Daphnia Pulicaria Research Articles

Absence of predation eliminates coexistence: experience from the fish–zooplankton interface

Examples from fishless aquatic habitats show that competition among zooplankton for resources instigates rapid exclusion of competitively inferior species in the absence of fish predation, and leads to resource monopolization by the superior competitor. This may be a single species or a few clones with large body size: a cladoceran such as Daphnia pulicaria, or a branchiopod such as Artemia franciscana, each building its population to a density far higher than those found in habitats with fish. The example of zooplankton from two different fish-free habitats demonstrates the overpowering force of fish predation by highlighting the consequences of its absence. Released from the mortality caused by predation, a population of a superior competitor remains at a density equal to the carrying capacity of its habitat, in a steady state with its food resources, consisting of small green flagellate algae, which are successful in compensating high loss rates due to grazing, by fast growth. In such a situation, the high filtering rate of Daphnia or Artemia reduces resources to levels that are sufficient for assimilation to cover the costs of respiration (threshold food concentration) in adults but not in juveniles. This implies long periods of persistence of adults refraining from producing live young, because production of instantly hatching eggs would be maladaptive. Severe competition for limiting resources imposes a strong selective pressure for postponing reproduction or for producing resting eggs until food levels have increased. Offspring can only survive when born in a short time window between such an increase in food levels and its subsequent decline resulting from population growth and intense grazing by juveniles. Such zooplanktons become not only a single-species community, but also form a single cohort with a long-lifespan population. The observations support the notion that diversity may be sustained only where predation keeps densities of coexisting species at levels much below the carrying capacity, as suggested by Hutchinson 50 years ago.

Variability in amino acid composition of alpine crustacean zooplankton and its relationship with nitrogen-15 fractionation

Amino acids (AAs) are critical biochemical compounds for living organisms. Because of the limited capacity for their de novo synthesis in many animals, the nutritional value of food largely depends on its AA composition relative to the animal's requirements. To improve present knowledge on AA variability in freshwater crustaceans, we studied the inter- and intraspecific variability in three contrasting species from an oligotrophic alpine lake (Daphnia pulicaria, cladocera; Cyclops abyssorum, cyclopoid copepod and Diaptomus cyaneus, calanoid copepod). Inter-species differences were larger than intraspecific variation, confirming a non-strict homeostasis in freshwater crustacean zooplankton. The intraspecific variability differed for each species: in Daphnia, it was mainly related with ontogenetic changes rather than reproduction; in Cyclops, both factors were equally important; and reproduction was the most relevant in Diaptomus. Reproduction changes were associated with serine and phenylalanine in the three species, while the AAs responsible for ontogenetic changes differed in each species. There were no gender differences in AA composition in any of the two copepod species. Free AAs formed a very low percentage of total AA pool (<2.7%). Taking advantage of the fact that Daphnia is the main prey for Cyclops in the lake studied, we further investigated to what extent the AA composition is related with Cyclops-Daphnia nitrogen stable isotope fractionation. Only those AAs that are both essential and are not trans-aminated during protein synthesis had a significant correlation with nitrogen stable isotope fractionation, supporting the hypothesis that an AA imbalance can be responsible for a variable nitrogen stable isotope fractionation.

Ecological consequences of intraspecific variation in lake Daphnia

Summary1. Although populations harbour considerable diversity, most ecological studies still assume they are homogeneous. However, mounting evidence suggests that intraspecific diversity is not only common, but also important for interactions with community members. Here, intraspecific variation in Daphnia dentifera in haemoglobin content is shown to be a marker of hypolimnion use.2. Hypolimnion use differed substantially within and among D. dentifera populations. Daphnia dentifera with haemoglobin resided primarily in the hypolimnion, while D. dentifera lacking haemoglobin migrated vertically. These ‘deep’ and ‘migratory’D. dentifera had different seasonal phenologies and dynamics.3. Deep and migratory D. dentifera had qualitatively different relationships with an important competitor, Daphnia pulicaria. Deep D. dentifera density was negatively correlated with D. pulicaria density, whereas migratory density was not correlated with D. pulicaria density.4. Given that D. pulicaria tends to reside in the hypolimnion, this negative correlation probably reflects competition between D. pulicaria and the deep D. dentifera. This pattern would have been missed if only the relationship between the overall lake populations of D. dentifera and D. pulicaria had been studied.5. Abundances of deep D. dentifera and D. pulicaria were both correlated with the size of the hypolimnetic refuge from fish predation, but in opposite directions. Lakes with large refuges generally had high D. pulicaria and low deep D. dentifera densities.

