Mesozooplankton of the Kosi Bay lakes,South Africa

The Kosi coastal lake system, a chain of four interconnected basins, is located in the subtropical north-eastern corner of South Africa. Little information is available on zooplankton of the system and the main aim of this study is to report on zooplankton samples collected during 2002 and 2003. The set of samples consists of seasonal, subsurface mesozooplankton samples that were collected during nighttime in each of the lakes. A well-developed salinity gradient was evident along the interconnected lakes in the subsurface water during all seasons, ranging from freshwater in the upper lake Amanzamnyama to a maximum of 22 recorded in Lake Makhawulani. The zooplankton community structures of the lakes reflected the salinity gradient of the system, with some coastal marine taxa recorded in the lakes closer to the mouth and only freshwater taxa recorded in Lake Amanzamnyama. Mesozooplankton diversity and abundance were relatively low compared to other estuarine systems along the eastern coast of South Africa. The dominant taxa were calanoid copepods Acartiella natalensis and Pseudodiaptomus stuhlmanni and the mysid Mesopodopsis africana in the lower lakes, whereas cyclopoids Mesocyclops sp. and Thermocyclops sp. dominated the freshwater lake Amanzamnyama.     

Plankton community and the relationship with the environment in saline lakes of Onon-Torey plain,Northeastern Mongolia

The plankton community of sixteen saline lakes located on Onon-Torey plain (Northeastern Mongolia) during the filling phase and the raising of the water level was investigated in July 2011. Thirty-five taxa of phytoplankton and thirty-one species of zooplankton were found. For phytoplankton, blue-green algae (Merismopedia elegans, Anabaenopsis elenkinii, Arthrospora fusiformis, Spirulina major, Lyngbya sp., Oscillatoria sp.) and green algae (Monoraphidium minutum, Tetrastrum komarekii, Ankyra ocellata, Oocystis sp.) were dominant. For zooplankton, Filinia longiseta, Brachionus plicatilis, B. variabilis, Hexarthra mira (Rotifera), Daphnia magna, Moina brachiata, M. mongolica (Cladocera), Arctodiaptomus bacillifer, Mixodiaptomus incrassatus, Metadiaptomus asiaticus (Copepoda) dominated. Mineralization, active hydrogen ratio, dissolved oxygen and water temperature were the main factors influencing the diversity, structure and distribution of plankton organisms in the steppe lakes during low water level. The RDA analysis for phytoplankton and zooplankton from different lakes was carried out for selected two groups which included lakes and a subset related species. The first group is of oligohaline and mesohaline lakes in which mostly green algae, rotifers and copepods inhabit. The second group is of mesohaline and polyhaline lakes with mainly blue-green algae, some crustaceans and rotifers inhabiting. High abundance and biomass of Spirulina major, Oscillatoria sp. and Brachionus variabilis were observed in lakes with high mineralization, pH and temperature.     

Ciliate community structure and interactions within the planktonic food web in two alpine lakes of contrasting transparency

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Salinity and the distribution of rotifers in the Lagos Harbour — Badagry Creek system,Nigeria

The waters of Lagos Harbour have a salinity variation of 30, as a consequence of the annual rainfall regime and of the influx of Atlantic Ocean waters. We made monthly plankton hauls at 10 stations for 11 months starting in October 1986, and found 51 species of rotifers, with Brachionus baylyi Sudzuki & Timms, Keratella hispida Lauterborn, Colurella obtusa Gosse, and Filinia pejleri Hutchinson recorded for the first time in Nigeria. In distribution, species numbers decreased with increasing salinity. Brachionus plicatilis, B. rubens, Hexarthra intermedia, Keratella americana, K. cochlearis and K. tropica were classed as euryhaline. Stenohaline species were Anuraeopsis fissa, Ascomorpha ovalis, Filinia longiseta, F. opoliensis, Gastropus sp., Lecane curvicornis and Monostyla stenroosi. All other monogononts were restricted to waters of salinity below 1.     

