Genetic diversity of picoeukaryotes in eight lakes differing in trophic status

The genetic diversity of picoeukaryotes (0.2-5.0 μm) was investigated in 8 lakes differing in trophic status in Nanjing, China. Denaturing gradient gel electrophoresis (DGGE) and cloning and sequencing of 18S rRNA genes were applied to analyze the picoeukaryotic communities. DGGE analysis showed that among the 8 lakes, the diversity of picoeukaryotes was greatest in the mesotrophic Lake Nan (24 bands) and least in the oligotrophic Lake Qian (12 bands). Cluster analysis of DGGE profiles revealed that the 8 lakes were grouped into 2 distinct clusters. Cluster 1 contained lakes Mochou, Zixia, Huashen, Nan, Pipa, and Qian, while cluster 2 contained lakes Xuanwu and Baijia. Clone libraries were constructed from the mesotrophic Lake Xuanwu and the oligotrophic Lake Zixia, and the 2 libraries were compared using the program LIBSHUFF. This analysis indicated that the picoeukaryotic community composition differed significantly between the 2 lakes (p = 0.001). A total of 25 operational taxonomic units were detected; 18 (62 clones) were related to known eukaryotic groups, while 7 (30 clones) were not affiliated with any known eukaryotic group. Alveolates and stramenopiles were the dominant groups in Lake Xuanwu, while alveolates and chlorophyta predominated in Lake Zixia. Multivariate statistical analysis indicated that the differences in the picoeukaryotic community composition of the 8 lakes might be related to trophic status and top-down regulation by metazooplankton.     

Genetic diversity of small eukaryotes in lakes differing by their trophic status

Small eukaryotes, cells with a diameter of less than 5 mum, are fundamental components of lacustrine planktonic systems. In this study, small-eukaryote diversity was determined by sequencing cloned 18S rRNA genes in three libraries from lakes of differing trophic status in the Massif Central, France: the oligotrophic Lake Godivelle, the oligomesotrophic Lake Pavin, and the eutrophic Lake Aydat. This analysis shows that the least diversified library was in the eutrophic lake (12 operational taxonomic units [OTUs]) and the most diversified was in the oligomesotrophic lake (26 OTUs). Certain groups were present in at least two ecosystems, while the others were specific to one lake on the sampling date. Cryptophyta, Chrysophyceae, and the strictly heterotrophic eukaryotes, Ciliophora and fungi, were identified in the three libraries. Among the small eukaryotes found only in two lakes, Choanoflagellida and environmental sequences (LKM11) were not detected in the eutrophic system whereas Cercozoa were confined to the oligomesotrophic and eutrophic lakes. Three OTUs, linked to the Perkinsozoa, were detected only in the Aydat library, where they represented 60% of the clones of the library. Chlorophyta and Haptophyta lineages were represented by a single clone and were present only in Godivelle and Pavin, respectively. Of the 127 clones studied, classical pigmented organisms (autotrophs and mixotrophs) represented only a low proportion regardless of the library's origin. This study shows that the small-eukaryote community composition may differ as a function of trophic status; certain lineages could be detected only in a single ecosystem.     

Changes in bacterial and ciliate densities with trophic status in Mediterranean shallow lakes

Ciliate and bacterial densities and their link with eutrophication were studied in fourteen shallow lakes in northwest Spain. Total phosphorus (TP) in these lakes varied between 30 μg l⁻¹ and 925 μg l⁻¹ and chlorophyll a concentration (chla) between 0.5 μg l⁻¹ and 107 μg l⁻¹. Bacterial abundance ranged from 1 × 10⁶ to 14 × 10⁶ cells ml⁻¹, while ciliate abundance ranged from 0.6 cells ml⁻¹ to 229 cells ml⁻¹. Lakes were classified into three trophic types from their TP and chla concentrations. Bacterial abundance was significantly correlated with trophic type, as well as with TP and with chla separately, whereas ciliate abundance was only correlated with chla. No significant relationship could be established between bacterial and ciliate abundance across the trophic gradient. A general pattern was observed in the ratios of bacterial abundance to TP and chla concentrations, of decreasing ratios with increases in the nutrient loading. This pattern was not found for ciliates. The dominant zooplankton group in 13 of the 14 lakes studied was Rotifera, which accounted for a mean of 71% of total zooplankton abundance (41% of zooplankton biomass). The positive correlation between bacteria and ciliates with this group, and the absence of any relationship with Cladocera suggest that top down control by cladocerans was weaker in our lakes than previously shown in northern European shallow lakes. Rotifers could be important predators of bacteria in the high-nutrient lakes of our study. Higher slopes of regressions on bacterial abundance towards the hypertrophic range indicate that top-down control was weaker in our lakes than in northern European shallow lakes.     

