Species distributions and shell characteristics of Pisidium (Mollusca: Bivalvia) in the Colorado Front Range: the role of abiotic factors

SUMMARY.

• 1 Forty lakes in the Colorado Front Range were sampled to examine the relationship between water chemistry and the distributions and shell characteristics of Pisidium species. Alkalinity, pH, and dissolved organic carbon contributed most to the classification of lakes according to the dominant Pisidium species. Alkalinity was also strongly correlated with mean clam weight for populations of P. casertanum, the most common of the five species collected.
• 2 Historical evidence shows a reduction in the number of lakes occupied by clams and a constriction in the distributions of certain species. These changes have occurred mainly in lakes that now have low alkalinity.
• 3 A role for acidification is suggested because the recorded changes in distributions have occurred during the last 20 years, coincident with a general decline in alkalinity in waters of the Front Range.

     

Growth of two Pisidium (Bivalvia,Sphaeriidae) species in the laboratory

Summary Growth of Pisidium casertanum and P. subtruncatum from the profundal of the eutrophic Lake Esrom was followed at four temperatures in the laboratory. The growth rate of both species increased with increasing temperature, but the maximum in P. casertanum was attained at 12° C, instead of at 20° C as in P. subtruncatum. The variation of Q₁₀ of growth in relation to animal weight was of similar form in both species at higher temperatures (6–12°C) but opposite at low temperatures (2–6°C). In the latter case Q₁₀ of P. casertanum had a peak and that of P. subtruncatum a low value at a corresponding size preceding maturity. At constant temperatures growth curves of the form y=a+bx+cx² were obtained, where y is weight in g and x is time in days. The growth of both species under natural conditions was simulated by use of the experimental growth rate equation and natural birth periods, sizes at birth, and temperature cycles. These simulations gave a maximum life span of 2.5–3 years for P. casertanum and 3.5–4.5 years for P. subtruncatum. The latter species showed slower growth rates at the low or medium temperatures prevailing in the profundal of this lake. A comparison with growth in nature reveals that the simulation overestimated growth in case of P. casertanum.     

Oscillations in density and diversity ofPisidium communities in two biotopes in Southern Poland

Summary From September 1979 to October 1980 in two types of biotopes (in the stream at Kali and in the Biaa Przemsza River near Jaroszowiec, southern Poland)P.amnicum, P.milium, P.subtruncatum, P.nitidum, P.personatum, P.casertanum andP. sp.non det. were found.P.amnicum was accepted as an indicator species for the river community, andP.milium for the stream. The irregular changes of species composition of the sphaeriid community in the stream at Kali are rather accidental and the only temporary occurrence ofP.nitidum, P.personatum andP. sp.non det. in the Biaa Przemsza River might be connected with seasonal migration.The dominance structure of thePisidium community in the stream agrees with the model described by CONNELL (1977) for the communities of large sedentary animals and plants. The dominance relations in the river community are similar to the situation described by CONNELL (1977) as characteristic for small lakes and islands.The density of the Sphaeriidae on the chosen habitat in the Biaa Przemsza River oscillated between 300 and 3.300 ind./m² and it reached almost 130.000 ind./m² in the stream at Kali.The SHANNON-WIENER's diversity index (H) and that of equitability or evenness of species abundances (J) had low values during the whole period of investigation.This study was performed as part of the Problem MRII/15 of the Polish Academy of Sciences.     

Lacustrine,profundal meiobenthic oligochaetes as indicators of trophy and organic loading

Meiobenthic and macrobenthic oligochaetes were collected from two depths in Lake Päjänne and examined together with environmental data in which two environmental gradients can be seen. Numbers of Piguetiella blanci, Aeolosoma quaternarium and Aeolosoma hemprichi were positively correlated with oxygen saturation and negatively correlated with total phosphorus and COD suggesting that these species are indicators of oligotrophic conditions. Numbers of Amphichaeta leydigii, Specaria josinae and Vejdovskyella comata were positively correlated with total phosphorus which implies that these species prefer eutrophic conditions. The last three species and Dero digitata showed positive correlations with COD and organic sedimentation, which suggests that they also indicate organic loading. A decrease in aeolosomatid biomass was seen with increasing phosphorus and organic loading. The average body size of detritus-feeding naidids in the upper part of the profundal zone was reduced by organic loading, and more clearly eutrophication, which emphasizes the importance of including meiobenthic oligochaetes in pollution investigations. Of the naidid and aeolosomatid species, Chaetogaster langi preferred the greatest depths of the profundal zone while most of the other species were more abundant at 20 m rather than at the maximum depth. Clustering of the oligochaete taxa supported these observations and clustering of sampling stations based on oligochaete variables generally corresponded to clustering based on environmental data.     