A comparison of zooplankton communities in saline lakewater with variable anion composition

Although salinity and aquatic biodiversity are inversely related in lake water, the relationship between types of salts and zooplankton communities is poorly understood. In this study, zooplankton species were related to environmental variables from 12 lakes: three saline lakes with water where the dominant anions were SO₄ and CO₃, four saline lakes with Cl-dominated water, and five dilute, subsaline (0.5–3 gl^?1 total dissolved solids) lakes of variable anion composition. Although this study comprised only 12 lakes, distinct differences in zooplankton communities were observed among the two groups of chemically defined saline lakes. Canonical correspondence analysis identified total alkalinity, sulphate, chloride, calcium, sodium, potassium, and total phosphorus as all contributing to the first two ordination axes (λ₁ = 0.97 and λ₂ = 0.62, P<0.05). The rotifer Brachionus plicatilis and the harpactacoid copepod Cletocamptus sp. prevailed lakes with Cl-dominated water. In contrast, the calanoid copepods Leptodiaptomus sicilis and Diaptomus nevadensis were dominant in the SO₄/CO₃-dominated lake water with elevated potassium (79–128 mg l^?1) and total phosphorus concentrations (1322-2915 μg l^?1). The contrasting zooplankton species distribution among these two saline lake types is likely explained by variable selective pressure on zooplankton and their predators from differing physiological tolerances to salt stress and specific ions. While inland saline lakes with Cl as the dominant anion are relatively rare in Canada and SO₄/CO₃ are the common features, our study provided an opportunity to compare zooplankton communities across the two groups of lakes.     

Zooplankton Seasonal Abundance of South AmericanSaline Shallow Lakes

The central provinces of Argentina are characterized by the presence of a high number of shallow lakes, located in endorheic basins, many of which have elevated salinities as well as eutrophic or hypereutrophic condition. The zooplankton of four saline shallow lakes of the province of La Pampa was studied on a monthly basis during a 2‐year period to determine its temporal and spatial variation. The surface of these shallow lakes (<2.5 m depth) varied between 56.8 and 215.9 ha, and some have from 8.4 to 20.8 g · l^–1. The more saline lakes have “clear” water and the less saline lakes “turbid” water. Fishes, Jenynsia multidentata , were present in only two lakes during the last two months of the studied period. The zooplankton was composed of 17 taxa of Rotifera, 5 taxa of Cladocera and 4 taxa of Copepoda. The low diversity and the faunistic composition are characteristic of saline environments. Although the studied lakes share 38% of the species, the faunistic similarity was higher between the two least saline lakes. The lowest diversity was found in the two most saline lakes. All four shallow lakes were characterized by their very high zooplankton density, especially in the least saline lakes (<80000 ind · l^–1). The abundance is significantly correlated with the water transparency but not with salinity. The zooplankton temporal variation was characterized by the alternation of macro‐ and microzooplankton, probably regulated by competition and intrazooplanktonic predation. In each lake, the spatial abundance distribution of the macro‐ and microzooplankton was homogeneous. It was related to the shallow depht of the lakes and their polymictic condition. The Scheffer model on alternative states in shallow lakes acknowledges that it cannot be applied to saline lakes because Daphnia , the main responsible for the clear water state, is not tolerant to high salinity. Our study shows that the most saline lakes, where the halophylic Daphnia menucoensis is abundant, have also the most clear waters. Another difference that we found with regards to the mentioned model is that, in turbid lakes, it could not have had a top‐down control on macrozooplankton exerted by fishes because in these lakes fishes were practically absent. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Community structure and bottom-up regulation of heterotrophic microplankton in arctic LTER lakes

Microplankton community structures and abundance was assessed in lakes at the Toolik Lake LTER site in northern Alaska during the summers of 1989 and 1990. The microplankton community included oligotrich ciliates, but rotifers and zooplankton nauplii comprised greater than 90% of total estimated heterotrophic microplankton biomass. Dominant rotifer taxa included Keratella cochlearis, Kellicottia longispina, Polyarthra vulgaris, Conochilus unicornis and a Synchaeta sp. Microplankton biomass was lowest in highly oligotrophic Toolik Lake (< 5 μgCl⁻¹ at the surface) and highest (up to 55 μCl⁻¹) in the most eutrophic lakes, experimentally fertilized lakes, and fertilized limnocorrals, consistent with bottom-up regulation of microplankton abundance.     