Salinity regulation of zooplanktonic abundance and vertical distribution in two saline meromictic lakes in south central British Columbia

Zooplankton species abundance and vertical distribution were followed in two south-central British Columbia saline meromictic lakes during the recent decade of their declining water levels. The lower salinity mixolimnion (0–7 m) of both lakes circulates partially most years down to their primary chemoclines marked by a 10–15 cm layer of purple sulfur bacteria (Amoebobacter purpureus) heavily grazed upon mainly by late copepodite stages and adults of Diaptomus connexus, possibly by some rotifers, but apparently not by cladocerans. Vertical distribution profiles, over an 8-year period, are presented mainly for the rotifer Brachionus plicatilis, two cladocerans Ceriodaphnia lacustris, and Daphnia pulicaria, as well as copepodite and adult D. connexus during normal periods of unimeromixis and a period of weak bimeromixis caused by shallow upper low salinity layers coming from drainage basin inputs.

Extended lifespan traded for diapause in Daphnia

Summary1. Most freshwater crustaceans of the genus Daphnia are cyclically parthenogenetic organisms that are well adapted to unstable habitats due to their short life cycle, wide phenotypic plasticity, and the ability to produce protective diapausing eggs in anticipation of environmental deterioration. Short lifespan and heterogonic reproduction are typical features of Daphnia in a broad spectrum of freshwater habitats, from small temporary pools to large permanent lakes. However, in some locations, departures may be observed from this typical life history pattern to obligate asexuality or extended lifespan.2. A 3‐year field study in a deep ultraoligotrophic fish‐free alpine‐type lake (Czarny Staw in the Tatra mountains in southern Poland) revealed the coexistence of two closely related asexual lineages of Daphnia of the pulex complex, which differ in body colour (transparent versus orange) and in their strategies for surviving long winters.3. The ‘transparent’ clone of European origin exhibits an ephemeral lifestyle. It completes its life cycle within a single season, forming two generations of active specimens during the short summer and producing diapausing eggs late in the season. Transparent individuals live no longer than 5–6 months in this cold lake and survive winter exclusively in the form of diapausing eggs.4. Individuals of the ‘orange’ lineage, which are closely related to eastern Nearctic Daphnia pulicaria, exhibit a biennial lifestyle unique to this genus. They do not form diapausing eggs or produce them only occasionally. Instead, they are active throughout the year and live for up to 13–14 months. Reproduction is postponed until the second year of life when food conditions have improved. Rich lipid reserves accumulated in the first season fuel them during the long winter and permit early reproduction the following spring.5. Possible reasons for the evolution of obligatory parthenogenesis and long lifespan in Daphnia are discussed.

Genome size evolution and polyploidy in the Daphnia pulex complex (Cladocera: Daphniidae)

Genome size was estimated in 49 clones of the Daphnia pulex complex from temperate and subarctic locations using flow cytometry and microsatellite DNA analyses. Significant genome size differences were found in diploid species belonging to the two genetically distinct groups (the pulicaria and the tenebrosa groups), with clones from the tenebrosa group having genome sizes 22% larger than those in the pulicaria group. Combined flow cytometry and microsatellite DNA analyses revealed that nearly all polyploid clones in the D. pulex complex are triploid and not tetraploid, as was previously suggested. Sequencing analyses of the ND5 gene to position clones in their respective clades within the D. pulex complex have uncovered three triploid clones of Daphnia middendorffiana with a D. pulex maternal parent. This result was unexpected because Daphnia pulicaria has always been identified as the maternal parent of these hybrid polyploid clones. Triploid clones likely owe their origins to interactions between sexual and asexual populations. Further interactions in the tenebrosa group have generated tetraploid clones but these events have been rare. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97, 68–79.