Amino acids as determinants of host preference for the xylem feeding leafhopper,Homalodisca coagulata (Homoptera: Cicadellidae)

Summary Homalodisca coagulata is a highly polyphagous xylem feeder with distinct seasonal patterns in it's selection of host plants. These patterns were examined in relation to the amino acid content of the xylem for four common host species; Lagerstroemia indica, Baccharis halimifolia, Prunus persica, and Prunus salicina. Xylem fluid was collected from each host species at times when numbers of feeding leafhoppers were both low and high. In each case, concentrations of amino acids were greatest when numbers were high. Similarly, comparisons between host species at given times showed that concentrations of amino acids were positively correlated with host selection. In a second study, amino acids of xylem were manipulated by budding scions of a non-preferred host (P. persica) on rootstocks of preferred (P. salinica) and non-preferred (P. persica) hosts. Morphology and phenology of the budded trees were similar to that of the scion species yet the xylem composition of amino acids was primarily dependent on the rootstock. Concentrations of amino acids and the preference of leafhoppers were roughly two-fold greater for scions of the preferred than the non-preferred rootstock. In both studies, amides (glutamine plus asparagine) were the amino acids most highly correlated with host selection. These compounds are the predominant amino acids in xylem fluid, have high nitrogen to carbon ratios, and account for a high percentage of the caloric value in xylem fluid. Many of the less abundant amino acids were positively correlated with host preference, but the correlations were less consistent and correlation coefficients were generally lower.Florida Agricultural Experiment Station Journal Series No. 9672     

Freshwater bryozoans (Bryozoa) of Norway III: distribution and ecology of Plumatella fruticosa

Bryozoans were investigated during field studies of 601 lakes and other surface water bodies throughout Norway from 1960 to 1978. The frequency of occurrence of Plumatella fruticosawas evaluated in relation to 12 environmental variables. Statistically significant deviations from the frequencies expected on the basis of random distribution were described using the categories preference, avoidance, and absence. According to our material P. fruticosais one of the most common bryozoan species in Norwegian fresh water, represented in samples from 251 localities. This species occurred frequently all over the country, north to 71° 06 N (the northernmost record globally). Maximum elevation above sea level was 1179 m (maximum for Northern Europe).In most studies of freshwater bryozoans from the holarctic region, Plumatella fruticosahas been reported as widely distributed, but locally rare. In Norway, however, P. fruticosa is frequently found all over the country, even far to the north. The species apparently finds an optimal climate in the cold temperate and cold regions of Norway. Plumatella fruticosapreferred lakes, avoided slow-flowing rivers and ponds, and was absent in smaller water bodies. The species preferred somewhat higher elevation (400–900 m above sea level), but avoided lakes with the lowest summer temperatures (below 11 °C). P. fruticosa also preferred oligotrophic conditions with poor aquatic vegetation, stony shores, lakes poor in calcium and magnesium (sometimes almost comparable with distilled water), where the water was clear and colourless, and slightly acidic to neutral. The species also preferred lakes surrounded by Sphagnum bogs and was absent when pH was below 5.2. P. fruticosa avoided eutrophic conditions with rich aquatic vegetation, alkaline water (pH above 7.0), lakes with a higher content of calcium and magnesium and those with strongly coloured water (above 100 mg Pt l^–1). In spite of its preference for ion-poor conditions, it was also found in a brackish water lake. For most environmental variables, the species had a wider tolerance range than reported from elsewhere.     

