Daphnia Population Dynamics Research Articles

Thermal dependence of Daphnia life history reveals asymmetries between key vital rates

Temperature variation affects virtually every aspect of ectotherms' ecological performance, such as their foraging rate, reproduction, and survival. Although these changes influence what happens at higher levels of organizations, such as populations and communities, qualitative changes in dynamics usually require some degree of asymmetry between key vital rates, i.e. that different vital rates, such as growth, development, fecundity and mortality rates, respond differently to temperature. In order to identify possible asymmetries among vital rates and/or life stages, we characterized the thermal response of individuals a clone of Daphnia sinensis, drawn from a high-mountain environment in Taiwan, and examined the temperature dependence of growth, maturation, reproduction, and mortality rates, as well as fitness measures (r and R0) at eight temperatures. Daphnia sinensis was able to maintain reproductive success over a broad range of temperatures, much wider than the one experienced in its environment. However, negative effects of temperature were perceptible at temperatures much lower than the highest one at which they can achieve reproductive success. Adult mortality greatly increased for temperatures above 23 °C, and other vital rates started to decelerate, resulting in a large drop in lifetime reproductive success. This finding implies that D. sinensis may be able to persist over a wide range of temperatures, but also that it may become more sensitive to the detrimental effect of species interactions at increased temperatures. Different vital rates responded relatively similarly at low temperatures, but the degree of asymmetry among these rates was much more pronounced at higher temperatures. In particular, rates associated with adult performance decelerated more strongly than juveniles’ rates. These findings indicate that elevated temperatures affect the balance between juvenile and adult performance, which is known to have a crucial role in Daphnia population dynamics. We discuss the implications of these results for the dynamics of structured populations.

Investigating the effect of pesticides on Daphnia population dynamics by inferring structure and parameters of a stochastic model

Identifying sublethal pesticide effects on aquatic organisms is a challenge for environmental risk assessment. Long-term population experiments can help assessing chronic toxicity. However, population experiments are subject to stochasticity (demographic, environmental, and genetic). Therefore, identifying sublethal chronic effects from “noisy” data can be difficult. Model-based analysis can support this process.We use stochastic, age-structured population models applied to data from long-term population experiments with Daphnia galeata in 1L aquaria with and without chronic pesticide treatments (diazinon and diuron) at sublethal concentrations. Posterior analysis following Bayesian inference of model parameters and states helped choosing an adequate description of life-history characteristics under the specific experimental conditions (a zero-inflated negative binomial distribution for reproduction and mortality without density dependence). For the insecticide treatments, the inferred marginal posterior parameter distributions indicated the need for a mortality rate that increases with time, indicating cumulative chronic toxic effects of diazinon on Daphnia populations. With this study, we demonstrate how stochastic models can be used to infer mechanisms from population data to help identifying sublethal pesticide effects.

Reconstruction of 100-year dynamics in Daphnia spawning activity revealed by sedimentary DNA

Environmental DNA (eDNA) is currently developing as a powerful tool for assessing aquatic species dynamics. However, its utility as an assessment tool for quantification remain under debate as the sources of eDNA for different species is not always known. Therefore, accumulating information about eDNA sources from different species is urgently required. The objective of our study was to evaluate whether sedimentary DNA targeting two Daphnia species, D. galeata and D. pulicaria, could track Daphnia population dynamics and resting egg production. Applying a quantitative PCR targeting the mitochondrial 12S rRNA gene on sediment cores collected in Lake Biwa, Japan, we compared sedimentary DNA concentration of Daphnia with the abundance of remains and ephippia, reflecting their abundance and resting egg production, respectively. We found that the sedimentary DNA concentrations of Daphnia for the past century were inconsistent with their population abundance. However, the concentration was highly correlated with the resting egg production. Our results provide evidence that ephippia with resting eggs, released during spawning activities, was a significant source of Daphnia DNA archived in sediments. Our work provides critical insights for using sedimentary DNA as a monitoring tool for egg production dating back 100 years.

