Water column oxygen demand and sediment oxygen flux: patterns of oxygen depletion in tidal creeks

Low dissolved oxygen (DO) levels often occur during summer in tidal creeks along the southeastern coast of the USA. We analyzed rates of oxygen loss as water-column biochemical oxygen demand (BOD₅) and sediment oxygen flux (SOF) at selected tidal creek sites monthly over a 1-year period. Ancillary physical, chemical and biological data were collected to identify factors related to oxygen loss. BOD₅ rates ranged from 0.0 mg l^?1 to 7.6 mg l^?1 and were correlated positively with organic suspended solids, total suspended solids, chlorophyll a concentrations, temperature, and dissolved oxygen, and negatively with pH and nitrate + nitrite. SOF rates ranged from 0.0 to 9.3 g O₂ m^?2 d^?1, and were positively correlated with temperature, chlorophyll a, and total suspended solids, but negatively with dissolved oxygen. Both forms of oxygen uptake were seasonally dependent, with BOD₅ elevated in spring and summer and SOF elevated in summer and fall. Average oxygen loss to sediments was greater and more variable than oxygen loss in the water column. Oxygen deficits at three of five locations were significantly related to BOD₅ and SOF, but not at two sites where ground water discharges were observed. Correlation and principal component analyses suggested that BOD₅ and SOF responded to somewhat different suites of environmental variables. BOD₅ was driven by a set of parameters linked to warm season storm water inputs that stimulated organic seston loads, especially chlorophyll a, while SOF behaved less strongly so. Runoff processes that increase loads of organic material and nutrients and ground water discharges low in dissolved oxygen contribute to occurrences of low dissolved oxygen in tidal creeks.     

Calibration of a complex hydro-ecological model through Approximate Bayesian Computation and Random Forest combined with sensitivity analysis

An automated calibration method is proposed and applied to the complex hydro-ecological model Delft3D-BLOOM which is calibrated from monitoring data of the lake Champs-sur-Marne, a small shallow urban lake in the Paris region (France). This method (ABC-RF-SA) combines Approximate Bayesian Computation (ABC) with the machine learning algorithm Random Forest (RF) and a Sensitivity Analysis (SA) of the model parameters. Three target variables are used (total chlorophyll, cyanobacteria and dissolved oxygen concentration) to calibrate 133 parameters. ABC-RF-SA is first applied on a set of simulated observations to validate the methodology. It is then applied on a real set of high-frequency observations recorded during about two weeks on the lake Champs-sur-Marne. The methodology is also compared to standard ABC and ABC-RF formulations. Only ABC-RF-SA allowed the model to reproduce the observed biogeochemical dynamics. The coupling of ABC with RF and SA thus appears crucial for its application to complex hydro-ecological models.     

Synechococcus as an indicator of trophic status in the Cochin backwaters,west coast of India

Eutrophication is a major problem in coastal water bodies. Information about the trophic status of water bodies will enable proper management of coastal ecosystems. In this regard, biological organisms which are sensitive to environmental changes can serve as indicators of ecosystem trophic status. In this study, seasonal and spatial variations of picophytoplankton (PP; <3 μm size) community structure was assessed in the Cochin backwaters (CB) with respect to the prevailing environmental conditions during three seasons, post-monsoon (PM-I; October 2011 and PM-II; November 2012), pre-monsoon (PrM; May 2012) and monsoon (MON; August 2012). CB, along the west coast of India, receives continuous load of nutrients throughout the year through anthropogenic wastes. Trophic status index (TRIX) scores showed that CB is highly eutrophic with a high phytoplankton biomass. Synechococcus was the dominant PP observed in the study area. Seasonal and spatial salinity variations influenced the PP distribution, especially Synechococcus where PE-rich Synechococcus (SYN-PE) were dominant in higher saline (>30) and PC-rich Synechococcus (SYN-PC) in lower saline (<30) waters. SYN-PC showed a significant positive relation with chlorophyll a suggesting that this group contributes substantially to the total phytoplankton biomass. TRIX scores and SYN-PC: SYN-PE abundance ratio were negatively correlated with salinity suggesting an influence of the tidal amplitude. SYN-PC correlated positively and SYN-PE negatively with TRIX scores suggesting that these groups occupy contrasting ecological niches. These findings imply that PP distribution pattern can serve as an indicator of the trophic status of coastal water bodies.     

