Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

Epizooic ciliates (Vorticella sp.) compete for food with their host Daphnia longispina in a small polyhumic lake

Summary Clearance rates of epizooic ciliates (Vorticella sp.) were measured together with their host, a planktonic cladoceran Daphnia longispina by using fluorescent latex beads as tracers of food. Vorticellans and their host graze on food of same size range (nanoplanktonic algae and bacteria). Individual clearance rates of Vorticella averaged 6.9 and 7.0 l ind^-1 h^-1 and those of Daphnia 463 and 708 l ind^-1 h^-1 for beads with diameter 2.00 and 3.92 m. On the average, epizooic vorticellans together on the carapace of Daphnia cleared particles with rates representing 25–33% of that the host cleared, the maximum rates being 50–80%. In a steeply stratified polyhumic lake vorticellans take advantage of following Daphnia to food patches and they can severely compete for food with their host.     

Growth of Daphnia longispina L. in a polyhumic lake under various availabilities of algal,bacterial and detrital food

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d^–1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a life-support system enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 g C 1^–1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.     

Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina

The vertical distributions of bacteria and algae in a steeply stratified, highly humic lake were studied during three 24 h periods in summer. The highest bacterial and algal densities and biomasses were recorded in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobium) which occurred at very low light intensity (< 1.5 μmol m⁻² s⁻¹). The numbers and biomasses of bacteria, both in the epilimnion and hypolimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Because of vertical migration, the diurnal vertical distribution of algae was more variable than that of bacteria. Particularly in July and August, when cryptomonads were abundant, the biomass of algae was much higher in the epilimnion during the day than at night. The flagellated chlorophytes, Chlamydomonas spp. and Scourfieldia cordiformis, stayed mainly in the upper hypolimnion close to the oxic-anoxic boundary zone where only a small proportion of Daphnia longispina was continuously present. Unpalatable Mallomonas chrysophytes with silicified plates and bristles, and small, presumably heterotrophic, flagellates stayed in the oxic epilimnion together with a dense (up to 300 ind l⁻¹) population of D. longispina. The results indicated that, besides the physical and chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorganisms in this polyhumic lake.     

Effects of Daphnia longispina on rotifer populations in a natural environment: relative importance of food limitation and interference competition

To test the relative importance of exploitative and interferencecompetition from Daphnia limiting rotifer populations in Bermejalesreservoir, birth rates of rotifers, food abundance, zooplanktonbiomass and size distribution of Daphnia were analysed fromhigh frequency sampling at two depths. A depletion in food abundanceby Daphnia feeding should be reflected in the birth rates ofrotifers susceptible to exploitative competition, while interferenceshould produce direct negative relationships between the numberof individuals of the interacting species. Results show thatsignificant relationships between food (chlorophyll-a) and birthrates of rotifers are found when the ratio chlorophyll-a/herbivorouszooplankton biomass is used as a measure of food availability.In our study, Anuraeopsis fissa was only affected through exploitativecompetition by Daphnia, while only Synchaeta oblonga showedevidence of being suppressed through interference competitionby this cladoceran.     

Ecology of predator-induced morphological defense traits in Daphnia longispina (Cladocera,Arthropoda)

Oecologia - Inducible defenses against predators are widespread among plants and animals. For example, some Daphnia species form neckteeth against predatory larvae of the dipteran genus Chaoborus....     

Productivity of Daphnia longispina in a highly humic boreal lake

The development of the Daphnia longispina (O. F. Müller)population in a highly humic boreal lake was followed throughoutone growing season, and the amount of secondary production wasestimated in relation to primary production and available foodresources. The growth rate method was applied in the secondaryproduction measurements. Daphnia longispina did not appear inthe water column until 16 May, after which the animals werepresent throughout the growing season. The population showedthree density peaks; the first appeared in early June, and thesecond and third in mid-July and at the beginning of September,respectively. Somatic production followed a seasonal pattern,with highest production rates in midsummer. The maximum valueof 127 mg C m^–2day^–1 was recorded at the beginningof July. The total annual net production of D. longispina was7.9 g C m^–2. During most of the growing season, the primaryproductivity in the lake was well below 100 mg C m^–2 day^–1and the total annual productivity of photosynthetic algae was5.0 g C m^–2. We conclude that in this lake the zooplanktonpopulation did not rely on phytoplankton primary productionas a sole carbon source, but that most of the carbon must haveoriginated from bacterial production either directly or througha microbial loop.     

