Observations on cyanobacterial population collapse in eutrophic lake water

In two laboratory-scale enclosures of water from the shallow, eutrophic Lake Loosdrecht (the Netherlands), the predominating filamentous cyanobacteria grew vigorously for 2 weeks, but then their populations simultaneously collapsed, whereas coccoid cyanobacteria and algae persisted . The collapse coincided with a short peak in the counts of virus-like particles. Transmission electron microscopy showed the morphotype Myoviridae phages, with isometric heads of about 90 nm outer diameter and >100-nm long tails, that occurred free, attached to and emerging from cyanobacterial cells. Also observed were other virus-like particles of various morphology. Similar mass mortality of the filamentous cyanobacteria occurred in later experiments, but not in Lake Loosdrecht. As applies to lakes in general, this lake exhibits high abundance of virus-like particles. The share and dynamics of infectious cyanophages remain to be established, and it is as yet unknown which factors primarily stabilize the host–cyanophage relationship. Observations on shallow, eutrophic lakes elsewhere indicate that the cyanophage control may also fail in natural water bodies exhibiting predominance of filamentous cyanobacteria. Rapid supply of nutrients appeared to be a common history of mass mortality of cyanobacteria and algae in laboratory and outdoor enclosures as well as in highly eutrophic lakes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Targeted deep sequencing reveals high diversity and variable dominance of bloom-forming cyanobacteria in eutrophic lakes

Cyanobacterial blooms in eutrophic lakes are severe environmental problems worldwide. To characterize the spatiotemporal heterogeneity of cyanobacterial blooms, a high-throughput method is necessary for the specific detection of cyanobacteria. In this study, the cyanobacterial composition of three eutrophic waters in China (Taihu Lake, Dongqian Lake, and Dongzhen Reservoir) was determined by pyrosequencing the cpcBA intergenic spacer (cpcBA-IGS) of cyanobacteria. A total of 2585 OTUs were obtained from the normalized cpcBA-IGS sequence dataset at a distance of 0.05. The 238 most abundant OTUs contained 92% of the total sequences and were classified into six cyanobacterial groups. The water samples of Taihu Lake were dominated by Microcystis, mixed Nostocales species, Synechococcus, and unclassified cyanobacteria. Besides, all the samples from Taihu Lake were clustered together in the dendrogram based on shared abundant OTUs. The cyanobacterial diversity in Dongqian Lake was dramatically decreased after sediment dredging and Synechococcus became exclusively dominant in this lake. The genus Synechococcus was also dominant in the surface water of Dongzhen Reservoir, while phylogenetically diverse cyanobacteria coexisted at a depth of 10 m in this reservoir. In summary, targeted deep sequencing based on cpcBA-IGS revealed a large diversity of bloom-forming cyanobacteria in eutrophic lakes and spatiotemporal changes in the composition of cyanobacterial communities. The genus Microcystis was the most abundant bloom-forming cyanobacteria in eutrophic lakes, while Synechococcus could be exclusively dominant under appropriate environmental conditions.     

Effects of food type on the life history of Daphnia clones from lakes differing in trophic state. II. Daphnia cucullata feeding on mixed diets*

1. The effects of feeding on suboptimal foods were investigated in Daphnia cucullata a zooplankton common in many types of lakes. Eleven clones of D. cucullata were collected from four lakes of varying trophic levels and fed a high (1 mg C l^–1) concentration of one of two diets: (i) a 1 : 9 mixture of the cryptophyte Cryptomonas pyrenoidifera with the green alga Scenedesmus obliquus ; and (ii) a 1 : 9 mixture of the cryptophyte C. pyrenoidifera with the filamentous cyanobacterium Oscillatoria limnetica.
2. As expected, the diet dominated by Oscillatoria was found to be significantly poorer compared with the diet dominated by Scenedesmus. All D. cucullata clones were, however, able to achieve positive population growth rates on both diets, supporting the view that daphnids can grow and reproduce on a diet dominated by filamentous cyanobacteria.
3. Clones originating from different types of lakes differed in their responses to the two diets. On the Scenedesmus -dominated diet, clones originating from moderately eutrophic lakes had lower population growth rates than clones from hypertrophic lakes. In contrast, on the Oscillatoria -dominated diet, the clones from moderately eutrophic lakes had higher population growth rates than clones from hypertrophic lakes. Contrary to expectation, clones originating from lakes dominated by filamentous cyanobacteria had more difficulty utilizing filamentous cyanobacteria as food than clones from less eutrophic lakes which contain less filamentous cyanobacteria.
4. As the reactions of clones originating from same type of lake resembled each other, it is hypothesized that the life histories of D. cucullata populations are locally adapted to environmental factors other than abundance of filamentous cyanobacteria, which are correlated with trophic levels.     