Influence of environmental variables on macroinvertebrate community structure in Lianhuan Lake

The structural characteristics of the macroinvertebrate community can effectively reflect the health status of lake ecosystems and the quality of the lake ecological environment. It is therefore important to identify the limiting factors of macroinvertebrate community structure for the maintenance of lake ecosystem health. In this study, the community composition of macroinvertebrate assemblages and their relationships with environmental variables were investigated in 13 small lakes within Lianhuan Lake in northern China. A self‐organizing map and K‐means clustering analysis grouped the macroinvertebrate communities into five groups, and the indicator species reflected the environmental characteristics of each group. Principal component analysis indicated that the classification of the macroinvertebrate communities was affected by environmental variables. The Kruskal–Wallis test results showed that environmental variables (pH, total phosphorus, nitrate, water temperature, dissolved oxygen, conductivity, permanganate index, and ammonium) had a significant effect on the classification of the macroinvertebrate communities. Redundancy analysis showed that mollusks were significantly negatively correlated with pH and chlorophyll a, while annelids and aquatic insects were significantly positively correlated with chlorophyll a and dissolved oxygen. Spearman correlation analysis showed that the species richness and Shannon''s diversity of macroinvertebrates were significantly negatively correlated with total phosphorus, while the biomass of macroinvertebrates was significantly negatively correlated with pH. High alkalinity and lake eutrophication have a serious impact on the macroinvertebrate community. Human disturbances, such as industrial and agricultural runoff, negatively impact the ecological environment and affect macroinvertebrate community structure. Thus, macroinvertebrate community structure should be improved by enhancing the ecological environment and controlling environmental pollution at a watershed scale.     

Effect of summer weather on internal loading and chlorophyll a in a shallow lake: a modeling approach

The effect of weather on the eutrophication of a shallow lake was estimated by a hydrodynamic lake model coupled with a simple water quality module. The model was applied to Lake Villikkalanjärvi in southern Finland. This shallow, agriculturally loaded lake may stratify during warm and calm periods in summer and as a result oxygen is often consumed from the hypolimnion, causing high internal loading of phosphorus. Vertical mixing and temperature distribution in the lake were simulated by a one-dimensional, horizontally integrated hydrodynamic model. State variables included in the water quality model were dissolved reactive phosphorus, chlorophyll a and dissolved oxygen. The model was first calibrated against observations from 1989 and 1990. Thereafter, simulations were carried out using weather data from the years 1961 to 1988. The results indicated that warm summer periods may cause high chlorophyll a concentrations due to high internal loading. In four years with exceptionally warm summers the model predicted maximum chlorophyll a concentrations almost twice as high as in years without remarkable internal loading. The model simulates accurately temperature and mixing but the reliability of water quality predictions could be improved by adding more factors regulating algal biomass and sediment phosphorus release.     

Interrelationships Between Microbiological and Chemical Parameters of Sandy Beach Sediments, a Summer Aspect