The thermal regime modifies the response of aquatic keystone species Daphnia to microplastics: Evidence from population fitness, accumulation, histopathological analysis and candidate gene expression

The water bodies are greatly influenced by chemical contamination and global increasing temperature. As an emerging pollutant, microplastics are widely distributed in the freshwater environment, raising concerns regarding their potential toxicity to organisms. Especially for zooplankton filter feeders, many of microplastics are in similar size as their food. Individually, both microplastics and temperature have profound effects on zooplankton populations and their function in ecosystems. However, the strength and direction of their interactive effects are still not clear. Here, we performed a comprehensive biotoxicity assessment providing empirical evidence that the temperature played a key role in shaping the sensitivity of the zooplankter, Daphnia magna, against microplastic toxicity. We found that exposure to microplastics generally caused negative effects on Daphnia individual fitness, such as increased lethality, declined fecundity and reduced population growth rate. This microplastic toxicity was more prominent at 30 °C than at 20 °C, and was rather minor at 15 °C. Moreover, the warming accelerated the ingestion of microplastics, and triggered abnormal ultrastructure of intestinal epithelial cells. In addition, the expression profiling of candidate genes revealed oxidative damage, fecundity impairment and energy retardation by microplastics were amplified with increasing temperature, which may contribute to the enhancement of microplastic toxicity under warming. Given that high temperature fluctuations are becoming more common and difficult to predict, the interactive effects of microplastics and climate warming on Daphnia population dynamics and biomass production may become increasingly aggravated in nature. Collectively, extrapolation for environmental risk assessment studies conducted under different temperature contexts may broaden our knowledge microplastic toxicity on aquatic organism fitness.

Daphnia mitsukuri traits responding to predation cues alter its population dynamics

Abstract Individual traits responding to predation cues are widespread in many aquatic and terrestrial organisms. However, linking individual traits to population dynamics in response to predation cues is poorly understood despite its important role in shaping ecosystems. In this study, we investigated the individual traits associated with population dynamics of Daphnia mitsukuri in response to two different cues from fish predation. Two clones of D. mitsukuri were reared under alarm signal, kairomone, and their combination treatment until the populations reached maximum abundances. Individual inducible traits, grazing rates, and population parameters were recorded for the growing populations. Our data showed the population dynamics were highly correlated with the individual traits, and some mechanical correlations were discussed in the present study. Specifically, in the combination treatment, the decreased individual size contributed to enhancing the individual grazing rate at a high population density, which ultimately resulted in a higher (75%) maximum population density compared to the control. Besides, diversified links between individual traits and population dynamics were observed in different clones, especially in dormant egg production. In summary, inducible changes in individual morphological characteristics and life history traits were associated with variation in population dynamics of Daphnia in response to cues from predation. This study is helpful to understand the effects of signals of predation risk on the individual and population levels.

The use of an individual-based model to clarify the role of biological information in daphnia population dynamics.

The use of an individual-based model to clarify the role of biological information in daphnia population dynamics.

Responses of algae, bacteria, Daphnia and natural parasite fauna of Daphnia to nutrient enrichment in mesocosms

Understanding responses of parasites to changes in nutrient regimes is necessary for prediction of their role in aquatic ecosystems under global change in nutrient loading. We studied the response of the natural parasite fauna of Daphnia longispina to nutrient enrichment in mesocosms in a small humic lake. We measured the concentrations of inorganic phosphorus and nitrogen in the water, total nutrients in the seston, algal and bacterial biomass, Daphnia population dynamics, Daphnia stoichiometry, Daphnia stable isotope values and the presence and abundance of parasites in treated mesocosms as compared to three control ones. Incorporation of the nutrient enrichment in the food web was seen as increased nutrient concentrations in the epilimnion and as a decrease in carbon:nutrient ratios and δ15N values in Daphnia. Nutrient enrichment did not significantly influence algal, bacterial or Daphnia biomass. One of the four parasite species observed, unidentified small gut parasite, had a higher prevalence (percentage of Daphnia infected) in treated mesocosms, but its intensity (number of parasites per infected host) remained the same among treatments. Our results suggest that the effect of nutrient enrichment on host–parasite dynamics is dependent on complex interactions within food webs and on the epidemiological traits of parasites.

Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP ≅ 0.5 μM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day−1) was higher than for defended Oocystis (0.62 ± 0.03 day−1), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.