Ecological speciation in a cyclic parthenogen: Sexual capability of experimental hybrids between Daphnia pulex and Daphnia pulicaria

Daphnia pulex and Daphnia pulicaria are cyclically parthenogenetic crustaceans that are distributed widely in North American freshwaters. Hybrids of D. pulex and D. pulicaria are common in the wild, where population genetic analyses have indicated that they are invariably obligately asexual. In this study, we characterize three sexual aspects (sexual egg production, male production, and sexual offspring production) of life history in experimentally bred hybrids to determine the degree of reproductive isolation. In striking contrast to natural hybrids, out of the 53 F1 hybrids we bred, none were obligately asexual. Neither male production nor sexual egg production was limited by hybridization. Our experimental crosses showed that there was little difficulty in generating or backcrossing the F1 generation. In addition, we were able to intercross F1s successfully to produce F2 progeny. We found no evidence that these taxa are reproductively incompatible, despite other reports of apparent adaptive genetic divergence. We conclude that reproductive isolation between D. pulex and D. pulicaria is due to ecological separation, not genetic incompatibility. In light of our results and published data on the biogeography and breeding system of these taxa, neither sterility due to hybridization nor direct inheritance of a sex‐limited meiosis suppressor appears to provide a satisfactory explanation for the asexuality of wild hybrids. We propose an alternate hypothesis in which one parental taxon invades a population of the other, the resulting F1 hybrids cross back to a carrier of a meiosis suppressor, and out of the resulting progeny, a high‐fitness asexual clone eventually displaces the rest of the population.

FESTERING FOOD: CHYTRIDIOMYCETE PATHOGEN REDUCES QUALITY OFDAPHNIAHOST AS A FOOD RESOURCE

When parasitic infections are severe or highly prevalent among prey, a significant component of the predator's diet may consist of parasitized hosts. However, despite the ubiquity of parasites in most food webs, comparisons of the nutritional quality of prey as a function of infection status are largely absent. We measured the nutritional consequences of chytridiomycete infections in Daphnia, which achieve high prevalence in lake ecosystems (>80%), and tested the hypothesis that Daphnia pulicaria infected with Polycaryum laeve are diminished in food quality relative to uninfected hosts. Compared with uninfected adults, infected individuals were smaller, contained less nitrogen and phosphorus, and were lower in several important fatty acids. Infected zooplankton had significantly shorter carapace lengths (8%) and lower mass (8-20%) than uninfected individuals. Parasitized animals contained significantly less phosphorus (16-18% less by dry mass) and nitrogen (4-6% less) than did healthy individuals. Infected individuals also contained 26-34% less saturated fatty acid and 31-42% less docosahexaenoic acid, an essential fatty acid that is typically low in cladocera, but critical to fish growth. Our results suggest that naturally occurring levels of chytrid infections in D. pulicaria populations reduce the quality of food available to secondary consumers, including planktivorous fishes, with potentially important effects for lake food webs.

The relationship between zooplankton community structure and lake characteristics in temperate lakes (Northern Wisconsin, USA)

