The development of Hexarthra spp. in a shallow alkaline lake

In Neusiedler See, a shallow alkaline lake with fluctuating water level and salinity, four species of Hexarthra occur: H. mira, H. fennica, H. jenkinae (occasional) and H. polyodonta. The analysis of longterm data reveals a general phenological pattern which does not change from year to year. They first occur in May, develop a maximum in June/July, sometimes a second one in August/September and disappear in October. But the species succession is different in the various years, occasionally only one species (H. mira or H. polyodonta) being present. There is a fairly consistent relation between the chemical conditions and the prevalent species; an increase in salinity favours the development of H. polyodonta. Low temperature and wind generated suspended particles have a negative influence on the development of the Hexarthra populations. Smaller populations of Hexarthra are in a relation to the occurrence of Leptodora indicating predation pressure of the latter species. In Neusiedler See the Hexarthra populations seem to be controlled to a great extent by abiotic factors, but predation by Leptodora and most probably by young fish seems to play an important role too.     

Susceptibility of Ephemeral Pool <Emphasis Type="Italic">Hexarthra</Emphasis> to Predation by the Fairy Shrimp <Emphasis Type="Italic">Branchinecta mackini</Emphasis>: can Predation Drive Local Extinction?

Isolated rock pools in the US desert southwest often develop dense populations of a lithophilic species of the rotifer Hexarthra. I hypothesized that rotifers persist in these isolated ponds due to the absence of either competition with or predation by potentially sympatric crustaceans, especially anostracans. I tested the latter idea with laboratory and field experiments, in each case exposing rotifers to adult fairy shrimp (Branchinecta mackini, the dominant anostracan in this region in winter ponds) in 200 ml microcosms. In␣most cases individual female fairy shrimp had distinct negative effects on rotifer suspensions due to direct predatory consumption of the smaller animals. Estimated effective water processing rates ranged from ca. 50 to over 300 ml ind⁻¹ h⁻¹ and rotifer consumption rates by female B. mackini were between 280 and > 600 Hexarthra consumed per individual predator per hour. Male B. mackini never significantly reduced rotifer numbers in either laboratory or field microcosms. The results indicate that, while perhaps not the sole determinant of rotifer distribution in these ephemeral pools, fairy shrimp predation can have a strong negative influence on natural Hexarthra populations.     

Elevational isolation of red fox populations in the Greater Yellowstone Ecosystem

Foxes in the Greater Yellowstone Ecosystem are reported to show high frequencies of blonde and gray coat colors. A survey of park sighting records showed that the frequency of the novel coat colors significantly increases at elevations greater than 2300 m, suggesting some form of elevational isolation. We evaluated the degree of genetic separation between the high-elevation foxes (>2300 m) and contiguous populations of foxes at mid-elevations (1600–2300m). Low-elevation (>1600 m) foxes from North Dakota, >1000 km straight line distance from our populations, were used as a control group. We genotyped 15 high-elevation, 15 mid-elevation, and 10 low-elevation foxes at 10 microsatellite loci each. Heterozygosity was significantly lower in both the high-elevation and mid-elevation populations compared to the low-elevation foxes. The genetic differentiation was significantly greater between the high-elevation and mid-elevation foxes than between the mid-elevation and low-elevation foxes. Similarly, estimates of R_ST and F_ST suggest less gene flow occurs between the contiguous high- and mid-elevation fox populations than between the mid- and low-elevation fox populations separated by > 1000 km. The assignment test further supports this hypothesis. Although further work is needed, we suggest that the high-elevation foxes are remnant populations from the Wisconsin glaciation and should be managed as a unique population.     

