Daphnia invasion: population dynamics of Daphnia assemblages in two eutrophic lakes with particular reference to the introduced alien Daphnia ambigua

The population dynamics of Daphnia species (D.cucullata, D.ambigua,D.galeata) in two small, productive lakes were studied and experimentswith field enclosures were carried out to elucidate the ecologicalniches of the introduced, alien species D.ambigua. Daphnia ambiguocoexisted temporally with the similarly sized D.cucullata, butlived in deeper water. Although both species had the same clutchsizes, similar proportions of ovigerous females, and death rates.D.cucullata was nurnerically dominant over D.ambigua. This suggeststhat the observed difference in abundance is simply a resultof the slower population growth rates of D.ambigua at lowertemperatures in greater depths. The experiments with field enclosuresshowed that when D.culcullata and D.ambigua were forced to livein the same layer of water, D.ambigua was numerically dominantover D.culullata both in the enclosures with and without fish.As the difference between the abundances of the two speciesdecreased in the enclosure with fish, D.cucullata seems to bebetter protected against fish predation than D.ambigua. In theenclosures without fish, D.ambigua and D.cucullata were numericallysurpassed by the larger species D.galeara. The present resultsimply that in some productive lakes with a high fish stock,D.mbigua invaded the niche of D.galeara and that the speciescan become an important component of the native zooplankton.The results also support the opinion that co-existing Daphniacan have very similar dynamics and niches. ¹ Present address: Zoologisches Institut, Christian-Albrechts-UniversitätKiel, Olshausenstrasse 40, D-24098 Kiel, FRG     

Daphnia dominance and zooplankton community structure in fishless ponds

Predation by fish has commonly been viewed as a primary driverof spatial and seasonal variation in Daphnia dominance and thesize structure of zooplankton communities. Yet, previous researchsuggests that large Daphnia do not always dominate in the absenceof predation. As alternatives to the planktivory model, numerousmechanisms have been put forth, including the effect of resourcecompetition and its interaction with resource quantity and qualityand abiotic factors (e.g. temperature). Here results are presentedof a field survey of 18 fishless, permanent ponds in southwestMichigan in which spatiotemporal variation in Daphnia pulexabundance and several potential determinants of this variationare explored. Results revealed a large amount of variation inD. pulex incidence and relative biomass, with some ponds exhibitingseasonal losses, some having few or no Daphnia, and some beingdominated by D. pulex for the entire sample period. Redundancyanalysis of zooplankton composition and pond environmental variables(biotic and abiotic) showed no relationship between D. pulexbiomass and measures of Chaoborus abundance, algal resourceproduction, or algal resource quality (including seston C:N:P).Instead, pH and temperature (both of which covaried) showedthe strongest relationship with D. pulex biomass.     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA     

Resource depression in Daphnia galeata, Daphnia cucullata and their interspecific hybrid: life history consequences

In this study, the effect of the food concentration on severallife history parameters of Daphnia galeata, Daphnia cucullataand the hybrid between these two species was examined. The foodconcentration had a significant effect on both growth and reproductionof all three taxa. Mortality was highly species specific, andto a lesser extent dependent on the food concentration. Changesin life history parameters resulted in changes in the intrinsicrate of population increase (r). The food dependence of thedifferent species was found to be different. Daphnia cucullatahad a relatively constant and low r. The r values for the hybridvaried most, with low values compared with the parental specieswhen grown at low food levels and relatively high values athigher food concentrations. Our results suggest that when foodconcentrations are high the hybrid will have a selective advantageover both parental species, but that at lower food levels D.galeatawill have an advantage over both D.cucullata and the hybrid.     

