Conochilus Unicornis Research Articles

The effect of increasing temperature and pCO2 on experimental pelagic freshwater communities

AbstractAs the global climate is changing, average water temperatures and the supply of CO2 to water bodies are increasing. To determine how the effects of these changes on freshwater communities interact, we ran a month‐long factorial mesocosm experiment, in which we manipulated water temperature (heated, ambient) and pCO2 (preindustrial, ambient, future). We found that the total phytoplankton biomass responded positively to the pCO2 and temperature treatments but no interactive effects were detected. Green algae were positively affected by temperature and, over the course of the experiment, responded to pCO2 first positively, then negatively. Heterokonts, on the other hand, were unaffected by temperature but responded positively to pCO2. pCO2 enrichment also led to increases in seston C : N stoichiometry, although the experiment ended before we could observe any effects of pCO2 on the zooplankton community composition. Warming caused shifts in zooplankton community composition, primarily through higher abundances of the cladoceran Bosmina longirostris and the rotifer Conochilus unicornis, and lower abundances of the rotifer Polyarthra vulgaris. We found that, in contrast to the effects of temperature, which can be explained by temperature‐dependent plankton growth curves, the responses of algal groups to pCO2 enrichment were difficult to anticipate, despite the availability of priors from previous pCO2 enrichment experiments in the same lake mesocosms. We conclude that climate change‐induced increases in aquatic pCO2 and temperatures are likely to affect pelagic ecosystems, though further research in freshwater systems is needed before generalized claims regarding the simple and interactive effects of pCO2 can be made.

Using the Diversity, Taxonomic and Functional Attributes of a Zooplankton Community to Determine Lake Environmental Typology in the Natural Southern Boreal Lakes (Québec, Canada)

Herein, we used zooplankton as a study model for determining how biodiversity components as well as taxonomic and functional attributes reflect lake typology in the natural southern boreal lakes. We estimated the regional and local variation in zooplankton diversity and the community structure across a set of fourteen lakes within a national park. Regional diversity (γ diversity) accounted for 40 species including 20 rotifers, 10 cladocerans and 8 copepods. Local diversity (α diversity) averaged 15 species per lake. Spatial variation in β diversity was inversely related to spatial variation in α diversity. Inter-lake variation in zooplankton communities based on taxonomy, functional traits and biotic indices was explained by two major limnological gradients: namely lake trophic status and fish community. The community structure reflected a gradient of rotifer to calanoid copepod dominance in response to trophic status. Several key species of rotifers (Kellicottia longispina and Conochilus unicornis) and of small (Bosmina and Diaphanosoma birgei) or large (Daphnia catawba and Holopedium gibberum cf glacialis) cladocerans were good indicators of lake zooplankton typology, as in other boreal lakes. We distinguished two main groups of lakes: (1) oligotrophic lakes inhabited by brook trout and dominated by the calanoid copepods and (2) mesotrophic lakes inhabited by northern pike and dominated by rotifers. Overall, our study can help managers better define monitoring and conservation strategies for lake ecosystems in natural parks.

Zooplankton Diversity of a Soft-water and Highly De-mineralized Reservoir of Meghalaya (Northeast India): The Spatio-temporal Variations and Influence of Abiotic Factors

Hydrobiological survey of a ‘soft-water’ and ‘highly de-mineralized’ reservoir of Meghalaya state of northeast India is undertaken to analyze zooplankton diversity with reference to the spatio-temporal variations and influence of abiotic factors. The littoral and limnetic zooplankton assemblages of this subtropical reservoir without aquatic vegetation reveal total 36 species, and record lower abundance, quantitative dominance of Rotifera, sub-dominance of Cladocera and Copepoda and moderate species diversity. Keratella cochlearis, Bosmina longirostris, Polyarthra vulgaris, Mesocyclops leuckarti, Conochilus unicornis and Asplanchna priodonta influence abundance, species diversity, dominance and equitability of zooplankton. We report differential spatial influence of individual abiotic factors with the relatively more importance at the limnetic region, and the canonical correspondence analysis registers 72.5% and 78.8% cumulative influence of 10 abiotic factors on the littoral and limnetic assemblages, respectively. The spatial differences of various diversity aspects and the influence of abiotic factors suggest habitat heterogeneity amongst the two regions. This study is a useful contribution to zooplankton diversity of the subtropical environs, and soft and de-mineralized waters in particular. Our results mark a distinct contrast to the lowest richness and abundance of zooplankton noted from India vide the preliminary 1990-91 survey of this reservoir.