Genetic evidence for the persistence of the critically endangered Sierra Nevada red fox in California

California is home to both the native state-threatened Sierra Nevada red fox (Vulpes vulpes necator), which historically inhabited high elevations of the Sierra Nevada and Cascade mountains, and to multiple low-elevation red fox populations thought to be of exotic origin. During the past few decades the lowland populations have dramatically expanded their distribution, and possibly moved into the historic range of the native high-elevation fox. To determine whether the native red fox persists in its historic range in California, we compared mitochondrial cytochrome-b haplotypes of the only currently-known high-elevation population (n = 9 individuals) to samples from 3 modern lowland populations (n = 35) and historic (1911–1941) high-elevation (n = 22) and lowland (n = 7) populations. We found no significant population differentiation among the modern and historic high-elevation populations (average pairwise F _ST = 0.06), but these populations differed substantially from all modern and historic lowland populations (average pairwise F _ST = 0.52). Among lowland populations, the historic and modern Sacramento Valley populations were not significantly differentiated from one another (F _ST = −0.06), but differed significantly from recently founded populations in the San Francisco Bay region and in southern California (average pairwise F _ST = 0.42). Analysis of molecular variance indicated that 3 population groupings (mountain, Sacramento Valley, and other lowland regions) explained 45% of molecular variance (F _CT = 0.45) whereas only 4.5% of the variance was partitioned among populations within these groupings (F _SC = 0.08). These findings provide strong evidence that the native Sierra Nevada red fox has persisted in northern California. However, all nine samples from this population had the same haplotype, suggesting that several historic haplotypes may have become lost. Unidentified barriers have apparently prevented gene flow from the Sacramento Valley population to other eastern or southern populations in California. Future studies involving nuclear markers are needed to assess the origin of the Sierra Nevada red fox and to quantify levels of nuclear gene flow.     

Decline of a sylvatic focus of Schistosoma mansoni in Guadeloupe (French West Indies) following the competitive displacement of the snail host Biomphalaria glabrata by Ampullaria glauca

Summary Grand Etang lake in Guadeloupe is a focus of intestinal schistosomiasis characterized by the virtual absence of human contact and the presence of a heavily infested rat population. The lake, with a surface area of 0.03 km², is situated in rain forest at an altitude of 450 m. Originally it contained an extensive belt of the floating plant Pistia stratiotes which provided a favourable habitat for Biomphalaria glabrata, the intermediate host snail of Schistosoma mansoni. In 1974 and 1975, a low prevalence (about 3%) of S. mansoni was observed in B. glabrata while a high prevalence (about 60%) was recorded in rats. The rat population was infected with another trematode, Ribeiroia guadeloupensis, which requires two intermediate hosts, B. glabrata and the fish Tilapia mossambica. In 1976, the prosobranch snail, Ampullaria glauca, was accidentally introduced into the lake. By 1980, this snail had spread over half of the lake and had caused marked regression of the P. stratiotes layer. Concomitantly, the population of B. glabrata declined to such a point that in 1985 only a small colony remained in the northern section of the lake. The consequences of these changes are reflected in a rapid decline of S. mansoni in the rat population from 61.3% in 1980 to 34.8% in 1981, 6.5% in 1982, 2.8% in 1983, 11.6% in 1984, and 0% in 1985 and 1986.     

Effect of salinity on competition between the rotifers Brachionus rotundiformis Tschugunoff and Hexarthra jenkinae (De Beauchamp) (Rotifera)

We studied the effect of different concentrations (0, 3, 6, 9 and 12 g l^–1) of sodium chloride at one food level of Chlorella (1×10⁶ cells ml^–1) on competition between the rotifers B. rotundiformis and H. jenkinae, both of which were isolated from a saline lake. The population growth experiments were conducted for 3 weeks. Both the rotifer species did not survive beyond one week at a salinity of 0 g l^–1. Regardless of salt concentration and the presence of a competitor, H. jenkinae reached higher densities than B. rotundiformis. When grown alone, both B. rotundiformis and H. jenkinae showed optimal peak population densities at the salinity of 6 and 9 g l^–1. Since biomass wise, B. rotundiformis was larger than H. jenkinae, it showed a lower numerical abundance. Thus, the maximum peak population densities of B. rotundiformis and H. jenkinae recorded in this study were 107±3 and 203±28 ind. ml^–1. The maximal rates of population increase for B. rotundiformis and H, jenkinae when grown alone were 0.264±0.003 and 0.274±0.004, respectively. Our results also indicated that B. rotundiformis and H. jenkinae coexisted better at a salinity of 6 and 9 g l^–1 of sodium chloride while a salinity of 3 g l^–1 favoured Hexarthra over B. rotundiformis. At 12 g l^–1, both the rotifer species grown alone or together showed lower growth rates compared to those at lower salinity levels. Except 0 g l^–1, in all other salinity treatments, H. jenkinae was a superior competitor to B. rotundiformis.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Age structure and body size of the Chuanxi Tree Frog Hyla annectans chuanxiensis from two different elevations in Sichuan (China)

