The productivity and carbon budget of a natural population of Daphnia lumholtzi Sars

D. lumholtzi in Lake Samsonvale, Queensland, Australia, is a small species (max. size approx. 7 µgC) that occurs in low abundance (max. abundance 6400 m^–3), with an average daily biomass of 3.32 mgC m^–3. Its annual rates of carbon assimilation, production and respiration, are 166, 110, and 56 mgC m^–3 y^–1 respectively. Annual biomass turnover (annual production/average daily biomass) is 33 and production efficiency is 50–66%. The population may consume 1.65–2.20 mgC m^–3 daily, equivalent to about 1% of the average daily standing crop of phytoplankton. Clutch size is small, 2 eggs, but represents 30–80% of a female's weight. A female may only produce 8–10 offspring in a full lifespan, nevertheless egg production may account for 56% of total production. The population shows autumn and spring peaks in abundance, and is believed to oversummer (4 months) as ephippia.     

Elemental composition of two coexisting Daphnia species during the seasonal course of population development in Lake Constance

Summary In 1986 and 1987 the seasonal course of population density, life history traits and biomass composition (CHN-elemental and ash) of two coexisting Daphnia species from L. Constance (Überlinger See) were studied. The two daphnids are morphologically similar, but demonstrate different seasonal variation of behaviour (i.e. diurnal vertical migration). With the exception of nitrogen content, the mean biomass composition of adults and that of eggs were significantly different: the dry biomass of eggs (adults) contained on average 55.6 (46.1) % C, 7.9 (6.5) % H, 9.7 (9.7) % N, and 10.0 (23.8) % ash. These values were similar to those of other daphnids; it may be concluded that the average daphnid composition is not species specific, but is determined mainly by the position in the life cycle and by the nutritional state of the animal. A distinct seasonal variation in the elemental composition of the biomass of adult daphnids was observed. It related significantly to the concentration of food, as indicated by the epilimnetic chlorophyll-a concentrations and by Secchi depth. From the amplitude of seasonal variation of elemental composition and from the shape of relationships between the elemental composition of daphnids and their food, it can be concluded that in summer the non-migrating D. galeata lives under stronger food limitation than the migrating H. hyalina. In spring, there were no differences in the biomass composition of either species. In summer, the adult fcmales of D. galeata contained significantly less carbon, and hydrogen than those of D. hyalina. In most parameters of biomass (i.e. biomass composition of adults) and of life history (i.e. clutch size, mean adult weight), D. galeata displayed higher coefficients of variation. A tendency in D. galeata to have peaks of population development carlier in the season and in D. hyalina to have them towards autumn was observed. These contrasting responses of the populations development, the seasonal variability of the biomass composition seem to support the view of Geller (1986) who called D. galeata and D. hyalina exploitative and conservative strategists respectively.This research was supported by the Deutsche Forschungsge-meinschaft within the Sonderforschungsbereich Cycling of Matter in Lake Constance (SFB 248), and by scholarship to R.B. from the Deutscher Akademischer Austauschdienst (DAAD)     

Molecular genetic analysis of Daphnia in the English Lake District: species identity, hybridisation and resting egg banks

• 1 Taxonomic issues in many Daphnia species complexes are often confused by a high degree of phenotypic plasticity and by interspecific hybridisation. Here, we employ molecular genetic tools to confirm the species composition and incidence of hybridisation in extant and resting egg populations of Daphnia from Windermere and Esthwaite Water in the English Lake District.
• 2 A combination of species‐diagnostic allozymes and mtDNA, confirms that contemporary populations are dominated by a single species, D. galeata.
• 3 We present the first account of genetic characterisation of dated ‘resting’ eggs using microsatellites and mtDNA, employing PCR‐based DNA recovery, thus providing a temporal dimension to taxonomic patterns. Thirty years ago, two species were present in Esthwaite, D. galeata and D. hyalina, but Windermere populations were dominated by D. galeata only.
• 4 The use of PCR‐based mtDNA RFLP analysis as a species‐diagnostic tool, and microsatellites to monitor clonal diversity, provide a valuable approach for long‐term studies, especially in populations free from the complicating effect of frequent hybridisation. The detailed limnological records available for many large lakes, and associated changes in land‐use, pollutants and climate, combined with long‐term ephippial molecular genetic data, provide opportunities for exploring natural and anthropogenic impacts on genetic and community structure.