Comparative analysis of cyanobacterial communities from polluted reservoirs in Korea

Cyanobacteria are the dominant phototrophic bacteria in water environments. Here, the diversity of cyanobacteria in seven Korean reservoir waters where different levels of algal blooms were observed during the summer of 2002, was examined by T-RFLP analysis. The number of T-RF bands in the HaeIII T-RFLP profiles analyzed from those water samples ranged from 20 to 44. Of these, cyanobacteria accounted for 6.1 to 27.2% of the total bacteria. The water samples could be clustered into 2 groups according to the Dice coefficient of the T-RF profiles. The eutrophic Dunpo and oligotrophic Chungju reservoirs were selected, and several representative clones from both reservoir waters analyzed for the nucleotide sequences of their 16S rDNA. The major clones were found to belong to the Microcystis and Anabaena species in the waters from the Dunpo and Chungju reservoirs, respectively, which was in agreement with the T-RFLP result. That is, the Microcystis and Anabaena species were dominant in the eutrophic and polluted Dunpo and oligotrophic Chungju reservoir waters, respectively. These results indicated that there is a correlation between prevalence of cyanobacterial species and levels of pollution in reservoir waters.     

汤溪水库富营养化特征与水质管理对策

通过2003年对广东省汤溪水库水生态学进行调查,分析了该水库的富营养化特征,并从流域管理和水动力学两个方面对水库水质管理对策进行了探讨。汤溪水库为中度富营养化水体,具有明显时空分布差异,丰水期富营养程度高于枯水期,溪头河流区高于新桥河流区。蓝藻是水库浮游植物的主要优势类群,其中引发水华的铜绿微囊藻(Microcystis aeruginosa)为优势种。水库上游集水流域分布近30万居民,大量的生活排污和农业污染形成了主要的外源污染源,这些外源污染的输入是水库水体营养盐的主要来源;合理处理上游居民生活垃圾、减少生活污染中磷含量、增加流域和库区周围植被覆盖、合理使用农业化肥是降低水库表层水体中营养盐浓度的主要途径。由于该水库的蓝藻和富营养化水平直接与水体的稳定性有关,利用对水库排水的可控制性调节水库水体的稳定性,可作为控制蓝藻水华发生的有效途径。     

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

Lake Breukeleveen (180 ha, mean depth 1.45 m), a compartment of the eutrophic Loosdrecht lakes system, was selected to study the effects of whole-lake foodweb manipulation on a large scale. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970–1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2 g m⁻² y⁻¹ to 0.35 g m⁻² y⁻¹. The water transparency (Secchi-depthca. 30 cm), however, has not improved. The aim of the food-web manipulation in Lake Breukeleveen was not only to improve the light climate of the lake, but also to study if the successfull effects observed in small lakes (a few ha) can be upscaled. In March 1989 the standing crop of planktivorous and bentivorous fish populations was reduced by intensive fishery, fromca. 150 kg ha⁻¹ toca. 57 kg ha⁻¹. The lake was made unaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0⁺ pike. However, water transparency did not increase in the following summer and autumn 1989, which is in contrast with great improvement in the light conditions previously observed in smaller lakes. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem.     