Age, body size, and growth patterns in the subtropical anuran Hyla annectans chuanxiensis from high (Dengchigou Protection Station) and low (Lingguan Town) elevations in Baoxing County of Sichuan province (China) were described using skeletochronology. Females were significantly older than males at the low-elevation site, but there was no significant difference between the sexes at the high-elevation site. Age at sexual maturity of both males and females was 2 years at the high-elevation site, whereas males matured at 1 year and females at 2 years at the low-elevation site. Males and females from the low-elevation population reached a maximum age of 3 and 4 years, respectively, whereas males and females from the high-elevation population reached a maximum age of 4 and 5 years, respectively. At both sites, females were significantly larger than males. Females and males from the high-elevation population were larger than individuals from the low-elevation population. When the effect of age was controlled, the differences in body size of the two populations were significant only for females. Von Bertalanffy growth curves indicated that the growth rates in males was greater than in females in both populations. They also showed that the growth of both sexes slowed at an earlier age in the low-elevation population than in the high-elevation population. The findings suggest that age is a major factor underlying body size patterns for both sexes, but that the elevation of the locality affects the body size of females.     

Genetic structure of natural populations of Nitrobacter in an aquatic environment

Nitrification has been essentially studied as a chemical process and the studies of the microbial population have been slightly worked out. This work constitutes a first attempt to study the diversity of Nitrobacter strains with the aim of working out the genetic structure of natural populations. The Nitrobacter population structure was compared between freshwater and sediments of the same lake. Nitrobacter isolates were identified and characterized by DNA/DNA hybridization, restriction pattern of rRNA genes, PCR/RFLP analysis of the ribosomal intergenic spacer and plasmid patterns. It was shown that this lake population was split into 3 subpopulations: a specific freshwater subpopulation, a specific sediment subpopulation and a nonspecific one. A 60 MDa plasmid was detected in 3,3% of freshwater isolates. A 37 MDa plasmid was detected in 59% of sediments isolates. This 37 MDa plasmid was not associated with a genomic group, but associated with a localization: the sediments. These results suggest that this 37 MDa plasmid could be implicated in the adaptation to sediment environment.     

Ecology of rotifers of Surinsar,a subtropical,fresh water lake in Jamme (J & K), India

17 species of rotifers have been recorded from Lake Surinsar, Jammu (J & K), India of which some are either exclusively limnetic (Brachionus angularis, Hexarthra sp., Filinia opliensis), or littoral (Brachionus patulus, M. ventralis, Trichotoria sp., Platyias quadricornis, Lecane (Monostyla) decipiens, L (M), bulla and Lecane sp.) and others (Keratella tropica, Anuraeopsis fissa, Brachionus quadridentatus, B. calyciflorus, Trichocera sp., T. similis, and Polyarthra sp.) seem to be wandering species. Seasonal maxima for both littoral and limnetic zones are reported. Most population maxima are contributed mainly by one or at best two species.Physico-chemical factors like temperature, pH, dissolved oxygen, free carbon dioxide, calcium, magnesium, and total alkalinity have been studied and their infuence on these rotifer species are discussed. On their thermal responses, the available rotifer species have been classified as warm stenothermal, cold stenothermal or eurythermal. The importance of Mytilina ventralis as a biological indicator for dissolved oxygen in this lake has been pointed out.     