At 12 stations located in sandy beach sediments of the brackish water Kiel Fjord and Kiel Bight (Baltic Sea, FRG), variations and interrelationships of microbiological, chemical, and physicochemical parameters were monitored. Depending upon location, wide variations of a number of parameters reflecting dissolved organic and inorganic nutrients, chlorophyll a, microbial number, and uptake activity of glucose were measured. Whereas most of the parameters generally showed the tendency to decrease from the inner to the outer Kiel Fjord, individual parameters (oxygen, particulate nitrogen, ribose, chlorophyll a, glucose/fructose ratio) increased with increasing distance from the inner Kiel Fjord. Similarities in the local variation pattern demonstrated various relationships between individual parameters. Among those, dissolved organic nutrients on the one hand and inorganic nutrients on the other hand were closely linked together. Variations of organic and inorganic nutrients corresponded to variations of microbial activity and physicochemical parameters. By comparing standing stock carbon with carbon production, a microbial biomass turnover time of about 100 h was calculated. Approximately 50% of the microphytobenthos primary production was fixed by microorganisms. Daily microbial carbon production (43 mg of C per m²) was in the range of meiofauna carbon (35 mg of C per m²).     

Assessment of nutrient limitation in Rijeka Bay, NE Adriatic Sea, using miniaturized bioassay

In this paper we consider the trophic state and ambient nutrient limitation by the trophic index (TRIX) calculation and nutrient enrichment bioassay using the diatom Phaeodactylum tricornutum at two differently eutrophicated NE coastal Adriatic locations (in the Rijeka Bay area). The first station in Rijeka Port had significantly higher concentrations of dissolved inorganic nutrients and TRIX indices (up to 7.53) than the station near Opatija. The DIN/TP ratios in Rijeka Port were lower (11.03 to 44.47) than those in Opatija (38.67 to 82.00), indicating P limitation in Opatija. The nutrient enrichment bioassays revealed P as the key limiting nutrient for the diatom growth at both sites. In addition, silica limitation was found in all water samples with the exception of the surface water samples in Rijeka Port. Nitrogen limitation was rarely evident.     

Compensation effect of phytoplankton on nutrients from a sewage outfall in summer

This paper investigates the relationship between nutrients, chlorophyll a and physical variables in the upper Saronikos Gulf, an oligotrophic marine environment south of the Greater Athens Metropolitan Area. Phosphate, silicate, ammonia, nitrite, nitrate, temperature, salinity and dissolved oxygen were determined at eight stations on 9 occasions during summer 1982. A thermocline led to the stratification of the water column and the pycnocline was related to the thermocline. The values of oxygen were more or less normal. The Eutrophication of the seawater in the vicinity of the sewage outfall was demonstrated by surface levels of chlorophyll a being forty to two hundred times above background. This parameter provides evidence for a high phytoplankton standing stock. However, there were no appreciable differences between the nutrients in the outfall area and those in the background. This suggests rapid uptake of nutrients and/or effective dispersal from the outfall.     

Modeling daily chlorophyll a dynamics in a German lowland river using artificial neural networks and multiple linear regression approaches

Phytoplankton biomass is an important indicator for water quality, and predicting its dynamics is thus regarded as one of the important issues in the domain of river ecology and management. However, the vast majority of models in river systems have focused mostly on flow prediction and water quality with very few applications to biotic parameters such as chlorophyll a (Chl a). Based on a 1.5-year measured dataset of Chl a and environmental variables, we developed two modeling approaches [artificial neural networks (ANN) and multiple linear regression (MLR)] to simulate the daily Chl a dynamics in a German lowland river. In general, the developed ANN and MLR models achieved satisfactory accuracy in predicting daily dynamics of Chl a concentrations. Although some peaks and lows were not predicted, the predicted and the observed data matched closely by the MLR model with the coefficient of determination (R ²), Nash–Sutcliffe efficiency (NS), and the root mean square error (RMSE) of 0.53, 0.53, and 2.75 for the calibration period and 0.63, 0.62, and 1.94 for the validation period, respectively. Likewise, the results of the ANN model also illustrated a good agreement between observed and predicted data during calibration and validation periods, which was demonstrated by R ², NS, and RMSE values (0.68, 0.68, and 2.27 for the calibration period, 0.55, 0.66 and 2.12 for the validation period, respectively). According to the sensitivity analysis, Chl a concentration was highly sensitive to dissolved inorganic nitrogen, nitrate–nitrogen, autoregressive Chl a, chloride, sulfate, and total phosphorus. We concluded that it was possible to predict the daily Chl a dynamics in the German lowland river based on relevant environmental factors using either ANN or MLR models. The ANN model is well suited for solving non-linear and complex problems, while the MLR model can explicitly explore the coefficients between independent and dependent variables. Further studies are still needed to improve the accuracy of the developed models.     