     

The use of biomarkers in Daphnia magna toxicity testing. IV. Cellular Energy Allocation: a new methodology to assess the energy budget of toxicant-stressed Daphnia populations

The Cellular Energy Allocation (CEA) methodology wasdeveloped as biomarker technique to assess the effectof toxic stress on the energy budget of testorganisms. This short-term assay is based on thebiochemical assessment of changes in the energyreserves (total carbohydrate, protein and lipidcontent) and the energy consumption (electrontransport activity). The CEA methodology was evaluatedusing Daphnia magna juveniles exposed for 96hto sublethal lindane and mercury chlorideconcentrations. The ecological relevance of the CEAassay was assessed by comparing the sub-organismalresponse with population level parameters (obtainedfrom 21 day life table experiments) such as theintrinsic rate of natural increase (r_m) and themean total offspring per female. Two differentmethodologies were used to assess the effect levels:the no (lowest) observed effect level (NOAECs-LOAECs)approach and the regression-based approach. Bothtoxicants caused a significant decrease in the netenergy budget of D. magna, with a LowestObserved (Adverse) Effect Concentration (LOAEC) of0.18 mg/l and 5.6 µg/l for lindane andHgCl₂,respectively. Changes in the lipid content of theorganisms were detected at toxicant concentrationslower than those affecting the total carbohydrate andprotein content. Toxicant specific effects wereobserved on the electron transport activity.Comparison of the CEA results with those of thepopulation level tests revealed that for mercury theCEA based LOAEC was a three times lower than thatbased on r_m and the total brood size(18 µg/l). For lindane the CEA based LOAEC was twotimes lower than the LOAEC based on r_m(0.32 mg/l) but was higher than that based on thetotal number of offspring produced (0.1 mg/l).Using the regression-based approach, EC₁₀ valueswere calculated using three parameter sigmoid orlogistic models. Comparison between the CEA andr_m based EC₁₀ values demonstrates that forboth chemicals similar effect concentrations areobtained: the CEA-based EC₁₀ (0.20 mg/l) forlindane is 1.5 times higher than the r_m-basedEC₁₀ threshold (0.13 mg/l), while for mercury thebiomarker-based EC₁₀ value (9 µg/l) was 1.4times lower than the population-based EC₁₀ value(12.5 µg/l).From these results, we suggest that the short-term CEAassay may be useful for predicting long-term effectsat the population level. The consequences of theobserved effects on the energy budget of the testorganism are discussed in the context of the effectsemerging at the population and community level.     

Side effects of water liming on body size and eggs development of Daphnia species in Lake Orta (N. Italy)

Life-tables of the Daphnia species (D. longispina and D. obtusa), currently inhabiting the previously acidified Lake Orta, were followed to study the side effects of liming on the demographic parameters (e.g. longevity, body size increment, eggs and new-borns production), of both species. Different media were prepared to simulate the lake water pH's (5, 6 and 7), and copper concentrations (40 μg l^-1), before and after liming, using GF/C filtered lake water in such simulation and as a reference medium. The obtained results have shown that, the individuals of D. longispina were only able to survive after the liming and the consequent reduction in copper concentrations. However, due to shift occurred in the different media pH's after liming toward slightly alkaline conditions, no significant variations in body size increment or in eggs and newborns production were detected of this species in the different media (p = 0.38, p = 0.29, p = 0.09 respectively). On the other hand, the effect of the variations in pH on the body size increment of the B. obtusa individuals was marginally significant (p = 0.07). A highly significant (p = 0.002), increase in eggs production by D. obtusa individuals was detected in the different media after the CaCO₃ addition. Conversely, a corresponding highly significant (p = 0.0007), decrease in the production of new-borns, accompanied by the degeneration of about 78% of the produced eggs, was detected in the different media after liming. Our results suggest that in Lake Orta the actual irregular abundance of the studied Daphnia species and the eggs degeneration phenomenon, observed in our study, could be largely attributed to the interspecific competition resulting from species-specific sensitivity to the continuous variations in the lake water Chemistry after the liming operation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Absence of intrinsic post‐zygotic incompatibilities in artificial crosses between sympatric coregonid species from upper Lake Constance

A full factorial crossing experiment with five females and five males of each of two coregonid species from upper Lake Constance was used to test for intrinsic post‐zygotic incompatibilities during early ontogeny. Up until shortly before hatching, there was no difference in embryo mortality between homo and heterologous crosses. A maternal effect on mortality was found in both species, but paternal effects and female–male interactions were absent. Thus, genetic incompatibility during early ontogeny does not appear to prevent introgressive hybridization, suggesting that genetic divergence between these species is maintained primarily by pre‐zygotic barriers. The recent genetic homogenizations of coregonid species flocks in European alpine lakes may have been caused by a flattening of adaptive landscapes through eutrophication, but intensive stocking with larvae obtained in hatcheries from artificially fertilized eggs is also likely to be a contributing factor. To safeguard diversity among sympatric coregonids, it is important to re‐establish ecological conditions conducive to species divergence and to revise traditional management strategies.     