Ultrastructure of extracorporeal secretions of four sessile species of Rotifera (Gnesiotrocha), with observations on the chemistry of the gelatinous tube

AbstractSecretion of an extracorporeal hydrogel is a common defensive strategy employed by many aquatic invertebrates including several species of sessile rotifers. Here, we provide a comparative study of the ultrastructure of the gelatinous secretions of four species of Superorder Gnesiotrocha (Monogononta): Conochilus unicornis, Collotheca ferox, Stephanoceros fimbriatus, and Stephanoceros millsii. Additionally, we use differential staining and enzyme degradation assays to explore the chemistry of the gelatinous tube of S. fimbriatus. At least three types of secretions are produced by these four species: an external gelatinous tube, a thick glycocalyx, and an adhesive peduncle. These three different secretions all have a fibrous appearance, but each has different electron densities, fiber thicknesses, and glandular origins. The gel tube, which is likely secreted by pedal glands, has a highly hydrated framework of mesh‐like fibers with thick internal and external laminae that differ slightly among species. Chemical staining suggests that the gel tube is composed of glycoproteins with a high content of sulfonic groups and limited or no obvious lipids or cationic proteins. This tube is resistant to chemical and enzymatic digestion. The proteinaceous glycocalyx is secreted by the syncytial integument and has various densities and thicknesses among species. The peduncle is an adhesive attachment that connects a rotifer's foot to a substratum, or, in some species, to conspecifics as part of a colony. The secretion is highly electron dense and contains membrane‐bound vesicles; it appears to be derived from a different set of pedal glands than the hydrogel tube. The different chemistries and anatomical origins of the three gelatinous secretions suggest an independent evolutionary history for each.

The distributional pattern of zooplankton community in beratan lake, bali

Zooplankton is an important component in water ecosystem that has a distribution in spatial and time scale. The distribution and diversity of zooplankton as one biological indicator of quality of the water way. This study was aimed at looking at the description of the distributional pattern and the abundance of zooplankton and Beratan Lake. Five sampling stations were determines based on the deeps of the lake using the field survey method, i.e., LB 1, LB 2, LB 3, LB 4 and LB 5. The zooplankton sample was taken from each stations by using Chaoborus net NXX 10 (the eye of the net size was 132 µ, then the zooplankton was preserved with 4% formalin solution. After being identified, there was five species, 4 Cladocera species and 8 rotifera species. The highest of abundance for copepod group was Thermocyclops. Diaphanosoma was Cladocera group which was mostly found. Conochilus unicornis was the most dominant rotifer.

Species composition, abundance and distribution of zooplankton in a tropical eutrophic lake: Lake Catemaco, México

Entre abril de 1992 y mayo de 1993, se realizaron mensualmente recolectas subsuperficiales de zooplancton, con red, en nueve localidades del lago, en donde también se determinaron la temperatura superficial, la visibilidad del disco de Secchi, el pH y la concentraciòn de oxígeno disuelto. Entre las 31 formas de plancton detectadas en las distintas submuestras se registraron 14 especies de rotíferos, tres de copépodos, cinco de cJadòceros y un ostrácodo, además de protozoarios, principalmente vorticélidos y ciliados. Los rotíferos fueron los organismos más abundantes; Brachionus havanaensis fue la especie dominante, seguida por Conochilus unicornis, Brachionus angularis, Keratella cochlearis y Conochilus dossuarius

Rotifer communities in the Laurentian Great Lakes, 1983–2006 and factors affecting their composition

