Chaoborus predation and zooplankton community structure in a rotifer-dominated lake

During summer, Chaoborus punctipennis larval densities in the water column of fishless, eutrophic Triangle Lake become very high, and coincidently, the spined loricate rotifer Kelfcottia bostoniensis becomes the dominant zooplankter. Research was done to test the hypothesis that selective predation by Chaoborus on soft-bodied rotifers controls species dominance in the mid-summer zooplankton of this lake. In situ predation experiments showed positive selection by Chaoborus for the soft-bodied Synchaeta oblonga, negative selection for K. bostoniensis, and intermediate selection for Polyarthra vulgaris, a species with rapid escape tactics. However, during a 21 day in situ mesocosm experiment, zooplankton dominance and succession in Chaoborus-free enclosures was identical to that in enclosures with Chaoborus at lake density. Despite the selective predation, Chaoborus larvae may not exert significant top-down control on rotifers, whose intense reproductive output during mid-summer in temperate eutrophic lakes results in new individuals at rates that exceed predatory losses.     

The potamoplankton of the Middle Loire and the role of the ’moving littoral‘ in downstream transfer of algae and rotifers

The seasonal succession of the potamoplankton of the Middel Loire was studied fortnightly from June to October 1995, a period of low water flow. The increase in dissolved oxygen, suspended matter, BOD₅ and chlorophyll a, associated with the decrease in nutrients downstream, are typical of eutrophic rivers. Algae were the principal components of the suspended matter flux in the river. Their density increased from the upper site of Dampierre-en-Burly to the lower site of Saint-Laurent des Eaux, with respective maxima of 68 ×10⁶ cells l^-1 and 106 × 10⁶cells l^-1. A large rotifer community developed in association with the algal growth, with densities up to 3700 ind l^-1 at Dampierre-en-Burly and up to 5800 ind l^-1 at Saint-Laurent des Eaux. They had a high Shannon-Weaver diversity, while grazing appeared to control the algae. As the year progressed to the warmer season, the biomass of the β and β α mesosaprobic rotifer species increased parallel with algae. The water theoretically transported potamoplankton at 0.2 ms^-1 during mid-summer, which meant that the biomass of the algae doubled in minimum 17 km and that of the rotifers in 34 km. From these values and the distances between sampling stations, we deduce that inoculation points were located in areas which could be as short as a hundred meters. Not only algae, but also rotifers are capable of restoring an inoculum of organisms from place to place. Summer hydrodynamics, close to localised lentic areas, were responsible for an increase in plankton abundance in the stream. These movements had an ecotone effect at the boundary between standing and running water, creating pulses. The lower regions were progressively enriched by successive inoculates, dilution and seeding inputs, which did not prevent the growth of algae and of parthenogenetic species in the river. Such processes, which occur in years of low flow, are responsible for the large variations in plankton density observed between years. This revised version was published online in August 2006 with corrections to the Cover Date.     

Distribution and abundance of zooplankton in the Waikato River,New Zealand

Potamoplankton is often a well developed component in large lowland rivers, yet little is known about its structure in New Zealand's longest river, the Waikato River. To redress this gap we sampled bimonthly at seven sites along the length of the river over 12 months. Rotifers were the dominant zooplankton in the Waikato River making up 85% of the total densities. Cladocerans represented 9% and copepods only 6%. Rotifers were also the most taxonomically rich group with 41 species in 20 genera identified throughout the study. Thirty rotifer species and nine genera represent new records for the river – two cladoceran species were also recorded for the first time. The highest densities of crustaceans and rotifers were found in the hydro lakes. Densities of crustaceans decreased with increasing distance downstream and densities of rotifers were on average 15 times greater than crustaceans in the lower river. The seasonality of Crustacea was similar to that in New Zealand lakes and rivers with high densities in summer and minimum densities over the winter period. Total rotifer densities showed a similar trend although there were marked seasonal differences between individual species.     

