Distribution of the bathypelagic perch fry layer along the longitudinal profile of two large canyon-shaped reservoirs

The distribution of the bathypelagic perch Perca fluviatilis fry (BPF) layer and its qualitative and quantitative changes were studied along the longitudinal profiles of the large canyon-shaped Slapy and Orlík Reservoirs (Czech Republic), using acoustic methods (SIMRAD EY500 echosounder, split-beam transducer, Sonar5 post-processing software). In Slapy Reservoir (sampled in late May), the BPF layer created by the mass of non-shoaling perch larvae and juveniles (average total length, L _T, 10·4 mm) was recorded from the dam for 29 km upstream. The BPF layer only vanished in the upper third of the reservoir due to the extremely cold hypolimnetic water discharged from Orlík Reservoir, which is situated upstream in the cascade. Both abundance and size of BPF increased significantly upstream following, in Slapy Reservoir, the trophic gradient. In Orlík Reservoir (sampled in late June), the BPF layer was created predominantly by shoaling perch fry individuals (average L _T 31·8 mm), recorded along the whole longitudinal profile of the reservoir (>50 km, filled with relatively warm water). Both the sizes of BPF and their shoaling activity again increased significantly from the dam towards the inflow following, in Orlík Reservoir, the trophic gradient. Pooling the data from both reservoirs, it was evident that the tendency to flock in a dense layer and, much later, to create distinct shoals, increased continuously with the size of BPF. A number of variables describing the BPF layer in Slapy and Orlík Reservoir are given.     

流溪河水库敞水区轮虫多样性与群落的动态特征

流溪河水库是位于北回归线上的大型峡谷型供水水库,于2013—2014年对该水库敞水区轮虫群落及其环境因子进行观测,分析敞水区轮虫多样性与群落动态特征。流溪河水库介于贫营养到中营养水平之间,丰水期(5月—9月)的总磷、透明度均高于枯水期(10月—4月),两年的总磷的平均值为0.019mg/L,透明度为2.55m。两年共检出轮虫40种,单月检出物种数波动范围为8—19种,月均检出轮虫13种。螺形龟甲轮虫、真翅多肢轮虫、沟痕泡轮虫和胶鞘轮虫是主要优势轮虫。2013、2014年轮虫年均总丰度分别为68.9、66.9个/L,两年物种丰富度在丰水期大于枯水期,但枯水期轮虫群落Simpson多样性指数均大于丰水期。采用Bray-Curtis距离测度群落之间的相异性,计算表明两年的枯水期轮虫群落之间的平均距离均大于丰水期,即丰水期时轮虫群落相似性较高,枯水期时轮虫群落变异较大。丰水期的水温和水质参数的变化相对稳定,种类的优势度更为明显,导致丰水期群落之间更为相似。主成分分析表明,无柄轮属、胶鞘轮属、多肢轮属、异尾轮属、泡轮属、晶囊轮属和皱甲轮属的主要种类的全年分布有较大的差异,而其余多数轮虫种类全年分布差异较小,反映了我国南亚热带地区水温全年变幅小,可维持较多的全年性种类共存。群落的方差解析与RDA分析表明,相对于生物与化学变量,物理环境变量主导了流溪河水库敞水区轮虫群落的变异,就单个变量而言,水温和食物是影响流溪河水库敞水区轮虫群落结构的重要因素,降雨则是影响轮虫群落结构变异的宏观因素。绝大多数轮虫为滤食性的,处于食物链底端,个体小、生活史短,轮虫群落在对环境因子变化的响应上与浮游植物具有相似性。     