Mysis in the Okanagan Lake food web: a time-series analysis of interaction strengths in an invaded plankton community

Mysis introductions to the lakes of western North America have shown they are important predators on zooplankton, especially daphnids, and intercept energy flows that would otherwise be available to pelagic fishes. However, understanding of the ecological roles of Mysis within invaded communities following their establishment remains weak. We analyzed zooplankton and phytoplankton data collected from Okanagan Lake, British Columbia, within a time-series framework to evaluate the strength of ecological interactions between Mysis and the other dominant plankton. Top-down effects of Mysis in the plankton community were only detected on cyclopoid copepods and cyanophytes. Mysis dynamics were mostly driven by bottom-up effects from diatoms and from small cladocerans whose dynamics were driven primarily by the abundance of edible phytoplankton. This result supports the growing appreciation of the importance of omnivory in mysids and was consistent between the two main basins of the lake. We also analyzed published stable C and N isotope data from the plankton of Okanagan Lake with an isotope mixing model to estimate the relative importance of various potential energy sources to Mysis. This analysis supported the time-series results suggesting the importance of diatoms and small zooplankton to Mysis. However, the isotopes also suggested important resource flows from Daphnia to Mysis, an interaction not detected in the time-series analysis. Taken together, these results suggest that Mysis is a strong interactor in the Okanagan Lake food web, relying in part on energy flow through Daphnia. However, subsidies from diatoms likely decouple seasonal Mysis population dynamics from the seasonal population dynamics of Daphnia.

The effects of global warming on Daphnia spp. population dynamics: a review

Various species of Daphnia usually play a key role in the food web of temperate freshwater systems. There is much evidence to show that climate change may influence Daphnia population dynamics, consequently altering both predator–prey interactions and the efficiency of algal biomass control in these ecosystems. This review will analyse and discuss the current knowledge on Daphnia responses to climate warming based on an analysis of selected papers. The presented results indicate that warming may have important direct and indirect effects on Daphnia biology and ecology via its influence on their life-history processes (metabolism, growth, reproduction) and the properties of their habitats. The plasticity of daphnids in terms of adaptive responses is generally high and includes phenotypic adaptations and changes in genotypes, although it also depends upon the strength of selection and the available genetic variation. The seasonal timing and magnitude of temperature increases are important for seasonal biomass fluctuations of Daphnia and similarly influence the potential synchrony of daphnids and phytoplankton succession (the timing hypothesis). In light of the most recent studies on this topic, even a minor warming during short but critical seasonal periods can cause factors that disturb Daphnia population dynamics to coincide, which may destabilize lake food webs by decoupling trophic interactions. Both winter and spring are important critical periods for determining future seasonal fluxes of Daphnia spp. and, consequently, the time of the clear-water phase and the occurrence and duration of Daphnia midsummer decline. Winter conditions may also affect the impact of fish predation on daphnids during summer months. However, the effects of global warming on Daphnia population dynamics and on ecosystem functioning are often difficult to predict due to their complexity and the presence of both antagonistic and synergistic drivers. Thus, the diverse responses of daphnids to climate anomalies depend on both biotic (predator abundance and seasonal phytoplankton succession) and abiotic factors (e.g. hydrodynamics, intensity and duration of thermal stratification, trophic state or geomorphology) of lakes, which are directly influenced by weather changes. The analysis and quantification of such complex interactions require the involvement of different kinds of specialists and the development of accurate research approaches, such as molecular genetic methods or mathematical modelling.

Mathematical modelling as a tool for predicting the intensity of eutrophication symptoms based on zooplankton and fish density

In temperate zones, algae biomass may be efficiently controlled by Daphnia sp., which is the most significant grazer among filter-feeding zooplankton. Daphnia population dynamics depends mostly on the trophic state of the ecosystem, which determines both fish community structure and food quality and availability. Relatively invariable water inflow to a reservoir with small amplitudes supports balanced biotic interactions. Here, we present simulations using the DALIS model that well characterised behaviour in a temperate ecosystem under stable abiotic conditions. The model results allowed us to predict the extent to which predation by fish would disturb the dynamics of Daphnia under different food conditions. Therefore, mathematical modelling may be an important tool for establishing appropriate recovery strategies in eutrophic water bodies. Although the model in this study characterised temperate zone conditions, its application to tropical lakes, where the dynamics of zooplankton and fish are substantially different, is discussed.