Rotifer communities were examined from the open waters of the Laurentian Great Lakes between 1983 and 2006 in the first inter-lake comparison involving all five lakes. Seventy rotifer taxa were found in summer samples, although most (> 80%) individuals were drawn from the following taxa: Conochilus unicornis, Polyarthra vulgaris, Keratella cochlearis, Kellicottia longispina, Keratella crassa, Synchaeta spp., Polyarthra major, Ploesoma truncatum, Keratella earlinae, and Keratella quadrata. Distributions of Filinia longiseta and several species of Brachionus and Trichocera were restricted to regions of higher trophy, notably the western basin of Lake Erie, but most taxa were widely distributed, differing in relative dominance rather than presence or absence. Rotifer community structure appeared to respond in tractable ways to differences in trophic state, with the four most common genera exhibited the following preferences, in order of decreasing trophy: Polyarthra, Keratella, Conochilus, and Kellicottia. Recent oligotrophication in Lake Huron was reflected in decreases in Keratella dominance and increased dominance of Conochilus, while in Lake Michigan oligotrophication was associated with decreases in Polyarthra and Keratella and increases in Conochilus, and Kellicottia. In Lake Ontario, the appearance of the exotic predatory Cercopagis pengoi was strongly associated with dramatic reductions in Polyarthra, while more gradual sequential increases in the dominance of Keratella, Conochilus and Kellicottia were probably the result of decreasing trophic state. Bythotrephes was identified in multivariate analyses as correlating strongly with rotifer composition, although only in Lake Michigan was a clear relationship between the predatory cladoceran and rotifer species shifts observed.

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

AbstractFrom 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)

Zooplankton fluctuations in Jurumirim Reservoir (São Paulo, Brazil): a three-year study

During three consecutive years, monthly samples of zooplankton were taken in the lacustrine (dam) zone of Jurumirim (São Paulo, Brazil). The seasonal effect on basic limnological features (thermal regime, oxygen distribution, phytoplankton biomass, etc.) was also examined. The influence of the seasonality on the fluctuation of the zooplankton composition and abundance was not clearly detected (low degree of recurrent patterns). Rotifers (32 taxa) were the most abundant organisms during almost the entire study period with some seasonal alternations in the maximum abundance peaks of the main taxa (Conochilus unicornis, Keratella americana, K. cochlearis and Hexarthra spp.), except for Polyarthra (mainly P. vulgaris). Only occasionally copepods were numerically dominant. Higher copepod abundance was positively associated to periods of increase in the water retention time. Among the Copepoda (10 taxa) the calanoids (mainly Notodiaptomus iheringi) were more abundant, especially in warmer periods. Conversely, cyclopoids had higher abundance in autumn and winter. The species Thermocyclops minutus and T. decipiens co-occurred, but the first attained higher abundance. Some evidence of co-existence strategies between both species are considered. Cladocera (17 taxa) was never numerically dominant and the main taxa (Bosmina spp., Ceriodaphnia spp. and Diaphanosoma spp.) occurred almost the whole study period and did not present a seasonal pattern of fluctuation. Diaphanosoma (mainly D. birgei) attained the highest abundance among cladocerans. Most organisms were always found at the surface, but they also occupy the whole water column, even in periods of stratified conditions and low oxygen concentration in the bottom layers. Among the main zooplanktonic taxa, only Hexarthra avoids deep layers. An exceptionally high concentration of Copepoda nauplii on the surface was influenced by low transparency, high concentration of phytoplankton at this layer and low oxygen concentration at the bottom. In periods of higher retention timevariability there was a more heterogeneous distribution of the zooplankton in the water column. The increase in the retention time seems also to favor the copepod development. Finally, some inter-decade changes are considered on the basis of zooplankton assemblage structure observations.

Indirect food web effects of Bythotrephes invasion: responses by the rotifer Conochilus in Harp Lake, Canada

As a recent invader of North Ameri- can lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton com- munities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis sig- nificantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were ob- served in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1-2 years after invasion by Bythotrephes. Ultimately, it ap- pears that both reduced competition followed by a loss of native invertebrate predators were nec- essary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it ap- pears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for tro- phic relationships.