广东省水库轮虫分布特征

在2000年调查了广东省18座水库的轮虫种类分布特征。在此基础上,于2001～2003年研究了滞留时间对飞来峡水库、流溪河水库和新丰江水库三座典型水库(平均滞留时间分别为14d、172d和644d)轮虫分布的影响。共检到轮虫101种,其中大多数种类来自臂尾轮科(Brachionidae)、腔轮科(Lecanidae)、异尾轮科(Trichcercidae)和疣毛轮科(Synchaetidae)。常见属为龟甲轮属(Keratella)、臂尾轮属(Brachionus)、腔轮属(Lecane)、多肢轮属(Polyarthra)、异尾轮属(Trichocerca)、晶囊轮属(Asplanchna)、聚花轮属(Conochilus)和无柄轮属(Ascomorpha)。从不同生活习性种类组成看．每次采样均以浮游种类为主。在直流型的飞来峡水库中,底栖和着生种类占种类数的41％;而在长滞留时间的新丰江水库中只占18％;中等滞留时间的流溪河水库则介于它们之间。从不同食性种类的丰度看,在新丰江水库和流溪河水库中,轮虫以滤食细小悬浮颗粒物的种类(螺形龟轮虫、独角聚花轮虫和广生多肢轮虫)为优势种;而在直流型的飞来峡水库中,以劫掠粒径较大食物的种类(疣毛轮虫等)为优势种。聚类分析表明,不同营养水平水库轮虫的组成并不是绝然不同的,而是随着营养水平的上升逐渐发生变化的。在营养水平较低水库中以广栖性的螺形龟甲轮虫为优势种,随着营养水平的上升,螺形龟甲轮虫优势度逐渐降低,而异尾轮虫或臂尾轮虫优势度逐渐上升,并在中富营养和富营养水库中成为优势种。     

Temporal and Spatial Distributions of Rotifers in Xiangxi Bay of the Three Gorges Reservoir,China

From July 2003 to June 2005, investigations of rotifer temporal and spatial distributions were car‐ried out in a bay of the Three Gorges Reservoir, Xiangxi Bay, which is the downstream segment of the Xiangxi River and the nearest bay to the Three Gorges Reservoir dam in Hubei Province, China. Thirteen sampling sites were selected. The results revealed a high species diversity, with 76 species, and 14 dominant species; i.e., Polyarthra vulgaris, Keratella cochlearis, Keratella valga, Synchaeta tremula, Synchaeta stylata, Trichocerca lophoessa, Trichocerca pusilla, Brachionus angularis, Brachionus calyciflorus, Brachionus forficula forficula, Ascomorpha ovalis, Conochilus unicornis, Ploesoma truncatum and Anuraeopsis fissa. After the first year of the reservoir impoundment, the rotifer community was dominated by ten species; one year later it was dominated by eight species. The community in 2003/2004 was dissimilar to that in 2004/2005, which resulted from the succes‐sion of the dominant species. The rotifer community exhibited a patchy distribution, with significant heterogeneity observed along the longitudinal axis. All rotifer communities could be divided into three groups, corresponding to the riverine, the transition and the lacustrine zone, respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aquastella gen. nov.: A new genus of saprolegniaceous oomycete rotifer parasites related to Aphanomyces, with unique sporangial outgrowths

The oomycete genus Aquastella is described to accommodate two new species of parasites of rotifers observed in Brooktrout Lake, New York State, USA. Three rotifer species – Keratella taurocephala, Polyarthra vulgaris, and Ploesoma truncatum – were infected, and this is the first report of oomycete infection in these species. Aquastella attenuata was specific to K. taurocephala and Aquastella acicularis was specific to P. vulgaris and P. truncatum. The occurrence of infections correlated with peak host population densities and rotifers were infected in the upper layers of the water column. Sequencing of 18S rRNA and phylogenetic analysis of both species placed them within the order Saprolegniales, in a clade closely related to Aphanomyces. The Aquastella species were morphologically distinct from other rotifer parasites as the developing sporangia penetrated out through the host body following its death to produce unique tapered outgrowths. Aquastella attenuata produced long, narrow, tapering, finger-like outgrowths, whilst A. acicularis produced shorter, spike-like outgrowths. We hypothesize that the outgrowths serve to deter predation and slow descent in the water column. Spore cleavage was intrasporangial with spore release through exit tubes. Aquastella attenuata produced primary zoospores, whereas A. acicularis released spherical primary aplanospores, more typical of other genera in the Aphanomyces clade.     

