Resource limitation and food quality for cladocerans in a tropical Brazilian lake

Food quality of the natural food (seston) from Lake Monte Alegre was evaluated throughout a series of life-table experiments with cladocerans from the same lake carried out in summer and autumn. Experiments were performed using cohorts of newborns originated from ovigerous females cultured for several generations in the lab or taken directly from the lake. For these tests of food limitation, juveniles of different species were submitted to one of the following treatments: (1) lake seston and (2) lake seston + green algae. The age at first reproduction, mean clutch size, total offspring and the intrinsic rate of natural increase (r) were estimated. Cladocerans responded to both seasonal changes in food resources and to additions of green algae. In summer, food levels were higher and cladocerans grew better than in autumn. The low fecundity and population growth rates of Daphnia gessneri, together with high C:P ratios (>500) in the seston in summer, suggested P limitation. The other cladocerans (Moina micrura, Ceriodaphnia cornuta and Simocephalus mixtus) were apparently less sensitive than D. gessneri to P-limitation in summer. However, energy limitation cannot be disregarded even in summer, although in a lower degree compared to autumn. In this season, the predominance of large diatoms and dinophyceans of low nutritional value and the low food concentration suggested both quantitative and qualitative food limitation. Phytoplankton composition, nutritional value and particle size appeared to be important factors in determining cladocerans reproduction rates with seston diets.     

Seasonal patterns of food limitation in Daphnia galeata: separating food quantity and food quality effects

Food limitation for the cladoceran Daphnia galeta was measuredgrowing them with natural lake seston for one growing seasonunder standardized conditions in the laboratory. Growth rateswere related to several measures of food quantity. Particulateorganic carbon (POC) <30 µm was the best predictorof total food availability measured. It was better than chlorophyll-a(Chl-a) of the same size fraction, algal volume calculated fromcell counts or total particle volume from Coulter Counter measurements.Daphnids were also grown with Scenedesmus acutus under the sameconditions and their growth rates were compared. Sestonic carbonconcentrations were related to Scenedesmus carbon concentrations,that provided equal growth rates. The ratio of the carbon concentrationsof both food types (seston, Scenedesmus) represented the foodquality of seston. Thus, the nutritional quality of naturallake seston relative to a standard alga was determined overone growing season. Growth rates of seston fed animals revealedthat they were food limited during long periods of the year.This was also ascertained by standard clutch sizes of fieldanimals. However, the causes of food limitation varied withinthe duration of the studied period. Whereas during the clear-waterphase the food quantity was limiting (while the quality washigh: 100% Scenedesmus), shortly after on 10 June and 17 Junefood conditions were close to threshold concentrations due tolow food quality. For the remainder of the summer, a POC of0.3–0.6 mg C I^–1 with a quality of     

Growth Responses of Tropical Cladocerans to Seston from Lake Monte Alegre (Brazil) Supplemented with Phosphorus, Fatty Acids, a Green Algae and a Cyanobacterium

The cladocerans Ceriodaphnia richardi, Daphnia ambigua, D. gessneri and Moina micrura were used to access food quality of Lake Monte Alegre’s seston. Experiments were carried out in summer and autumn as growth assays with lake seston only (control) and seston supplemented with phosphate, fatty acids or Synechococcus, and Scenedesmus. In summer, high C:P ratios in seston suggested strong phosphorus limitation, however, contrary to the expectations of stoichiometric theory, the addition of phosphate to seston did not improve cladoceran growth. Addition of PUFA increased growth rates and clutch size of D. gessneri, suggesting a possible deficiency in essential fatty acids in summer. Addition of Scenedesmus increased significantly growth rates of the cladocerans D. gessneri and C. cornuta, suggesting energy limitation in summer. In autumn, C:P ratios were lower than in summer, but still above the threshold ratio for Daphnia. At this time, addition of phosphate increased significantly growth rates of Daphnia suggesting strong P limitation, especially in D. gessneri. However, energy limitation was still important in autumn, as suggested by a further increase in growth rates in +Syn and +Sce treatments. Energy limitation was especially strong for Moina micrura, which is a fast-growing species, with high P content. Algal digestion resistance is a plausible hypothesis for energy limitation, since carbon concentrations in both seasons were above incipient limiting levels. These results show that the seston C:P ratio was not a consistent predictor of cladoceran P limitation and that factors other than P and energy limitation seem to be also important, such as PUFA or other biochemical factors. An erratum to this article is available at .     