Population dynamics of Chaoborus flavicans and Daphnia spp.: effects on a zooplankton community in a volcanic eutrophic lake with naturally high metal concentrations (L. Monticchio Grande, Southern Italy)

The response of Daphnia populations to invertebrate predators involves morphological or behavioural changes. Few studies suggest that contaminant aqueous metals, like Cu or Ni at environmentally relevant concentrations, interfere with invertebrate chemical communication systems, such as that which operates between Daphnia and Chaoborus. The objective of our study was to determine if this interference could be also observed in lakes naturally rich in dissolved metals, such as volcanic lake (Lago Grande di Monticchio). This study aimed to assess if natural dissolved metals (e.g., Fe, Mn and Sr) could impair the ability of Daphnia pulex and D. galeata × hyalina × cucullata 'complex' populations to respond to Chaoborus kairomones by producing morphological defenses against potential predation, and to understand how Chaoborus predation might affect zooplankton community composition and overall zooplankton density. The predator impact did not result in: i) any morphological changes; ii) any apparent shift in body size pattern of the prey population; iii) any shift in life history traits. Chaoborus accounted for high mortality rates in Cladocera and strongly reduced the chance of individuals to reach maturity. Moreover, highly significant negative correlations between abundance of dominant taxa of zooplankton and C. flavicans were found. The last larval instars of C. flavicans seem to reduce the number of crustaceans, particularly cladocerans and copepod adults and could play an important role in structuring zooplankton communities. Our results suggest that metal inhibition of defence strategies induction probably occurs along the signal transduction pathway in Lake Grande di Monticchio. Impairment of chemosensory response to predatory chemical cues may have widespread ecological consequences in aquatic systems. Chaoborus predation effects can greatly affect both zooplankton biomass and community composition, impact interactions at lower trophic levels and generate an ecological cascade leading to a rapid eutrophication.

Influence of Allelochemicals from Submergent Plants for Population Dynamics of Daphnia

大型水槽（容量1トン）に沈水植物19種を種別に1年間栽培した。各沈水植物が代謝した他感作（アレロパシー）物質が包含された水槽表層水（沈水植物19種および対照系1）の濾液を小型水槽に湛水し、ミジンコの休眠卵からの飼育試験を実施した。各系ミジンコ休眠卵100個からの孵化率（％）は、タヌキモ2％、ホザキノフサモ3％であり、無植栽の対照系4％よりも低かった。一方、ガシャモク85％、ヒロハノエビモ73％、インバモ62％、センニンモ58％などは著しく高い孵化率が達成された。これらの相違は、水生植物の代謝した物質にミジンコ休眠卵が反応し、ミジンコの孵化が促進・阻害されたものと考えられる。孵化後のミジンコ個体数の推移に着目すると、ホザキノフサモ、リュウノヒゲモ、ヤナギモ、タヌキモ、オオトリゲモの5種では、実験開始7週目までにミジンコが全て死滅。孵化率の高かったガシャモク、ヒロハノエビモ、インバモ、センニンモでは400個体/l－1以上のミジンコ生息密度が維持された。次いでセキショウモ、コウガイモ、エビモ、ササバモの4種において100個体/l－1程度の高いミジンコ生息密度が確認された。対照系（植栽無し）におけるミジンコ個体密度30～40個体/l－1よりも明らかに低い密度を示したのが、シャジクモ、ヒメフラスコモの2種であった。対照系と同等のミジンコ密度だったのはハゴロモモ、ツツイトモ、イトモ、クロモの4種であった。これらの事実から、自然水域におけるミジンコの生息個体密度には、水質・底質・水流などのさまざまな環境要因の他にも、同地に繁茂する沈水植物が直接影響を及ぼしうることが明らかとなった。

Daphnia body size and population dynamics under predation by invertebrate and fish predators in Lago Maggiore: an approach based on contribution analysis

Predation on Daphnia is size-specific: while zooplanktivorous fish select large, ovigerous females, carrying larger clutches, predation by invertebrates, particularly Cladocera, is generally regarded as acting mainly on young, small, non-ovigerous females. The two types of predators therefore produce different effects on the population of their prey: fish predation results in decreases in Daphnia fecundity, proportion of adults, and maximum body size, while predation by the invertebrate predators leads to decreases in the number and proportion of juvenile Daphnia. We investigated the effect of predation by three predators (one vertebrate, two invertebrates) on Daphnia. The study combines contribution analysis of Daphnia birth rate dynamics, with a body-size-oriented analysis of both predator and prey populations. Contribution analysis showed that during April-May, when Bythotrephes densities were low, changes in Daphnia birth rate were due to both changes in fecundity (ConF) and in the proportion of adults (ConA), with ConA being much larger (by absolute value) than ConF; such a pattern is expected under fish predation. Whereas during mid May to June, when Bythotrephes densities were high, changes in Daphnia birth rate were again mainly due to both ConF and ConA, but now the difference between these contributions was less pronounced. Although Bythotrephes predation produced similar effects on Daphnia population dynamics as fish predation, affecting both fecundity and the proportion of adults, it can be distinguished in two ways from fish predation. Firstly, the ConF-ConA pattern is more uniform under invertebrate predation than under fish predation and secondly, the mean body size of Daphnia increased continuously whereas the mean size of ovigerous females remained approximately constant except for the last 3 weeks of June, indicating a high mortality of juveniles during the whole period and an increased mortality of young females with eggs during the last 3 weeks. We conclude that both zooplanktivorous fish and invertebrate predators affect population dynamics of Daphnia in Lago Maggiore. However, the invertebrate predator Bythotrephes has the largest impact on Daphnia dynamics in June, both because of its high densities and its relatively large size at that time. Predation pressure exerted by Bythotrephes is high enough to reduce the Daphnia densities at the end of June to low values.

