigate the impact of anthropogenic eutrophication, which affects redox chemistry and organic matter (OM) sources in sediments, on microbial communities across five lakes in central Switzerland. Lipid biomarkers and distributions of microbial respiration reactions indicate strong increases in aquatic OM contributions and microbial activity with increasing trophic state. Across all lakes, 16S rRNA genes analyses indicate similar depth-dependent zonations at the phylum- and class-level that follow vertical distributions of OM sources and respiration reactions. Yet, there are notable differences, such as higher abundances of nitrifying Bacteria and Archaea in an oligotrophic lake. Furthermore, analyses at the order-level and below suggest that changes in OM sources due to eutrophication cause permanent changes in bacterial community structure. By contrast, archaeal communities are differentiated according to trophic state in recently deposited layers, but converge in older sediments deposited under different trophic regimes. Our study indicates an important role for trophic state in driving lacustrine sediment microbial communities and reveals fundamental differences in the temporal responses of sediment Bacteria and Archaea to eutrophication.     

Seasonal changes in sediment and water chemistry of a subtropical shallow eutrophic lake

A field study was conducted (May 1981 to June 1982) to develop a data-base on seasonal changes of water and sediment chemistry of Lake Monroe (4 000 ha surface and ca. 2 m deep) located in central Florida, USA. This shallow eutrophic lake is a part of the St. Johns River. Quantitative samples of lake water and sediments were collected on a monthly basis from 16 stations and analyzed for various physico-chemical parameters. Relatively high levels of dissolved solids (mean electrical conductivity (EC) = 1832 µS cm¹) prevailed in the lake water, and seasonal changes in EC were probably associated with hydrologic flushing from external sources, such as incoming water from upstream as well as precipitation. Average monthly levels of total N and P during the study period were 1.82 and 0.21 mg l^–1, respectively. Nutrient concentrations in the water did not show any strong seasonal trends. Organic matter content of lake sediments ranged from 1 to 182 g C kg^–1 of dry sediment, reflecting considerable spatial variability. All nutrient elements in the sediments showed highly significant (P < 0.01) correlations with sediment organic C, though little or no significant relationship appeared at any sampling period between water and sediment chemistry of the lake. Temporal trends in water and sediment chemical parameters may have been concealed by periodic hydrologic flushing of the St. Johns River into Lake Monroe.Florida Agricultural Experiment Stations Journal Series No. 7836.     

Chlorophylla and adenosine triphosphate levels in Antarctic and temperate lake sediments

Analysis of adenosine triphosphate (ATP) from surficial sediment layers in two antarctic lakes and two temperate lakes showed a high degree of similarity in spite of differences between trophic state, mictic state, or geographic location. Adenosine triphosphate was found at all levels sampled in temperate lake sediment cores but occasionally was present only in surficial layers of antarctic cores. Surficial sediment layers from antarctic lakes contained high chlorophylla (Chla) levels due to the extensive benthic algal mats which occur there. In some antarctic cores, Chla was detectable in deep, old mat layers, whereas Chla was not found in any of the temperate lake cores. Antarctic lake sediments appear to be unique environments where Chla molecules can remain intact for long periods of time due to low light, temperature, and microbial activity. As such, these lakes are important natural laboratories where a long history of microbial interactions can be studied without metazoan perturbation effects. Although there was much variability in concentration of Chla and ATP between samples, there appears to be no relationship between Chla or ATP levels to mictic or trophic states of the lakes. These data suggest that sediment microbial communities may be independent of environmental and biological properties of the overlying water masses.     

Contribution of Sediment Respiration to Summer CO2 Emission from Low Productive Boreal and Subarctic Lakes

We measured sediment production of carbon dioxide (CO₂) and methane (CH₄) and the net flux of CO₂ across the surfaces of 15 boreal and subarctic lakes of different humic contents. Sediment respiration measurements were made in situ under ambient light conditions. The flux of CO₂ between sediment and water varied between an uptake of 53 and an efflux of 182 mg C m⁻² day⁻¹ from the sediments. The mean respiration rate for sediments in contact with the upper mixed layer (SedR) was positively correlated to dissolved organic carbon (DOC) concentration in the water (r² = 0.61). The net flux of CO₂ across the lake surface [net ecosystem exchange (NEE)] was also closely correlated to DOC concentration in the upper mixed layer (r² = 0.73). The respiration in the water column was generally 10-fold higher per unit lake area compared to sediment respiration. Lakes with DOC concentrations <5.6 mg L⁻¹ had net consumption of CO₂ in the sediments, which we ascribe to benthic primary production. Only lakes with very low DOC concentrations were net autotrophic (<2.6 mg L⁻¹) due to the dominance of dissolved allochthonous organic carbon in the water as an energy source for aquatic organisms. In addition to previous findings of allochthonous organic matter as an important driver of heterotrophic metabolism in the water column of lakes, this study suggests that sediment metabolism is also highly dependent on allochthonous carbon sources.     

Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from <0.1 ng gdw^-1 sediment in the ENR to 5 ng gdw^-1 in WCA3 sediments; and MeHg constituted <0.2% of total Hg (Hg_T) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimated using tracer-level injections of²⁰³ Hg(II) into intact sediment cores, ranged from 0 to 0.12 d^-1, or about 1 to 10 ng g^-1 d^-1when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controls concentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota, suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments.Sulfate concentrations in surficial pore waters (up to 400 µm), microbial sulfate-reduction rates (up to 800 nm cc^-1 d^-1) and resultant pore water sulfide concentrations (up to 300 µm) at the eutrophic northern sites were all high relative to most freshwater systems. All declined to the south, and sulfate concentrations in WCA2B and in central WCA3 resembled those in oligotrophic lakes (50–100 µm). MeHg concentration and production were inversely related to sulfate reduction rate and pore water sulfide. Control of MeHg production in the northern Everglades appears to mimic that in an estuary, where sulfate concentrations are high and where sulfide produced by microbial sulfate reduction inhibits MeHg production.     

Deepwater sediments and trophic conditions in Florida lakes

Sediment cores were taken from near maximum depth in 15 Florida lakes representing a wide range of trophic conditions. Chemical analyses of surface sediments showed Al, Fe, and Ca to be the most abundant elements in all samples, and the ratio of Al to Ca to be smaller for eutrophic lakes. Sediment organic matter increased with trophic state, as did the degree to which it was enriched in nitrogen. Corresponding sediment C/N ratios decreased with increasing lake trophic state and showed significant negative correlation with chlorophylla, total N, and total P in the water column. Concentrations of sedimentary chlorophyll derivatives showed some relation to trophic state but differences in basin morphometry hinder its use as an inter-lake index of chlorophyll production.     

Microbial Diversity in Water and Sediment of Lake Chaka, an Athalassohaline Lake in Northwestern China

We employed culture-dependent and -independent techniques to study microbial diversity in Lake Chaka, a unique hypersaline lake (32.5% salinity) in northwest China. It is situated at 3,214 m above sea level in a dry climate. The average water depth is 2 to 3 cm. Halophilic isolates were obtained from the lake water, and halotolerant isolates were obtained from the shallow sediment. The isolates exhibited resistance to UV and gamma radiation. Microbial abundance in the sediments ranged from 10⁸ cells/g at the water-sediment interface to 10⁷ cells/g at a sediment depth of 42 cm. A major change in the bacterial community composition was observed across the interface. In the lake water, clone sequences affiliated with the Bacteroidetes were the most abundant, whereas in the sediments, sequences related to low G+C gram-positive bacteria were predominant. A similar change was also present in the archaeal community. While all archaeal clone sequences in the lake water belonged to the Halobacteriales, the majority of the sequences in the sediments were related to those previously obtained from methanogenic soils and sediments. The observed changes in the microbial community structure across the water-sediment interface were correlated with a decrease in salinity from the lake water (32.5%) to the sediments (approximately 4%). Across the interface, the redox state also changed from oxic to anoxic and may also have contributed to the observed shift in the microbial community.     

USING EPILITHIC ALGAL COMMUNITIES TO ASSESS TROPHIC STATUS IN IRISH LAKES1

Relationships between taxonomic composition of shallow epilithic algal communities and nine environmental variables in 32 lakes of different trophic states in Ireland were explored using gradient analysis. A canonical correspondence analysis using four representative environmental variables, alkalinity (correlated with pH and conductivity), maximum phytoplankton chl a (CHL_max) (correlated with total P, total N, and chl), turbidity, and water color explained 21% of the variance in taxa distributions. The first two axes were significant and accounted for 77% of the variance in the periphyton–environmental relationship. The first axis was strongly related to alkalinity and color, which reflected geology and land use in the watersheds. The second axis was most correlated with CHL_max, and separation of lakes corresponded to their Organization for Economic Cooperation and Development (OECD) trophic classification based on water chemistry. Eutrophic lakes were characterized by cyanobacteria taxa and Stigeoclonium sp. Diatoms and desmids were generally more abundant in oligotrophic and mesotrophic lakes. Values for diatom trophic indices were poor indicators of trophic state. Weighted averaging regression and calibration techniques were used to develop transfer functions between 84 taxa and total P, total N, and CHL_max. The total P inference model predicted OECD trophic classification correctly for 84% of the lakes. Values for taxa preferences resulting from such models can provide the foundation for biomonitoring schemes using extant periphyton communities. The turnover time of periphyton taxa should integrate changes in environmental conditions at a temporal scale intermediate to surface‐sediment fossil diatom assemblages and water column variables, which may be more appropriate for detecting annual changes.     

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large,shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability. Handling editor: Luigi Naselli-Flores