Resource quality effects on Daphnia longispina offspring fitness

The present study addressed the question: is the fitness ofDaphnia longispina neonates related to the quality (species)of the resource their mothers consumed? A 2 2 factorial designwith two maternal and neonate resource types was employed, withRhodomonas and Microcystis used as high- and low-quality resources,respectively. Neonate fitness was assessed using demographicparameters obtained from a life-table study. The resource typeneonates consumed always had a stronger effect on the neonatesthan did the maternal type. Neonate consumption of Rhodomonas.relative to Microcystis. increased fitness, as estimated bythe instantaneous population growth rate, by 25–28%. whilematernal consumption of Rhodomonas increased fitness by 6-8%.Neonate Rhodomonas consumption also reduced the age at reachingthe primiparous instar; increased brood size at the primiparousinstar and body length immediately after the primiparous instar;increases average dutch size, number of clutches produced, totalegg production. and mean body length up to and on age 30 days.Regardless of resource type, maternal Rhodomonas consumptionresulted in neonates which reached maturity at an earlier ageand had a larger individual size immediately after reachingmaturity. The mean clutch size, total egg production, and lengthup to and at age 30 days were all significantly affected bymaternal diet when neonates consumed Microcystis; however, thesevariables were not influenced by maternal type when neonatesconsumed Rhodomonas. This is the first study to document maternallymediated effects of resource quality on the fitness of crustaceanzooplankton neonates.     

Calcium limitation in Daphnia magna

The role of ambient calcium concentrations on survival, moulting,growth and egg production was assessed in the cladoceran Daphniamagna. A threshold for survival was found in the range 0.1–0.5mg Ca l^–1, even when ionic strength of the medium waskept constant. Accumulated length and length specific dry weightwas retarded at low Ca (0.5–1.0 mg Ca l^–1) at foodconcentrations above incipient limiting level. For lower foodlevels, the effect of Ca on growth was less clear. The effectof low Ca on growth rate was most manifest during the firstdays after hatching, reflecting the higher Ca demands of theearly juveniles. Age-specific egg production was strongly reducedat Ca concentrations <10 mg Ca l^–1. This was partlyan indirect effect of reduced growth rates, but could also bean effect of higher energetic costs associated with Ca uptakein a Ca-poor medium. The high Ca demands in D.magna may notbe representative of other Daphnia species. Nevertheless, highspecific Ca content seems to be a common property of Daphniaspp. and Ca deficiency could be a major determinant of speciessuccess and community structure among crustacean zooplankton;it also puts constraints on carbon sequestration in the pelagicfood web of softwater lakes.     

Interrelations between the vertical distribution of Daphnia and chlorophyll a in two large limnetic enclosures

This paper summarises the major factors influencing the daytimevertical distribution of Daphnia hyalina var. laciutris (Sars)in two large experimental enclosures (Lund Tubes). In both tubes,the depth at which the Daphnia aggregated during the day wasclosely related to sub-surface ir-radiance. On a few occasionsaggregations of Daphnia were found in ‘dark’ waterassociated with a deep chlorophyll maximum. On many occasions,however, the animals' light response per se ensured their aggregationat depths of maximum phytoplankton abundance. The most importantfactor influencing the dispersion of animals in the water columnwas the steepness of the light gradient. In turbid water verticalaggregations were well defined, whereas in clear water the animalswere widely dispersed around their depth of maximum abundance.Daphnia also tended to disperse in the water column when theirpopulation density was high or when food was scarce. A simplemodel based on surface irradiance, water turbidity and populationdensity explained the basic pattern of vertical distributionthroughout the season. The implications of these findings arediscussed in relation to the effects of zooplankton grazingand speculations on the adaptive significance of depth regulation.     

Can phaeopigments be used as markers for Daphnia grazing in Lake Constance?

The formation of chlorophyll a degradation products was measuredwith natural phytoplankton from Lake Constance and Daphnia magnaand native Daphnia as grazers in grazing experiments duringspring bloom conditions using high-pressure liquid chromatography(HPLC). Chlorophyll a start concentrations were between 1.2and 16.3 µg l^–1; phaeopigment weights constituted5% of chlorophyll a weight. Only phaeophorbide a was a markerfor Daphnia grazing; concentrations of other phaeopigments (phaeophytina, chlorophyllide a and two unidentified phaeopigments) didnot increase during Daphnia grazing. Conversion efficiencies(chlorophyll a to phaeophorbide a) were between 0 and 43% ona weight basis, and between 0 and 65% on a molar basis. Conversionefficiencies were highest at high grazer density (40 Daphnial^–1) and after a 24 h exposure time. Grazing by microzooplanktonprobably led to the formation of the two unidentified phaeopigments.In Lake Constance, Daphnia density was significantly positivelycorrelated with the phaeophorbide a/chlorophyll a ratio whenit was <5000 Daphnia m^–3. However, when higher Daphniadensities were included in calculations, then Daphnia densitywas positively, but insignificantly, correlated with the phaeophorbidea/chlorophyll a ratio. This suggests that when the level offood per Daphnia is low, then grazing is more efficient withless production of phaeophorbide a and a higher production ofcolourless products.     

