Daphnia lumholtzi and Daphnia ambigua: population comparisons of an exotica and a native cladoceran in Lake Okeechobee, Florida

The population dynamics of an exotic cladoceran (Daphnia lumholtzi Sars) and a native cladoceran (Daphnia ambigua) were studied over a 12 month period in subtropical Lake Okeechobee, Florida (USA), to quantify the extent of invasion of the exotic species and compare ecological niches. Daphnia lumholtzi accounted for up to 70% of the Daphnia assemblage during the summer months (June-August), while D.ambigua accounted for up to 97% of the Daphnia assemblage from fall to spring (October-April). The densities of the two species were inversely corelated. The exotic species was most concentrated in the shallower, warmer, north and south ends of the lake during the summer. It also was present, but at much lower densities, in the central lake region during the fall. The native species displayed a ubiquitous distribution throughout the lake during spring and winter, but was concentrated in the deeper, cooler, central region during the summer. Relationships of the two species with environmental conditions indicate that water column temperature might affect the seasonal and spatial distribution of the two Daphnia species. The results also indicate that D.lumholtzi may be filling a 'vacant' seasonal or spatial niche when conditions are unfavorable for D.ambigua.      

A model for temperature correction of size-specific respiration in Bythotrephes cederstroemi and Daphnia middendorffiana

Respiration rates for the cladocerans Bythotrephes cederstroemi and Daphnia middendorffiana were measured over a broad range of temperatures. The data were fit to the model f(K) W^b where f(°K) is a formulation based on enzyme activity as a function of temperature. The enzyme kinetic model better described the biological observations of respiration as a function of temperature than standard empirical models. The results indicate that Bythotrephes has characteristics of a stenotherm and that the arctic Daphnia middendorffiana has characteristics of a eurytherm based on the linear region of Arrhenius plots (where enzymes are fully active). Above 23°C, Bythotrephes was limited by inactivation of respiratory enzymes. Also, below 14°C, enzyme inactivation became significant and not fully compensated for. Daphnia middendorffiana has a temperature region from 10 to 26°C in which the respiratory process was linear on an Arrhenius plot. Temperature-dependent and weight-specific respiration was described for each species with a formulation over a complete range of environmental temperatures. The formulation for temperature inactivation is applicable to all enzyme processes and should be used in aquatic studies requiring a broad temperature range.      

Notes on the systematics and ecology of Daphnia pulex LEYDIG in Northern Canada

Daphnia populations from the region of Yellowknife (N. W. T.) on the Great Nave Lake, from Tuktoyaktuk (N. W. T.) on the coast of the Beaufort Sea east of the Mackenzie-Delta, and from Shingle Point (Y. T.) west of the Mackenzie-Delta were described. Population diagrams were given for the beginning of September 1974. The populations found at the above mentioned sites were compared with Daphnia pulex from Spitsbergen and the German island of Spiekeroog. Though some characteristics of Daphnia middendorffiana FISCHER occurred, mainly in individuals from very northern ponds, these were always found in individuals which also possessed some of the characteristics of Daphnia pulex. In every pond, specimens with characteristics of Daphnia middendorffiana lived together with other individuals resembling Daphnia pulex. Males were found in three out of the four visited ponds on the shore of the Arctic Sea, and were apparently Daphnia pulex. All the material sampled in northern Canada was regarded as belonging to Daphnia pulex LEYDIG 1860 emend. RICHARD 1896. There was evidence, that characteristics of Daphnia middendorffiana develop under the influence of environmental conditions, such as food supply. Daphnia magna STRAUS was found far north of the arctic circle.     

A major shift in Daphnia genetic structure after the first ice‐free winter in a German reservoir