Distribution, ecology, mode of reproduction and karyology of freshwater planarians (Platyhelminthes; Turbellaria; Tricladida) in the springs of the central Pyrenees

The distribution, reproductive modality and karyology of populations of freshwater planarians were studied together with physico-chemical variables m 149 springs in the central Pyrenees Diversity was limited to four species (Dugesia (Dugesia) subtentaculata, Polycelis felina. Crenobia alpina and Phagocata vitta), already known from streams and lakes Planarians were found rarely (only a third of springs contain them), m low-moderate abundance, and any single spring did not harbour more than one species The distribution of potential predators and preys among the springs sampled was fairly uniform So. we assume that whereas predation is unlikely to influence triclad distribution, the oligotrophic character of these habitats may explain, through food shortage and ensuing interspecific competition, the presence of a single species in each spring Besides interspecific competition, the distribution of each species can also be influenced by particular abiotic parameters Temperature and current velocity appeared to be the dominant factors, the first separating C alpina from the rest of species, and the second favouring the presence of P felina in a large set of springs These factors also seem to determine planarian distribution in streams and rivers Other environmental factors such as calcium bicarbonated water (for D subtentaculata) and preference for karst systems (for Phagocata vitta) are of importance Some peculiarities of distribution, like the scarcity of C alpina in reophile habitats and the thermal tolerance of Phagocata vitta, may be due both to competition from P felina and to historical factors in the former and to morphological (presence of pigment) and ecological (epigean habits) features m the latter, respectively Karyological analyses demonstrated triploidy in all populations of D subtentaculata. a polyploid origin for C alpina and the finding in Phagocata vitta of a new polyploid series leading to the highest chromosome number in turbellarians (14n = 238) Furthermore, the basic chromosome number of C alpina (n = 21) and Phagocata vitta (n = 17) differs from those found in northern European populations (n = 7 in both, respectively) This suggests that under the specific names C alpina and Phagocata vitta more than one species actually occurs, this being a problem for further studies Whereas polyploidy has often been related to living in cold, harsh habitats, asexual reproduction has been linked to environments where biotic stress is low The preeminence of asexual reproduction (by fissiparity) and polyploid forms in freshwater planarians inhabiting the unproductive, biotically unsaturated springs of the central Pyrenees adds new evidence to strengthen such links     

On the ecology of Trichocercidae (Rotifera)

A material consisting of 46 trichocercid rotifers from diverse waters in south and central Sweden was analyzed to reveal their relationships to their substrate. Most species were found in a wide variety of periphytic environments, but a few seemed to be specialized on bogs or psammon, respectively. Several trichocercids are generally regarded as plankters, but some of them obviously invade the open water only on certain occasions. A few species of Trichocerca are registered as indicators of eutrophy, but in some cases only a planktic occurrence may be regarded as indicating. On the other hand, hypertrophic environments are generally avoided. Trichocerca similis prefers polyhumic environments, T. uncinata oligotrophic and running-water conditions. The occurrence is often best explained by studying the food choice.     

Carbon dioxide output in the respiration of three Pisidium species (Bivalvia,Sphaeriidae)

Summary The carbon dioxide output in respiration of Pisidium casertanum, P. henslowanum and P. conventus was studied in the laboratory. A closedbottle method was used, and the amount of dissolved inorganic carbon was measured with an infrared gas analyser. The mean value of the constant b in equation R=aW ^b, where R is the respiration rate and W the ash-free dry weight of the animal, was ca. 0.75 in both the eurytherm P. casertanum and the cold-stenotherm P. conventus at five temperatures, but the temperature dependence of b was opposite in these species. The temperature dependence of metabolism increased with increasing size in P. casertanum and P. henslowanum but decreased in P. conventus. A reversed acclimatization pattern was found, where the metabolism rate and its temperature dependence was lower in the cold-adapted stenotherm species.     

Dissolved Organic Carbon as Major Environmental Factor Affecting Bacterioplankton Communities in Mountain Lakes of Eastern Japan

Relationships between environmental factors and bacterial communities were investigated in 41 freshwater lakes located in mountainous regions of eastern Japan. Bacterioplankton community composition (BCC) was determined by polymerase chain reaction-denaturing gradient gel electrophoresis of the 16S rRNA gene and then evaluated on the basis of physicochemical and biological variables of the lakes. Canonical correspondence analysis revealed that BCC of oligotrophic lakes was significantly influenced by dissolved organic carbon (DOC) content, but its effect was not apparent in the analysis covering all lakes including mesotrophic and eutrophic ones. The generalized linear model showed the negative association of DOC on the taxon richness of bacterioplankton communities. DOC was positively correlated with the catchment area per lake volume, suggesting that a large fraction of DOC supplied to the lake was derived from terrestrial sources. These results suggest that allochthonous DOC has a significant effect on bacterioplankton communities especially in oligotrophic lakes. The genus Polynucleobacter was detected most frequently. The occurrence of Polynucleobacter species was positively associated with DOC and negatively associated with total phosphorus (TP) levels. In addition, TP had a stronger effect than DOC, suggesting that oligotrophy is the most important factor on the occurrence of this genus.     