Induction of helmet development by a Chaoborus factor in Daphnia ambigua during juvenile stages

Daphnia ambigua juveniles developed spike-like helmets inducedby a water-soluble factor from Chaoborus, when reared in Chaoborusflavicans-conditioned medium. A study was made of the time atwhich the Chaoborus factor affects helmet development. Juvenilesdeveloped the helmets when individuals were exposed to the Chaoborusfactor after being born, but did not develop the helmets whenthe individuals were exposed to the factor only in the egg orembryo stages. These results indicate that D.ambigua respondsto the Chaoborus factor during the juvenile stages.     

Instar-dependent mortality rates of coexisting Daphnia species in Lake Vechten, The Netherlands

In Lake Vechten, population parameters were determined for twocoexisting Daphnia species, D.hyalina and D.cucullata, fromApril 23 to December 10, 1986. Daphnia hyalina is dominant inspring and D.cucullata in summer and autumn. Size frequencydistributions were converted into instar frequency distributionsby using experimental growth curves and field measurements onnewborn size and the size at first reproduction. A discreteevent computer model was used to calculate the instar mortalityrates. Relative importances of instar mortality rates for thepopulation mortality rate were calculated and discussed. Themortality of the juvenile stages was found to be more importantthan adult mortality during most of the sampling season, despiteegg mortality when egg-bearing females are killed. Only in Septemberand October was the mortality of adult instars more important.Correlation coefficients between differences in the rates ofincrease and differences in mean brood size and instar mortalitiesshow that the former are caused mainly by differences in juvenilemortality, the main factor determining the replacement of D.hyalinaby D.cucullata during early summer.     

Effects of Daphnia on microzooplankton communities

Intact phytoplankton and microzooplankton communities from eutrophicStar Lake were incubated for 4 days with and without Daphniapulex, Daphnia galeaia mendotae, or a natural assemblage ofDaphnia species. They were sampled at the onset and terminationof the experiment for bacterial, phytoplankton, ciliate, rotifer,copepod and cladoceran densities. The cladocerans had variedeffects on the rotifers, ranging from significant suppressionof most rotifer species (Keratella cochlearis, Polyarthra remata,Keratella crassa) in the D.pulex jars, to the suppression ofone (K.crassa) or no species in the D.galeata mendotae and StarLake Daphnia assemblage jars, respectively. Small ciliates (<30µm, longest dimension), such as Strobilidium sp. and Pseudo-cyclidiumsp., were adversely affected by most of the cladoceran treatments,while several larger ciliates (>81 µm) were unaffectedin all such treatments. Ciliates were not consistently morevulnerable to cladoceran suppression than similarly sized rotifers.The suppression of ciliates and rotifers was attributable toboth direct effects (predation, interference, or both) and indirecteffects (e.g. resource competition) of the cladocerans. ¹Present address: Department of Biology, University of Louisville,Louisville, KY 40292, USA     

Range expansion and potential dispersal mechanisms of the exotic cladoceran Daphnia lumholtzi

. Recently the exotic cladoceran Daphnia lumholtzi has invadedfreshwater systems throughout the southern and midwestern UnitedStates. We conducted regional surveys of eastern Kansas reservoirsto document the range expansion of D.lumholtzi. Daphnia lumholtziwas found in five of 35 reservoirs sampled in 1994, and 11 ofthe 35 reservoirs when re-sampled in 1997. In addition, we sampled40 small ponds inaccessible to recreational boats, within thewatershed of an invaded reservoir. We did not find D.lumholtziin any of these ponds, suggesting that non-human dispersal mechanismsplay an insignificant role in the range expansion of D.lumholtzithroughout the United States. Further experimentation, however,is needed to determine if the absence of D.lumholtzi from thesesmall ponds is due to insufficient dispersal mechanisms or theinability of this invader to successfully colonize followingarrival. Daphnia lumholtzi has broad limnological tolerances.However, invaded reservoirs tend to be larger in area, havehigher Secchi disk depths, and lower total phosphorus, totalnitrogen and chlorophyll a levels relative to non-invaded reservoirs.Analyses of pre-invasion zooplankton communities indicate thatD.lumholtzi may be invading reservoirs in which native Daphniaspecies are rare. While the long-term effects of the invasionof D.lumholtzi are unknown, it has the potential to dominatelate summer zooplankton communities in eastern Kansas reservoirs.Therefore, we need to continue to survey and monitor invadedreservoirs to document the range expansion of D.lumholtzi anddetermine the long-term implications of the introduction ofthis invader.     