1. Climate warming may cause disruption of trophic linkages in aquatic ecosystems and lead to changes in abundance and genetic structure of zooplankton populations. We monitored the community of the Daphnia galeata‐hyalina hybrid complex in the Saidenbach Reservoir (Saxony, Germany) using allozyme electrophoresis for three consecutive years (2005–07), including one (2007) following an unusually warm winter that prevented the formation of ice cover for the first time in the history of the reservoir. 2. Genetic composition during the 2007 season differed substantially from the two preceding years that experienced the usual 3‐month ice period. Three abundance peaks in June, July and October 2007 were dominated by hybrids of Daphnia galeata x hyalina, whereas in the 2005 and 2006 seasons two peaks in June and September were dominated by Daphnia hyalina genotypes. 3. The genetic composition of the pool of diapausing eggs produced in autumn and the rate of change of genotype abundance during the following spring indicate recruitment of the D. hyalina subpopulation from ex‐ephippial animals during the spring population increase. 4. The differing potential to contribute to the overwintering animal pool or to the inoculum from diapausing eggs was confirmed by results from laboratory life‐table experiments. Daphnia galeata clones survived longer and produced parthenogenetic offspring under winter conditions, whereas D. hyalina clones showed a shorter lifespan and produced resting eggs. 5. Our results indicate a profound role of recruitment strategy in the observed shift in genetic composition. Increasing winter temperatures predicted in the context of climate change may thus favour overwintering animals, leading to an increase in the contribution of these genotypes to the population. Such microevolutionary processes may dampen possible seasonal mismatches between daphnid populations and their food or predator populations.     

Is competition important to arctic zooplankton community structure?

1. Daphnia pulex and Daphnia middendorffiana are commonly found in the Toolik Lake region of arctic Alaska. These two species are very similar morphologically, although their natural distributions differ markedly: D. pulex is restricted to shallow ponds, while D. middendorffiana is widely distributed and found in a variety of ponds and lakes. We compared the reproductive capabilities of D. pulex and D. middendorffiana grown under similar resource conditions and in the absence of the invertebrate predator Heterocope septentrionalis. In situ life table and mesocosm experiments were conducted in Toolik Lake and Dam Pond, habitats that have historically contained natural populations of D. middendorffiana, but never D. pulex. 2. Daphnia pulex exhibited a significantly higher net growth rate than D. middendorffiana in both life table and mesocosm experiments although D. pulex has never been found in either Toolik Lake or Dam Pond. Daphnia middendorffiana exhibited a negative net growth rate in Dam Pond, which had lower resource levels then Toolik Lake. Therefore, the smaller D. pulex appears to have a lower food threshold concentration than the larger D. middendorffiana. 3. Our results indicate that D. pulex is a superior resource competitor in the Toolik Lake region. These results combined with distributional patterns suggest that the restricted distribution of D. pulex in these arctic lakes and ponds cannot be explained by resource competition alone. We suggest that in the presence of H. septentrionalis, predation is an important factor structuring arctic zooplankton communities in the Toolik Lake region.     

Effects of winter conditions on Daphnia dynamics and genetic diversity in a dimictic temperate reservoir

1. Winter conditions shape plankton dynamics and community composition in temperate regions, but their effect on dynamics and genetic composition of cyclical parthenogens like Daphnia is largely unclear. 2. For 5 years, we studied the dynamics, hatching from resting eggs and genetic structure of a D. galeata × longispina hybrid complex in a dimictic, temperate reservoir. Our main hypothesis was that higher spring densities and an earlier population peak will be observed after warmer winters, with a lower genetic diversity because of a lower contribution of resting eggs to population growth. 3. The study period could clearly be categorised into cold‐winter years (n = 3) and warm‐winter years (n = 2). Daphnia densities at the end of spring overturn were ～10‐fold lower after cold winters than after warm ones, but no pattern emerged concerning the timing and the height of the population peak in early summer. 4. Hatching intensity from resting eggs was higher and contributed up to 8.5% to Daphnia abundance in a cold‐winter year compared to a negligible contribution in a warm‐winter year. Consistent with this finding, new multilocus genotypes (MLGs) adding to the overwintering stock after the end of spring overturn and presumably originating from resting eggs increased genetic diversity and attained high frequencies within the population only after a cold winter. New MLGs were recorded also after warm winters, but they never gained dominance and no shift in genetic diversity was observed. However, genetic diversity was not generally reduced after warm winters. 5. Our results confirm earlier findings that winter conditions have only a limited effect on the main growth phase and the peak of Daphnia during late spring and early summer. However, winter conditions determine the contribution of resting eggs to the population development, which may profoundly alter the genetic composition of the population compared to the previous season.     