Musculature of two bdelloid rotifers, Adineta ricciae and Macrotrachela quadricornifera: organization in a functional and evolutionary perspective

Organization of muscles in microinvertebrates has often been studied to answer functional questions and understand phylogenetic relationships among taxa. In this study, the musculature of two bdelloid species, Adineta ricciae and Macrotrachela quadricornifera , was illustrated, and their organization was compared with other rotifer taxa to generate possible hypotheses of evolutionary relationships among Rotifera. The two species share a common habitat but differ from each other in feeding and locomotion. A. ricciae feeds on the biofilm by scraping it, is unable to swim, and slides on the head cilia using the foot to propel over the substratum. M. quadricornifera feeds by filtration, can swim, and advances by looping in a leech-like motion. Their musculature, stained with TRITC-phalloidin, was observed using confocal laser scanning microscopy. Major differences between the two species were observed in the muscles of head and foot, possibly reflecting differences in their life style. Muscles of the trunk were similarly arranged: circular muscles surrounded longitudinal bands, which were inserted at different points on the body wall. In both bdelloids, circular muscles of the trunk were incomplete ventrally, a condition also present in Seison and in soft-bodied monogononts from benthos. Within rotifers, circular muscles in the form of complete rings are present in acanthocephalans and in soft-bodied planktonic monogononts but are absent in loricate monogononts, which generally possess dorsoventral bands. The diversity of muscle organization among rotifers was interpreted and discussed.     

Spatio-temporal Investigations on the Planktonic Organisms of the Middle Loire (France), During the Low Water Period: Biodiversity and Community Dynamics

The objective of this work was to analyse the distribution of the planktonic communities involved in the functioning of a 255-km river stretch and to get a better understanding of the influence of the river morphology on the diversity and dynamics of the micro-organisms. The planktonic communities (phytoplankton, bacterioplankton, proto- and metazooplankton) scarcely considered together in fluvial systems, were analysed at four sites of the Middle Loire during the low water period, in parallel to physical and chemical analyses. Physical and chemical variables such as turbidity, pH, suspended matter and chlorophyll a concentration were high, illustrating the classical, productive summer period. The algae played a major role in the water oxygenation until end-summer, then the algal drop concomitant to the bacterial sustained abundance appeared responsible for oxygen depletion. The downstream site enriched by nutrients inputs of two tributaries, carried the highest algal and bacterial densities. Situated in a meanders zone, the Middle Loire is characterised by a high habitat heterogeneity, the up- and downstream sites were wide and spread of vast standing zones and vegetated islands, whereas the two intermediate ones were narrower and more uniform. This morphological variability strongly impacted the micro-organisms diversity and distribution. Indeed, the algae and zooplankton composition were clearly influenced by the physical habitats of the river, the Cyanophyta were favoured by the lentic conditions and the Bacillariophyta by the turbulent ones, while the young stage of copepod and the large rotifer predators were indicator of a lentic origin. Thereafter, the river heterogeneity interfered with the zooplankton dynamics, the standing conditions enhancing the rotifer predation. In that way, we hypothesise that two opposite patterns characterised the wide sites spread of lentic water and the more uniform channels. In the first case, the zooplankton could prey on the ciliates protozoan, which in return favoured the flagellate ones; conversely in the second situation the zooplankton limited by the physical constraints did not impact the ciliates which could depress the flagellates. Thus, the similar geomorphology of the distant upstream and downstream sites (255-km apart) induced relatively close organisms distribution. Hence, disagreeing with the river continuum concept, this assertion shows the strong influence played by the local morphological characteristics of the Middle Loire in potamoplankton composition and dynamics.     

The rotifer fauna of the River Loire (France), at the level of the nuclear power plants

In the heated affluents of nuclear power plants of the river Loire, rotifers are abundant. Cosmopolitan species are numerically dominant, but a tropical fauna is also present, among which the genus Brachionus, representing 20% of the total species, is best represented. From a comparison between water upstream and downstream of the power plants, it further appears that in downstream warmed-up waters, some species show an important development, but not in colder upstream waters.     