Use of a mathematical model to study the role of maternal effects in population dynamics and diapause formation in Daphnia

An individual-based model of a population of Daphnia longispina, a common species of zooplankton of lakes and temporary water-bodies of the temperate zone of the Palearctic is described. The concept of this model is based upon an individual's potential for growth and reproduction and for the change from parthenogenesis to gamogenesis, which was controlled by the life conditions of three consecutive generations of Daphnia. This was achieved by simulating the effects of temperature, food conditions and photoperiod experienced by grandmothers and mothers on an individual's potential for growth and reproduction. The model was used to test hypotheses on the significance of maternal transmission of information in population dynamics of Daphnia. The results supported several important conclusions, e.g. the corroboration of the importance of maternal effects for seasonal adaptations of crustaceans. The possibility of accumulation of maternal effects in successive generations apparently increases the resistance of the population to inter-annual fluctuations of environmental parameters. The model also confirms that maternal effects along with intra-population polymorphism are a mechanism providing stability of biological systems on the species (population) level.

Complementary population dynamics of exotic and native Daphnia in North American reservoir communities

During its invasion of North America, the exotic Daphnia lumholtzi has rap- idly colonized numerous reservoirs, natural lakes, and large rivers. In the current study, we examine the overlap between D. lumholtzi and native Daphnia species through analysis of two data sets: co-occurrence in reservoirs of the south-central United States and population dynamics in one reservoir. In 171 reservoirs and oxbow lakes, D. lumholtzi was among the most prevalent species and its distribution was independent of other Daphnia species. Over a 28-month period in Stockton Lake, Mis- souri, D. lumholtzi was abundant only in late summer, a period when the epilimnion was warm (25 - 30 uC) and cyanobacteria were common. Native Daphnia (D. mendo- tae, D. parvula, and D. retrocurva) complemented this pattern, being generally rare in summer and most abundant during winter and spring. Peak densities and average fecundities of native Daphnia during the cooler months were typically greater than densities and fecundities of D. lumholtzi in summer. The complementary population dynamics between native and exotic species may be significant for the food webs in warm reservoirs, with D. lumholtzi providing another food resource when the abun- dance of large zooplankton is ordinarily low.

Life history consequences of sterol availability in the aquatic keystone species Daphnia

The absence of essential biochemical nutrients, such as polyunsaturated fatty acids or sterols, has been considered as a mechanism determining trophic interactions between the herbivore Daphnia and its phytoplankton food source. Here, we experimentally quantify the sensitivity of two Daphnia species to decreasing amounts of dietary sterols by measuring variations in life history traits. The two species Daphnia magna and D. galeata were fed different mixtures of the sterol-containing green alga Scenedesmus obliquus and the sterol-free cyanobacterium Synechococcus elongatus; a higher proportion of Synechococcus in the food is equivalent to a decrease in dietary sterols. To address the significance of sterol limitation, the Daphnia species were also fed Synechococcus supplemented with cholesterol. In both species, somatic and population growth rates, maternal dry mass, the number of viable offspring, and the probability of survival were significantly reduced with the lower availability of sterols. A high correlation between the sterol content of the mixed diet and the somatic and population growth rates was found, and growth on cholesterol-supplemented Synechococcus fitted well into this correlation. Somatic growth of first-clutch neonates grown on 100% Synechococcus exhibited a pattern similar to that of somatic growth of their mothers grown on the different food regimes, which demonstrated the significance of maternal effects for sterol-limited population growth. Daphnia galeata had a twofold higher incipient limiting sterol level than D. magna, which indicated interspecific differences in sterol requirements between the two Daphnia species. The results suggest a strong impact of dietary sterols on life history traits and therefore, population dynamics of the keystone species Daphnia.