Factors affecting the survival and growth of bacteria introduced into lake water

The populations of Pseudomonas sp. B4, Escherichia coli, Klebsiella pneumoniae, Micrococcus flavus, and Rhizobium leguminosarum biovar phaseoli declined rapidly in lake water. The initially rapid decline of the two pseudomonads and R. phaseoli was followed by a period of slow loss of viability, but viable cells of the other species were not found after 10 days. The rapid initial phase of decline was not a result of Bdellovibrio spp., bacteriophages, or toxins in the water since Bdellovibrio spp. were not present and passage of the lake water through filters that should not have removed bacteriophages or soluble toxins led to the elimination of the rapid phase of decline. The addition of 250 g of cycloheximide and 30 g of nystatin per ml eliminated viable protozoa form the lake water, and the population of Pseudomonas sp. B4 did not fall and the decline of E. coli and K. pneumoniae was delayed or slowed under these conditions. Pseudomonas sp. L2 proliferated rapidly in lake water amended with glucose, phosphate, and NH₄NO₃, but its numbers subsequently fell abruptly; however, in water amended with cycloheximide and nystatin, which killed indigenous protozoa, the population density was higher and the fall in numbers was delayed. Of the nutrients, the chief response was to carbon, but when glucose was added, phosphorus and nitrogen stimulated growth further. Removing other bacteria by filtering the lake water before inoculation with Pseudomonas sp. L2 suggested that competition reduced the extent of response of the pseudomonad to added nutrients. We suggest that the decline in lake water of bacteria that are resistant to starvation may be a result of protozoan grazing and that the extent of growth of introduced species may be limited by the supply of available carbon and sometimes of nitrogen and phosphorus, and by predation by indigenous protozoa.     

The predation on Asellus aquaticus (L.) by perch,Perca fluviatilis (L.), in a small forest lake

Asellus aquaticus (L.) was the most important benthic food item for perch, Perca fluviatilis (L.), in a small, extremely humic forest lake in southern Finland. The proportion of A. aquaticus in the diet of perch varied according to the former's availability, which, in turn, depended on its life cycle. Perch 11.0–12.9 cm (total length) most frequently fed on A. aquaticus; smaller and larger perch fed more frequently on crustacean zooplankton and aquatic insect larvae, respectively. The high density of the perch population, the importance of A. aquaticus in the benthos of the study lake and the general high activity level of the prey resulted in a high predation (0.1–1.8% per day). The population of A. aquaticus was also limited by the scarcity of macrophytic vegetation and the small area of oxygenated littoral in the lake.     

Genetic diversity and morphological differentiation in Liatris helleri (Asteraceae), a threatened plant species

Liatris helleri (Asteraceae) is an insect-pollinated herbaceous perennial endemic to several high-elevation sites in the Blue Ridge Mountains of North Carolina. Allozymes were used to describe the genetic diversity and population structure in nine populations of this rare, federally listed species. Differences in leaf morphology were also examined for greenhouse-grown plants representing several populations. The proportion of the total genetic diversity found among populations, as indicated by the allozyme data, was 16%. Higher levels of population differentiation were found for differences in leaf shape; population of origin accounted for 37% of the variation in maximum leaf width, while families within populations accounted for 7%. In contrast to many endemic species, L. helleri maintains fairly high levels of genetic diversity. For the species, the percent polymorphic loci was 87.5, the average number of alleles at variable loci was 3.00 and the gene diversity was 0.276. Mean population values were percent polymorphic loci =58.4, mean number of alleles per polymorphic locus =2.59 and gene diversity =0.219. The estimated gene flow was low (Nm=0.85–1.32) and a relatively high correlation (r=0.55; p<0.005) was found between linear geographic and genetic distance. This suggests that the populations are partially isolated by distance, despite the limited range (<60 km) of the species. We recommend that population distinetiveness be maintained in restoration efforts.     

Diatom associations and succession in Lake Kariba,South Central Africa

Diatom Associations formed by 1% or more of species, found by the Thomasson analysis method are discussed in relation to the morphology and limnology of the four basins of Lake Kariba. A brief comparison is made with associations in two stretches of the Zambesi river. A dominant association formed by certain species of Melosira demonstrated a pattern of succession, in relation to the thermal cycle in the lower, middle and upper lake by variations of the percentage occurrence of species forming it: Melosira granulata dominated during homothermy and remained so throughout stratification in the lacustrine conditions at the barrier or wall; M. granulata v. angustissima dominated during overturn and became the dominant species in the riverine conditions of the two uppermost basins. Results from the Central basin illustrated the intermediate position of this area of the lake. The pattern was disrupted in the far N.E sub-basin of the Sanyati basin, which in general showed individuality and where accidental and ambivalent plankters became co-dominant with the Melosira association during homothermy.A second association, formed by species of Cyclotella, Fragilaria and Synedra was a regular feature of the diatom population, it was sub-dominant to the melosira association at the barrier but gradually decreased up-lake and was superseded by accidental and ambivalent planktonic species. Except for the N/E Sanyati sub-basin, the diatom associations showed a gradual change from upper to lower lake, thus basin individuality was not seen.     