Zooplankton of turbid and hydrologically dynamic prairie rivers

1. Compared with rivers in more humid, forested ecoregions of eastern and midwestern U.S.A., rivers in semi‐arid grassland of the U.S. Great Plains tend to be relatively shallow, more variable in discharge, and characterised by high suspended sediment loads. Although critical life stages of fish in prairie rivers probably depend at least partially on zooplanktonic food, data on community and distributional patterns of potamoplankton in these widespread ecosystems are almost entirely absent. 2. We examined summer zooplankton distribution in five prairie rivers (Arkansas, Kansas, Platte, Elkhorn, and Niobrara Rivers) spread over six degrees of latitude during 2003–2004. We compared our results from 126 samples with previously collected data from the Ohio and St Lawrence Rivers in forested ecoregions and correlated differences with abiotic environmental conditions. 3. The importance of hydrological retention zones to stream biota has been recently demonstrated for rivers with quasi‐permanent islands and slackwater regions, but the importance of slackwaters formed by ephemeral sandbar islands in prairie rivers is unknown. We evaluated the role of hydrological retention for planktonic rotifers, cladocera, and copepods in the Kansas River during the summer of 2004. 4. Zooplankton assemblages were extremely similar among prairie rivers (Sorensen Dissimilarity Index: mean = 0.07) but moderately disparate for comparisons of prairie versus forested‐basin rivers (mean = 0.50). 5. Total zooplankton densities in prairie rivers (approximately 81 L^?1) were intermediate between the Ohio (approximately 92 L^?1) and St Lawrence Rivers (approximately 43 L^?1), but relative abundances were significantly different. Rotifers represented >99% of zooplankton individuals in grassland rivers, but only approximately 37–68% in other rivers. Rotifer species richness was lower in prairie rivers, but relative abundances of common genera were much less skewed compared with eastern rivers where Polyarthra dominated rotifer assemblages (41–73%). 6. For comparisons among rivers, rotifers were significantly more abundant in turbid rivers, while microcrustaceans were less dense. However, for comparisons within the Kansas River over time, rotifer densities were inversely related to turbidity. We hypothesise that rotifers indirectly benefit from river turbidity because their food competitors (cladocera) and predators (e.g. cyclopoid copepods and visually feeding fish) are relatively more susceptible to suspended sediments. 7. Crustacean densities were positively related to the degree of hydrological retention (negatively to current velocities) throughout the study, but rotifer densities were significantly depressed by current velocities only when river discharge was high, making slackwaters that much more valuable. Ephemeral sandbars may not provide sufficient hydrological retention in time and space to sustain viable crustacean populations, but they are adequate to help sustain growth of rotifer populations.     

Diversity and Abundance of the Planktonic Rotifers in Different Environments of the Upper Paraná River Floodplain (Paraná State – Mato Grosso do Sul State,Brazil)

This study proposes that diversity and abundance of rotifers show spatial and temporal variations in the Upper Paraná River floodplain due to heterogeneity of the environment and hydrological level fluctuations of the main river. The structure and dynamics of rotifer assemblages were investigated by samplings carried out during the rainy (February) and dry period (August) of the year 2000, in 36 environments (rivers, channels, backwaters, open and isolated floodplain lakes). The influence of phytoplankton biomass on rotifer diversity and abundance was also investigated. 104 taxa of rotifers were identified. The highest species richness was found in rivers and open floodplain lakes, the highest abundances in the isolated floodplain lakes, and the highest values of species diversity in the channels, especially during the rainy period. β₂-diversity values were higher in the channels, especially during the dry period. Flow differences and food availability were predominant factors influencing the structure and dynamics of the rotifer communities.     