EVALUATION OF ENVIRONMENTAL FACTORS ON CYANOBACTERIAL BLOOM IN EUTROPHIC RESERVOIR USING ARTIFICIAL NEURAL NETWORKS1

Cyanobacterial blooms are a common issue in eutrophic freshwaters, and some cyanobacteria produce toxins, threatening the health of humans and livestock. Microcystin, a representative cyanobacterial hepatotoxin, is frequently detected in most Korean lakes and reservoirs. This study developed predictive models for cyanobacterial bloom using artificial neural networks (ANNs; self‐organizing map [SOM] and multilayer perceptron [MLP]), including an evaluation of related environmental factors. Fourteen environmental factors, as independent variables for predicting the cyanobacteria density, were measured weekly in the Daechung Reservoir from spring to autumn over 5 years (2001, 2003–2006). Cyanobacterial density was highly associated with environmental factors measured 3 weeks earlier. The SOM model was efficient in visualizing the relationships between cyanobacteria and environmental factors, and also for tracing temporal change patterns in the environmental condition of the reservoir. And the MLP model exhibited a good predictive power for the cyanobacterial density, based on the environmental factors of 3 weeks earlier. The water temperature and total dissolved nitrogen were the major determinants for cyanobacteria. The water temperature had a stronger influence on cyanobacterial growth than the nutrient concentrations in eutrophic waters. Contrary to general expectations, the nitrogen compounds played a more important role in bloom formation than the phosphorus compounds.     

Warming promotes cold-adapted phytoplankton in temperate lakes and opens a loophole for Oscillatoriales in spring

The effects of the recent warming trend in many northern temperate lakes on the species composition of spring phytoplankton remain poorly understood, especially because a recent change in nutrients has complicated efforts, and previous studies have defined spring according to the calendar. We analysed data from 1979 to 2004 from Lake Müggelsee (Berlin, Germany), using physical and biological parameters to define the spring period. We show that a change in timing of spring plankton events in warm years led to the paradox of lower mean water temperatures during the growth period, favouring cold-adapted diatoms over cyanobacteria, and within the diatoms, some cold-adapted centric forms over pennate forms. Under high P : Si ratios, the increased time between phytoplankton and cladoceran peaks opened a loophole for filamentous cyanobacteria (Oscillatoriales) in warm years to establish dominance after the diatoms, which are silicate limited. Therefore, the warming trend promotes filamentous cyanobacteria, a well-known nuisance in eutrophic lakes, and surprisingly, cold-adapted diatoms.     

Seasonal changes of phytoplankton and cyanobacteria/cyanotoxin risk in two shallow morphologically altered lakes: Effects of water level manipulation (Wieprz-Krzna Canal System,Eastern Poland)

Seasonal development of phytoplankton was compared over two years in two eutrophic, morphologically altered lakes. During study, the water level in Lake Tomaszne were periodically regulated whereas in Lake Mytycze, the water level manipulation was not performed. We assumed that the supply with fertile waters and subsequent discharge can be reflected in seasonal changes of phytoplankton taxa composition and biomass, especially of toxigenic cyanobacteria. In Lake Tomaszne, the development of toxigenic Nostocales (Aphanizomenon gracile and Dolichospermum planctonicum) was supported by the entrance of water from a canal (due to supply of water containing NH₄⁺-N). After water discharge, the replacement of Nostocales by the toxigenic Planktothrix agardhii was associated with low light conditions and a supply of phosphates from the bottom sediments. In Lake Mytycze, microalgae (Chlorococcales) were predominant throughout the study period. The rapid growth of toxigenic cyanobacteria (A. gracile, Planktolyngbya limnetica and Microcystis spp.) occurred only at extremely low water levels. In Lake Tomaszne, the lake with periodic water level regulation, the high cyanobacteria/cyanotoxin risk and decline of its ecological status were more conspicuous than in Lake Mytycze, the lake with natural water level changes.     

Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers’ diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.     

On the annual variation of phytoplankton biomass in Finnish inland waters

Annual variations in phytoplankton biomass in 63 lakes in Southern and Central Finland are discussed. Biomass is rather small during winter (January–April), usually <0.05 mg l^–1 (fresh weight) and there are no differences between oligotrophic and eutrophic lakes. In early spring and in autumn biomass varies widely, depending mainly on water temperature. Phytoplankton biomass is smaller in July than in June and August in oligotrophic lakes (biomass <0.20 mg l^–1 fresh weight) and mesotrophic (biomass 1.0–2.5 mg l^–1) lakes, but greater in eutrophic (biomass 2.5–10.0 mg l^–1) and hypereutrophic (biomass >10.0 mg l^–1) lakes. The standard deviation of phytoplankton biomass in Finnish inland waters is usually smallest in July, which facilitates the comparison of phytoplankton between different kinds of lakes.     