Seasonal changes in the biochemistry of lake seston

1. The quantity of seston was measured and the elemental carbon, nitrogen and phosphorus (C, N, P) and biochemical composition (carbohydrate, protein, lipid) of the < 53 μm size fraction in three temperate lakes during one year was analysed. The lakes differed in nutrient concentration and were characterized as oligotrophic, mesotrophic and eutrophic. Linear regression analyses defined associations between seston composition and either lake trophic status, depth or season. 2. The concentration of particulate organic seston was greatest during spring and autumn and lowest during the clear water period in early summer. Seasonal patterns in seston elemental and biochemical percentage composition (quality) were observed to be independent of differences in seston quantity. 3. Concentrations of seston C, N and P were high in most cases in the spring and autumn and low in summer. Concentrations of P were particularly high during late summer and early autumn in the metalimnion, perhaps because of recovery of P from anaerobic sediments and hypolimnetic waters. Because seston C and N did not increase as markedly as P, C : P and N : P ratios both declined in the autumn. Primary production was thought to be co-limited by N and P in all three of these lakes; however, the data suggested that N might be more important as a major limiting nutrient in the eutrophic lake as the metalimnion increased in depth in late summer and autumn. 4. Concentrations of protein, carbohydrate, polar lipid and triglyceride generally increased with lake type as expected (greatest in the eutrophic lake), but showed no relationship with water depth. As the year progressed, no significant changes were measured in protein and carbohydrate concentrations; however, the concentration of polar lipid decreased and triglyceride increased significantly with time of year. 5. The biochemical composition of seston varied during the year and among lakes; for example, in Lake Waynewood the proportion of protein composing the seston (percentage protein by weight) varied from < 10% to > 40%. No statistically significant patterns in the percentage protein or carbohydrate were found. However, the proportion of seston comprised of triglyceride decreased with lake type and increased during the year; whereas the proportion of seston as polar lipid increased with lake type and decreased during the year. Triglyceride comprised most of the lipid. Both protein : lipid and protein : carbohydrate ratios tended to be greatest in summer and lowest in the spring and autumn. 6. Relationships between samples and biochemical composition analysed by Canonical Correspondence Analysis (Canoco) indicated similar patterns in seasonal changes in seston biochemistry for the three lakes, with samples separated primarily by vectors for lake type (oligotrophic to eutrophic) and the percentage polar lipid (proportion of total lipid) and secondarily by vectors for date and water depth (epilimnion or metalimnion). 7. These seasonal biochemical changes in the seston food base were compared with biochemical changes known to occur in algae grown under N-or P-limited conditions in the laboratory, and the resultant quality of this algal food for suspension-feeding consumers (zooplankton). It was concluded that zooplankton were likely to be physiologically challenged by these distinct seasonal shifts in the quality of lake seston.     

Effects of algal food quality on fecundity and population growth rates of Daphnia

1. Food quality was at least as important as food quantity for both fecundity and population growth responses of the cladoceran Daphnia pulicaria fed the green alga Ankistrodesmus falcatus grown under N limitation, P limitation, or non-limited condition.
2. The fecundity of D. pulicaria was reduced under conditions of low food quality (low N or low P) compared with that for animals fed control non-limited algae regardless of ration size. The reduced fecundity of D. pulicaria fed P-limited food could be partially alleviated by increasing the ration (hence, compensation), but such was not the case for animals fed N-limited food.
3. Population growth rates of D. pulicaria ( r _max) were significantly reduced under conditions of low-quality food for both N-limited and P-limited algae. Population growth rates were unaffected by ration size, indicating no compensation.     