Zooplankton assemblages in two reservoirs: one subjected to accentuated water level fluctuations,the other with more stable water levels

The abundance, composition and dynamics of zooplankton were followed in two reservoirs of the River Douro catchment. The Serra Serrada Reservoir is subject to marked fluctuations in water levels. The highest values of total phosphorus, soluble reactive phosphorus, nitrate, water colour and chlorophyll a were found during the minimum level phase. Rotifera was dominant except in late summer and autumn when the cladoceran Ceriodaphnia quadrangula or the copepod Tropocyclops prasinus replaced them as the dominant zooplankton. Among the rotifers the most common taxa were Keratella cochlearis, Conochilus sp. and Asplanchna priodonta. Maximum rotifer density was about 80,000 ind m⁻³ in 2000, 200,000 ind m⁻³ in 2001 and 100,000 ind m⁻³ in 2002. Among the crustacean zooplankton C. quadrangula achieved densities of up to 45,000 ind m⁻³ and T. prasinus, up to 80,000 ind m⁻³. Canonical correspondence analysis revealed a strong contribution of the variation in the stored water volume, temperature, total phosphorus, chlorophyll, nitrates, and water transparency to the observed, significant association between zooplankton assemblage and environmental variables. In the Azibo Reservoir, fluctuations in water level are smaller. Only total phosphorus, cholorophyll and conductivity varied seasonally. Cladocera and Copepoda were dominant during the whole study period. The most abundant taxa were Ceriodaphnia pulchella, Daphnia longispina, Diaphanosoma brachyurum, Bosmina longirostris and Copidodiaptomus numidicus. Cladocera achieved densities of up to 25,000 ind m⁻³ and Copepoda up to 15,000 ind m⁻³. Rotifera in general reached densities of up to 6,000 ind m⁻³. On the basis of canonical correspondence analysis only temperature and conductivity were significantly associated with zooplankton assemblage.     

The Spatial Segregation of Zooplankton Communities with Reference to Land Use and Macrophytes in Shallow Lake Wielkowiejskie (Poland)

The spatial distribution of zooplankton in relation to two types of land‐use (forested and pastoral‐arable) of a lake's surroundings and to various habitats (helophytes, elodeids, nymphaeids and open water) was examined along 16 parallel transects on a macrophyte‐dominated lake (area – 13.3 ha; mean depth – 1.4 m). The type of habitat was the main determinant of zooplankton community structure. Dissected‐leaved elodeids harboured the richest and most abundant community with typically littoral (e.g., Colurella uncinata) and pelagic species (e.g., Keratella cochlearis). Two species (Polyarthra major and P. vulgaris) selectively chose the open water and one (Lecane quadridentata) the Typha stand. No spatial differentiation in zooplankton abundance was recorded between the two types of the catchment area. One possible explanation may be the shallowness and small area of this lake which may support full mixing and no difference in physical‐chemical gradients. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Field and experimental studies on the combined impacts of cyanobacterial blooms and small algae on crustacean zooplankton in a large,eutrophic, subtropical,Chinese lake

Field and experimental studies were conducted to evaluate the combined impacts of cyanobacterial blooms and small algae on seasonal and long-term changes in the abundance and community structure of crustacean zooplankton in a large, eutrophic, Chinese lake, Lake Chaohu. Seasonal changes of the crustacean zooplankton from 22 sampling stations were investigated during September 2002 and August 2003, and 23 species belonging to 20 genera were recorded. Daphnia spp. dominated in spring but disappeared in mid-summer, while Bosmina coregoni and Ceriodaphnia cornuta dominated in summer and autumn. Both maximum cladoceran density (310 ind. l⁻¹) and biomass (5.2 mg l⁻¹) appeared in autumn. Limnoithona sinensis, Sinocalanus dorrii and Schmackeria inopinus were the main species of copepods. Microcystis spp. were the dominant phytoplankton species and formed dense blooms in the warm seasons. In the laboratory, inhibitory effects of small colonial Microcystis on growth and reproduction of Daphnia carinata were more remarkable than those of large ones, and population size of D. carinata was negatively correlated with density of fresh large colonial Microcystis within a density range of 0–100 mg l⁻¹ (r = −0.82, P< 0.05). Both field and experimental results suggested that seasonal and long-term changes in the community structure of crustacean zooplankton in the lake were shaped by cyanobacterial blooms and biomass of the small algae, respectively, i.e., colonial and filamentous cyanobacteria contributed to the summer replacement of dominant crustacean zooplankton from large Daphnia spp. to small B. coregoni and C. cornuta, while increased small algae might be responsible for the increased abundance of crustacean zooplankton during the past decades.     

EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of many Great Basin lakes has resulted from the diversion of tributary streams for agricultural or municipal uses. At Mono Lake, an alkaline salt lake in eastern California, salinities have risen from 50 to 100 g·L^?1 in just 50 years. Experimental mesocosms were established to simulate some of the potential ecological effects that could have accompanied this change. The influence of salinity on diatom diversity, taxonomic structure, and primary production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0.5 m deep, with open tops covered by 1 mm mesh net. Five treatments (50, 75, 100, 125, and 150 g·L^?1) with four replicates per treatment were used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g·m^?2) and diversity (from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities ≥75 g·L^?1. Photosynthetic oxygen production also was significantly lower at salinities ≥75 g·L^?1. Diatom indicator taxa for these shifts included Denticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Stephanodiscus oregonicus increasing in relative abundance in higher salinity treatments, accompanied by decreases in Achnanthes minutissima, Cymbella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting dominance at moderate salinity levels (75 to 125 g·L^?1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophyte, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occurred only in salinity treatments from 50 to 100 g·L^?1. Diversion of tributary stream flow and resulting salinity increases in this lake threaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision of the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to around 75 g·L^?1 and is expected to increase the diversity and productivity of the benthic algae of this ecosystem.     

A description of pelagic zooplankton in the large lakes of the Todzha Depression (Bolshoi Yenisey River basin, Tuva)

The zooplankton structure of large lakes (Noyon-Khol’, Shuram-Khol’, Borzu-Khol’, Many-Khol’, Kadysh, and Todzha) of the Todzha Depression has been studied. A comparison of the species composition, abundance, and biomass of zooplankton in the lakes was made. Rotifers have the largest species diversity: from 10 to 16 species, with Conochilus sp., Collotheca sp., Kellicottia longispina (Kellicott), and Asplanchna priodonta Gosse being the most abundant. Copepods are represented mostly by Arctodiaptomus paulseni (Sars), Eudiaptomus graciloides (Lilljeborg), Cyclops scutifer Sars, and Mesocyclops leuckarti Claus. Daphnia galeata Sars and Bosmina longispina Leydig are the dominant cladoceran species. It is shown that the zooplankton communities of the lakes in the Todzha Depression are characterized by a high degree of species composition similarity. Factors forming the taxonomic structure of pelagic zooplankton in the system of these lakes are displayed.     

Diet,food preference,and algal availability for fishes and crabs on intertidal reef communities in southern California

Synopsis Herbivory by wide-ranging fishes is common over tropical reefs, but rare in temperate latitudes where the effects of herbivorous fishes are thought to be minimal. Along the west coast of North America, herbivory by fishes on nearshore reefs is largely restricted to a few members of the Kyphosidae, distributed south of Pt. Conception. This paper presents information on natural diets and results from feeding choice experiments for two abundant kyphosids from intertidal habitats in San Diego, California —Girella nigricans andHermosilla azurea, and similar data for the lined shore crab,Pachygrapsus crassipes, which also forages over intertidal reefs. These results are compared with the availability of algae in intertidal habitats measured during summer and winter, on both disturbed and undisturbed habitats. The diets of juveniles ofG. nigricans andH. azurea collected from nearshore habitats were dominated by animal prey (mainly amphipods), but adults of these fishes, andP. crassipes, consumed algae nearly exclusively, with 26, 10, and 14 taxa of algae identified fromG. nigricans, H. azurea, andP. crassipes, respectively. Algae with sheet-like morphologies (e.g.Ulva sp.,Enteromorpha sp., members of the Delesseriaceae) were the principal algae in the diets of the fishes, and calcareous algae (e.g.Corallina sp.,Lithothrix aspergillum) and sheet-like algae (Enteromorpha sp.) comprised the greatest identifiable portion of the shore crab's diet. Feeding choice experiments indicated that the fishes preferred filamentous algae (e.g.Centroceras clavulatum, Polysiphonia sp.,Chondria californica) and sheet-like algae (e.g.Enteromorpha sp.,Ulva sp.,Cryptopleura crispa) over other algal morphologies, whereas the shore crab chose jointed calcareous algae (e.g.Lithothrix aspergillum, Corallina vancouveriensis, Jania sp.) most frequently. The diets and preferences for algae by the fishes were generally most similar to the assemblage of algae available in early successional (disturbed) habitats during summer when sheet-like and filamentous algae are abundant. The shore crab exhibited the opposite trend with a diet more similar to late successional (undisturbed) habitats.     