Segregation of habitat and prey in two sympatric carnivorous plant species,Drosera rotundifolia and Drosera intermedia

Summary Labelled nitrogen was used to evaluate the effects of intensive forest management on soil nitrogen transformations. The total release of N into inorganic forms (ammonium plus nitrate) was much greater than net N mineralization in all treatments. Immobilization of N by microbes was greatest in minimally-treated harvested plots, while the turnover of N within soil microbes was greatest in intensively-treated plots. Ammonium was immobilized 2.4–3.2 times more rapidly than nitrate in havested plots; nitrification in disturbed sites could thus increase the availability of N to regrowing vegetation.     

The response of European Daphnia magna Straus and Australian Daphnia carinata King to changes in geomagnetic field

This study investigates the effects of lifelong exposure to reversed geomagnetic and zero geomagnetic fields (the latter means absence of geomagnetic field) on the life history of Daphnia carinata King from Australia and Daphnia magna Straus from Europe. Considerable deviation in the geomagnetic field from the usual strength, leads to a decrease in daphnia size and life span. Reduced brood sizes and increased body length of neonates are observed in D. magna exposed to unusual magnetic background. The most apparent effects are induced by zero geomagnetic field in both species of Daphnia. A delay in the first reproduction in zero geomagnetic field is observed only in D. magna. No adaptive maternal effects to reversed geomagnetic field are found in a line of D. magna maintained in these magnetic conditions for eight generations. Integrally, the responses of D. magna to unusual geomagnetic conditions are more extensive than that in D. carinata. We suggest that the mechanism of the effects of geomagnetic field reversal on Daphnia may be related to differences in the pattern of distribution of the particles that have a magnetic moment, or to moving charged organic molecules owing to a change in combined outcome and orientation of the geomagnetic field and Earth's gravitational field. The possibility of modulation of self-oscillating processes with changes in geomagnetic field is also discussed.     

Early life-history dynamics of Caribbean coral species on artificial substratum: the importance of competition, growth and variation in life-history strategy

The development of a coral community was monitored for 6 years (1998–2004) on 46 m² of artificial settlement substrate in Curaçao, Netherlands Antilles. Growth and survival of recruits (n=1385) belonging to 16 different species were quantified in relation to characteristics of the benthic community developing around them. The early life history dynamics (i.e. growth rate, growth strategy and survival) of corals differed among species although these differences were small for species occupying similar habitats (i.e. underside versus topside of substratum). In contrast to recruit survival, juvenile growth rates were highly variable and unrelated to benthic community structure, at least at the scale of this study. Competing benthic organisms affected coral recruitment success through space preemption (mainly by macroalgae) or recruit overgrowth (mainly by sponges). The results highlight the small spatial scale (mm–cm) at which the processes responsible for recruitment success or failure occur and emphasize the need to include such small-scale observations in studies of coral early life-phase dynamics.     

Chemical and mechanical defense against herbivory in two sympatric species of desert Acacia

Abstract. Most African Acacia trees occur in semi-arid savannas, but some species grow in deserts at the periphery of their range in northern Africa and the Middle East. We studied Acacia raddiana and A. tortilis in the southern Negev desert of Israel. According to the resource availability hypothesis, plants in poor habitats invest heavily into anti-herbivore defense because losses to browsers are costly when growth rates are low. Few plants invest simultaneously in different categories of defense. This suggests that trade-offs exist, although little is known about how individuals allocate resources to defence in natural populations. In a field experiment, we studied chemical and mechanical defences as a response to differential browsing for more than 10 yr. Both Acacia species increased spinescence but there was no clear response in the production of total polyphenols, condensed tannins, and protein-precipitating tannins (only A. raddiana may increase secondary compounds under very high browsing levels). There was an age effect in spinescence but not in secondary compounds for both species, with younger trees investing more into anti-herbivore defense than older individuals. We were unable to find negative correlations between traits of chemical and mechanical defense. We conclude that inducibility and age effects suggest a cost for mechanical but not necessarily for chemical defense. Contrary to the predictions of the resource availability hypothesis, concentrations of secondary compounds were not higher in this desert environment than in savannas.     