The physicochemical limnology of a stretch of the Guadalupe River,Texas, with five main-stream impoundments

Summary A study of limnological conditions was conducted on a 168-km stretch of the Guadalupe River in central Texas for one year beginning in February, 1969. This river stretch was unique in that it had a sequence of five shallow, nonstratified, main-stream reservoirs. Flow through the study area was controlled by releases from these five reservoirs and from Canyon Reservoir, a deep-storage reservoir, located 30 km upstream. Physicochemical properties measured monthly on a diel basis at 16 stations were: dissolved oxygen, chlorophyll a, pH alkalinity, specific conductance, water temperature, turbidity, transparency, water velocity, and discharge.Seasonal and diel changes in limnological conditions were of greater magnitude in impoundments than in lotic areas. Periods of low flow and low velocity resulting in long water retention periods in impoundments coupled with greater solar input associated with long days contributed to the greatest fluctuations. Increased discharge, particularly from Canyon Reservoir, caused more uniformity of conditions throughout the study area so that conditions at all points approached average seasonal conditions.Longitudinal variations in physicochemical conditions were influenced by the impoundment of water, effluent of sewage disposal plants, reaeration through turbulence, and was greatly affected by seasonal periods of drought and flooding.High dissolved oxygen in impoundments was accompanied by high chlorophyll a, high pH, and low bicarbonate alkalinity. These conditions indicate the importance of bicarbonate ions as a carbon source for phytoplankton in this system.     

Distribution and special traits of naturalization of new and rare zooplankton species in waterbodies of the Upper Volga basin

More than ten new zooplankton species were registered in reservoirs of the Upper Volga during 2002–2005. Since 2003, the cladoceran Diaphanosoma orghidani Negrea, had intensively settled in different biotopes of Ivankovo, Uglich, Rybinsk, and Sheksna reservoirs and is the most abundant (16 000 ind./m³) in the latter one. The rotifer Asplanchna henrietta Langh. was found for the first time in Rybinsk Reservoir in 1985, and by 2004, this species had also occurred in Ivankovo, Uglich reservoirs, Lake Nero, and some small waterbodies of the basin. In Lake Nero its density reaches especially high values (>1 million ind./m³). Causes, mechanisms, and ecological consequences of the expansion of new zooplankton species are discussed.     

上海饮用水源地青草沙水库轮虫群落结构的特征及其环境影响因子

青草沙水库是位于长江河口的一座江心水库,是现今上海最大的水源地,作为一个半自然半人工水体,其生态系统具有特殊性,既会受到来水影响,又有自身独特性,轮虫作为最有效的水质监测生物之一,能很好地反应水库的水体现状,为水库管理提供生物学参考依据。于2014~2015年对青草沙水库轮虫群落结构的特征进行了研究,依据水库轮虫水平分布和时间分布特征,分析了该水库轮虫的种类组成、优势种、污染指示种、数量分布。结合多样性指数运用聚类分析方法分析轮虫群落结构特征,采用冗余分析方法研究轮虫和环境因子的关系。两年共发现轮虫32种,2014年为20种,优势种7种;2015年为26种,优势种2种。其中针簇多肢轮虫(Polyarthra trigla)为两年共有优势种。寡污型种类共鉴定出8种。2014年年均生物密度比2015年低,分别为(156.37±119.48)ind/L和(237.12±252.86)ind/L。2014年年均生物量比2015年高,分别为(0.1316±0.1146)mg/L和(0.1229±0.1371)mg/L。聚类分析表明,轮虫的水平分布与水库的形状、地理特征水文情况有着很大关系。冗余分析(RDA)表明,水温、溶氧、总氮、总磷、浮游植物生物密度为主要影响因子。     