Pleistocene glaciations and polyphyletic origins of polyploidy in an arctic cladoceran

RFLP analysis of the ND4-ND5 genes of the mtDNA genome in Daphnia middendorffiana and three closely allied species was used to investigate its origin and age. Populations of D. middendorffiana from arctic Canada were found to possess three distinct mtDNA lineages, only one of which appears unique to this species. The other two mtDNA lineages are either closely allied or identical to haplotypes in D. pulicaria, suggesting that it is the maternal parent of many clones of D. middendorffiana. Within D. pulicaria, mtDNA lineages have largely disjunct distributions, suggesting that populations of this species persisted in three glacial refugia (arctic, western, eastern) during the Pleistocene. Hybridizations between these refugial stocks and other species such as D. melanica and D. pulex likely generated many of the polyploid lineages of D. middendorffiana following the Wisconsinan glaciation. The presence of one unique mtDNA lineage in D. middendorffiana suggests that at least some of its clones are more ancient, but further studies are needed to rule out the possibility of their recent derivation from an as yet undetected sexual species. As a general result, this study suggests that polyploid cladocerans are unlikely to predate the Pleistocene.     

Zooplankton,and related phytoplankton cycles,in a eutrophic lake

The seasonal cycles of zooplankton were determined for 18 consecutive months in a sewage-enriched lake in northern Canada and were related to algal availability and utilization, food consumption, temperature and the density of predators. Most of the common species (Daphnia pulex, Daphnia middenorffiana, Keratella cochlearis, Keratella quadrata, Polyarthra vulgaris) increased in abundance in May and June, reached a mid-summer maximum, and declined sharply in the fall. Phytoplankton densities increased sharply in May, peaked early in June and gradually decreased through the summer and fall. Since the quantity of algae in the guts remained constant during this period, algal availability and utilization had no direct impact on the seasonal cycles of any species. Furthermore, the amount of ingested material in D. pulex and D. middendorffiana was similar regardless of time of year, implying that the total quantity of food in the environment did not restrict development. Although temperature was the most important factor influencing variations in the densities of all species, predation by Cyclops spp. probably had little effect on the population dynamics of the fauna.     

The effect of suspended sediments on Lake Texoma Daphnia: field distributions and in situ incubations

1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma‐Texas, U.S.A.) on 12 dates over the course of 5 years. 2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions. 3. To test the hypothesis that interference with foraging excludes clear‐water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.     

Population dynamics and production of Daphnia hyalina and Bosmina longirostris in a shallow, eutrophic reservoir

SUMMARY. 1. The population densities, dynamics and production of Daphnia hyalina and Bosmina longirostris were studied over a 2-year period in a shallow eutrophic reservoir in eastern England. The diet of the two species was assessed and their ecology was compared in relation to environmental factors. 2. Daphnia hyalina was characterized by small overall body size, probably due to heavy size-selective predation by fish. The first generation produced from the overwintering population appeared in April and Depopulation peaked in late spring and early summer. Adult survival was poor. Peak egg production varied between years, while mean brood size fell in summer probably due to food limitation during the cyanobacterial bloom. At this time selection for individual algal species was high and many guts were empty. Reproductive ratios were inversely related to population density. Bosmina longirostris exhibited a similar annual cycle. 3. Patterns of production were very different in the 2 years in D. hyalina. In 1981 production was high from August to early October, but in 1982 the peak was in spring. Annual production was 32.4 g C m^?2 in 1981 and 13.3 g C m^?2 in 1982. In 1981 B. longirostris production peaked in spring and late summer; in 1982 there was a high production in summer only. Annual production was 3.2 g C m^?2 in 1981 and 2.2 g Cm^?2 in 1982. 4. The number of adult D. hyalina was highly correlated with water temperature and total phytoplankton biomass, but these were negatively correlated with the number of eggs, mean brood size and instantaneous birth rate. In contrast, clutch size of B. longirostris was positively correlated with water temperature while birth rate was positively correlated with both temperature and phytoplankton biomass. The biomass of both species was positively related to total phytoplankton biomass, but neither relationship explained much of the variation in zooplankton biomass. 5. The significance of the observations on plankton in terms of managing the water quality in eutrophic reservoirs is discussed.     