湖北四湖泊营养类型与轮虫群落的关系

对湖北梁子湖水系不同营养类型(中营养型、富营养型)4个湖泊中轮虫的群落结构和物种多样性进行了周年研究,分析比较了不同营养类型湖泊的轮虫种类组成、分布、优势种组成、密度、生物量和多样性指数。结果表明：轮虫的种类数、物种多样性与营养水平呈负相关关系,轮虫密度大体上随营养水平提高而增大,富营养化引起轮虫空间异质性降低,受污染湖泊与非污染湖泊轮虫种类数、寡污性种类数及分布差异尤为明显。用多样性指数评价湖泊营养状态与TLIc方法一致。     

Freshwater Copepods and Rotifers: Predators and their Prey

Three main groups of planktonic animals inhabit the limnetic zone of inland waters and compete for common food resources: rotifers, cladocerans and copepods. In addition to competition, their mutual relationships are strongly influenced by the variable, herbivorous and carnivorous feeding modes of the copepods. Most copepod species, at least in their later developmental stages, are efficient predators. They exhibit various hunting and feeding techniques, which enable them to prey on a wide range of planktonic animals from protozoans to small cladocerans. The rotifers are often the most preferred prey. The scope of this paper is limited to predation of freshwater copepods on rotifer prey. Both cyclopoid and calanoid copepods (genera Cyclops, Acanthocyclops, Mesocyclops, Diacyclops, Tropocyclops, Diaptomus, Eudiaptomus, Boeckella, Epischura and others) as predators and several rotifer species (genera Synchaeta, Polyarthra, Filinia, Conochilus, Conochiloides, Brachionus, Keratella, Asplanchna and others) as prey are reported in various studies on the feeding relationships in limnetic communities. Generally, soft-bodied species are more vulnerable to predation than species possessing spines or external structures or loricate species. However, not only morphological but also behavioural characteristics, e.g., movements and escape reactions, and temporal and spatial distribution of rotifer species are important in regulating the impact of copepod predation. The reported predation rates are high enough to produce top-down control and often achieve or even exceed the reproductive rates of the rotifer populations. These findings are discussed and related to the differences between the life history strategies of limnetic rotifer species, with their ability to quickly utilize seasonally changing food resources, and adjust to the more complicated life strategies of copepods.     

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.     

The Influence of Biotic and Abiotic Factors on Psammic Rotifers in Artificial and Natural Lakes

The effect of sand grain size, porosity and the abundance of algae on the community structure of psammic rotifers was tested in three anthropogenic lakes (Upper Silesia) and compared with three natural ones (West Pomerania). The structure of grain size in the studied beaches of artificial and natural lakes was similar but differences were found in the abundance of algae. The structure of the psammic rotifer community in Upper Silesian lakes was different from natural lakes in West Pomerania. The structure of psammic rotifer communities in anthropogenic reservoirs and lakes (West Pomerania) differed in numbers of species and numbers of psammobiont species, and also diversity index. The most altered structure of rotifer assemblages was observed in the psammon rotifers of the strongly contaminated Dzierżno Duże Lake.     

The potential for population growth of Ascomorpha ecaudis

The intrinsic rate of natural increase (r_m) of the rotifer Ascomorpha ecaudis was found to be a hyperbolic function of food concentration. The threshold food concentration and r_max/2-food concentration determined for this species were significantly lower than values predicted from allometric models. These growth characteristics may be related to the mucus house in which Ascomorpha lives and/or the symbiotic algae living in its body tissues. The maximum rate of population growth recorded (0.71 d^–1) was similar to that of other soft-bodied rotifers of similar body mass. These population growth characteristics and the resistance of this species to invertebrate predation should allow it to become a dominant member of freshwater zooplankton communities. However, field observations suggest that it is not. Reasons for this are suggested.     