Winter phytoplankton community structure in three shallow temperate lakes during ice cover

The general model of seasonal phytoplankton succession in temperate lakes suggests that winter phytoplankton growth is minimal under ice-cover. However, some studies have found diverse phytoplankton communities during winter. The primary objectives of this study were to determine the species composition and the changes in the winter phytoplankton community structure under the ice. For 2 consecutive winters, phytoplankton samples were collected under ice-cover at 4 sites on 3 lakes in Arrowwood National Wildlife Refuge (ANWR), near Pingree, North Dakota. Ninety taxa were identified and enumerated. Densities of several of these taxa frequently exceeded 10⁶ cells l^–1. The winter phytoplankton communities in these lakes were dominated by flagellates, principally cryptomonads, a synurophyte (Synura uvella), small chrysophytes (Chrysococcus spp., Kephyrion spp.) and a dinoflagellate (Peridinium aciculiferum), as well as non-flagellate microchlorophytes (Monoraphidium spp., Ankistrodesmus spp., and Pseudodictyosphaerium sp.), a cyanobacterium (Gloeocapsa aeruginosa) and centric diatoms (Stephanodiscus minutulus, S. parvus and Cyclotella meneghiniana).     

Zooplankton community composition of lakes in the Yukon and Northwest Territories (Canada): relationship to physical and chemical limnology

We analysed associations between zooplankton species composition and local abiotic factors in 30 lakes located along a 900 km south-north transect from Whitehorse (Yukon Territory) to Inuvik (Northwest Territories). The lakes were situated in three broadly defined vegetation zones: (i) Boreal forest (between Whitehorse and Dawson City), (ii) alpine tundra (Ogilvie mountains north of Dawson City) and (iii) subarctic forest-tundra (near Inuvik). Lakes in the alpine tundra were characterised by lower conductivity, temperature, chlorophyll a and nutrients than those in the other two zones. Those in the forest-tundra were generally small and shallow, and had higher chlorophyll a concentrations than lakes further south. Lakes in forested catchments spanned a larger latitudinal range and exhibited a greater variety of physical and chemical characteristics. However, they were generally deeper, with higher conductivity, temperature and ionic concentrations. Forty-one zooplankton taxa were identified from the 30 lakes, of which the most frequently occurring were the rotifers Conochilus unicornis, Kellicottia longispina, Keratella cochlearis and Polyarthra vulgaris, the cladocerans Daphnia middendorffiana and Bosmina longirostris, and the copepods Leptodiaptomus pribilofensis, Heterocope septentrionalis and Cyclops spp. The lakes contained between two and fifteen species (mean = 6.9). Alpine tundra lakes contained slightly less species (mean = 5.8) than those at lower elevations; in particular the cladoceran fauna was depauperate or absent. Relationships among the lakes, species and environmental factors were examined using canonical correspondence analysis, with forward selection and associated Monte Carlo permutation tests. Chloride, silica and temperature showed statistically significant relationships with species distribution, and together these abiotic factors explained 25% of the variation in zooplankton communities within Yukon and Northwest Territories lakes.     