Effects of experimental greenhouse warming on phytoplankton and zooplankton communities in fishless alpine ponds

The impacts of global warming on aquatic ecosystems are expected to be most pronounced at higher trophic levels in cold­water environments. Therefore, we hypothesized that warming of fishless alpine ponds would suppress large‐bodied consumers (e.g., cladocerans, copepods) and stimulate fast‐growing microorganisms (e.g., phytoflagellates, rotifers), thereby altering the community composition and total abundance of zooplankton and phytoplankton. This hypothesis was tested using three blocks of four experimental mesocosms (1000‐liter capacity) that were located next to alpine ponds in Banff National Park, Canada. Each block received unfiltered pond water and sediment from a pond following ice out in June 2000. A warming treatment (control vs. 3.6°C warmed) was achieved by controlling the ventilation of greenhouse canopies that were suspended over each of the mesocosms. By the end of our 50‐d experiment, warming significantly suppressed total zooplankton biomass because large cladocerans (Daphnia pulex) declined while rotifer (Keratella cochlearis, Conochilus unicornis) abundance increased during the second half of the experiment. In contrast, warming did not affect total phytoplankton biomass but significantly altered community composition by favoring phytoflagellates (Mallomonas, Synura, Trachelomonas) over larger filamentous green algae (Mougeotia, w). Warming did not significantly increase dissolved nitrogen and phosphorus concentrations. Therefore, warmer growing conditions and reduced grazer biomass best explained the increased abundance of more edible, fast‐growing phytoflagellates in the warmed mesocosms. Our findings support the hypothesis that moderate warming can destabilize plankton dynamics, thereby potentially reducing the reliability of water quality and food resources for higher trophic levels (e.g., planktivorous fish) in shallow cold‐water ecosystems.

Distribution of rotifer assemblages in North Island, New Zealand, lakes: relationships to environmental and historical factors

1. The geographical distribution of planktonic rotifer species was investigated in 31 lakes in the North Island of New Zealand.2. A total of 78 species was recorded. Species richness, previously thought to be low in New Zealand, was found to be comparable with that of northern temperate lakes with an average of 21.1 species found per lake. The large, deep oligotrophic Lake Taupo had the lowest richness, and the artificial reservoirs the highest.3. The distribution of species, investigated using multivariate techniques [Cluster analysis, canonical correspondence analysis (CCA)], was most strongly associated with trophic state gradients. For example, Conochilus unicornis, C. dossuarius and Ascomorpha ovalis were associated with more oligotrophic conditions, and Brachionus budapestinensis, B. calyciflorus and Keratella tropica with more eutrophic conditions. Inorganic turbidity was also of importance in determining rotifer distributions in some shallower lakes.4. Some species, for example K. australis and C. exiguus, appear to be limited in distribution by poor dispersal abilities.

Grazing by a dominant rotifer Conochilus unicornis Rousselet in a mountain lake: in situ measurements with synthetic microspheres

Grazing rates of zooplankton were analysed in the summer of 1999 in Yellow Belly Lake, an oligotrophic system in the Sawtooth Mountains of Idaho (U.S.A.). The colonial rotifer Conochilus unicornis was a dominant species in the epilimnion, with densities reaching 20 colonies 1−1 (ca. 400 ind. 1−1). Clearance rates were measured with an in situ Haney Grazing chamber and synthetic microspheres 5, 9 and 23/μm in diameter. At epilimnetic temperatures of around 14 °C, mean clearance rates for 5µm particles ranged from 30 to 65 µ1 ind.−1 h −1. Clearance rates were 2–9 times higher on the 5µm spheres than on the 9 µm spheres, and C. unicornis almost never fed on the 23 µm spheres. Grazing rates did not change over the diel cycle. Clearance rates declined more than 10-fold as temperatures declined from 14 °C in the epilimnion to 7 °C in the metalimnion. In the epilimnion, grazing by C. unicornis was more important than grazing by crustaceans in the community, at least on particles ≤9µm. The results show the importance of grazing by rotifers in lakes, and the significance of spatial variations that influence grazing rates.