The susceptiblity of Keratella cochlearis to interference from small cladocerans

SUMMARY. 1. Even at high population densities (300–2000 ind. 1⁻¹), only one of five small cladoceran species (adult body length <1 mm) significantly suppressed population growth of the rotifer Keratella cochlearis through interference (encounter) competition. At 500 ind. H, adults of D. ambigua (0.96 mm body length) imposed an instantaneous per capita death rate of 0.21 day⁻¹ on this rotifer. These short-term experiments may have underestimated cladoceran interference because newborn rotifers were rarely present.
2. Newborn rotifers (<12 h old) were much more susceptible than adult rotifers (> 24 h old) to interference from Ceriodaphnia dubia. All of the small cladoceran species tested were very much less likely than large Daphnia (body lengths >1.2 mm) to interfere with K. cochlearis , but perhaps at high population densities they could suppress population growth of susceptible rotifer species by damaging, and possibly eating, relatively small and soft-bodied newborn individuals.
3. K. cochlearis of the tecta form, without a posterior spine, produced offspring of the typica form, with a posterior spine, in the presence of C. dubia. This developmental response is stimulated by at least several, and possibly all, cladocerans and probably reduces the susceptibility of the rotifer to cladoceran interference.     

Effect of two microalgae concentrations on body size and egg size of the rotifer <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis>

The rotifer, Brachionus calyciflorus, was grown with two algae species (Chlorella sp. and Scenedesmus obliquus) at different concentrations (0.1, 1 and 10 × 10⁶ cells ml⁻¹). The body size (lorica biovolume) of individual rotifer and their egg size were measured when the populations were roughly in the exponential phase of population growth. The body size of the rotifers differed significantly (P < 0.05) among the two algae species used, however this effect was not observed for egg size. The body size of rotifers fed on higher densities of Chlorella sp. (10 × 10⁶ cells ml⁻¹) was significantly larger than for those fed on lower and medium densities (0.1 and 1 × 10⁶ cells ml⁻¹). Body size and egg size of rotifers fed with different amounts of Scenedesmus did not differ significantly. The egg size was significantly larger at higher food level of Chlorella. A significantly positive correlation was observed between the adult rotifer body size and their egg size.     

Rotifers in different environments of the Upper Paraná River floodplain (Brazil): richness, abundance and the relationship with connectivity

Rotifers were collected in the open water of twenty-eight locations (3 rivers, 12 connected lagoons and 13 isolated lagoons) of the upper Paraná River floodplain during the high water (February) and low water (August) period of 2001. Greatest species richness was found in rivers during the high water period. Isolated lagoons had the lowest species richness. Abundance was highest in connected lagoons followed by isolated lagoons and then rivers, but did not show marked spatial or temporal variation. Some species dominated in isolated lagoons during high water and others in connected lagoons during the low water. These results were contrary to our expectations; we expected to observe highest species richness and abundance of rotifers in isolated lagoons during both extremes of the hydrological cycle. Our findings suggest the importance of connectivity among environments to rotifer species richness. The absence of an intense flood during 2001 facilitated development of rotifer populations during the high water period due to lack of dilution and high availability of food resources (phytoplankton).     

Contribution of rotifers to the diet and fitness of Boeckella (Copepoda: Calanoida)

1. The calanoid copepods, Boeckella triarticulata Sars and Boeckella hamata Brehm, are major components of the freshwater zooplankton of New Zealand. It was not known whether these copepods ingest rotifers, nor whether the inclusion of rotifers in their diets might improve their fitness. The present study aimed to identify rotifer taxa which are eaten by each copepod species, and to examine the fitness consequences of the inclusion of one species of rotifer in the diet of B. triarticulata .
2. In feeding experiments using natural rotifer assemblages, both species of copepod ingested the rotifer Anuraeopsis fissa (0.4–4% of daily carbon intake), and B. triarticulata also ingested Polyarthra dolichoptera (6–30% of daily carbon intake) and Keratella cochlearis tecta (1% of daily carbon intake).
3. The contribution of rotifers to the fitness of adult female B. triarticulata was assessed by comparing survival and reproduction among five diets that contained varying densities of algae ( Cryptomonas sp.) and/or Polyarthra dolichoptera (4 μg C L⁻¹). Boeckella triarticulata produced fewer clutches on a rotifer-only diet than on a solely algal diet, and addition of rotifers to a threshold algal diet did not affect copepod fitness relative to the solely algal diets. The present results suggest that Polyarthra at 4 μg C L⁻¹ is not a high-quality food for B. triarticulata .     