Zooplankton community structure can be affected by within-lake and by watershed ecological factors, including water chemistry (related to landscape position), lake morphology and human activity in the watershed. We hypothesized that all three groups of drivers would be correlated with zooplankton species richness and species composition for lakes in northern Wisconsin. Data collected from 52 lakes allowed us to explore the relationship of zooplankton community structure with ecological drivers. We found that crustacean zooplankton species richness was not significantly correlated with independent environmental variables derived from PCA ordination, nor with measures of community structure based on NMS ordination. However, species composition was correlated with environmental gradients. Larger zooplankton species (Daphnia pulicaria, Epischura lacustris, Skistodiaptomus oregonensis, Mesocyclops americanus) occurred in large and deep lakes low in the landscape gradient, whereas the smaller species Ceriodaphnia dubia, Daphnia retrocurva and Leptodiaptomus minutus tended to occur high in the landscape. This shift in species composition was correlated with increased conductivity, primary productivity and the hypolimnetic refuges characteristic of larger deeper lakes lower in the landscape. Riparian housing development and littoral zone habitat (measured as building density and by abundance of logs in littoral zones) were not correlated with zooplankton community structure. In these relatively low-impact lakes, natural drivers are still the most significant determinants of zooplankton community structure.

Both Costs and Benefits of Sex Correlate With Relative Frequency of Asexual Reproduction in Cyclically ParthenogenicDaphnia pulicariaPopulations

Sexual reproduction is generally believed to yield beneficial effects via the expansion of expressed genetic variation, which increases the efficiency of selection and the adaptive potential of a population. However, when nonadditive gene action is involved, sex can actually impede the adaptive progress of a population. If selection promotes coupling disequilibria between genes of similar effect, recombination and segregation can result in a decrease in expressed genetic variance in the offspring population. In addition, when nonadditive gene action underlies a quantitative trait, sex can produce a change in trait means in a direction opposite to that favored by selection. In this study we measured the change in genotypic trait means and genetic variances across a sexual generation in four populations of the cyclical parthenogen Daphnia pulicaria, which vary predictably in their incidence of sexual reproduction. We show that both the costs and benefits of sex, as measured by changes in means and variances in life-history traits, increase substantially with decreasing frequency of sex.

Maternal and direct effects of natural-like changes of photoperiod and food condition manipulation on life history parameters in Daphnia

This study analyzed changes of life history parameters of a Daphnia pulicaria clone in response to increasing (spring conditions) versus decreasing (fall conditions) photoperiod and low and high food concentrations in maternal and offspring environments. One treatment increased the photoperiod from 12 to 16 h at a rate of 3 min per day, while the other treatment decreased the photoperiod from 16 to 12 h at the same rate. We also used two food concentrations: low (0.2 mg C/l) and high (0.5 mg C/l). Maturation time, first clutch size, survivorship, and somatic growth in offspring depended on the direction of photoperiodic changes for Daphnia grown under high food concentration. Natural-like changes of photoperiod on life history parameters and abrupt changes resulted in similar effects on life history parameters of Daphnia (see Alekseev and Lampert 2004 Hydrobilogia 526:225–230).

The genetic legacy of polyploid Bolivian Daphnia: the tropical Andes as a source for the North and South American D. pulicaria complex

We investigated genetic variation in asexual polyploid members of the water flea Daphnia pulex complex from a set of 12 Bolivian high-altitude lakes. We used nuclear microsatellite markers to study genetic relationships among all encountered multilocus genotypes, and combined this with a phylogenetic approach using DNA sequence data of three mitochondrial genes. Analyses of mitochondrial gene sequence divergence showed the presence of three very distinct clades that likely represent cryptic undescribed species. Our phylogenetic results suggest that the Daphnia pulicaria group, a complex of predominantly North American species that has diversified rapidly since the Pleistocene, has its origin in South America, as specific tests of topology indicated that all three South American lineages are ancestral to the North American members of this species group. A comparison between variation of nuclear and mitochondrial markers revealed that closely related polyploid nuclear genotypes sometimes belonged to very divergent mitochondrial lineages, while distantly related nuclear genotypes often belonged to the same mitochondrial lineage. This discrepancy suggests that these South American water fleas originated through reciprocal hybridization between different endemic, sexually reproducing parental lineages. It is also likely that polyploidy of the investigated lineages resulted from this hybridization. Nevertheless, no putative diploid parental lineages were found in the studied region.