Phytoplankton ecology in the Southern Benguela current. I. Biochemical composition

Investigations on phytoplankton communities in a nearshore region off the Cape Peninsula revealed three types of upwelled water. During active upwelling temperatures were < 10 °C and concentrations of inorganic nutrients were high (Type 1). Maturing upwelled water was characterized by temperatures > 10°C and nitrate concentrations varying between 2 and 15 μg-at. NO₃-N · 1^?1 (Type 2), while aged upwelled water (Type 3) contained low concentrations of nitrate (<2 μg-at. NO₃-N · 1^?1) at temperatures > 10°C. During the summer of 1978–1979 diatoms dominated the communities from October to January but microflagellates were dominant in February and March. In both types of community, low concentrations of ATP, chlorophyll a, protein and carbohydrate were measured in Type 1 water with protein/carbohydrate ratios being > 1. In Type 2 water concentrations of chlorophyll a, ATP and protein were high and the protein/carbohydrate ratio was > 1. Concentrations of chlorophyll a and ATP remained high in Type 3 water but the protein/carbohydrate ratio decreased to < 1 due to an increase in the concentration of acid-soluble glucan. It was concluded that the communities were in an active phase of growth in Type 1 and Type 2 water when adequate nutrients were available, but were in a slow-growing phase in Type 3 water when nitrate concentrations were low. Correlation coefficients, simple linear regressions and stepwise multiple regressions between biochemical and environmental variables confirmed that nitrate was the nutrient most closely related to the biochemical composition of phytoplankton. Using linear regression equations of biochemical variables on glucan it was estimated that chlorophyll a existed in a ratio of ≈ 1: 1 between living phytoplankton and bacteria/detritus, while the percentage of ATP was high in the phytoplankton component of Type 1 water but low in that of Type 2 water. The percentage of protein in detritus was greater than in living phytoplankton, and the carbohydrate content of living phytoplankton increased as the upwelled water matured from Type 1 and Type 2 to Type 3.     

Estimating fish community diversity from environmental features in the Tagus estuary (Portugal): Multiple Linear Regression and Artificial Neural Network approaches

Relationships between environmental variables and diversity (Shannon‐Weaver index) of the fish communities in the Tagus estuary and adjacent coastal areas were analyzed. The focus was on the linearity or nonlinearity of these abiotic/biotic characteristics, with the aim to obtain an accurate short–medium term time‐scale diversity prediction from habitat variables alone. Multiple Linear Regressions (MLR) were used for the linear approach and Artificial Neural Networks (ANNs) for the nonlinear approach. MLR results in the external validation phase indicated a lack of model accuracy (R² = 0.0710; %SEP = 47.5868; E = ?0.0217; ARV = 1.0217; N = 43). Results of the best of the Artificial Neural Networks used in this study (12‐15‐15‐1 architecture) in the external validation phase (ANN: R² = 0.9736; %SEP = 7.8499; E = 0.9722; ARV = 0.0278; N = 43) were more accurate than those obtained with MLR. This indicates a clear nonlinear relationship between variables. In the best ANN model, nitrate concentration, depth, dissolved oxygen and temperature were the most important predictors of fish diversity in the Tagus estuary. The sensibility analysis indicated that the remaining variables (silicate, nitrite, transparency, salinity, slope, phosphate, water particulate organic matter, and chlorophyll a) played lesser roles in the model.     