Diversity,taxonomy and autecology of autochtonous epipelic cyanobacteria of the genus <Emphasis Type="Italic">Komvophoron</Emphasis> (Borziaceae,Oscillatoriales): a study on populations from the Czech Republic and British Isles

Epipelic cyanobacteria were studied in bottom sediments from ponds and lakes in the United Kingdom and the Czech Republic, covering a trophic gradient from deep, oligotrophic, glacial lakes to shallow, eutrophic, urban manmade ponds. Although many planktic species were observed, the most frequently occurring cyanobacteria were the motile filamentous genera Oscillatoria, Phormidium, Pseudanabaena and Komvophoron. These genera represent autochtonous epipelic assemblages, occurring in upper surface layers of the sediments throughout the year and exhibiting seasonal variation in their abundancy. The occurrence of individual Komvophoron species was influenced by sediment quality, particularly the proportions of fine mud and organic detritus. A new species Komvophoron hindakii sp. nov. is described. Morphological features, autecology and occurrence of all the Komvophoron-like morphospecies found are discussed.     

Restoration and recovery of shallow eutrophic lake ecosystems in The Netherlands: epilogue

During the symposium Restoration and recovery of shallow lake ecosystems in The Netherlands studies on restoration of eutrophic lakes were addressed and discussed. Many Dutch shallow lakes have received high external loadings of phosphorus through supply water that is influenced by the River Rhine and loadings in The Netherlands. Two important Action Plans (the Rhine Action Plan, the North Sea Action Plan) are now in operation to reduce nutrient emissions. The targets set are not likely to be fully reached, so that supplementary reduction of phosphorus supplied to inland fresh waters will be required. In several shallow lakes such a reduction has been achieved recently, but without leading to discernible recovery. The main causes of delay are phosphorus storage and its subsequent release from sediments and foodweb; however, the remaining extraneous phosphorus supply is often still too high. Supplementary actions are, therefore, called for. A further reduction of phosphorus inputs is suggested, besides supplementary measures proposed, viz. dredging, flushing, biomanipulation, chemomanipulation. Restoration to the past situation via upwelling groundwater appeares to be feasible in some cases. There is a common consensus that each lake behaves differently depending to its morphology, hydrology and history of eutrophication. Therefore each lake has to be studied before restoration measures can be applied. Besides, the ecosystem should not only be studied as a separate entity, but as a part of systems of a higher integration level.     

Effects of resuspension and eutrophication level on summer phytoplankton dynamics in two hypertrophic areas of Lake Taihu,China

The composition of summer phytoplankton communities in two highly eutrophic areas of Lake Taihu, a large, shallow water body in China (area 2,338 km²; mean depth 2.0 m), was studied in relation to selected environmental variables. Both areas have multiple uses, including drinking water supply, fisheries, and recreation. The first area, Meiliang Bay, is much closer to central lake and subjected to wind-driven sediment resuspension, while the second, at Wuli Lake, is less affected by wave action but is considered more nutrient-rich due to its proximity to human inhabitation. By comparing two lake areas, we attempt to advance the understanding and management of different regions of shallow lakes, mainly differing in resuspension levels and eutrophication levels. A comparison of these two lake areas indicated a higher standing crop of phytoplankton in Wuli Lake than in Meiliang Bay. Canonic Correspondence Analysis indicated that nutrient status and sediment resuspension were the principal factors influencing phytoplankton composition in both areas. The dominant algal group in Meiliang Bay comprised immobile R-type strategists (Reynolds in Growth and reproductive strategies of freshwater phytoplankton, 1988), including filamentous diatoms and green algae (Aulacoseira granulate and Planctonema lauterbornii), and recruitment-beneficial RS-type strategists, mainly the colonial cyanobacteria (Microcystis spp.). The community of Wuli Lake was dominated by S-strategists, principally motile flagellates (Cryptomonas spp.) and C-strategists including small nonmotile centric diatoms and chlorococcal green algae.     

Lake restoration with and without dredging of phosphorus-enriched upper sediment layers

Human activity has been the cause of continuing decline of water quality in most Dutch lakes. Development of lake restoration programmes must take into account the lake functions. Major reduction of the nutrient and pollutant loading is the primary step in lake restoration. Still, the recovery of eutrophic lakes is retarded frequently because of internal phosphorus loading by the lakes' sediments. Sediment dredging, as an additional tool for water quality management to accelerate accomplishing the desired water quality, is studied. In this paper we evaluate the preliminary results of eight lake restoration projects in the Netherlands. The lakes are compared in order to estimate the magnitude of the internal phosphorus loading. Dredging as an additional measure was carried out twice in the peatlake Geerplas. In the Nieuwkoop Lakes only the external phosphorus loading was substantially reduced from 0.9 to 0.2 g P m^–2 y^–1. Provisional results of these two shallow peatlake restoration projects focussed on eutrophication abatement with and without dredging, are presented. Both show a decrease in phosphorus concentration in the lakes. The necessity to dredge the lakes is discussed.     