Incorporating life histories and diet quality in stable isotope interpretations of crustacean zooplankton

Summary1. Stable isotope studies have been extremely useful for improving general food web descriptions due to their ability to simultaneously summarize complex trophic networks and track the energy flow through them. However, when considering trophic relationships involving only two or few species, application of general isotopic interpretations based on average fractionation values may easily lead to misleading conclusions. In these cases a more accurate consideration of the current processes involved in the isotopic fractionation should be considered.2. We investigated the trophic relationships of the crustacean zooplankton assemblage in an alpine lake (Lake Redon, Pyrenees) by means of stable isotopes of carbon and nitrogen and applied information on their life history and biochemical composition in the interpretation.3. The three species occurring in the lake had distinct isotopic signatures: the two copepod species (the cyclopoid Cyclops abyssorum and the calanoid Diaptomus cyaneus) had higher nitrogen isotopic composition than the cladoceran (Daphnia pulicaria), indicative of a higher trophic position of the two copepods. Most intra‐specific isotopic variability was associated with growth, while the effect of metabolic turnover was negligible. The effects of changes in the proportion of lipids was restricted to the adults of the two copepods.4. Daphnia Juveniles showed ontogenetic shifts in their carbon, and nitrogen isotopic composition. Cyclops copepodites only showed changes in carbon isotopic composition. These isotopic shifts with changes in size were the result of: (i) the prevalence of growth over metabolic turnover as the main factor for isotopic variability and (ii) feeding, during the growth period, on isotopically depleted food in the case of Daphnia, and on isotopically enriched food in the case of Cyclops.5. The carbon isotopic variation in Cyclops juveniles could be explained by fitting an isotopic growth model that considered that they fed entirely on Daphnia. However this was not the case for nitrogen isotopic variability. Cyclops nitrogen isotopic composition variation and the Cyclops to Daphnia nitrogen isotopic enrichment were closely correlated to the quantity of Daphnia protein and to the dissimilarity in the essential amino acid composition between the two species, which can be interpreted as an indication of consumer nitrogen limitation.

Initial conditions mediate the interaction between Daphnia and bloom‐forming cyanobacteria

To assess whether Daphnia populations in eutrophic lakes can increase when bloom‐forming cyanobacteria dominate the phytoplankton assemblage and whether such an increase can result in strong suppression of phytoplankton biomass, I created contrasting initial conditions (high Daphnia pulicaria, low cyanobacteria vs. low D. pulicaria, high cyanobacteria) via fish manipulation in large enclosures, then removed fish from some enclosures and subsequently monitored zooplankton and phytoplankton abundance for 48 days. After being released from fish predation, D. pulicaria was apparently able to reduce cyanobacteria and total phytoplankton biomass to very low levels despite the fact that the phytoplankton assemblage was initially composed of about 90% Microcystis aeruginosa, a species that has inhibited Daphnia growth and reproduction in many laboratory studies. Thus, it appears possible for Daphnia to graze down an established bloom of cyanobacteria. In contrast, in enclosures where fish were never present, M. aeruginosa was eventually able to increase from low levels despite initially high D. pulicaria biomass. As a result, the apparent effect of D. pulicaria on M. aeruginosa at the end of the experiment was very different across enclosures with different initial conditions.

Influence of food quality on depth selection of Daphnia pulicaria

We studied the habitat choice of juvenile and adult Daphnia pulicaria in thermally stratified water columns (plankton towers) with a deep water algal maximum (DCM). The DCM consisted of either filamentous cyanobacteria (Planktothrix agardhii), non-filamentous Chlorophyceae (Scenedesmus obliquus) or a mixture of both. Adult D. pulicaria spent more time at colder temperatures in the presence of P agardhii than in the presence of S. obliquus, either as the sole food source or when mixed with P agardhii. Juvenile D. pulicaria did not show a different habitat choice in the three food treatments. In a fourth treatment, we also determined Daphnia distribution in the absence of food. Comparing the habitat choice of juveniles and adults in each of the four treatments, the latter spent more time at colder temperatures when food was absent or when in the sole presence of P agardhii. Additional grazing and stable isotopic marker experiments showed that D. pulicaria ingested and assimilated Planktothrix filaments. The results suggest that the differences in habitat choice between adult D. pulicaria in the presence of different food types were influenced by food quality effects: adult Daphnia which move to colder waters in the presence of low quality P agardhii decrease their metabolic rate and might thus be able to invest more resources into reproduction when environmental conditions improve.