Structural variations of phytoplankton in the coastal seas of Yucatan,Mexico

A study was done of the relationship between hydrographic variables and the composition, abundance, community structure and biomass spectrums of coastal phytoplankton at scales greater than 100 km on the Yucatan Peninsula (SE Gulf of Mexico). This was done during the season of greatest environmental instability in the region, the northwind season (late fall to winter). Samples were collected at stations in the west (Campeche), north (Yucatan), and east (Quintana Roo) zones of the Peninsula. Measurements were taken of temperature, salinity, dissolved oxygen, dissolved inorganic nutrients (ammonia, nitrite, nitrate and phosphate) and chlorophyll a, and samples were taken for phytoplankton analysis. The hydrographic results showed the Campeche zone as having the lowest salinity (<35 psu) values, as well as the highest inorganic nutrient and chlorophyll a values, all of which are related to continental water contributions. The Yucatan zone had the lowest temperatures and the lowest inorganic nutrient values, indicating influence from the Yucatan Current and the Gulf of Mexico. A total of 159 phytoplankton species were identified, dominated by diatoms (>80%) and dinoflagellates. Phytoplankton exhibited greater concentration, richness, equitability and diversity in Campeche, while the lowest community structure values were had in the Quintana Roo zone. The ordination analysis demonstrated that the dominant genera were the diatoms Chaetoceros, Pseudonitzschia and Thalassionema. The biomass spectrums exhibited the lowest slope in environments of higher heterogeneity, with Campeche being the most disturbed and heterogeneous zone and Quintana Roo that with the least heterogeneity.     

Distinct age and landscape influence on two reservoirs under the same climate

With the purpose of finding out whether different landscape occupation could affect water quality in two reservoirs of distinct age and subjected to the same climatic influence, several factors were investigated in a study lasting from January 2000 till December 2001. Total phosphorus, orthophosphate, chlorophyll a concentrations and water colour were determined monthly in winter and biweekly in summer, in two reservoirs located in the Portuguese part of River Douro catchment. Complementarily, variables such as nitrate, ammonium ion and ammonia gas, as well as water temperature, dissolved oxygen, conductivity, and transparency were measured. Trophic state of both reservoirs was assessed by computation of Carlson's Trophic State Index. The potential allochthonous sources of phosphorus and nitrogen to both reservoirs were identified and estimated. Differences between reservoirs were found for conductivity, water temperature, transparency and water colour. According to Carlson's Trophic State Index both reservoirs were classified as meso-eutrophic from winter to the beginning of summer, and as eutrophic from this period onwards. Intrinsic factors such as age of reservoir, organic matter inputs from decomposition of flooded terrestrial vegetation and exposure of littoral sediments to cycles of drying and wetting, as well as extrinsic factors such as grazing and frequent land fires, can explain the trophic state of S. Serrada Reservoir. Allochthonous sources of nutrients originated from agriculture and grazing in the catchment area, and recreational activities in the reservoir probably are the factors with greatest influence on Azibo Reservoir trophic state. Based upon the obtained data, management measures are suggested to prevent further eutrophication and water quality degradation in both reservoirs.     

Spatial and temporal variations of hydrological conditions,nutrients and chlorophyll <Emphasis Type="Italic">a</Emphasis> in a Mediterranean coastal lagoon (Mar Menor,Spain)