Differential use of food resources by the instars of Chaoborus punctipennis

SUMMARY. 1. Differential use of food resources by instars I-1V of Chaoborus punctipennis was examined in a mesotrophic New Hampshire lake during midsummer when all instars coexisted. Diet composition and prey preference were quantified first, at night when all instars were feeding at the same depth, and second, during the day when the early instars (I-II) and instar III were segregated by depth. Relative abundance, per cent biomass, and per cent frequency of occurrence of soft and hard-bodied rotifers, flagellated phytoplankton, protozoans, and crustaceans in C. punctipennis diets were quantified using crop content analyses. 2. All four instars ingested large, flagellated phytoplankton (mainly Dinobryon and Ceratium). This unexpected result suggests that the effects of phytoplankton on Chaoborus growth and survival should be investigated. All instars also consumed rotifers (mainly Kellicottia, Gastropus, Polyarthra), but only instars III and IV fed upon crustaceans (mainly Daphnia, Bosmina and Diaphanosoma). Small rotifers (Gastropus spp., Keratella cochlearis, Trichocerca similis) occurred more frequently and were more abundant in early instar diets than late instar (III and IV) diets, whereas large rotifers (Asplanchna priodonta and Keratella crassa) were eaten only by instars III and IV. Zooplankton with gelatinous sheaths (e.g. Ascomorpha, Collotheca and Holopedium) were rarely ingested. 3. Developmental increase in gape diameter of C. punctipennis seemed to be the major proximate mechanism causing dietary differences among instars. Body widths of hard-bodied prey in C. punctipennis crops were always less than gape diameter. The relationship between prey body width and Chaoborus gape diameter, coupled with knowledge of prey escape behaviour, should be useful for predicting the presence of hard-bodied prey taxa in diets of other Chaoborus species. 4. All four instars of C. punctipennis selected soft-bodied, or weakly loricate, rotifer prey over crustaceans and phytoplankton. Early instars preferred the small rotifer T. similis and the protozoan Difflugia sp. to other rotifers and phytoplankton when feeding in the epilimnion or thermocline during the day or night. Late instars positively selected the rotifer, Asplanchna. Prey value (prey weight ingested per unit handling     

Impact of turbulence on riverine zooplankton: a mesocosm experiment

1. With increases in river discharge over time and space, zooplankton generally encounter increased turbulence, turbidity, hydraulic forces, downstream advection and food limitations, all of which should affect species diversity and densities. Of these factors, the role of turbulence on the distribution of zooplankton is least known along longitudinal and lateral dimensions in river networks. 2. We tested the factorial effects of turbulence and grazing level on Ohio River potamoplankton in spring and summer using twelve 1600‐L, outdoor mesocosms. Turbulence was calculated using the Froude number for equal depths but with current velocities of 0.064 and 0.32 m s^?1. Grazing levels corresponded to a high density treatment (=ambient river densities of rotifers, copepods and cladocerans) and a low density treatment (initially no zooplankton >64 μm). All tanks had the same water residence time, and hydraulic stress was minimized by circular flow patterns. 3. Zooplankton densities and population growth rates were significantly affected by turbulence level and season. In general, rotifer populations grew faster in high turbulence tanks (though Keratella and Brachionus populations flourished in both treatments in summer) and microcrustaceans thrived better in low turbulence environments. The larger, calanoid copepods handled more turbulent conditions much better than cyclopoids or nauplii. Zooplankton had no detectable effects on particulate organic carbon concentrations in either month (values were higher in spring), but rotifers reduced chlorophyll concentrations in both months. 4. The relative importance of turbulence in controlling potamoplankton is probably to vary not only on a longitudinal basis in river networks but also with both the hydrogeomorphic complexity of river reaches and the type and amount of river regulation. Plans for river rehabilitation and management should incorporate non‐turbulent habitats in large rivers as a means of enhancing zooplankton populations and providing an important food web component for planktivores.