The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (N_tot 700, P_tot 40 μg l^?1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Võrtsjärv (N_tot 1600, P_tot 54 μg l^?1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total zooplankton abundance or biomass), whereas in L. Võrtsjärv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Võrtsjärv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Võrtsjärv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Võrtsjärv (biomass varied from 0.133 to 1.570 g m^?3; mean value 0.800 g m^?3 in L. Peipsi and from 0.201 to 0.706 g m^?3, mean 0.400 g m^?3 in L. Võrtsjärv; the corresponding data for abundances were: 8,000–43,000 ind. m^?3, mean 30,000 ind. m^?3 for L. Peipsi, 50,000–100,000, mean 52,000 ind. m^?3 for L. Võrtsjärv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Võrtsjärv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.     

The trophic state of shallow lakes in The Netherlands

A study has been carried out on the development of an ecological assessment method for shallow lakes in The Netherlands. Analyses of eutrophication characteristics of 93 lakes with, in total, 127 sampling localities gave insight into some of the steering variables, such as total-P, total-N, chlorophyll-a and transparency. On the basis of phytoplankton species composition, three main groups of lakes could be distinguished. The first group, characterized by persistent filamentous Cyanobacteria, had the highest summer means of chlorophyll-a and total-P. The second group was characterized by non-persistent filamentous Cyanobacteria, occurring in low abundancies, and at lower chlorophyll-a and total-P concentrations. The third group of lakes was characterized by the absence or very low abundance of the filamentous Cyanobacteria. As a result of these analyses, criteria for the abatement of eutrophication in shallow lakes could be refined.     

Modeling the role of wind and warming on Microcystis aeruginosa blooms in shallow lakes with different trophic status

This study focuses on the role of wind exposure and storm events, in interaction with trophic status and temperature, on the competition between two species: Microcystis aeruginosa and a typical green alga. It is based on a water column model containing ecological and fluid mechanic features including mixing and shear stress at the bottom. This model addresses for the first time the impact of storm events (inducing sediment and nutrient resuspension) on algal dynamics. Simulations with realistic environmental forcings were performed with different sets of wind, temperature, and trophic conditions. With normal temperatures, conditions for dominance and bloom formation of M. aeruginosa in summer are restricted to hypertrophic waters with low wind exposure. Higher wind exposure (above 2 m s^?1) impairs the formation blooms even in favorable trophic conditions and enhances the dominance of green algae. Hotter temperatures allow the dominance of M. aeruginosa for lower phosphorus conditions and higher wind exposure and lead to the exclusion of green algae for high phosphorus content and low wind exposure. Nevertheless, high wind exposure (above 3 m s^?1) still prevents dense bloom formation and allows the coexistence of both species. Storm events bring two counterbalancing features: sediment and nutrient resuspension. The first leads to a decrease of phytoplankton density in response to high turbidity, and the second leads to an increase and better maintenance of M. aeruginosa blooms due to high phosphorus concentration released in the water. This result depends on the timing of the event and on general wind exposure as phosphorus release only benefits M. aeruginosa if exposure to wind is low.     

Stable isotope analysis of the origins of zooplankton carbon in lakes of differing trophic state

Carbon stable isotope analysis was carried out on zooplankton from 24 United Kingdom lakes to examine the hypothesis that zooplankton dependence on allochthonous sources of organic carbon declines with increasing lake trophy. Stable isotope analysis was also carried out on particulate and dissolved organic matter (POM and DOM) and, in 11 of the lakes, of phytoplankton isolates. In 21 of the 24 lakes, the zooplankton were depleted in ¹³C relative to bulk POM, consistent with previous reports. δ¹³C for POM showed relatively little variation between lakes compared to high variation in values for DOM and phytoplankton. δ¹³C values for phytoplankton and POM converged with increasing lake trophy, consistent with the expected greater contribution of autochthonous production to the total organic matter pool in eutrophic lakes. The difference between δ¹³C for zooplankton and that for POM was also greatest in oligotrophic lakes and reduced in mesotrophic lakes, in accordance with the hypothesis that increasing lake trophic state leads to greater dependence of zooplankton on phytoplankton production. However, the difference increased again in hypertrophic lakes, where higher δ¹³C values for POM may have been due to greater inputs of ¹³C-enriched organic matter from the littoral zone. The very wide variation in phytoplankton δ¹³C between lakes of all trophic categories made it difficult to detect robust patterns in the variation in δ¹³C for zooplankton. Received: 2 November 1998 / Accepted: 3 December 1999     