UV-B induced mortality and antioxidant enzyme activities in Daphnia magna at different oxygen concentrations and temperatures

Survival of adult Daphnia magna was assessed after acute (<96h) exposure to UV₃₁₂ under various temperatures (6, 12 and 18°C)or oxygen concentrations (5.6, 8.5 and 14.1 mg O₂ l^–1)in the laboratory. The surviving animals were screened for theenzymes catalase (CAT) and glutathione transferase (GST), whichmay protect against UV-induced oxidative damage. In addition,the same two enzymes were assayed in separate experiments afteracute exposure to UV₃₁₂ (6 h, 0.014 mW cm^–2) and the differentlevels of temperature and oxygen. No differences were observedin either CAT or GST activity after exposure to the three levelsof oxygen, but there was a tendency for decreasing specificactivity with decreasing temperature for both enzymes. CAT activitywas not influenced by UV radiation, whereas GST activity displayeda slight increase. Oxygen concentration did not influence survivalduring UV exposure but, contrary to expectations, survival testsat different temperatures clearly showed that reduced temperatureincreased survival. The results indicate that temperature effectsmust be considered when comparing dose effect relationshipsin situ, and suggest that low temperature is not a major causeof UV susceptibility in cold-adapted alpine and Arctic populationsof Daphnia.     

Long-term temperature acclimation in Daphnia magna: effects on filtering rates

Populations of a laboratoiy clone of Daphnia magna were acclimatedat 5, 10, 15 and 24C, and a varying temperature regin for aminimum of 70 days. The effect of temperature (5–25C)on filtering rates was measured for animals acclimated to theconstant temperatures and for a wild population collected froma reservoir. Acclimation temperature strongly influenced ffltcrmgrates at test temperatures. Animals with acclimation temperaturesclosest to the test temperature tended to have highest filteringrates at that temperature.     

Combined effects of food shortage and oxygen deficiency on life history characteristics and filter screens of Daphnia

Daphnia magna were reared in environments with different concentrationsof food and dissolved oxygen, and their life history characteristicsand filter-screen areas were analyzed. Both low food concentrationand low oxygen concentration degraded Daphnia fitness (as measuredby individual growth and reproduction), and synergism was detectedstatistically for the effects of the two factors together. Lowfood concentration induced the development of large filter screensin Daphnia, but this development was suppressed by low oxygenconcentration, suggesting that low oxygen concentration reducesthe filtering rate of Daphnia in a food-limited environment.Therefore, energy uptake by Daphnia seems to be reduced by foodshortages and oxygen deficiencies jointly, and this reductionin energy uptake may be a possible mechanism causing the synergisticeffects of the two environmental stresses on the life historyof Daphnia.     

影响武夷山景区松墨天牛种群动态变化的因素分析

为了监测武夷山景区松墨天牛Monochamus alternatus Hope的发生,防范松材松材线虫Bursaphelonchus xylophilus病的入侵,于2005-2007年每年4-9月,在武夷山景区10个地点进行了调查,研究了气象因素和环境因素对松墨天牛数量变化的综合影响规律及其程度,对调查结果进行了矢量递进因子分析。结果表明：环境因子和气象因子是松墨天牛羽化孔数和诱集成虫数变化的主要因素,影响松墨天牛成虫诱集数的环境因子按重要性排列依次为：郁闭度、地被物覆盖率、海拔、林龄、坡位、坡向和树高;影响羽化孔数的环境因子按重要性排列依次为：郁闭度、地被物覆盖率、树高、林龄、海拔、坡位和坡向。影响松墨天牛诱集成虫数和羽化孔数的气象因子按重要性排列依次为：月平均温度、最高与最低温差、总降雨量（mm）、气压（hpa）和平均湿度（%）。单从环境因子和气象因子的统计数据分析,环境因子的重要性次序是：坡位、郁闭度、地被物覆盖率、海拔（m）、 林龄（a）、树高（m）、坡向;气象因子的重要性次序是：气压（hpa）、 平均温度 （℃）、最高与最低温差（℃）、 总降雨量（mm）、平均湿度（%）。按月分析和按年度分析时其相对重要性基本上是一致的。结论认为,环境因子和气象因子对松墨天牛的数量变化有重要影响。     