The species composition of rotifers in the water reservoirs of Central Vietnam

The rotifer fauna has been studied in four water reservoirs in the Khanh Hoa Province, Central Vietnam. A total of 65 species and subspecies and 22 genera representing 15 families have been found; 25 species and subspecies are identified as new to Vietnam. Morphometric characteristics are given for Filinia camascela Myers, Brachionus donneri Brehm, and B. dichotomus reductus Koste et Shiel. The rotifer fauna of Central Vietnam and those of other countries of Southeast Asia are compared. Some of the tropical and subtropical species recorded have been repeatedly reported from Russia.     

Monthly occurrence and feeding habits of larval and juvenile Ryukyu-ayu <Emphasis Type="Italic">Plecoglossus altivelis ryukyuensis</Emphasis> in an estuarine lake and coastal area of the Kawauchi River,Amami-oshima Island,southern Japan

We examined monthly occurrence patterns and feeding habits of larvae and juveniles of the critically endangered Ryukyu-ayu Plecoglossus altivelis ryukyuensis in the lower reach of the Kawauchi River, Amami-oshima Island, southern Japan, for two successive years. The study area was classified into a brackish lake (Lake Uchiumi) and a coastal zone (Sumiyo Bay) based on monthly variations in water temperature and salinity. Larvae and juveniles occurred from December to April in the first year and from January to February in the second year, respectively. Smaller-sized and earlier developmental-staged P. altivelis ryukyuensis individuals tended to emerge in larger numbers in Lake Uchiumi than in Sumiyo Bay. The present data suggest that Lake Uchiumi, where the physical environment is characterized by lower temperature and salinity than in Sumiyo Bay, would be suitable as a nursery ground for this fish, especially for earlier developmental-staged larvae. No individuals ≥25.0 mm body length, which is the size at which they begin their upstream migration, occurred in Lake Uchiumi or Sumiyo Bay in April or May, when upstream migration is assumed to reach a peak based on previous studies, in either year. The peak of upstream migration, therefore, may have occurred several months earlier in the years covered in this study due to higher water temperature than in typical years. To assess the feeding habits of this fish, the gut contents were observed. In both areas, copepods were abundant in P. altivelis ryukyuensis diet with calanoids and Oithona spp. being the most abundant. The diet composition differed between Lake Uchiumi and Sumiyo Bay and clearer ontogenetic variation in the composition of the diet was observed in Lake Uchiumi. Small individuals fed on tintinnid ciliates, but the proportion of these organisms in the diet gradually decreased with growth. Large individuals fed exclusively on harpacticoid copepods and insects including demersal species. Overall, larval and juvenile P. altivelis ryukyuensis exhibited a generalist feeding habit that tended to increase with growth. It is indispensable to adequately manage the environmental conditions in the lower reach of the Kawauchi River, especially Lake Uchiumi, to conserve P. altivelis ryukyuensis population.     

Annual dynamics and production of rotifers in an eutrophication gradient in the Baltic Sea

Spatial and temporal fluctuations in rotifer abundance have been monitored along a trophic gradient in the northern Baltic. The most common rotifer was Synchaeta spp., which had one abundance peak in June and one in September–October. Only during the latter period was the abundance significantly higher in the eutrophic basin compared to the reference area. The annual production of Synchaeta spp. was about double in the eutrophic basin. A positive correlation between Synchaeta spp. biomass and phytoplankton biomass was obtained during the autumn, but not during the early summer peak, although the phytoplankton community was dominated by the same species. Keratella quadrata, K. cochlearis and K. cruciformis were most abundant in August–September, and all three species had increased abundance in the eutrophic basin.     