The Mar Menor is a sheltered and hypersaline lagoon, with salinity ranges between 38 and 51 psu. The lagoon is threatened by several pressures and in the last decades detrimental impact on the natural community structure and dynamics have increased. In the watershed, agricultural practices are rapidly evolving from extensive dry crop farming to intensively irrigated crops, with increasing loads of nutrient and pollutants to the lagoon. Hydrological conditions, nutrients and chlorophyll a concentrations were analysed in 1997 and 2002–2003 in a grid of 20 stations in the lagoon. Different time scales, from daily to inter-annual, were considered. In the considered periods, the dissolved inorganic nitrogen (DIN) increased whilst phosphate decreased significantly. These contrasting patterns depended upon the increased agricultural loading for DIN and were due to the implementation of the wastewater works for phosphates. In 1997 and 2002, the highest nitrate concentrations were usually found on the west coast of the lagoon, close to the mouths of the main watercourses. In parallel, the lowest concentrations were detected at the inner coastline along “La Manga” sandy bar and “El Estacio” channel. Based on weekly data, correlations between chlorophyll a concentrations and environmental variables disagreed with traditional eutrophication models. Relationships between chlorophyll a and nutrients were negative, suggesting that in the short term phytoplankton controlled nutrient concentrations. Moreover, nitrate and phosphorous seemed to alternate as limiting factors. The relationships between chlorophyll a became positive when considering time lags and analysed at longer time scales (monthly or seasonal means), thus suggesting a very rapid response of primary producers to nutrient enrichment. A significant correlation between chlorophyll a concentration and fish larvae density was also found at all time scales analysed, suggesting a top-down control of the trophic web.     

Short-term analysis of the phytoplankton structure and dynamics in two ponds with distinct trophic states from Cierva Point (maritime Antarctica)

Phytoplankton communities dominating Musgos and Papúa ponds with differing trophic states were sampled over 3 days enabling the detection of the physiological and population responses of microalgae to short-scale changes in biotic and abiotic factors, rather than frequently analyzed changes in community composition responses to long-scale environmental changes. We hypothesized that both environments undergoing diel changes would be dominated by phytoplankton with generalist strategies, while community structure would be mostly dictated by the trophic state of each water body. The phytoplankton biovolumes of both ponds were strongly dominated by euplanktonic nanoflagellated Chlorophyta, while phycocyanin-rich picocyanobacteria dominated the picophytoplankton. Parallel diel cycles of air and water temperatures were more pronounced on a sunny, warm day which prompted algal photosynthesis, revealed by strong increases in dissolved oxygen and pH. Nutrient and phytoplanktonic chlorophyll a confirmed the hypertrophic condition of Papúa pond. This accounted for the distinct community composition encountered in each pond, which remained stable throughout the study, as revealed by the SIMI index. The inverse relationship between the chl a/abundance ratio and the abundances of dominant species together with varying net growth rates (k′) showed algal reproduction, yet densities remained rather stable in both cases. In Musgos pond, fluctuations in k′ for small and median ciliates shadowed those of pico- and nanophytoplankton, respectively, strongly suggesting that they can control algal growth in these 2-level trophic chains.     

Population fluctuation and vertical distribution of meiofauna in the Red Sea interstitial environment

The composition and distribution of the benthic meiofauna assemblages of the Egyptian coasts along the Red Sea are described in relation to abiotic variables. Sediment samples were collected seasonally from three stations chosen along the Red Sea to observe the meiofaunal community structure, its temporal distribution and vertical fluctuation in relation to environmental conditions of the Red Sea marine ecosystem. The temperature, salinity, pH, dissolved oxygen, and redox potential were measured at the time of collection. The water content of the sediments, total organic matters and chlorophyll a values were determined, and sediment samples were subjected to granulometric analysis. A total of 10 meiofauna taxa were identified, with the meiofauna being primarily represented by nematodes (on annual average from 42% to 84%), harpacticoids, polycheates and ostracodes; and the meiofauna abundances ranging from 41 to 167 ind./10 cm². The meiofaunal population density fluctuated seasonally with a peak of 192.52 ind./10 cm² during summer at station II. The vertical zonation in the distribution of meiofaunal community was significantly correlated with interstitial water, chlorophyll a and total organic matter values. The present study indicates the existence of the well diversified meiofaunal group which can serve as food for higher trophic levels in the Red Sea interstitial environment.     