Adaptability to elevated temperature and nitrogen addition is greater in a high-elevation population than in a low-elevation population of Hippophae rhamnoides

Hippophae rhamnoides L. is a broadleaf deciduous woody shrub occurring in southwest China, where it has been widely used in ecological restoration. In this study we investigated growth and physiological responses of 2-year-old healthy seedlings to elevated temperature, nitrogen (N) addition and their combination in two contrasting populations from high and low elevations. In closed-top chamber experiments, two populations were subjected to two temperature conditions (ambient temperature and temperature elevated by 2.2 ± 0.2°C) and two N levels (0 and 25 g N m⁻² a⁻¹). Compared with the control, increases in total leaf area (TLA), total chlorophyll content (TC), light-saturated photosynthetic rate (P _max), guaiacol peroxidase activity (POD), catalase activity (CAT) and carbon isotope composition (δ¹³C) were greater in the high-elevation population than in the low-elevation population under elevated temperature. On the other hand, decreases in root and shoot biomass ratio (RS), TC, P _max, light saturation point (L _SP), light compensation point (L _CP), superoxide dismutase (SOD), POD, CAT and δ¹³C were lower in the high-elevation population than in the low-elevation population under N addition. Moreover, the combination of elevated temperature and N addition decreased RS, P _max, apparent quantum efficiency (Φ), SOD, POD and δ¹³C significantly more in the low-elevation population than in the high-elevation population. These results demonstrated that there are different adaptive strategies among H. rhamnoides populations, the high-elevation population exhibiting higher adaptability to elevated temperature and N addition than the low-elevation population.     

Some peculiar elements in the rotifer fauna of the atlantic sahara and of the atlas mountains

In addition to ubiquitous forms, the Adrar of Mauretania has a number of rare tropicopolitan species, which occur here in relative abundance. In the Moroccon Atlas mountains, West-African forms meet boreo-alpine forms.A population of Hexarthra from Atar, Mauretania, was intermediate between the fennica-group and the jenkinae-group.Contribution n° 28 from project Limnology of the Sahara, under contract n° 2.0009/75 with the Fonds voor Kollektief Fundamenteel Onderzoek, Belgium     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

A regression analysis of the summer population dynamics of Polyarthra vulgaris in a northern Michigan bog lake

The population dynamics of a planktonic rotifer (Polyarthra vulgaris) were examined in a brown water, acid lake in northern Michigan, U.S.A. Predation by Chaoborus punctipennis and low food (Navicula spp. and Cyclotella spp.) concentrations were the main factors limiting P. vulgaris populations of all factors examined. The data presented here support a hypothesis for zooplankton limitation by an invertebrate predator.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

The co-occurrence of Artemia parthenogenetica and Branchinella spinosa (Branchiopoda: Anostraca) in a saline pond of south eastern Italy

The co-occurrence of Artemia parthenogenetica Bowen & Sterling, 1978 and Branchinella spinosa Milne-Edwards, 1840, was previously only supposed, on the basis of finding their cysts in the same lagoon, but active stages were never recorded together. In a saline lake in south eastern Italy, populations of the two species were observed for two years. A. parthenogenetica was typical of the early wet season (October–January), while B. spinosa, even if it was present before, developed in the late part of the wet season (February–May). Adult A. parthenogenetica were recorded only in the season 1999–2000. The seasonal distribution of A. parthenogenetica is probably controlled by abiotic conditions (i.e., photoperiod, temperature, conductivity, and rainfall); however, competition between specimens of similar body size (e.g., adults of A. parthenogenetica, and juveniles of B. spinosa) could explain the lower numbers of B. spinosa population in winter. The discovery of cysts of both species deep in the sediment of the lake suggests a long-lasting co-occurrence of these two Anostraca.