Influence of successional stage of Eucalyptus regnans (mountain ash) on habitat use by reptiles in the Central Highlands,Victoria

Abstract Eleven sites, representing different successional stages of Eucalyptus regnans-dominated forest in the Central Highlands of Victoria, were selected to identify potentially important habitat variables for heliothermic reptiles, three species (Eulamprus tympanum, Niveoscincus coventryi, Pseudemoia spenceri) of which were common at these sites. Analyses reveal that many forest attributes are strongly correlated with stand age and that some are significant habitat variables for some of these species. ‘One-at-a-time’ modelling of habitat variables reveals that counts of E. tympanum are most strongly correlated with the proportion of grass as substrate (?ve), moss cover of logs (?ve) and litter depth (+ ve); N. coventryi counts are most significantly related to litter depth (?ve), numbers of logs (?ve), log diameter (?ve), the proportion of bare ground as substrate (?ve) and length of sunny patches (+ ve); P. spenceri counts are most significantly related to the number of sunny patches (?ve) and moss cover of logs (?ve). The total reptile count is most significantly correlated with stag diameter (?ve) and log diameter (?ve). Predictive equations for each of the common species are also presented, using measured habitat variables, and these, along with ‘one-at-a-time’ models, indicate that a number of factors determine reptile occurrence or abundance and that no single predictor variable is apparent.     

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

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

Increased Diversity of Predacious <Emphasis Type="Italic">Bdellovibrio</Emphasis>-Like Organisms (BLOs) as a Function of Eutrophication in Kumaon Lakes of India

Predation by Bdellovibrio-like organisms (BLOs) results in bacterial community succession in aquatic ecosystems. The effects of nutrient loading on the distribution and phylogeny of BLOs remain largely unknown. To this end, we present our findings on BLO diversity from four north-Indian lakes that are variable in their trophic status; Nainital is eutrophic, both, Bhimtal and Naukuchiatal are mesotrophic and Sattal remains oligotrophic, respectively. Initially, total heterotrophic bacteria and BLOs were quantified by most probable number (MPN) analyses using Pseudomonas putida and Escherichia coli as prey bacteria. Total bacterial numbers were at least two-logs higher in the eutrophic lake samples compared with oligotrophic lake. Similarly, BLO numbers were approximately 39-fold higher using Pseudomonas sp., which is likely the preferred prey within these lakes. Conversely, significant differences were not observed between mesotrophic and oligotrophic BLO numbers when E. coli was used as the prey. PCR-RFLP of small subunit rDNA (SSU rDNA) of BLOs, followed by cloning, sequencing, and taxonomic categorization revealed distinct differences such that, eutrophic lake consisted of higher BLO diversity compared with mesotrophic and oligotrophic lake, most likely due to both, higher numbers and availability of a diverse population of prey bacteria resulting from nutrient loading in this ecosystem.     

Production and turnover ratio of Pisidium casertanum (Pelecypoda: Sphaeriidae) in the Ottawa River near Ottawa-Hull,Canada

Production of the sphaeriid clam Pisidium casertanum (Poli) was measured at several locations in the Ottawa River. Maximum production occurred in locations where sediment grain size was 3–4 phi units (0.250–0.0625 mm diameter, fine sand to very fine sand) and current velocity was near 0.1 m/sec. The turnover ratio of P. casertanum in the Ottawa River is a constant 3.8.     

Factors affecting the occurrence of Dendrocoelum lacteum (Turbellaria) in Swedish lakes

By discriminant analysis on data from 115 lakes in South Sweden it was shown that the distribution and abundance of the triclad Dendrocoelum lacteum was predominantly governed by the density of the isopod Asellus aquaticus, its main food. The occurrence of the latter was mainly correlated with the nutrient richness of the lakes, most probably via presence of appropriate detritus, chiefly allochtonous material. As opposed to British oligotrophic lakes, most Swedish nutrient-poor lakes are surrounded by deciduous vegetation, which indirectly promotes the existence of A. aquaticus and consequently D. lacteum. Presence of mat-forming plants generally seemed to depress triclad density, whereas smaller stones promoted their occurrence, at least in nutrient-rich lakes.     