Water level fluctuations and dynamics of amphibious plants at Lake Constance: Long-term study and simulation

Inundations of lakeshores are classical examples of how disturbance can influence community diversity and composition. As the occurrence and intensity of flooding are predicted to change dramatically as a result of climate change, predicting the consequences of such changes has become a major task for community ecology. Here we present abundance data of five species that comprise a species-poor community of high conservation value at lakeshores of Lake Constance over 17 years, during which one of the longest flood periods and the lowest water levels since 1890 occurred. We used simple regression models and increasingly sophisticated Markov chain models plus non-linear parameter estimation to put down abundance changes to direct effects of flooding on population-dynamic parameters and to indirect effects of flooding through modification of interspecific competition. We found a negative effect of flood duration on abundance changes for the non-specialist species Agrostis stolonifera and Phalaris arundinacea, but no effect on Carex acuta. The specialist species, Ranunculus reptans but not Littorella uniflora showed a positive effect of flooding. Data analysis revealed an unambiguous competitive hierarchy with the two graminoid species (C. acuta, P. arundinacea) being superior, and the habitat specialists being most sensitive to interspecific competition. We used estimated parameters to project the community dynamics under different flooding regimes. Long-term projection showed that the original community is threatened by two non-specialist species (C. acuta and P. arundinacea). Even if this forecast was influenced by various model limitations, it may indicate irreversible changes in soil fertility during the phase of high eutrophication between 1950 and 1980. Our study demonstrated that long-term abundance relevés combined with Markov modelling and predictive simulations are an important counterpart to detailed short-term studies. The combination of empirical and theoretical methods elucidates the interaction of biotic and abiotic factors in community change.     

Trophic specialization and morphological divergence between two sympatric species in Lake Catemaco,Mexico

The association of morphological divergence with ecological segregation among closely related species could be considered as a signal of divergent selection in ecological speciation processes. Environmental signals such as diet can trigger phenotypic evolution, making polymorphic species valuable systems for studying the evolution of trophic‐related traits. The main goal of this study was to analyze the association between morphological differences in trophic‐related traits and ecological divergence in two sympatric species, Astyanax aeneus and A. caballeroi, inhabiting Lake Catemaco, Mexico. The trophic differences of a total of 70 individuals (35 A. aeneus and 35 A. caballeroi) were examined using stable isotopes and gut content analysis; a subset of the sample was used to characterize six trophic and six ecomorphological variables. In our results, we recovered significant differences between both species in the values of stable isotopes, with higher values of δ¹⁵N for A. caballeroi than for A. aeneus. Gut content results were consistent with the stable isotope data, with a higher proportion of invertebrates in A. caballeroi (a consumption of invertebrates ten times higher than that of A. aeneus, which in turn consumed three times more vegetal material than A. caballeroi). Finally, we found significant relationship between ecomorphology and stable isotopes (r = .24, p < .01), hence, head length, preorbital length, eye diameter, and δ¹⁵N were all positively correlated; these characteristics correspond to A. caballeroi. While longer gut and gill rakers, deeper bodies, and vegetal material consumption were positively correlated and corresponded to A. aeneus. Our results are consistent with the hypothesis that morphological divergence in trophic‐related traits could be associated with niche partitioning, allowing the coexistence of closely related species and reducing interspecific competition.     

Fitness optimization of Daphnia in a trade-off between food and temperature

In thermally stratified lakes with a deep chlorophyll maximum (DCM), Daphnia face a trade-off between food availability and optimum development temperatures. We hypothesize that Daphnia optimize their fitness by allocating the time spent in the different vertical habitats depending on the distribution of algal resources and the temperature gradient. We used the plankton towers (large indoor mesocosms) to study the vertical distribution of a population of Daphnia hyalina×galeata in three different temperature gradients with a DCM. Additionally, we determined the fitness of Daphnia in the epilimnion and hypolimnion by transferring water from these layers into flow-through systems where we raised Daphnia and assessed their juvenile growth rate as a measure of fitness. The fitness distribution was correlated with the vertical distribution. The vertical distribution most likely reflected the proportions of time Daphnia allocated to dwelling in the two vertical habitats.     

Contrasting energy allocation strategies of two sympatric Merluccius species in an upwelling system

This study investigated the somatic growth and energy allocation strategy of two sympatric hake species (Merluccius polli and Merluccius senegalensis), coexisting under the strong influence of the Mauritanian upwelling. The results revealed that ontogeny, bathymetry, geography and reproduction shaped the differences found between the condition dynamics of the two species. Aside from species‐specific differences, individuals were observed in better condition in the northernmost area (more influenced by the permanent upwelling) and in the deepest waters, probably the most favourable habitat for Merluccius spp. Both species also displayed contrasting trade‐offs in energy allocation probably due to the dissimilarity of their habitats, which favours the existence of divergent adaptive strategies in response to different ontogenic requirements. It was hypothesized that M. polli invests in mass and energy reserves while sacrificing growth, as larger sizes may not provide an ecological advantage in a deeper and more stable environment. Moreover, M. senegalensis capitalizes on a steady growth without major disruptions, enabling earlier spawning at the expense of a lower somatic mass, which is fitting to a less stable shallower environment. This study sheds new light on differences in the biological traits and life strategies of Merluccius spp., which permit their overlap in a complex upwelling system and may contribute to the long‐lasting scientific‐based management of these species.     