Benthic resources are the key to Daphnia middendorffiana survival in a high arctic pond

1. Shallow arctic lakes and ponds have simple and short food webs, but large uncertainties remain about benthic–pelagic links in these systems. We tested whether organic matter of benthic origin supports zooplankton biomass in a pond in NE Greenland, using stable isotope analysis of carbon and nitrogen in the pond itself and in a ¹³C‐enrichment enclosure experiment. In the latter, we manipulated the carbon isotope signature of benthic algae to enhance its isotopic discrimination from other potential food sources for zooplankton. 2. The cladoceran Daphnia middendorffiana responded to the ¹³C‐enrichment of benthic mats with progressively increasing δ¹³C values, suggesting benthic feeding. Stable isotope analysis also pointed towards a negligible contribution of terrestrial carbon to the diet of D. middendorffiana. This agreed with the apparent dominance of autochthonous dissolved organic matter in the pond revealed by analysis of coloured dissolved organic matter. 3. Daily net production by phytoplankton in the pond (18 mg C m^?2 day^?1) could satisfy only up to half of the calculated minimum energy requirements of D. middendorffiana (35 mg C m^?2 day^?1), whereas benthic primary production alone (145 mg C m^?2 day^?1) was more than sufficient. 4. Our findings highlight benthic primary production as a major dietary source for D. middendorffiana in this system and suggest that benthic organic matter may play a key role in sustaining pelagic secondary production in such nutrient‐limited high arctic ponds.     

Hydrodynamic Characteristics of Daphnia middendorffiana

Sinking terminal velocities of D. middendorffiana (with and without tail spine) were measured and correlated with morphological parameters and water temperature. The main dimensions of the body showed positive growths rate correlations with the body weight. In turn, negative allometric variation was observed in the tail-spine length. The density of D. middendorffiana was age specific, and decreased as the individual grew. No differences in the sinking terminal velocities and trajectories between individuals with and without tail spine were observed. The observed hydrodynamic characteristics of D. middendorffiana were simulated by means of a model based on equivalent ellipsoids and spheres. The analysis showed that the effects of shape and the body projections are negligible on the sinking rates of D. middendorffiana.     

Life history variation within a parthenogenetic population of Daphnia parvula (Crustacea: Cladocera)

Summary Evidence for genetically determined life history variability within a population or a species is rare. In this three year experimental examination of a parthenogenetically reproducing population of the planktonic crustacean Daphnia parvula, we found evidence for a succession of clones or groups of clones that exhibited distinctive body size and reproductive differences that were maintained after numerous generations under standardized conditions in the laboratory. The D. parvula population reached maximum density in the fall and maintained relatively high densities through the winter and spring. Isolates from this fall-winter-spring period all had a larger body size at death and higher fecundity when compared with summer isolates under natural food and temperature conditions. These differences could not be accounted for by differences in temperature and food abundance among the seasons. An additional difference in these experiments was a shift in reproductive effort by the summer isolate which produced a higher proportion of its offspring in the first two broods. The shift in life history characteristics and a summer decline of the Daphnia parvula population was correlated with both an increase in inedible and perhaps toxic blue-green algae and an increase in a dipteran predator Chaoborus. Comparison of the survivorship curves for all of the seasonal life history experiments indicated that D. parvula survivorship was not lower during the summer discounting a toxic effect from blue-green algae. Positive population growth on natural food in the laboratory at this time indicated food was not limiting and that predation was the probable cause of the population decline.Laboratory life history experiments under standardized food and temperature conditions were run with D. parvula isolates from the spring and summer plankton. Genetically based differences as determined in these experiments were smaller body size, lower fecundity, smaller brood size, and shorter life span for the summer animals relative to spring animals. Thirty seven percent of the summer animals also reproduced at an earlier age under standardized conditions. The shift in reproductive effort to earlier broods by summer animals rnder natural conditions appeared to be a phenotypic response as the summer isolate did not produce a higher proportion of its offspring in early broods under standardized conditions.When estimates of predatory mortality were added to the life tables of the standardized experiments, the earlier reproduction of some of the summer animals allowed a population increase under a regime of intense predation. Life tables for the spring animals predicted a population decline under these circumstances. Predictable seasonal changes in biotic factors such as predation suggest a mechanism whereby diverse life history patterns with corresponding differences in r may be maintained within a population.     