ENVIRONMENTAL CONTROL OF STALK LENGTH IN THE BLOOM‐FORMING,FRESHWATER BENTHIC DIATOM DIDYMOSPHENIA GEMINATA (BACILLARIOPHYCEAE)1

Blooms of the freshwater stalked diatom Didymosphenia geminata (Lyngb.) M. Schmidt in A. Schmidt typically occur in oligotrophic, unshaded streams and rivers. Observations that proliferations comprise primarily stalk material composed of extracellular polymeric substances (EPS) led us to ask whether or not the production of excessive EPS is favored under nutrient‐limited, high‐light conditions. We conducted experiments in outdoor flumes colonized by D. geminata using water from the oligotrophic, D. geminata–affected Waitaki River, South Island, New Zealand, to determine the relationship between D. geminata stalk length, cell division rates, and light intensity under ambient and nutrient‐enriched conditions. Stalk lengths were measured in situ, and cell division rates were estimated as the frequency of dividing cells (FDC). FDC responded positively to increasing light intensity and to nutrient additions (N+P and P). Under ambient conditions, stalk length increased as light level increased except at low ambient light levels and temperature. Nutrient enrichment resulted in decreased stalk length and negative correlations with FDC, with this effect most evident under high light. Our results are consistent with the hypothesis that extensive stalk production in D. geminata occurs when cell division rates are nutrient limited and light levels are high. Thus, photosynthetically driven EPS production in the form of stalks, under nutrient‐limited conditions, may explain the development of very high biomass in this species in oligotrophic rivers. The responses of FDC and stalk length under nutrient‐replete conditions are also consistent with occurrences of D. geminata as a nondominant component of mixed periphyton communities in high‐nutrient streams.     

Decline in zoobenthos densities in the profundal sediments of Lake Mendota (Wisconsin,USA)

High densities of zoobenthos inhabited Lake Mendota's profundal zone in the early 1900s through the mid-1940s. Chaoborus punctipennis was the most abundant organism during the winter, along with moderate densities of Chironomus spp., Pisidium sp., oligochaetes, and Procladius sp. By the early 1950s, Chaoborus punctipennis densities had declined to 10% of former levels, while Chironomus increased significantly. However, by the mid-1960s, Chaoborus, Chironomus, and Pisidium densities had decreased to very low population levels. By 1987–89, Pisidium was no longer found. Zoobenthos that had not decreased from earlier surveys were oligochaetes and Procladius, although further sampling of oligochaetes is needed to confirm current densities. These organisms are the most tolerant of severe anoxia.Four possible reasons for this decline were evaluated: (a) decline in food availability, (b) increase in fish predation, (c) use of toxic insecticides in the drainage basin, and (d) changes in the profundal sediment environment. Based on literature information and long-term data for Lake Mendota, a change in the profundal sediment environment is the most likely explanation for the decline in the less-tolerant zoobenthos species. Although the duration and extent of anoxia in the hypolimnion have not changed since the early 1900s, hypolimnetic ammonia and hydrogen sulfide concentrations apparently have increased as Mendota became more eutrophic after the mid-1940s. However, further study is needed to determine if these higher concentrations or other factors were responsible for the dramatic decline in lake Mendota's profundal zoobenthos.     

Occurrence of the marbled rose-chafer (Protaetia lugubris Herbst, Coleoptera, Cetoniidae) in rural avenues in northern poland

The occurrence of Protaetia lugubris, an endangered species developing in hollow trees, was studied in a network of rural avenues in northern Poland. We detected 1002 trees from nine species with hollows suitable for beetle development (25% of all trees inspected). Among them, 74 trees (7.4%) from seven species were occupied by P. lugubris. The distribution of P. lugubris was random with respect to tree species identity. The beetle preferred trees above 200 cm in circumference with a tendency towards higher occupancies of the bigger trees having circumferences above 300 cm. P. lugubris did not show any significant preferences according to hollow entrance area, exposition and road surface type. Our results indicate that P. lugubris is a generalist species colonizing all suitable hollows. Its occurrence indicates suitable conditions for many other species associated with tree cavities and decaying wood.     