ASSOCIATION OF A FREE‐LIVING EUGLENA WITH A COLONIAL ROTIFER

Euglena agilis (syn. Euglena pisciformis) is a free‐living euglenoid found in brackish or fresh water. This species can be identified by its fusiform shape, single long flagellum, and prominent anterior eyespot. It can be distinguished from other euglenoids in part by its two to three large chloroplasts that run parallel to the length of the body, each with a single pyrenoid capped on two sides by paramylon. Euglena agilis is commonly found in our collecting ponds in central New Jersey. In two consecutive summers, we have found large numbers of these cells living symbiotically with the colonial rotifer Conochilus unicornis. We do not know if either organism benefits or is harmed in this relationship. For example, while we've seen the rotifers of the colony ingest other algae, we have not seen predation of the E. agilis (neither those embedded in the mucilaginous secretions of the rotifer nor swimming outside of the colony). This association is interesting for several reasons: 1) E. agilis is normally free‐living. 2) There are few known symbiotic relationships between euglenoids and rotifers. 3) E. agilis lose their long single flagellum when in the colony. We plan to use light and electron microscopy to investigate the ultrastructure of the E. agilis and its relationship to the rotifer colony.

Seasonal Abundance and Vertical Distribution of Zooplankton in Lake Ikeda, Southern Japan.

Species composition, seasonal variation and vertical distribution of zooplankton in Lake Ikeda, located in the south of Kyushu Island, Japan, were investigated from July 1993 to December 1994. Twenty-six species were identified, of which fourteen were newly recorded for the lake, these being five rotifers, six cladocerans and three copepods. The zooplankton community was dominated by copepods, cladocerans and rotifers. As the mean of 3 stations, they represented 49.6%, 28.5% and 21.9%, respectively. Rotifera was the most diverse group (13 species); other groups were Cladocera (9 species) and Copepoda (4 species). The zooplankton population was predominantly rotifers in the summer of 1993, of which Polyarthra euryptera and Conochilus unicornis were abundant. The calanoid copepod Eodiaptomus japonicus appeared in May 1994 and became dominant from August onwards. Vertical distribution showed that the majority of the zooplankton population appeared in the upper water layer (0-15m) of the lake.

Are bacteria an important food source for rotifers in eutrophic lakes?

In situ grazing measurements using fluorescent particles of 0.5, 2.4 and 6.3 mu m diameter in eutrophic Lake Loosdrecht (The Netherlands) showed that Anuraeopsis fissa, a small rotifer, filtered the smallest, bacteria sized particles as efficiently or more efficiently than the larger particles. In contrast, three other rotifer species (Brachionus angularis, Filinia longiseta and Pompholyx sulcata) filtered the bacteria- sized particles less efficiently than the larger particles. Both Keratella cochlearis and Conochilus unicornis only ingested the bacteria-sized particles. Anuraeopsis fissa had a higher uptake of fluorescent bacteria-sized particles than K.cochlearis, both in 1 mu m filtrate of lake water and in lake water. Within both species, uptake did not differ between juveniles and adults. When cultured on three different size fractions of lake water (1, 3 and 15 mu m filtrate) in July, all rotifer species declined in numbers on the 1 and 3 mu m filtrates, while A.fissa and B.angularis increased in numbers on the 15 mu m filtrate. The high abundance of small bacteria in the lake water could not support rotifer populations. It is concluded that bacteria are not a suitable food source of high quality for A.fissa because its population does not grow even though the bacterial concentration was higher than its estimated threshold food concentration. In August, when individually cultured, the mortality was high for all species, but especially for F.longiseta. The lifespan of K.cochlearis was reduced in the 1 and 3 mu m filtrates of lake water, compared with in the 15 mu m filtrate. The lifespan of A.fissa was similar in all filtrates, but reproduction was reduced in the 1 and 3 mu m filtrates, as in Keratella. On the 15 mu m filtrate, their ages at first reproduction and growth rates did not differ. Individuals of A,fissa older than 4 days showed a higher survival in the 15 mu m filtrate than in the other two filtrates, as did K.cochlearis throughout its life. Hence, bacteria seem to be a more important food source for younger individuals of A.fissa than of K.cochlearis. [KEYWORDS: Fresh-water zooplankton; keratella-cochlearis;population-dynamics; resource utilization; natural-populations; planktonic rotifers; filinia-terminalis; seasonal patterns; clearance rates; size selection]