A taxonomic reappraisal of the European Daphnia longispina complex (Crustacea, Cladocera, Anomopoda)

The Daphnia longispina complex contains some of the most common water flea species in the northern hemisphere, and has been a model organism for many ecological and evolutionary studies. Nevertheless, the systematics and nomenclature of this group, in particular its Palaearctic members, have been in flux for the past 150 years; this hinders the correct interpretation of scientific results and promotes the erroneous use of species names. We revise the systematics of this species complex based on mitochondrial sequence variation (12S rDNA and COI) of representative populations across Europe, with a special focus on samples from type localities of the respective taxa. Combining genetic evidence and morphological assignments of analysed individuals, we propose a comprehensive revision of the European members of the D. longispina complex. We show that D. hyalina and D. rosea morphotypes have evolved several times independently, and we find no evidence to maintain these morphotypes as distinct biological species. Alpine individuals described as D. zschokkei are conspecific with the above-mentioned lineage. We suggest that this morphologically and ecologically plastic but genetically uniform hyalina–rosea–zschokkei clade should be identified as D. longispina (O. F. Müller, 1776). The valid name of Fennoscandian individuals labelled D. longispina sensu stricto in the recent literature is D. lacustris G. O. Sars, 1862. Additionally, we discovered another divergent lineage of this group, likely an undescribed species, in southern Norway. Our results present a solution for several prevailing taxonomic problems in the genus Daphnia , and have broad implications for interpretation of biogeographical patterns, and ecological and evolutionary studies.     

Relationships between heterotrophic nanoflagellates and the demographic response of Daphnia longispina in a eutrophic lake with poor food quality conditions

1. Based on both field data and laboratory studies, the summer population of Daphnia longispina living in a stratified eutrophic lake was examined in relation to the abundance of algae, nanoflagellates and picocyanobacteria.
2. In early July, the Daphnia population replaced Bosmina and remained the dominant zooplankter during summer 1994. Its development in July was concomitant with an increase of edible algae but, despite the apparent abundance in available food, the Daphnia population decreased throughout August suggesting poor food conditions.
3. From mid-August to the beginning of September, the biomass of inedible phytoplankton was greater than that of the smaller, edible fraction. Consequently the average rate of increase, birth rates and fecundity of Daphnia remained low. Although the biomass of heterotrophic nanoflagellates was consistently low, the demographic parameters of Daphnia were correlated throughout this period with these protozoans.
4. Life table experiments run in the laboratory showed that epilimnetic food supported both the growth in length of individual Daphnia and an increase in fecundity, but metalimnetic food supported only individual growth. D. longispina probably failed to reproduce because of the abundance of detritus mixed with the heterotrophic nanoflagellates in the metalimnetic water at that period of the year. The vertical migration of Daphnia into these deeper layers could be caused by a predator avoidance mechanism.     

UV-B treated algae exhibiting different responses as a food source for Daphnia magna

The response of green algae Scenedesmus quadricauda and Selenastrumcapricornutum to different levels of UV-B radiation was studied.Daphnia magna, the next step in the food web, was fed with UV-Btreated algae. UV-B radiation induced the synthesis of UV absorbingsubstances and photosynthetic pigments, enhanced the level ofrespiratory potential and suppressed growth of the treated algae.Biomass production in S. quadricauda was higher than in S. capricornutum,but the production of photosynthetic pigments and relative amountsof UV absorbing substances as well as respiratory potentialwere more pronounced in S. capricornutum. A short-term feedingexperiment with D. magna showed that in general, females ingestedhigher numbers of cells of algae treated with a high UV-B dose,but the biomass of ingested algae did not differ significantly.The exception was S. capricornutum treated with a high UV-Bdose; a lower biomass was ingested, probably reflecting thesmaller UV-treated cells.     

Emergence pattern and hatching cues of Daphnia pulicaria (Crustacea, Cladocera) in an alpine lake

We developed a methodology to predict brook trout (Salvelinus fontinalis) distribution using summer temperature metrics as predictor variables. Our analysis used long-term fish and hourly water temperature data from the Dog River, Vermont (USA). Commonly used metrics (e.g., mean, maximum, maximum 7-day maximum) tend to smooth the data so information on temperature variation is lost. Therefore, we developed a new set of metrics (called event metrics) to capture temperature variation by describing the frequency, area, duration, and magnitude of events that exceeded a user-defined temperature threshold. We used 16, 18, 20, and 22°C. We built linear discriminant models and tested and compared the event metrics against the commonly used metrics. Correct classification of the observations was 66% with event metrics and 87% with commonly used metrics. However, combined event and commonly used metrics correctly classified 92%. Of the four individual temperature thresholds, it was difficult to assess which threshold had the “best” accuracy. The 16°C threshold had slightly fewer misclassifications; however, the 20°C threshold had the fewest extreme misclassifications. Our method leveraged the volumes of existing long-term data and provided a simple, systematic, and adaptable framework for monitoring changes in fish distribution, specifically in the case of irregular, extreme temperature events.     