Abundance, body composition and reproductive output of Gammarus minus (Crustacea: Amphipoda) in ten cold springs differing in pH and ionic content

1. A survey of thirty-two rheocrene springs in central Pennsylvania revealed that, Like Gammarus in lakes and streams, Gammarus minus is absent from springs with pH <6.0 and conductivity <25μS cm^?1 (total range in pH = 4.6–7.7, and in conductivity = 14–411 μS cm^?1). 2. In ten springs G. minus density was positively correlated with Ca²⁺ and Mg²⁺ hardness, but not with pH, unless three springs with either an exceptionally high velocity or extremely high densities of the potentially competing snail Fontigens nickliniana were omitted. 3. Adults were larger in the springs with few or no large predators than in those with more predators. In all ten springs, adult dry mass was unrelated to spring pH and ionic content, but brooding female dry mass covaried positively with Ca²⁺ and Mg²⁺ hardness in the five predator-poor springs. 4. Body water, Na and Ca contents and body mass/length ratios varied independently of spring pH and ionic content. Water content was inversely correlated with fat content, but even when expressed as a percentage of fat-free wet mass, it was unrelated to water chemistry. 5. In juveniles, males and non-brooding females, fat content varied independently of spring pH and ionic content, but in brooding females it was correlated with alkalinity and Ca²⁺ and Mg²⁺ hardness. The cost of reproduction in brooding females may have been a factor here; they had significantly lower per cent fat than did non-brooding females. Juvenile fat content did not differ significantly among spring populations, whereas adult fat content did. The per cent fat of brooding females covaried positively with body size among springs, and this was marginally true for non-brooding females, as well. The residuals of brooding female per cent fat against dry mass were not related to water chemistry. 6. Brood size (number of embryos in a brood) and brood mass varied significantly among populations, but independently of spring pH and ionic content. Both covaried positively with maternal body size among springs. The residuals of these relationships were unrelated to water chemistry, as was the percentage of females brooding. 7. G. minus from a pH 6 spring survived better and lost less body mass in acidic soft water than did those from a pH 7.6 spring. However, although G. minus has apparently been able to adapt (or acclimate) to pH 6 water it has failed to adapt to more acidic waters. A physiological or structural constraint may be involved because this species has probably had ample opportunity to evolve resistance to dilute acidic water. This hypothesis is consistent with the threshold effect observed: above pH 6 G. minus shows very little evidence of osmotic or metabolic stress, but beiow pH 6 viable populations apparently cannot survive at all. However, the gradual linear decrease in population density of G. minus with decreasing alkalinity and Ca²⁺ and Mg²⁺ hardness suggests that other factors may also be involved (e.g. a decrease in food quality).     

Species richness of Protozoa in Japanese lakes

The protozoan fauna and species richness in the pelagic zone of 15 Japanese lakes were investigated in 1996 using polyurethane foam (PF) substrates. The most common species were flagellates, such as Cryptomonas erosa, Oikomonas termo, and Pleuromonas jaculans. Cinetochilum margaritaceum and Actinophrys sol were the most common species of the Ciliata and Sarcodina, respectively. The similarity of species occurrence was calculated from presence/absence data, but this revealed no clear trend with respect to the influence of lake properties such as trophic state, surface area, or mean depth. The occurrence pattern of Protozoa was most similar in L. Chuzenji and L. Biwa (north basin), two oligomesotrophic natural lakes. Log species richness was positively correlated with log total phosphorus (r = 0.54, P < 0.05) and negatively with log mean depth (r = −0.58, P < 0.05). The diversity index (Margalef's formula), highly correlated with the total species number (r = 0.85, P < 0.01), was negatively correlated with log lake area (r = −0.71, P < 0.01). The logarithm of Phytomastigophora number was positively correlated with log total nitrogen (r = 0.53, P < 0.05), and the logarithm of Ciliata number was negatively correlated with log lake area (r = −0.55, P < 0.05). The species richness of Protozoa on PF substrates was determined by both the nutrient status of the lake and the distance from the location of the suspended PF substrate to the lake bottom or shore. Received: September 25, 1999 / Accepted: January 6, 2000