Distribution patterns of fish biomass by acoustic survey in three Tunisian man‐made lakes

In order to test the hypothesis that fish distribution is related to water depth, three exploited reservoirs were sampled at day and at night using a Simrad EK60 echosounder. Acoustic surveys were carried out between February and June 2008, which focused on the fish size, species density (fish per ha) and biomass (kg ha^?1) along vertical and longitudinal gradients. It was evident in the surface layer (0–3 m) during daytime that the larger fish sizes occupy waters near the dam or the middle of Lakhmess and Sejnane reservoirs. In the upper layer at nighttime, a gradient of fish size is shown proportional to the depth at Lakhmess Reservoir and inversely proportional at the man‐made Sejnane Lake. At Lakhmess and Laabid reservoirs, fish density was significantly higher in deep water (3926 ± 1409 and 624 ± 258 fish per ha) rather than in the upper layer (988 ± 2 and 8 ± 2 fish per ha in daytime, respectively), while at Sejnane Reservoir the number of fish per area was similar along the vertical gradient. The biomass was significantly higher in waters deeper than 3 m at Sejnane Reservoir whereas at Lakhmess it was higher in the surface layer. Fish biomass increased from upstream (0.16 ± 0.05 kg ha^?1) to middle (3.07 ± 2.96 kg ha^?1) and downstream (3.82 ± 2.30 kg ha^?1 at night) areas in the man‐made Laabid Lake while a similar longitudinal gradient occurred in the deepest Sejnane Reservoir. The vertical gradient in fish distribution is discussed. From the dam to the tributary of the entire water column, it was concluded that fish biomass distribution was governed by depth and was most abundant in areas with deep waters.     

Temporal and spatial distribution of rotifera in a Chilean reservoir: A possible effect of impoundment hydrodynamics

Rotifers were sampled at monthly intervals for a year at four monitoring stations in Rapel Reservoir (a hydroelectric impoundment) in Central Chile (34°02S; 71°35W). Fifteen species were identified, but only Keratella cochlearis was consistently found at each station, it usually was the most abundant (> 50% of total rotifers). Marked differences were found among sites; stations I and 2, those nearest to the dam, showed greatest rotifer densities in spring and autumn. Station 4, that nearest to the inflowing rivers, had its highest rotifer abundance in summer. Station 8 had the highest mean density but the least marked seasonal changes in rotifer numbers.The impoundment water dynamics and the relative locations within the lake (distance from the dam or from the rivers) are suggested as determinants for different rotifer assemblages and, probably, for most other planktonic organisms.     

Selective recruitment and resurgence of tropical river phytoplankton: evidence from the Nile system of lakes,rivers, reservoirs and ponds

The recruitment of major species-components of phytoplankton is considered with reference to diverse water-bodies—rivers, headwater lakes, reservoirs and lateral waters—of the Nile system. The importance of massive ‘inocula’, carried in flowing and inter-communicating waters, is emphasised; the most direct evidence is from quantitative population dynamics in longitudinally sampled river-reservoir systems. Other indications of extraneous derivation are taken from geographical distribution and apparent invasions of newly recorded species. For five reservoirs, there are records of initial colonisation (in two) and of the annual colonisation of seasonally impounded water (in three). There was a selective recruitment of reservoir/river major species over 1 year to newly created ponds, from a massive inoculum of added river water, studied in relation to the novel and changing physical and chemical environment. Periods of predominant decomposition were there linked with the abundance of other species not observed in the original river water. Reservoir species can be carried to extend abundance far downstream, as observed in the Blue Nile where normal succession was altered by a new reservoir upstream; also in Egypt below the former Aswan Reservoir and the later High Dam Lake. The different and distinctive compositions of the phytoplankton communities in headwater lakes reflect large environmental differences that, with high downstream turbulence, probably make them insignificant for recruitment of most species typical of the downstream waters. There is evidence from hydrology and floristic similarities for recruitment from lateral standing waters along the river, especially those of one major swamp area.     