Reproductive strategy as population density varies in Daphnia magna (Cladocera)

• 1 In this study an experiment was carried out to test whether Daphnia magna (Cladocera) changed its reproductive strategy in response to varying population density but constant food supply.
• 2 As population density increased there was a reduction in the age at which females reproduced. Early reproduction was offset by smaller body length and clutch size (number of eggs per female) relative to later reproducing females grown at lower population densities.
• 3 This pattern was interpreted as a strategy carried out by females to reduce the time at which they release their offspring. One potential advantage of early reproduction is to reduce the risk of severe food limitation for neonates born during a period of rapid population increase.

     

Chronic responses of different clones of Daphnia longispina (field and ephippia) to different food levels

Resting eggs are a fundamental reproductive strategy among freshwater cladocerans. Under adverse environmental conditions, whole Daphnia populations can disappear from a lake and a new community will arise from ephippial eggs. Since these new populations are subjected to genetic variation, their responses to environmental stress or contaminants can be different from the “original” population. In the present study, life history responses (reproduction and growth) of Daphnia longispina to different food concentrations was studied. Two Daphnia populations were tested: (a) field clones and (b) ephippial clones. Food (Selenastrum capricornutum) concentration was the stressor tested (absence of food, and low to high food concentrations). The results showed that reproductive responses of D. longispina to the tested food concentrations varied among clones, independently from their origin.     

The relationship between planktonic algae and bacteria in a small lake

The seasonal changes and vertical distribution of the aerobic and anaerobic bacteria in a small edaphically eutrophic lake which exhibited thermal and chemical stratification are described. There was some correspondence between the phytoplankton and particularly the aerobic bacteria but this was not consistent. Increases in the numbers of anaerobic bacteria coincided with the low dissolved oxygen concentrations in the hypolimnion when algal populations were first senescent and then increasing actively in size.     

Relative importance of competition with Daphnia (Cladocera) and nutrient limitation on Anuraeopsis (Rotifera) population dynamics in a laboratory study

• 1 In the laboratory, the growth and reproduction of Anuraeopsis fissa were measured when fed on Scenedesmus species grown in nutrient‐sufficient, nitrogen‐limited and phosphorus‐limited media and in the presence or absence of one adult Daphnia longispina per vial.
• 2 Poor food quality may reduce the effect of competition on rotifers. Competition from Daphnia was stronger with nutrient sufficient algae than with nutrient limited algae. P‐limitation significantly reduced Anuraeopsis population growth rate and fecundity. The effect of nutrient limitation on Anuraeopsis was stronger than that of competition with Daphnia. The Anuraeopsis population declined with P‐limited food in both the presence and absence of Daphnia.
• 3 Exploitative competition by Daphnia on Anuraeopsis was stronger in the nutrient‐sufficient treatment than in the N‐limited one. Density, fecundity and population growth rate of Anuraeopsis were negatively affected by Daphnia in the nutrient‐sufficient treatment, while only fecundity was reduced by Daphnia in the N‐limited treatment. Consequently, in the N‐limited treatment, mortality should be lower in the presence of Daphnia. This result could suggest that Anuraeopsis lives longer when short of nitrogen.
• 4 Nutrient limitation may affect to the competitive interactions between zooplankton species. P‐limitation decreased the quality of algae as food for Anuraeopsis while N‐limitation decreased the susceptibility of this rotifer species to exploitative competition by Daphnia.

     

The relative importance of different ciliate taxa in the pelagic food web of lake constance

Abundance, biovolume, and species composition of pelagic ciliates in Lake Constance were recorded over two annual cycles (1987/88). Production was estimated from mean annual biovolumes and size-specific growth rates obtained from the literature. Cell concentrations and biovolumes ranged from 0.1 to 120 cells ml^–1 and from 3 to 1,200 mm³ m^–3, respectively. Mean annual values were, respectively, 6.8 cells ml^–1 and 94 mm³ m^–3 in 1987, and 12.0 cells ml^–1 and 130 mm³ m^–3 in 1988. In both years, prostome nanociliates (<20m) dominated numerically, while strobiliids in the size range 20–35m contributed most significantly to ciliate production. Ciliate community production, according to a crude calculation, yielded approximately 10–15 g C m^–2 year^–1.     