Complex effects of a kairomone of Chaoborus and an insecticide on Daphnia pulex

Daphnia pulex were reared in Chaoborus-conditioned water containingthe insecticide carbaryl, and their life history parametersand morphologies were investigated. The insecticide inhibitedthe animals' growth and reproduction and delayed their maturationtime more intensely in the chaoborus-conditioned water thanin the control Chaoborus-free water, indicating that a kairomoneof Chaoborus made the Daphnia more sensitive to the insecticide.The Chaoborus conditioned water induced neckteeth formationof D.pulex in instars 1–2 and elongated the intermoultingperiod of juveniles. The moulting to the spined morphs and elongationin duration of juvenile stages seemed to increase the risk ofdamage from the insecticide. The potential population growthrate of D.pulex in treatments was estimated as a possible fitnessindicator of the animals. It was reduced synergistically bythe kairomone of Chaoborus and the insecticide. Some individualskept neckteeth until the third or fourth instar stage when theywere exposed to sublethal concentrations of the insecticidein the Chaoborus-conditioned water. This was considered as aresult of synergistic effects of both the kairomone and theinsecticide. Insecticides may be a factor inducing further developmentof protuberant structures in cyclomorphic Daphnia in naturalwater bodies.     

Vulnerability of Daphnia middendorffiana to Parabroteas sarsi predation: the role of the tail spine

The predaceous calanoid copepod Parabroteas sarsi and Daphniamiddendorffiana co-exist in South Andes ponds. Daphnia middendorffianajuveniles have a tail spine with negative allometric development.A series ofexperiments was carried out with tailed and tail-removedjuveniles of three different instars. In all cases tested, feedingrates were significantly higher on tail-removed prey. Directobservations showed a higher frequency of unsuccessful attackson spined juveniles when compared with tail-removed juveniles.The proportion of dorsal attacks also increased in spined juveniles.Prey total length was a much better predictor of feeding ratethan prey body size. Three groups of juveniles with equal totallength, but differentage, body size and biomass, showed no significantdifferences in their vulnerability to predation.     

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodiedDaphnia pulicaria have a different effect from smaller-bodiedDaphnia galeata mendotae on the composition of summer algalassemblages in eutrophic lakes. Three separate cubitainer experimentswere run for 5 days in a replicated factorial design utilizingtwo algal community types and the two Daphnia species. Inorganicphosphorus and nitrogen were added to prevent nutrient limitationof the algae. Both edible and inedible size fractions of chlorophylla increased in cubitainers without Daphnia spp. Grazer additionusually resulted in a reduction in edible chlorophyll; reductionswere greater in D.pulicaria cubitainers. Grazing by Daphniaspp. on presumed inedible chlorophyll was variable. Algal sizewas not always a good predictor of grazeability. The resultsof this study indicate that D.pulicaria, because of its greaterfiltration potential and ability to ingest larger particles,provides a stronger control on inedible-sized algae when comparedto equal numerical densities of D.g.mendotae. However, Aphanizomenonincreased as a response to heavy grazing pressure by D.pulicariaon other algal species. This suggests that biomanipulation effortsthat promote large-bodied Daphnia may not produce desirableresults if nutrient inputs remain high.     

Keeping Daphnia out of the surface film with cetyl alcohol

Some Daphnia species are difficult to culture due to a tendencyto become trapped in the surface film. A low level of cetylalcohol (1.26 0.12 mg) was shown to be highly effective inreducing the surface film entrapment of a Daphnia species typicallysusceptible to such a problem (D.rosea). In a species that doesnot exhibit surface film problems (D.pulicaria), a low levelof cetyl alcohol had no effect, indicating that this substanceis not toxic to Daphnia in low quantities. However, a high level(10.6 1.0 mg) of cetyl alcohol was shown to decrease the survivalof both species and reproduction of the susceptible species.