大型溞引导的沉水植被生态修复对滴水湖水质的净化效果

2007年4月-2008年1月,在滴水湖D港中段长950m(水量为10000m3)的修复区内投放大型溞(Daphnia magna),以滤除水华藻类等颗粒有机物,然后移栽伊乐藻、苦草、轮叶黒藻、光叶眼子菜和菹草等沉水植物,逐月监测水体中总氮(TN)、铵态氮(NH4+-N)、硝态氮(NO3--N)、亚硝态氮(NO2--N)、总磷(TP)、活性磷酸盐(PO43--P)和COD等水质指标,分析沉水植被栽培对滴水湖水体水质的净化效果.结果表明:试验期间,修复区水体TN、TP、NO3--N、NO2--N、NH4+-N、PO43--P和COD显著低于对照区(P0.01),溶解氧(DO)增加了50.4%,水体透明度(SD)平均在3.4~3.7m,水质达到国家Ⅱ~Ⅲ类地表水水质标准;2008年3月应用已构建的沉水植被群落对富营养化流水水体水质进行净化试验,7d后修复区流水水体除BOD外,TN、TP、NO3--N、NO2--N、NH4+-N、PO43--P和COD均显著降低(P0.01),DO增加了17.98%,SD提高了30cm.利用大型溞控藻后移栽沉水植物对滴水湖水体水质的净化效果十分显著.     

Seasonality and Fecundity of Daphnia Lumholtzi Sars in Lake Phewa,Nepal

Monthly sampling of zooplankton in Lake Phewa, Nepal revealed 18 species of zooplankton excluding protozoans. The seasonality and fecundity of the largest common cladoceran Daphnia lumholtzi Sars was studied. Two distinct peaks of abundance were noted. No active stages were found during May and June. Egg ratio varied from zero to 0.88 in mature females. From the presence of only helmeted individuals of this species it is concluded that the predation pressure by fish was high.     

Population structure in two sympatric species of the Lake Tanganyika cichlid tribe Eretmodini: evidence for introgression

Patterns of genetic differentiation were analysed and compared in two sympatric species of the endemic Lake Tanganyika cichlid tribe Eretmodini by means of mitochondrial DNA (mtDNA) sequences of the control region and six microsatellite DNA loci. The sample area covers a total of 138 km of mostly uninterrupted rocky shoreline in the Democratic Republic of Congo and includes the entire distribution range of Tanganicodus cf. irsacae that stretches over a distance of 35 km. Both markers detected significant genetic differentiation within and between the two species. T. cf. irsacae contained lower overall genetic variation than Eretmoduscyanostictus, possibly due to its more restricted range of distribution and its smaller effective population sizes. Complete fixation of Tanganicodus mtDNA haplotypes was observed in Eretmodus at two localities, while at two other localities some Tanganicodus individuals possessed Eretmodus mtDNA haplotypes. Taking into account the relatively large average sequence divergence of 6.2% between the two species, as well as the geographical distribution of mtDNA haplotypes in the lake, the observed pattern is more likely to be a consequence of asymmetric introgression than of shared ancestral polymorphism. As there is significant population differentiation between sympatric Tanganicodus and Eretmodus populations, the events of introgressions may have happened after secondary contact, but our data provide no evidence for ongoing gene flow and suggest that both species are reproductively isolated at present time.     

Lowered female reproductive effort as an indicator for increased male production and sexuality in Daphnia (Crustacea: Cladocera)

Summary

The relationship between environmental factors, sex ratio, mode of reproduction, and female reproductive effort was examined in the cyclically parthenogenetic Daphnia magna. Female reproductive effort was found to decline before a significant production of males and sexual eggs takes place. Unlike factors such as crowding, poor nourishment, low (sometimes high) temperature and short or long day length, lowered reproductive effort is not a cue but rather an indicator of the selective advantage that would accrue from a changed sex ratio and mode of reproduction. The mechanism of the complex system of environmental sex determination in Daphnia is reviewed, and the adaptivity of ESD is discussed.