Effects of food quality and starvation on the optimal foraging behavior of Daphnia magna (Cladocera)

In the present work, we evaluated the feeding selectivity of starved Daphnia magna on two freshwater green algae Chlamydomonas sajao and Chlorellapyrenoidosa. Compared to C. pyrenoidosa, food quality of C. sajao are better in food palatability (cell size and digestibility), but poor in nutritional content (total carbon content). D. magna was starved for 0 and 8 d, and then was allowed to graze on a mixture of C. sajao and C. pyrenoidosa with following proportion: 5 × 10⁴: 35 × 10⁴ cells ml^?1, 20 × 10⁴: 20 × 10⁴ cells ml^?1 and 35 × 10⁴: 5 × 10⁴ cells ml^?1. The results indicated that the ingestion rate and filtration rate of starved D. magna, comparing with satiated groups, on C. pyrenoidosa increased significantly, while, inverse trends was observed in C. sajao. Base on selectivity coefficient of D. magna, we observed that when D. magna was in satiation C. sajao will be preferred, while, C. pyrenoidosa will be selected when D. magna was in starvation, and moreover, these foraging behaviors were not influenced by the relative food abundance of each green alga. Therefore, a tradeoff between food palatability (physical makeup) and food nutritional content (chemical composition) can be hypothesized in the foraging behavior of D. magna, which is modified by the starvation of feeder. High valuable food is always selected by D. magna as predicted by optimal foraging theory. However, when D. magna is in satiation food diets with adequate size and easy digestibility will be preferred, while, those foods with relatively higher lipid or total carbon content will be selected when D. magna is in starvation.     

Pokey,a New DNA Transposon in Daphnia (Cladocera: Crustacea)

We report the complete sequence of two representatives of the transposable element, Pokey, isolated from the ribosomal DNA of the cladoceran, Daphnia pulicaria. We describe the general features of this element, which confirms its classification as a DNA transposon. We show that Pokey is similar to piggyBac and, as such, is a member of the TTAA-specific family of elements. Pokey is putatively autonomous, possessing an open reading frame that encodes a putative protein with similarity to piggyBac's transposase, in addition to putative proteins from Drosophila and human LOOPER elements, and several human proteins of unknown function. We show that these proteins all contain amino acid motifs that are perhaps conserved for similar functions. We demonstrate that Pokey is carrying several regions of similarity to Daphnia pulex IGS sequences, likely the result of some sort of recombination event with the host genome. Pokey inserts into a conserved region of the large subunit ribosomal RNA gene known to contain other arthropod elements, suggesting that this location is a hot spot for insertional activity. However, Pokey is unique in that it is the only DNA transposon yet known to insert into this region, as other such insertions are non-LTR retrotransposons. It is now clear that this "ecological niche" has been effectively exploited by both retrotransposons and DNA transposons.     

Filtering rates on natural bacteria by Daphnia longispina and Eodiaptomus japonicus in Lake Biwa

Filtering rates on [³H]thymidine-labelled natural unattachedbacteria and that on [¹⁴C]bicarbonate-labelled natural planktonwhich pass through the 25 µm-mesh-size screen were measuredfor Daphnia longispina and Eodiaptomus japonicus in Lake Biwa.Errors associated with the radioisotope technique, i.e the lossof labels after feeding trials and the self-absorption of thebeta emittance of ³H, were checked and corrected for the calculationof the filtering rates. It was suggested that Daphnia collectsbacteria efficiently, although the efficiency is somewhat variabledepending on food particle composition (i.e. presence and absenceof larger particles) and feeding condition (i.e. animal densityand physical disturbance). By contrast, copepodites of Eodiaptomuswere suggested to be less efficient bacteria feeders. Food resourceexploitation strategies of these two co-existing zooplanktersare discussed.     

Some observations on the diet and food selection ofDaphnia hyalina (Cladocera) in an eutrophic lake

Summary The food composition in the guts ofDaphnia hyalina was compared with the phytoplankton composition in the lake water on several dates during the year. The guts contained almost exclusively algae; detritus was not an important food item. ExceptOscillatoria agardhii, all large algal forms were generally avoided. On a whole,Scenedesmus was the most preferred taxon, but as numbers or biomass ingestedO. agardhii was more important. The filamentous blue-green algae>60 m (range: 60–1500 m) were strongly avoided byDaphnia. The observed size selective feeding is probably the result of limitations in their filtering apparatus rather than active selection.