A direct test of cyanobacterial chemical defense: Variable effects of microcystin‐treated food on two Daphnia pulicaria clones

To determine the direct effects of microcystin on the fitness of herbivorous zooplankton, we experimentally added microcystin‐LR to freeze‐dried cells of palatable Chlorella and fed these compound‐treated cells to two clones of Daphnia pulicaria that had shown differing responses to a diet containing a strain of Microcystis aeruginosa that produces microcystin. The Daphnia that performed better on a diet containing live Microcystis showed reduced population growth when exposed to microcystin‐LR‐treated Chlorella, whereas the Daphnia that performed poorly on the diet containing live Microcystis was not affected by the experimental diet containing microcystin‐LR. This is the first study to unambiguously show that some Daphnia strains are and some strains are not harmed by the consumption of microcystin‐LR. These surprising results were not generated by interference from lipophilic secondary metabolites in Microcystis. When the crude lipophilic extract of Microcystis was added to dried Chlorella cells, it enhanced the fitness of both Daphnia clones. We hypothesize that the Daphnia clone more tolerant to live cells may upregulate resistance when cued by the presence of the live Microcystis cells, but not by microcystin‐LR alone. Alternatively, the Daphnia clone that grew well on a diet containing live Microcystis may sequester compounds from Microcystis that defend the cyanobacteria from autotoxicity; these compounds would have been unavailable to Daphnia consuming freeze‐dried Chlorella treated with microcystin‐LR alone. Thus, microcystin‐LR can suppress Daphnia fitness when consumed; however, the effects of microcystin vary across clones of herbivorous zooplankton and the consequences of this variance should not be overlooked when considering zooplankton‐cyanobacteria interactions.

Biochemical limitation of resting egg production in Daphnia

Many planktonic invertebrates can reproduce by immediately developing subitaneous eggs, which allow fast reproduction, and by resting eggs, which enable the survival of eggs in unfavorable environmental conditions. Since resting eggs can stay viable for decades or even centuries, they are likely to be richer in essential, easily degrading biochemicals than subitaneous eggs. Using the freshwater cladoceran Daphnia pulicaria we test whether the switch between these two qualitatively different reproductive modes is influenced by the availability of essential polyunsaturated fatty acids (PUFAs) in the food. We show that (1) when raised on a PUFA‐poor diet, resting eggs of D. pulicaria contain much more PUFAs than subitaneous eggs, and unlike subitaneous eggs, contain considerable amounts of eicosapentaenoic acid (EPA), which was not present in their diet; (2) supplementation with EPA or using EPA‐rich algae as food results in dramatically increased resting egg production; (3) when resting eggs are induced by starvation, their increased frequency in the presence of EPA‐rich food can be entirely explained by maternal effects. Since dormancy is a ubiquitous phenomenon among invertebrates, our result that its onset can be severely limited by the availability of essential biochemicals may hold for a variety of taxa.