Effects of essential fatty acids and N and P-limited algae on the growth rate of tropical cladocerans

SUMMARY 1. In this study, the effects of nutrient (N and P) deficiency and the importance of essential polyunsaturated fatty acids (PUFA) [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] to tropical cladocerans, growth and reproduction were determined in a growth bioassay.
2. The animals were fed N/P-sufficient, N-deficient and P-deficient algae, and also N and P-deficient algae supplemented with fish oil emulsions rich in EPA and DHA.
3. Cladocerans showed different responses to nutrient-deficient algae and also to supplements of fish oil emulsions. Moina micrura was most sensitive to P-deficient alga and, surprisingly, grew better and produced more eggs in N-deficient alga than in N/P sufficient alga. Ceriodaphnia cornuta was less sensitive, growing well in both N and P-deficient algae. This species, however, had a lower clutch size in N-deficient alga. On the other hand, Daphnia gessneri was the most sensitive to mineral limitation, showing decreased growth and clutch size in both nutrient-deficient algae.
4. The PUFA supplements to nutrient-deficient algae increased growth rates only for M. micrura and C. cornuta , suggesting that these fatty acids are important food requirements for these species.     

Influence of phytoplankton fractions on growth and reproduction of tropical cladocerans

The influence of two seston fractions, < 20 µm (nanoplankton) and 20 µm (microplankton), on growth and reproduction of cladoceran species with different sizes, from Lake Monte Alegre, was evaluated through individual growth and life table experiments. Size, shape and other features of the algae in the fractions were described. The procedure of filtering lake water through a 20 µm net for seston fractionation caused mutual contamination. The smallest cladoceran species, Ceriodaphnia cornuta Sars and Moina micrura Kurz, produced larger clutch sizes and exhibited higher intrinsic rates of population growth (r) in the nanoplankton, despite contamination of their food by inedible algae. The largest species, Simocephalus mixtus Sars, produced larger clutch sizes in the microplankton. There were no differences in juvenile biomass growth between treatments for M. micrura and Daphnia gessneri Herbst, but lower value of the exponential growth rate (g) in the microplankton was significant for M. micrura. Fecundity (eggs/total female) of M. micrura was significantly lower in the microplankton, while D. gessneri did not reproduce in this fraction, at the end of growth experiments. Spines, colonies, cenobium, filaments, hard cell wall, and gelatinous sheaths, represented constraints to cladoceran reproductive performance, besides algae size. Microplankton contamination by nanoplanktonic algae, in the experiments, probably minimized the negative effect of inedible algae. Nanoplankton was more suitable for the smallest species and microplankton for the largest one.     

Influence of phytoplankton fractions on growth and reproduction of tropical cladocerans

The influence of two seston fractions, < 20 μm (nanoplankton) and ≥ 20 μm (microplankton), on growth and reproduction of cladoceran species with different sizes, from Lake Monte Alegre, was evaluated through individual growth and life table experiments. Size, shape and other features of the algae in the fractions were described. The procedure of filtering lake water through a 20 μm net for seston fractionation caused mutual contamination. The smallest cladoceran species, Ceriodaphnia cornuta Sars and Moina micrura Kurz, produced larger clutch sizes and exhibited higher intrinsic rates of population growth (r) in the nanoplankton, despite contamination of their food by inedible algae. The largest species, Simocephalus mixtus Sars, produced larger clutch sizes in the microplankton. There were no differences in juvenile biomass growth between treatments for M. micrura and Daphnia gessneri Herbst, but lower value of the exponential growth rate (g) in the microplankton was significant for M. micrura. Fecundity (eggs/total female) of M. micrura was significantly lower in the microplankton, while D. gessneri did not reproduce in this fraction, at the end of growth experiments. Spines, colonies, cenobium, filaments, hard cell wall, and gelatinous sheaths, represented constraints to cladoceran reproductive performance, besides algae size. Microplankton contamination by nanoplanktonic algae, in the experiments, probably minimized the negative effect of inedible algae. Nanoplankton was more suitable for the smallest species and microplankton for the largest one.     

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso‐ to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed. 2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes. 3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non‐algal and detrital C to the POC pool was high. 4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition.     

Reproductive potential of Daphnia galeata in relation to food conditions: implications of a changing size-structure of the population

A field study was conducted in the highly eutrophic, biomanipulated Bautzen Reservoir to investigate the influences of food conditions on fecundity of Daphnia galeata during spring and early summer in four consecutive years. By defining a relation between body size and maximum brood size, the realised reproductive potential (RRP) of Daphnia was expressed as percentage of actual egg numbers of the population in relation to calculated maximum egg numbers. This parameter is independent of the size structure of the population. It was found to correlate generally with mean clutch size. However, deviations occurred if size at maturity (SAM) declined. Extremely low SAM values (0.8 mm) were recorded in 2 years, corresponding with increasing food concentrations. Low clutch sizes in these situations can be explained by small body sizes of ovigerous females, whereas RRP increased, thus reflecting good food conditions. From the comparison of different reproductive parameters, it is concluded that RRP is the most reliable one with regard to bottom-up influences in case of extremely variable size structure and possible predation impacts. Fecundity of D. galeata was clearly food limited during the clear-water stage. Relatively high POC-threshold concentrations of 0.26 mg C l^–1 suggest that food quality was low and may be equally important as food quantity in limiting egg production.     