Planktonic Fauna of Small Reservoirs in the Komi Republic

One hundred and eight species and forms of planktonic organisms have been revealed in three small reservoirs in the Komi Republic (the northeast of European Russia). Species that were not previously known for the water bodies of the Vychegda River Basin (Limnosida frontosa) and the Republic of Komi (Lecane mira, Macrochaetus subquadratus, Filinia longiseta limnetica, and Rhynchotalona falcata) have been found. It has been established that the richness of the planktonic fauna depends on the age of the water body: the maximum number of species is found in the oldest reservoir, namely, the Kazhim Reservoir. The planktonic communities of artificial reservoirs are distinguished by a low level of similarity of faunas both with each other and in comparison with the natural lakes of the region. The fauna distribution in the old reservoirs is uneven. The relatively large fauna richness of planktonic communities in the littoral zones is due to the water-level regime in the reservoirs.     

Comparative analysis of rotifer community structure in five subtropical shallow lakes in East China: role of physical and chemical conditions

Worldwide, there have been few comparative studies on rotifer communities in subtropical lakes. We studied changes in rotifer community structure over 1 year and its relationship to several physicochemical variables in five subtropical shallow lakes in East China, covering a nutrient gradient from mesotrophy to moderate eutrophy. In these lakes, the genera Brachionus, Lecane, and Trichocerca dominated the rotifer species composition, and Polyarthra dolichoptera, Keratella cochlearis, Filinia longiseta, T. pusilla, and Anuraeopsis fissa were the dominant species. With increased nutrient loading, total rotifer abundance and species dominance increased, indicating that rotifer abundance might be a more sensitive indicator of trophic state than species composition. Comparative analyses of the six rotifer community indices calculated in this study and redundancy analysis (RDA) revealed that the two slightly eutrophic lakes and the other two moderately eutrophic lakes exhibited a high degree similarity in community structure. This suggests that the trophic state of a lake determines the rotifer community structure. In contrast, in the two moderately eutrophic lakes, the mass ratios of TN:TP and the contents of TP suggested N-limitation and cyanobacteria dominance in phytoplankton communities might be possible. In these lakes TN played a more important role in shaping the rotifer community according to stepwise multiple regression and RDA. RDA analysis also suggested that rotifer species distribution was strongly associated with trophic state and water temperature, with water temperature being the most important factor in determining seasonality.     

Biomanipulation as a useful water quality management tool in deep stratifying reservoirs

In six deep, soft-water reservoirs, ranging from oligotrophic to eutrophic, fishery management has been guided by the use of biomanipulation to improve water quality and opportunities for recreational fishing. As evidenced by the establishment of larger-bodied daphnids, a low level of zooplanktivory could be maintained in the newly filled Grosse Dhünn and refilled Brucher and Lingese Reservoirs only by regular stocking of piscivores (Sander lucioperca, Esox lucius, Salmo trutta lacustris, Onchorhynchus mykiss) in combination with size and bag limitations for the recreational fisheries. However, in the mesotrophic Pre-Reservoir Grosse Dhünn, Bever Reservoir and the newly filled slightly eutrophic Wupper Reservoir it took between 8 and 10 years before the predator-resistant zooplankton community responded to management with a switch to larger daphnids. Except for oligotrophic Grosse Dhünn Reservoir where angling is prohibited, the expected enhancement of piscivore biomass through stocking not least was prevented by anglers. However, growth of perch (Perca fluviatilis) benefited from the changed fishery management relying upon stocking piscivores only allowing them to reach the size of piscivory. The appearance of larger daphnids in Pre-Reservoir Grosse Dhünn and Wupper Reservoir resulted in the biomass of the Daphnia spring peak to increase and occur earlier causing prolonged clear water conditions. Although the larger daphnids increased transparency, total summer mean chlorophyll concentrations in the euphotic zone only decreased in the slightly eutrophic reservoirs due to reduced phosphorus availability following unchanged external phosphorus loadings. Reduced phosphorus availability in these reservoirs caused a real oligotrophication. Although the edible seston fraction was controlled “top down” the results did not support the trophic cascade hypotheses because total phytoplankton remained controlled “bottom up”, admittedly triggered by “top down” forces. In general, the results support the importance of indirect (non-lethal) effects as the driving forces for the successful biomanipulations, particularly in slightly eutrophic reservoirs. Not least, the delayed zooplankton response provides an indication that the underlying change of internal feedbacks was not driven by external forces to stabilize the reservoirs trophic state in these deep stratifying reservoirs. Handling editor: D. Ryder     