Differences in food webs and trophic states of Brazilian tropical humid and semi-arid shallow lakes: implications of climate change

Global warming may intensify eutrophication of shallow lakes by affecting nutrient loading, evaporation rates, and water level and thus produce major changes in food webs. We investigated to what degree food webs in tropical humid lakes differed from those in more eutrophic semi-arid lakes of the same latitude. Our results indicate that the catchment area-to-lake area ratio, nutrients, chlorophyll a, suspended solids, abundances of phytoplankton, zooplankton, and omnivorous fish as well as total fish catch per unit effort were all higher in the semi-arid lakes, whereas inlet water-to-evaporation ratio (proxy for water balance), water transparency, percentage macrophytes cover, and the piscivores:omnivores ratio were higher in the humid lakes. Our results suggest that reduced inlet water-to-evaporation ratio will increase lake eutrophication, which, in turn, as in temperate regions, will alter trophic structure of the freshwater community.

     

Lability of organic carbon in lakes of different trophic status

1. We used first‐order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BOD_ult), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 μg L⁻¹, labile organic carbon (OC_L = 0.3BOD_ult) from 0.75 to 2.95 mg C L⁻¹ and k from 0.044 to 0.14 day⁻¹. 3. Our data showed that there were greater concentrations of OC_L but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OC_L was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27–55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long‐term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications.     

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.     

Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

The total nitrogen mass balance was studied over a two-year period in two reservoirs located in south-eastern Poland. Theobserved nitrogen retention (N_ret) inthe reservoirs ranged from 0.18 to 2.12g m^-2d^-1 (mean 0.83g m^-2d^-1) in case of theRzeszów Reservoir and from -0.02 to0.41 g m^-2d^-1 (mean 0.13g m^-2d^-1) in case of the SolinaReservoir, while the percentage retention(N_ret) ranged from 7 to 42% (mean22%) and -15 to 73% (mean 35%),respectively. A good relationship betweenN_ret and nitrogen loads was observedfor the Solina Reservoir. The data obtainedby the author and collected from theresearches of 6 other reservoirs located insouthern Poland were used to develop anempirical model suitable to predict thenitrogen seasonal dynamics in a reservoir.The model allows for the determination ofboth the annual mean N_ret value andits seasonal variation in a reservoir.     

Reconstructing the historical trophic status of northwestern Pennsylvania lakes using GIS

From each of 46 watersheds in glaciated northwestern Pennsylvania we estimated phosphorus export (kg P/ha/yr) from weekly or twice-weekly measured stream phosphorus concentrations and measured stream discharges, and determined land covers using GIS. Simple and step-down multiple regression analyses yielded models that explained 24% of the variation in P export using land cover within whole watersheds, and 64% of the variation using land cover within 200 m riparian buffers. We used these models to predict P loading to seven lakes and found that predicted lake [P] was consistent with measured lake [P]. To estimate pre-settlement lake [P] we reapplied the P export models with the assumption that human-impacted land covers were originally forests. Predicted (hindcast) pre-settlement lake [P] indicated that six of the seven lakes were edaphically mesotrophic (10 < [P] < 20 μg/l). Lake remediation targets set on the assumption that area lakes were historically oligotrophic ([P] < 10 μg/l) will be unattainable.     

The microbial loop in the planktonic communities in lakes with various trophic status

The structure of planktic trophic chains was studied in eight lakes of European Russia and five lakes in Central Asia. The lakes differed in the level of productivity, morphometric parameters, and the type of agitation and mineralization. It is found that the microbial loop of picophototrophic organisms, bacteria, heterotrophic flagellates, infusoria, and viruses constitutes 12.3-64.7% of the total plankton biomass. Positive correlation between the biomass of microbial community and the primary production of phytoplankton is observed, whereas no relation is revealed between the share of microorganisms in the plankton biomass and the trophic status of the water body. The presence of a great number of cladocerans decreased the role of the microbial loop in the structural organization of the planktic community. Heterotrophic flagellates consuming 3-81% of daily bacterial production were the principal cause of bacteria elimination only in some of the studied water bodies.     

Evidence for invasive carp mediated trophic cascade in shallow lakes of western Victoria,Australia

Hydrobiologia - Lakes of western Victoria are typically shallow (2–3 m), slightly saline (3–8 ppt) and eutrophic (total phosphorus 0.1–5&nbsp;mg l?1 and total nitrogen...     