Evidence of phosphorus-limited individual and population growth of Daphnia in a Canadian Shield lake

We performed bag experiments in a Canadian Shield lake with generally high seston (suspended food particles mainly composed of algae) carbon (C):phosphorus (P) ratios, and investigated the responses of individual and population growth of herbivorous Daphnia dentifera on their abundance with (+P) and without (−P) a phosphorus enrichment to lake water. In both treatments, increased abundance of D. dentifera reduced seston C concentration and was accompanied by decreases in population and individual growth rates. However, P-enrichment increased seston P concentration and then reduced seston C:P ratio from 400–700 to ca 100 (by atoms). As a result, both individual and population growth rates were significantly higher in the +P treatment at all animal abundances even though seston C concentrations were similar between the treatments. The magnitude of the growth enhancement by the P-enrichment was independent of animal abundance. Stepwise regression analyses revealed that 71 and 90% of the variance in the population and individual growth rates, respectively, were explained by seston C and P concentrations, and that the contribution of the seston P concentration was roughly the same as that of seston C. Such joint effects of seston C and P indicate that food quality (P content) as well as food quantity (C concentration) can influence Daphnia not only at the level of individual growth but also at the level of population dynamics in P-limited lakes. Our results thus strongly corroborate the hypothesis that the population development of a key herbivore Daphnia in P-limited Canadian Shield lakes is inhibited by the direct effects of P-limited food on individual growth, which weaken the strength of trophic cascading interactions starting from piscivorous fish through planktivorous fish and zooplankton to algae.     

Resource-mediated demographic variation during the midsummer succession of a cladoceran community

SUMMARY. 1. The midsummer succession of cladoceran zooplankton in Lake Texoma was examined by demographic characterization of lake populations and a parallel set of life table experiments in which the dominant cladoceran species were raised on naturally occurring lake phytoplankton at lake temperatures. Diaphanosoma leuchtenbergianum occurred throughout the study period (mid June-mid August 1981) but Daphnia parvula, D. galeata mendotae and Ceriodaphnia lacustris were rarely detected after 15 July.
2. Analyses of size structure and size-specific fecundity of the cladoceran populations showed that (1) size structure shifted to smaller individuals in mid July, (2) size-specific fecundity was relatively constant in the smaller size classes and was constant or declined in larger females of each species, and (3) size at maturation did not vary significantly during the study of any population.
3. Individual growth rates of the two Daphnia species and Ceriodaphnia in the life table experiments were significantly lower in mid July when populations were declining and when size structure of each population was more dominated by smaller Individuals. The reduction in individual growth rates was short-lived in the life table experiments; growth rates returned to pre-decline levels by the end of July.
4. Two effects of temporary reductions in individual growth rates when size-specific fecundity and size at maturation are constant are (1) that size structure is temporarily shifted to smaller individuals, and (2) the potential rate of population growth is reduced because of delays in reproduction. Because data on size structure and size-specific fecundity are commonly used to evaluate the relative effects of predators and resource changes on populations it is noteworthy that erroneous conclusions about the mechanism of population decline may be reached when individual growth rates vary.     