Effect of nutrient availability on the uptake of PCB congener 2,2′,6,6′-tetrachlorobiphenyl by a diatom ( Stephanodiscus minutulus) and transfer to a zooplankton ( Daphnia pulicaria)

The objective of this study was to examine the importance of nutrient status of a diatom ( Stephanodiscus minutulus) to the uptake of PCB congener #54 (2,2′,6,6′-tetrachlorobiphenyl) and the subsequent transfer of PCB to a pelagic grazing zooplankton ( Daphnia pulicaria). The algae, which were grown under different nutrient treatments, were then fed to a zooplankton to examine the subsequent food chain transfer of PCB. Algal cultures were grown for at least 2 weeks in a steady state condition in (1) non-limiting, (2) low-Si, (3) low-N or (4) low-P media. Steady state algal cultures were dosed with 0.2 μg L −1 PCB and were sampled for PCB uptake after 24 h. D. pulicaria were allowed to graze on these same cultures for 48 h before being analyzed for PCB body burdens. Low-Si (68% or 0.135 μg L −1 of PCB) and low-P cultures (62%) had significantly higher percentage uptake of total PCB than the non-limiting (55%) or low-N (52%) treatments. When these values were divided by biochemical or elemental parameters, PCB per lipids (μg μg −1) had one of the lowest coefficients of variation (CV) across the four treatments, indicating their importance in PCB uptake. When equal biovolumes of the four different treatment cultures were fed to zooplankton, both the low-N (13.9 ng PCB mg wet weight −1) and the low-P (9.6 ng PCB mg wet weight −1) grazing D. pulicaria had significantly higher PCB per wet weight than the low-Si (5.6 ng PCB mg wet weight −1) and non-limited (2.6 ng PCB mg wet weight −1) grazing D. pulicaria. There were no significant differences between algal nutrient treatments in PCB per wet weight of zooplankton grazing on clean algal food in PCB contaminated media. This study indicates that uptake of PCB by phytoplankton can be significantly altered by nutrient availability which subsequently affects transfer to zooplankton, potentially through such responses as grazing rate and lipid assimilation.

Variation in ephippial buoyancy in Daphnia pulicaria

Summary1. Freshwater zooplankton often produce diapausing eggs to survive environmental stress. The diapausing eggs of Daphnia (Crustacea, Cladocera) are encased in an ephippium that either floats at the surface or sinks to the sediment. These two types of ephippia may represent different strategies between spatial (floaters) and temporal (sinkers) dispersal of offspring.2. We observed floating and sinking characteristics of ephippia obtained from eight lakes. We then conducted an experiment with 26 Daphnia pulicaria clones obtained from six of these lakes and observed the production of buoyant and non‐buoyant ephippia under high and low food conditions.3. Ephippia were more often non‐buoyant than buoyant both from females caught in nature and those reared in the laboratory. The experiment revealed that each clone was able to produce both types of ephippia, but that there was considerable among‐clone variation in the percentage of non‐buoyant ephippia produced.4. We conclude that production of non‐buoyant versus buoyant ephippia may be driven by both genetic and environmental factors.

Bacterioplankton communities of Crater Lake, OR: dynamic changes with euphotic zone food web structure and stable deep water populations

The distribution of bacterial and archaeal species in Crater Lake plankton varies dramatically over depth and with time, as assessed by hybridization of group-specific oligonucleotides to RNA extracted from lakewater. Nonmetric, multidimensional scaling (MDS) analysis of relative bacterial phylotype densities revealed complex relationships among assemblages sampled from depth profiles in July, August and September of 1997 through 1999. CL500-11 green nonsulfur bacteria (Phylum Chloroflexi) and marine Group I crenarchaeota are consistently dominant groups in the oxygenated deep waters at 300 and 500 m. Other phylotypes found in the deep waters are similar to surface and mid-depth populations and vary with time. Euphotic zone assemblages are dominated either by β-proteobacteria or CL120-10 verrucomicrobia, and ACK4 actinomycetes. MDS analyses of euphotic zone populations in relation to environmental variables and phytoplankton and zooplankton population structures reveal apparent links between Daphnia pulicaria zooplankton population densities and microbial community structure. These patterns may reflect food web interactions that link kokanee salmon population densities to community structure of the bacterioplankton, via fish predation on Daphnia with cascading consequences to Daphnia bacterivory and predation on bacterivorous protists. These results demonstrate a stable bottom-water microbial community. They also extend previous observations of food web-driven changes in euphotic zone bacterioplankton community structure to an oligotrophic setting.