A model approach to evaluate the effect of temperature and food concentration on individual life-history and population dynamics of Daphnia

We present a model framework for the simulation of growth and reproduction of Daphnia at varying conditions of food concentration and temperature. The core of our framework consists of an individual level model that simulates allocation of assimilated carbon into somatic growth, maintenance costs, and reproduction on the basis of a closed carbon budget. A fixed percentage of assimilated carbon is allocated into somatic growth and maintenance costs. Special physiological adaptations in energy acquisition and usage allow realistic model performance even at very low food concentrations close to minimal food requirements. All model parameters are based on physiological measures taken from the literature. Model outputs were thoroughly validated on data from a life-table experiment with Daphnia galeata. For the first time, a successful model validation was performed at such low food concentrations. The escalator boxcar train (EBT) was used to integrate this individual level model into a stage-structured population model. In advance to previous applications of the EBT to Daphnia we included an additional clutch compartment into the model structure that accounts for the characteristic time delay between egg deposition and hatching in cladocerans. By linking two levels of biological organisation, this model approach represents a comprehensive framework for studying Daphnia both at laboratory conditions and in the field. We compared outputs of our stage-structured model with predictions by two other models having analogous parameterisation: (i) another individual level Daphnia model (Kooijman–Metz model) and (ii) a classical unstructured population model. In contrast to our Daphnia model, the Kooijman–Metz model lacks the structure to account for the optimisation of energy acquisition and maintenance requirements by individual daphnids. The unstructured population model showed different patterns of population dynamics that were not in concordance with typical patterns observed in the field. Thus, we conclude our model provides a comprehensive tool for the simulation of growth and reproduction of Daphnia and corresponding population dynamics.

Annual Patterns in Bacterioplankton Community Variability in a Humic Lake

Bacterioplankton community composition (BCC) was monitored in a shallow humic lake in northern Wisconsin, USA, over 3 years using automated ribosomal intergenic spacer analysis (ARISA). Comparison of ARISA profiles of bacterial communities over time indicated that BCC was highly variable on a seasonal and annual scale. Nonmetric multidimensional scaling (MDS) analysis indicated little similarity in BCC from year to year. Nevertheless, annual patterns in bacterioplankton community diversity were observed. Trends in bacterioplankton community diversity were correlated to annual patterns in community succession observed for phytoplankton and zooplankton populations, consistent with the notion that food web interactions affect bacterioplankton community structure in this humic lake. Bacterioplankton communities experience a dramatic drop in richness and abundance each year in early summer, concurrent with an increase in the abundance of both mixotrophic and heterotrophic flagellates. A second drop in richness, but not abundance, is observed each year in late summer, coinciding with an intense bloom of the nonphagotrophic dinoflagellate Peridinium limbatum. A relationship between bacterial community composition, size, and abundance and the population dynamics of Daphnia was also observed. The noted synchrony between these major population and species shifts suggests that linkages across trophic levels play a role in determining the annual time course of events for the microbial and metazoan components of the plankton.

Maternal Effects of Photoperiod and Food Level on Life History Characteristics of the Cladoceran Daphnia Pulicaria Forbes

The response of various life-history characteristics of Daphnia pulicaria to photoperiod and food concentration was measured in 16 combinations of maternal and offspring environments (long vs. short day, high vs. low food) in flow-through experiments. Response variables in offspring were time and survival to release of first offspring, clutch size and neonate mass in the first brood, mass of adult females after 30days and somatic growth rate during the course of the experiment. Most of these parameters were directly controlled by food concentration in the offspring environment, but maternal effects frequently modified the response. A long day length in the maternal environment resulted in a prolongation of the time to first clutch release in offspring similar to the direct effect of low food. Likewise, survival to maturation and female mass were affected by maternal photoperiod. Somatic growth rate and clutch size responded to combined effects of maternal food conditions and photoperiod. The laboratory results were used to predict the seasonal change of fecundity of Daphnia in the field. When data on clutch size are ordered in a sequence as the different combinations of maternal and offspring environment occur during the seasonal succession in a temperate lake, they show a bimodal distribution with a high peak in spring and a lower peak in fall. This pattern is consistent with field observations. We conclude that photoperiod and maternal effects are important factors influencing life history and population dynamics of Daphnia.