Size‐based interactions across trophic levels in food webs of shallow Mediterranean lakes

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Vertical distribution of attached algae in shallow fishponds of different trophic status

Spring and summer vertical distributions of attached algae were studied on artificial substrates (glass cover slips) in shallow fishponds of different trophic status. Differences were found in colonization process between two fishponds, two seasons and between two sites at each fishpond (central part, fishpond shore). Oligotrophic fishpond U třech krátkych can be characterized by better light conditions and lower temperatures. While equal vertical distribution of periphytic algae was observed at oligotrophic site, their relative cover decreased continuously in spring and strongly in summer with the increasing depth at eutrophic site. Oligotrophic fishpond can be characterized by very slow colonization rate and increasing abundance of Chrysophyceae. On the other hand Cyanobacteria were more frequent in eutrophic fishpond where periphyton displayed high colonization rate. Diatom species richness was the highest at oligotrophic site during spring experiments. The sites near the fishpond shore compared to sites in the central part of the fishpond can be characterized by higher rate of colonization, higher periphyton relative cover, lower proportion of Chrysophyceae and higher proportions of diatoms and cyanophytes indicating probably higher trophic level.     

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

The role of cladocerans in tracking long-term change in shallow lake trophic status

Shallow lakes have been affected by a variety of human activities profoundly altering their ecological structure and function. Cladocerans have been used to track change resulting from a variety of drivers at a number of time scales. Aquatic macrophytes are well recognised as reflecting the ecological condition of a lake. Here, we compare the plant macrofossils with the sub-fossil cladoceran assemblages from 20 dated sediment cores. Co-correspondence analysis was used to determine the degree of commonality of change in community composition of the two biological groups through time. This analysis revealed very high levels of agreement in the nature and timing of change at all the sites examined with very high correlation coefficients between the axis 1 scores for macrofossils and cladocerans. Furthermore, at all sites a high proportion of the variance (min 20%, max 54%) in the macrofossil data was explained by the change in the cladoceran assemblage. Sub-fossil macrofossil and cladoceran assemblages, from at least from 1700 AD onwards, were examined in more detail at three sites: Ormesby Great Broad, Felbrigg Lake and Lake Søbygaard. There was very good accord in the main shifts of the cladoceran and macrofossil assemblages at all three sites. This may reflect the long-term shift in the principal focus of primary production from the benthic to the pelagic habitat. We suggest that the combination of their central position in the food-web and the presence of both pelagic and benthic taxa make cladocerans a strong candidate as the single best indicator of (palaeo) ecological condition related to changing trophic status and alteration in food-web structure in shallow lakes.     

Zooplankton structure in the Loosdrecht lakes in relation to trophic status and recent restoration measures

A five-year zooplankton study (1982–86) on three shallow and highly eutrophic lakes in the Loosdrecht area (The Netherlands) did not reveal any significant changes following the considerable reduction in external P-loading (from about 1.0 g to 0.3 g P m^–2 year^–1) since mid-1984.The recent annual fluctuations in the rotifer and crustacean densities are within the range of those found before the restoration measure became operative. A decrease in the average size of the crustaceans and an absence of large-bodied forms reflects an increased fish predation rather than a change in the quality or quantity of their sestonic food ( < 150 µm) which continues to be dominated by filamentous cyanobacteria and Prochlorothrix hollandica, a prochlorophyte discovered in these lakes recently.     

A phytoplankton trophic index to assess the status of lakes for the Water Framework Directive

Despite improvements in wastewater treatment systems, the impact of anthropogenic nutrient sources remains a key issue for the management of European lakes. The Water Framework Directive (WFD) provides a mechanism through which progress can be made on this issue. The Directive requires a classification of the ecological status of phytoplankton, which includes an assessment of taxonomic composition. In this paper, we present a composition metric, the plankton trophic index, that was developed in the WISER EU FP7 project and demonstrate how it has been used to compare national phytoplankton classification systems in Northern and Central Europe. The metric was derived from summer phytoplankton data summarised by genus from 1,795 lakes, covering 20 European countries. We show that it is significantly related to total phosphorus concentrations, but that it is also sensitive to alkalinity, lake size and climatic variables. Through the use of country-specific reference values for the index, we demonstrate that it is significantly related to other national phytoplankton assessment systems and illustrate for a single European (intercalibration) lake type how it was used to intercalibrate WFD boundaries from different countries.