Zooplankton biomass in tropical reservoirs in southern Brazil

In order to test the hypothesis that zooplankton biomass distribution (total and taxonomic groups) was influenced by the nutrient concentration and primary productivity distribution in three tropical reservoirs, subsurface samples were taken in the fluvial, transitional and lacustrine regions of three reservoirs (oligotrophic, mesotrophic and eutrophic) in southern Brazil (Paraná State) in March and September 2002. Zooplankton biomass ranged from 0.04 to 264.47 mg DW m⁻³. Higher biomass values were observed for cladocerans (73.60%; 0.01–259.86 mg DW m⁻³), followed by copepods (22.05%; 0.01–69.69 mg DW m⁻³) and rotifers (4.35%; 0.01–11.52 mg DW m⁻³). In general, the total zooplankton, rotifer, cladoceran and copepod biomass, and chlorophyll-a and total nutrient concentrations showed a similar longitudinal distribution within the reservoirs. Total zooplankton, rotifer and cladoceran biomass were related to the chlorophyll-a concentration, and zooplankton biomass was related to the total phosphorus distribution. This may have been due to the significant multicolinearity between the chlorophyll-a and total phosphorus concentrations. Cyanobacteria influenced the taxonomic group biomass results by interfering with the filter feeding in larger zooplankton species, which favoured the dominance of smaller species. As regards the longitudinal distribution of copepod biomass, cyanobacteria biomass determined the displacement of the microcrustaceans to the fluvial region of Iraí Reservoir. Our results supported the hypothesis formulated and the primary productivity was the major predictor of the zooplankton biomass distribution in the reservoirs. Handling editor: S. Dodson     

Intraspecific spatial genetic differentiation of a Daphnia species within a long narrow reservoir

Environmental gradients influence the distribution and taxonomic composition of planktonic taxa, including Daphnia. In canyon-shaped reservoirs with pronounced horizontal gradients of food supply, predation pressure and other factors, not only species and interspecific hybrids but also clones within these taxa are non-randomly distributed. Using a long-term data set from a reservoir mostly dominated by a single Daphnia species, we evaluated whether intraspecific genetic differentiation can be frequently detected between upstream and downstream reservoir regions with different environmental conditions. We analysed variation at four allozyme loci (two species-specific and two polymorphic) to assess the taxonomic composition and intraspecific variation of Daphnia collected in different years (between 1995 and 2005) and periods of the growing season. D. galeata dominated in all samples, although other species and hybrids with D. galeata were also occasionally detected. Despite limited variation at the analysed loci, D. galeata from upstream and downstream regions were significantly genetically differentiated on seven out of twelve sampling dates. Although genetic drift in geographically distant subpopulations may contribute to differentiation, we presume that the observed patterns are primarily due to different selection regimes. We predict that a significant genetic differentiation within planktonic populations also occurs more frequently in natural water bodies.     

Notes on the rotifers of coal mine water in Eastern Poland

The species composition and quantitative structure of the rotifer fauna was investigated in a reservoir containing coal mine water. Only nine mainly planktonic species of rotifers, were found. Two of these were dominating: Brachionus angularis and B. rubens. They are typical indicators of eutrophic waters. Chlorides and sulphates may have an influence on the occurrence and quantitative structure of rotifer assemblages in the investigated reservoir.