Editorial [III-IV

The zooplankton of seven high altitude lakes in the Sierra Nevada, California, were sampled for three to six years. Periodic pH depression occurs in the lakes during spring snowmelt and infrequent, intense summer rains. The lakes contained many crustaceans and rotifers known or suspected to be acid intolerant, such as Daphnia rosea, Daphnia middendorffiana, Diaptomus signicauda, Conochilus unicornis, and Polyarthra vulgaris. Macrocyclops albidus, Bosmina longirostris and Diaptomus signicauda were the crustaceans most frequently encountered during snowmelt, while Polyarthra vulgaris, Keratella quadrata and Keratella taurocephala were the most fre- quently encountered rotifers. In several lakes, the zooplankton was numerically dominated by known acid resistant species during snowmelt (e.g. B. longirostris and H. gibberum). Daphnia rosea was found during snowmelt only three times during the entire study. Seasonal variation in species richness occurred in some of the lakes. No longterm trends in species richness were observed

Evaluation of bacterivory of Rotifera based on measurements of in situ ingestion of fluorescent particles, including some comparisons with Cladocera

Bacterivory of pelagic rotifers and cladocerans in eutrophic Lake Loosdrecht (The Netherlands) was determined by microscopic observation of in situ tracer particle uptake. Ingestion rates of rotifer species using 0.51 mu m microspheres or fluorescently labelled bacteria as tracers differed, with one exception. The ingestion rates depended on both the species and the tracer type. For cladocerans, fluorescently labelled bacteria seemed to underestimate grazing, presumably due to rapid digestion of tracer cells. Comparing results obtained with 0.51 mu m microspheres, rotifers were much more important grazers on bacteria than cladocerans in the study period (April-September). Based on microspheres, the rotifer populations with the highest uptake of bacteria were Filinia longiseta (May-July) and Anuraeopsis fissa (June-September). According to the uptake of fluorescently labelled bacteria, Conochilus unicornis had the highest uptake in June and A. fissa in July. [KEYWORDS: Fresh-water zooplankton; clearance rates; crustacean zooplankton; planktonic community; seasonal patterns; labeled bacteria; loosdrecht lakes; filtration-rates; natural bacteria; filtering rates]

Rotifer communities from some Araucanian lakes of southern Chile

During the last decade much information has been produced about the zooplankton communities in southern Chile; however, most of this is related to the crustacean assemblages. The present communication examines the spatial and temporal distribution of rotifer assemblages and their relation to the environmental variables during one-year period in four Araucanian lakes. A total of 19 species was found in these oligotrophic lakes. Keratella cochlearis, Synchaeta stylata, Trichocerca porcellus, Conochilus unicornis and Collotheca pelagica were widespread, and seven species exhibited a more restricted distribution among the lakes. Species richness varied from 6 to 12; similarly, one or two dominant species usually accounted for more than 80% of the total annual abundance. Similar dominant species occurred in two lakes, but their maximum peaks of abundance differed in time; in the remaining lakes the most important species were different. Calculated rotifer diversity showed a fluctuating pattern, with low values during the year in three lakes, and high ones in Lake Llanquihue. Species diversity was significantly related to species richness in all lakes. Discriminant analysis based on the occurrence and abundances of species throughout the year revealed that the rotifer assemblage in Lake Llanquihue was different from that in the rest of the Araucanian lakes. Furthermore, the same analysis using environmental variables showed that this lake is clearly discriminated from the others on the basis of the ionic composition of the water (i.e. Cl−, Na+, Mg2+). Rotifer abundances in these lakes were significantly influenced by a number of abiotic variables, including those related to water ionic composition. These relationships may imply that the small differences in chemical characteristics of these lakes influence the structure of the rotifer community.

The Structure and Function of Zooplankton in Thingvallavatn, Iceland

The zooplancton of Thingvallavatn is dominated by three species of rotifers and another three species of crustaceans. The rotifer Polyarthra dolichoptera (Idelson) constitutes up to 90% of the rotifer biomass in summer, while in autumn Keratella cochlearis (Gosse) and Conochilus unicornis (Rousselet) together contribute 55-80% of the routifer biomass. Cyclops abyssorum G. O. Sars s.l. varies from a minimum if 30% in Spetember to 100% if crustacean numbers, during winter. Leptodiaptomus minutus (Lillj.) reaches maximum or 50% by numbers in August and Daphnia longispina (O. F. Mull.) s.str 60% by numbers in September