Phototrophic Picoplankton in Temperate Lakes: Seasonal Abundance and Importance along a Trophic Gradient

The seasonal development of autotrophic picoplankton was investigated in seven Danish lakes representing a eutrophication gradient. Highest cell abundance between 1.5 to 6 × 10⁵ cells ml⁻¹ were found in mid-summer. Minor peaks were observed in spring. In winter, densities were below 10³ ml⁻¹. The highest relative picoplankton contribution to total autotrophic biomass also occurred in mid-summer. In the eutrophic lakes and one humic lake the average seasonal contribution of picoplankton to total chlorophyll was below 1% increasing to 5-8% in the meso- and oligotrophic clear water lakes. During short periods the proportion of picoplankton did reach 25%. The higher relative importance of picoplankton in less productive lakes was not due to higher actual chlorophyll concentrations, but due to a much more pronounced response by larger algae at higher nutrient loading. Both cyanobacteria and eukaryote organisms were present as picoplankton. Only eukaryotes were found in one eutrophic lake and an acidic, humic lake. In the eutrophic lakes eukaryote picoplankton was dominant; both with respect to cell densities and biovolume, whereas cyanobacteria dominated the two meso-oligotrophic lakes. Autotrophic picoplankton were present in all lake types, however their importance seemed to be less in most eutrophic lakes than in less productive, meso-oligotrophic lakes.     

Phytoplankton assemblages and steady state in deep and shallow eutrophic lakes – an approach to differentiate the habitat properties of Oscillatoriales

Ecological conditions and phytoplankton succession in two shallow hypertrophic lakes (Langer See and Melangsee) and a dimictic, eutrophic lake (Scharmützelsee) in a lake chain in Eastern Germany were analyzed from 1999 to 2001 in order to find situations of phytoplankton steady state assemblages and variables controlling the phytoplankton composition according to Reynolds et al. (2002). Long term background data from 1993 to 2001 suggest steady state conditions in shallow lakes, whereas the deep lake exhibited irregular fluctuations between various phytoplankton stages. Since the phytoplankton composition in the shallow lakes was similar in all the 3 years, it was highly predictable. Steady state conditions dominated by different species of Oscillatoriales were detected during the summer period 1999 and 2000 in Langer See and in Melangsee (see Mischke & Nixdorf, this volume). This dominant assemblage found in both lakes (group S ₁ acc. to Reynolds et al., 2002): Planktothrix agardhii (Gom.) Anagn. et Kom., Limnothrix redekei (Van Goor) Meffert, Pseudanabaena (Lauterb.) is typical in turbid mixed layers with highly light deficient conditions, but it is also regularly dominant in the dimictic lake Scharmützelsee as observed in 1999 and 2001 (Pseudanabaena limnetica (Lemm.) Kom. The Nostocales Cylindrospermopsis raciborskii (Wolz.) Seenayya et Subba Raju and Aphanizomenon gracile (Lemmerm.) Lemmerm. were important in the shallow lakes as well as in lake Scharmützelsee. Nevertheless, the occurrence of filamentous cyanobacteria in the dimictic lake was not regular and an unpredictable change in phytoplankton development was observed in 2000. It is discussed, whether this phenomenon of regular succession in shallow hypertrophic lakes is caused by adaptation to a resilient and an extreme environment or by the pool of species that can live or survive in that environment. This was checked through comparison of the depth of the mixed layer, the mean daily irradiance within this layer and the nutrient resources. Although the nutrient resources in both types of lake are near threshold levels, indicating growth inhibition by dissolved nutrients (DIP, DIN, TIC, DSi), the under water light supply seems to be the key factor favoring the dominance of filamentous cyanobacteria belonging to the functional group S ₁.     

Eutrophication of moderately deep Dutch lakes during the past century: flaws in the expectations of water management?