Influence of Physicochemical Parameters on the Abundance of Coliform Bacteria in an Industrial Site of the Hooghly River, India

Industrial effluents from jute, paper, pulp mills and sewage from households are regularly discharged into the Hooghly River. It generates a potential risk for both humans and animals of the area concerned. In the present study, water quality of the Hooghly River passing by the site of a growing township (Naihati, North 24 Parganas, West Bengal, India) was assessed throughout the year 2010 on the basis of the data collected on the physicochemical and microbiological parameters. The water samples collected on each month revealed the presence of higher amount of coliform bacteria, Streptococcus sp. and Escherichia coli, than the standard limit. Different physicochemical parameters like chemical oxygen demand, biological oxygen demand, dissolved oxygen (DO), total suspended solids, total dissolved solids (TDS), total hardness, alkalinity, chlorinity, nitrate and nitrite of the water at the sampling sites were found to be considerably higher than the levels standardized by WHO (2006). It was found that the relative abundance of Streptococcus and E. coli was influenced by two independent variables (water quality parameters), namely, DO and TDS. The abundance of coliform bacteria in the water sample warrants the adoption of proper measures to reduce the pollution level at the point source on way of scientific disposal of industrial effluents.     

Short-term variations in abundances and potential activities of viruses,bacteria and nanoprotists in Lake Bourget

Samples were collected at four depths every 6 h over a 42-h period during two contrasting seasons (June vs. December) from Lake Bourget, France, for evidence of circadian fluctuations in the concentrations and potential activities of viruses, prokaryotes and protists in relation to environmental conditions: temperature, chlorophyll a and dissolved organic carbon (DOC) concentrations. Considerable vertical and temporal fluctuations were observed for all variables. Circadian variations were noted for DOC and chlorophyll a concentrations. Despite the external abiotic forcing (light, water movements), the fluctuations of microbial variables (including viruses) in most cases were apparently linked to biotic factors and interactions. Standing stocks and activities, as well as the number and levels of correlations among the microbial components, were, surprisingly, higher in winter than in summer. We speculate that this was because trophic interactions prevailed over the seasonal forcing (i.e. temperature) in shaping the observed differences.     

Community structure of rotifers in two environments of the upper River Paraná floodplain (MS) - Brazil

We evaluate the influence of abiotic and biotic factors on the community structure of rotifers across a regional hydrological cycle in lotic and lentic environments of the upper River Paraná. Depth, transparency, temperature, pH, electrical conductivity, dissolved oxygen, chlorophyll a and densities of rotifers were measured at two stations in Lake Guaraná (littoral and open water regions) and at one station in the River Baía (open water region). Highest densities of rotifers were found at the lake littoral. Canonical correlation analysis related environmental variables with the densities of the most abundant rotifers. The strongest relationship was with chlorophyll a, dissolved oxygen, hydrological level and water temperature. Diversity of rotifers at each station was mainly explained by fluctuations in hydrological level. Results of grouping analysis suggested the formation of groups according to phases of the hydrological cycle.     

Trophic status determines the nirS-denitrifier community in shallow freshwater lakes

To explore whether trophic status is involved in identifying the denitrifier communities in shallow freshwater lakes, comparative studies regarding the variation of nirS-denitrifier communities were performed in water columns of six shallow freshwater lakes, ranging from mesotrophic to hypereutrophic status, in Jiangsu province of China. Restriction fragment length polymorphism (RFLP) analysis and cloning, followed by sequence analysis of selected samples were performed to examine the nirS-denitrifier communities. Results showed that a same predominant nirS unique operational taxonomic unit (OTU) appeared in mesotrophic, light eutrophic and middle eutrophic lakes, while a different nirS OTU appeared in the hypereutrophic lake. The nirS dendrogram could be divided into four clusters, and suggested a habitat-specific observation. The nirS libraries from lakes having mesotrophic and light eutrophic statuses grouped together, while those from lakes having middle eutrophic and hypereutrophic statuses diverged. The hypereutrophic lake had the lowest richness and evenness of nirS. Thus, the observed variation of nirS distribution and diversity was mainly affected by the trophic status, via some environmental factors such as pH, dissolved oxygen, nitrite, nitrate and chlorophyll-a concentrations.