The Effects of Stocking and Removal of a Nonnative Salmonid on the Plankton of an Alpine Lake

Bighorn Lake, a fishless alpine lake, was stocked with nonnative brook trout, Salvelinus fontinalis, in 1965 and 1966. The newly introduced trout rapidly eliminated the large crustaceans Hesperodiaptomus arcticus and Daphnia middendorffiana from the plankton. In July 1997, we began to remove the fish using gill nets. The population comprised 261 fish that averaged 214 g in wet weight and 273 mm in fork length. Thereafter, zooplankton abundance increased within weeks. Early increases were caused by the maturation of Diacyclops bicuspidatus, few of which reached copepodid stages before the removal of the fish because of fish predation. Daphnia middendorffiana, absent when fish were present, reappeared in 1998. Hesperodiaptomus arcticus, which had been eliminated by the stocked fish, did not return. The proportion of large zooplankton increased after fish removal, but their overall biomass did not change. Algal biomass was low and variable throughout the 1990s and correlated with water temperature but not with nutrient concentrations or grazer densities. Diatoms were the most abundant algal taxon in the lake, followed by Dinophyceae. Chrysophyceans and cryptophyceans were eliminated after the fish were removed. Chlorophyll a concentrations were unaffected. Gill netting is a viable fish eradication technique for smaller (less than 10 ha), shallow (less than 10 m deep) lakes that lack habitable inflows and outflows or other sensitive species. Further work is required to define appropriate removal methods for larger lakes and watersheds. Received 30 May 2000; Accepted 14 November 2000.     

Overcrowding,food and phosphorus limitation effects on ephipphia production and population dynamics in the invasive species <Emphasis Type="Italic">Daphnia lumholtzi</Emphasis>

Daphnia lumholtzi has been very successful in colonizing North America since its appearance in Texas in 1990. Although previous studies have sought to link its success as an invasive species with various aspects of its population biology, there is little experimental data linking the invasion success of D. lumholtzi with its autecology, specifically its reproduction strategy. In this study we sought to link food quality and quantity to diapause in D. lumholtzi through a variation in phosphorus (P) content of algae, food quantity, and light level. We also assessed the effect of Daphnia peak population densities on reproductive rates and production of resting eggs. We found that when food is abundant, per capita ephippia production may be limited by P, but under food limitation conditions, there is no significant effect of food quality on ephippia production. Our results suggest that a combination of food quality/quantity and population density may work together to induce the production of resting eggs in this invasive species. Handling editor: S. Declerk     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Clonal diversity and turnover in an overwintering Daphnia pulex population,and the effect of fish predation

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Microsatellite markers for European Daphnia

We present 32 polymorphic microsatellite markers for species of the European Daphnia longispina group: D. galeata, D. hyalina, D. rosea, D. cucullata and D. curvirostris. Microsatellite markers were either isolated from genomic libraries or optimized based on previously published sequence information of sister taxa. Cross‐species tests revealed that all but one of the polymorphic markers are applicable to more than one species, which allows intra‐ and interspecific genetic studies on, i.e. population structure, hybridization events and introgression.     

Resource dynamics and seasonal changes in competitive interactions among three cladoceran species

A new approach to measuring zooplankton feeding success on naturalseston assemblages enabled us to test laboratory studies inthe field without being confined to radiotracers and their associatedproblems. We compared changes in the nutritional status of threespecies of cladoceran - Daphnia rosea Sars, D. middendorffianaFischer and Holopedium gibberum Zaddach - exposing them to differentnatural seston compositions. A modified version of the lipid-ovary index was applied in time-series experiments at CastleLake, California, throughout the summer of 1982. We used thesame index in competition experiments which were designed todetect shifts in competitive interactions among all of the abovespecies as a consequence of changes in the resource base. Thetime-series experiments with various seston compositions indicatedthat temporal and vertical distribution patterns of grazerswere strongly affected by the availability of suitable food.Daphnia rosea and H. gibberum had no noticeable effect on eachother's growth in early and midsummer, whereas the decline ofthe D. rosea population appeared to be accelerated by H. gibberumlater in the season. Daphnia middendorffiana, spatially separatedfrom the other two species, was unable to survive on food preferredby D. rosea and H. gibberum, whereas the latter two cladoceranswere negatively affected by D. middendorffiana when placed inwater containing hypolimnetic seston. Our results suggest thatresource abundance and composition has a much larger influenceon some zooplankton communities than previously acknowledged. ¹Present address: Lockheed EMSCO, Environmental Programs Office,1050 E. Flamingo Road, Las Vegas, NV 89109, USA