We studied the trophic development of the past 30–100 years in eight moderately deep Dutch lakes based on their sedimentary fossil diatom assemblages. The dominant diatoms indicating meso- to eutrophic conditions were Aulacoseira subarctica, Cyclotella ocellata, C. cyclopuncta, C. meneghiniana, Puncticulata bodanica, Aulacoseira granulata, Cyclostephanos dubius, C. invisitatus, Stephanodiscus hantzschii, S. medius, and S. parvus. Ordination of diatom data separated the lakes into four groups according to their total phosphorus concentrations (TP), water supply, water management, and origin. The first group consists of dike-breach lakes, which were in stable eutrophic to hypertrophic conditions throughout the past century with diatom-inferred TP (DI-TP) concentrations of between 70 and 300 μg l⁻¹. The main factors influencing these dike-breach lakes are river management, ground water supply of riverine origin, and local land use. The second group are artificial lakes of fluctuating oligo- to mesotrophic conditions and DI-TP concentrations of 10–30 μg l⁻¹. Only one of the artificial lakes showed a DI-TP increase due to changes in catchment agricultural practice. A third group includes an artificial moat and an inland dike-breach lake with DI-TP concentrations of 50–100 μg l⁻¹. The fourth group contains an individual dike-breach lake with stable mesotrophic conditions of 50 μg l⁻¹ throughout the past century. Rather than showing a regional pattern, the studied lakes behave very individualistically with regard to their trophic history, reflecting changes in the local hydrology and in their nutrient sources.     

The state of the environment of the Loosdrecht lakes

The Loosdrecht lakes are a system of shallow, interconnected, peat lakes in the centre of The Netherlands. The main environmental functions of the Loosdrecht lakes are nature and recreation. From the point of view of the Dutch policy, a Specific Environmental Quality (Bijzondere Milieukwaliteit) should be set for these lakes.The most serious environmental problem of the area is eutrophication. The Loosdrecht lakes have, by increasing external phosphorus loading, changed, from clear lakes with few macrophytes, followed by a period of abundant characean growth, to turbid lakes dominated by cyanobacteria and detrital matter. Eutrophication was counteracted by use of sewerage systems and dephosporization of the supply water. The resultant decrease in external phosphorus loading did not result in a decrease of turbidity by suspended particles.The eutrophication of the lake ecosystems was described as a series of phases. One of those phases, the status around 1940, has been used as an ecological reference system.By means of a graphical presentation technique, the so-called AMOEBE-approach, the state of the environment of the Loosdrecht lakes has been visualized. Thirty-two ecological parameters, including both biotic and abiotic factors, have been selected and quantified. Concrete target values for these parameters have been derived from historical reports and from Lake Western Loenderveen, located close to the Loosdrecht lakes, but less eutrophic.The general conclusion is that the state of the environment of the Loosdrecht lakes is far from what is required with respect to a Specific Environmental Quality, as many of the selected parameters, like water transparency, total phosphorus, mineral nitrogen, cyanobacteria, bream, pike, macrophytes, birds and otter, deviate by over an order of magnitude from their desired levels.     

Genetic diversity of cyanobacteria in four eutrophic lakes

Recent studies indicate genetic diversity of cyanobacteria in eutrophic lakes is not represented well by culture collections or morphology. Yet, few studies have investigated genetic richness and evenness of cyanobacteria using culture-independent methods. We compared the genetic structure of cyanobacteria supported by four neighboring eutrophic lakes during the ice-free season. The partial phycobilincpcB/A genes plus intergenic spacer (PC-IGS) was used as a genetic marker.Sequences were phylogeneticallygrouped by maximum likelihood into genotypes representing sub-genera of the major taxa. Genotypes fell into genera commonly observed by microscopy in these lakes including Microcystis, Aphanizomenon, Chroococcus, Anabaena, and Cylindrospermopsis. Only three genotypes were shared among all four lakes, despite significant water flowage between lakes.A Parsimony P-test indicated lakes were significantly (p=0.01) clustered on the maximum likelihood tree. Pairwise differences using Unifrac distance were moderately or not significant. Analysis of molecular variance (AMOVA) indicated genetic variation among all genotypes (φ=0.06, p<0.001) and 94% of variability occurred within lakes rather than between lakes (6%), explaining the lack of pairwise differences between lakes. Lorenze curves of genotype abundance in each lake showed genetic structure was only moderately uneven (Gini coefficients of 0.37-0.5) indicating lakes did not support dominant genotypes. Overall, results from this study suggest diversity of cyanobacteria is shaped by heterogeneity within lakes (temporally or spatially) and relatively even population structures.