Influence of biochemical,mineral and morphological features of natural food on tropical cladocerans

Five laboratory experiments were performed to evaluate the effect of supplements of fatty acids and a green alga on the individual growth and reproduction of three species of tropical cladocerans Ceriodaphnia richardi, Daphnia ambigua, and D. gessneri feeding on natural seston from the Brazilian Lake Monte Alegre. Cohorts of newborns from cultivated females were submitted to one of the following treatments: (1) Natural seston, (2) Natural seston + microcapsules of EPA and DHA or linoleic and linolenic fatty acids, (3) Natural seston + oil-free microcapsules, and (4) Natural seston + green alga Scenedesmus spinosus (1 mg C l⁻¹). Particulate organic carbon, algal carbon, C:P ratios of seston and green alga, polyunsaturated fatty acids (PUFA) content of seston and cladocerans, as well as phytoplankton composition, size, and shape were measured. The addition of fatty acids to seston did not significantly enhance growth and reproduction of the cladocerans, suggesting that sestonic PUFA content is sufficient for promoting cladoceran development, even in the cool–dry season when the fatty acids used in the experiments were 5–10 times lower in the seston than in the warm–wet season. Despite high C:P molar ratios in most experiments, there was only one indication of growth limitation by P. Reproduction was more affected than individual growth on some occasions by food quantity (energy) caused apparently by algal size, morphology, and digestion resistance. Energy availability, which is affected by algae morphological characteristics, seems to prevail over PUFA and P in controlling growth and reproduction of cladocerans in tropical Lake Monte Alegre.     

Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

     

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.      

Responses of tropical cladocerans to a gradient of resource quality

1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with ³²P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities.     

Diel and vertical variability of seston food quality and quantity in a small subalpine oligomesotrophic lake

Vertical profiles of seston food quality and quantity were measuredin subalpine Castle Lake for particulate carbon, chlorophylla, fatty acids and phosphorus in addition to abiotic parametersincluding water temperature and dissolved oxygen levels. Fieldand laboratory incubation experiments were employed to manipulateDaphnia rosea growth environments. Sestonic eicosapentaenoicacid (EPA) content was much lower, whereas carbon to phosphorus(C:P) ratios were much higher in the epilimnion compared tothe deep-water chlorophyll maximum (16–20 m). In a temperature-controlledincubation, Daphnia grew faster when fed seston from the deep-waterchlorophyll maximum. In in situ cage incubations, D. rosea grewfastest in the epilimnion. Daphnia rosea in a migrating treatmentexhibited intermediate growth rates between the ones for epilimnionand hypolimnion. A projection of D. rosea growth rates by growthmodels without regard to water temperature showed much highergrowth potentials in the hypolimnion. However, with inclusionof water temperature, epilimnetic water always gave higher potentialgrowth rates. In this lake, warmer temperatures of epilimneticwater override the higher food quality and quantity providedby the deep-water chlorophyll maximum.     

洞庭湖区社鼠消化道长度和质量的季节变化

对洞庭湖区社鼠（Niviventer confucianus）自然种群四季的消化道各器官的长度和质量进行了测定。结果表明。消化道指标季节变化明显。总体消化道含内容鲜质量、净鲜质量与干质量均具有显著的季节变化。以冬、春季较高。夏、秋季较低。消化道各器官的变化与总消化道不尽相同。其中,以胃的变化相对比较稳定。仅长度的变化达显著水平,以冬季最短,夏、秋季较长。这是动物为适应繁殖季节能量需求而增加摄食量的反应,也与夏秋季食物丰富度有关。小肠、盲肠、大肠的长度和质量指标（内容鲜质量、净鲜质量和干质量）以冬、春季较高。夏、秋季较低。这是该鼠对冬、春季低温和食物匮乏的适应性反应。总体来说。洞庭湖区社鼠为适应夏、秋季繁殖盛期能量需求增加的主要对策是增加摄食量;而为适应冬、春季低温环境和食物数量和质量减少的主要对策是增大消化道。提高消化效率。     

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.     

Phenotypic Plasticity in Life-history Traits and Feeding Appendages in Two Species of Daphnia Fed a Natural Phytoplankton Assemblage

The ability of two cladoceran species – Daphnia pulicaria and D. mendotae – to utilize a natural phytoplankton assemblage dominated by cyanobacteria was investigated experimentally. Reference animals were fed high quality green algae. The natural phytoplankton assemblage originated from Oneida Lake (NY, USA). Both cladoceran species occur in the lake, but their densities vary temporally. As expected, lake phytoplankton were a poor food source. Fecundity decreased, and age at first reproduction and offspring size increased. The magnitude of these effects varied between and within species. The increase in offspring size differs from studies using toxic cyanobacterial strains, but agrees with expectations on adaptive responses to low food availability. Both species performed equally on the lake phytoplankton, but D. pulicaria appeared to be the superior species in the high quality food treatment.