Quantification of invertebrate predation and herbivory in food chains of low complexity

Zooplankton grazing impact on algae, heterotrophic flagellates and bacteria, as well as invertebrate predation on herbivorous zooplankton, were investigated in two sub-Antarctic lakes with extremely simple food chains. The two species of herbivorous zooplankton present in the lakes (the copepods boeckella michaelseni and Pseudoboeckella poppei) exerted substantial grazing pressure on algae. However, the dominant algal species exhibited properties that enabled them to avoid (large size or extruding spines, e.g. Staurastrum sp., Tribonema sp.) or compensate (recruitment from the sediment, Mallomonas sp.) grazing. There are only two potential invertebrate predators on the herbivorous copepods in the two lakes: the copepod Parabroteas sarsi and the diving beetle Lancetes claussi. Vertebrate predators are entirely abscent from sub-Antarctic lakes. Based on our experiments, we estimated that the predators would remove at most about 0.4% of the herbivorous copepods per day, whereas planktivorous fish, if present in the lakes, would have removed 5–17% of the zooplankton each day. Consequently, the invertebrate predators in these high-latitude lakes had only a marginal predation impact compared to the predation pressure on zooplankton in the presence of vertebrate predators in temperate lakes. The study of these simple systems with only two quantitatively functionally important trophic links, suggests that high grazing pressure foreces the algal community towards forms with grazer resistant adaptations such as large size, recruitment from another habitat, and grazer avoidance spines. We propose that due to such adaptations, predictions from food web theory are only partly corroborated, i.e. algal biomass actually increases with increasing productivity, although the grazer community is released from predation. In more species-rich and complex systems, e.g temperate lakes with three functionally important links, such adaptations are likely to be even more important, and, consequently, the observable effects of trophic interactions from top predators on lower trophic levels even more obscured.     

Dominant rotifers of Võrtsjärv (Estonia)

Rotifers form 71% of the zooplankton of the strongly eutrophic (total N 2 g m^–3, total P 53 mg m^–3) Võrtsjärv (Estonia). Altogether 150 taxa of rotifers occur. Species characteristic of oligo- and mesotrophic waters have totally disappeared during the last 30 years, or are disappearing now. Species whose numbers and biomass reached 20% or more of total zooplankton were considered dominants. These were Anuraeopsis fissa, Keratella cochlearis, K. quadrata frenzeli, Polyarthra dolichoptera, P. luminosa, Synchaeta verrucosa and Trichocerca rousseleti. The contribution of dominant rotifers to total zooplankton and its biomass is as follows: S. verrucosa 25% and 39%, P.dolichoptera 34% and 25%, K. cochlearis 25% and 7%, K. quadrata frenzeli 9% and 7%, A. fissa 28% and 0.4%, T. rousseleti 20% and 0.6%, respectively.     

Horizontal distribution of cladocerans in arctic Greenland lakes – impact of macrophytes and fish

Submerged macrophytes may play an important role as a refuge for zooplankton against predators. However, a recent study suggests that their importance depends on the trophic state of the lake. We studied the impact of fish and macrophytes on the horizontal distribution of pelagic cladocerans in 56 oligotrophic arctic Greenland lakes. In north-east and western Greenland, zooplankton was sampled in the near-shore (littoral) and central (pelagial) part of all lakes and fish were sampled with multiple mesh-sized gill nets. Macrophytes were visually estimated in the littoral. In north-east Greenland, 5 taxa of cladocerans were found, while 14 taxa were recorded in western Greenland. Daphnia pulex occurred only in fishless lakes in both northeast and western Greenland and avoided the near-shore areas in the shallow and deep lakes. Bosmina spp. and Holopedium gibberum were evenly distributed between the littoral and the pelagial in the deep and shallow fishless lakes. However, their near-shore density was lowest in the presence of fish. Macrophyte-related and benthic cladocerans concentrated either in the littoral or were evenly distributed between the littoral and the pelagial, irrespective of depth and fish presence or absence. Macrophytes had no impact on the horizontal distribution of pelagic cladocerans. Thus, it is concluded that horizontal heterogeneity of Bosmina spp. and Holopedium gibberum might be affected by the presence of fish.     

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 longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l^–1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.