Stoichiometry in an ecological context: testing for links between <Emphasis Type="Italic">Daphnia</Emphasis> P-content,growth rate and habitat preference

We used laboratory experiments with ten Daphnia taxa to test for links between Daphnia P-content, growth rate and habitat preference. The taxa represent a wide range of body sizes and most show distinct preferences for one of three habitats: shallow lakes, deep, stratified lakes or fishless ponds. Previous studies show that taxa from shallow lakes and fishless ponds experience high predation risk and rich food resources, whereas taxa from deep lakes experience low predation risk, strong food limitation and potentially P-deficient resources. Thus, we predicted higher P-content and higher maximal growth rates in taxa from ponds and shallow lakes and lower P-content, lower maximal growth but reduced sensitivity to P-limitation in taxa preferring stratified lakes. In each of 25 experiments, a clonal Daphnia cohort was cultured for 4 days on a P-sufficient (molar C:P ratio 70) or a P-deficient (C:P 1,000) diet of a green alga at a high concentration (1 mg C l^–1). The P-content of adult Daphnia fed the P-sufficient diet ranged from 1.52 to 1.22% mass. Small-bodied taxa from shallow lakes had higher P-content than larger-bodied taxa from deep lakes or fishless ponds. However, we found a nonsignificant negative correlation between P-content and growth on the P-sufficient diet, rather than the positive relationship predicted by the growth rate hypothesis. The P-deficient diet resulted in declines in both growth rate and P-content compared with the P-sufficient controls and the extent of the declines differed between taxa. Taxa from ponds showed a marginally greater decline in growth with the P-deficient diet compared with taxa from shallow or deep lakes. However, contrary to stoichiometric theory, no relationship was found between a species P-content and growth depression on the P-deficient diet. Although we found evidence for habitat adaptations, our results show that factors other than Daphnia P-content are important in determining differences between Daphnia species in both maximal growth rate and sensitivity to P-limited growth.     

Foraging behavior by <Emphasis Type="Italic">Daphnia</Emphasis> in stoichiometric gradients of food quality

Mismatches in the elemental composition of herbivores and their resources can impact herbivore growth and reproduction. In aquatic systems, the ratio of elements, such as C, P, and N, is used to characterize the food quality of algal prey. For example, large increases in the C:P ratio of edible algae can decrease rates of growth and reproduction in Daphnia. Current theory emphasizes that Daphnia utilize only assimilation and respiration processes to maintain an optimal elemental composition, yet studies of terrestrial herbivores implicate behavioral processes in coping with local variation in food quality. We tested the ability of juvenile and adult Daphnia to locate regions of high-quality food within a spatial gradient of algal prey differing in C:P ratio, while holding food density constant over space. Both juveniles and adults demonstrated similar behavior by quickly locating (i.e., <10 min) the region of high food quality. Foraging paths were centred on regions of high food quality and these differed significantly from paths of individuals exposed to a homogeneous environment of both food density and food quality. Ingestion rate experiments on algal prey of differing stoichiometric ratio show that individuals can adjust their intake rate over fast behavioral time-scales, and we use these data to examine how individuals choose foraging locations when presented with a spatial gradient that trades off food quality and food quantity. Daphnia reared under low food quality conditions chose to forage in regions of high food quality even though they could attain the same C ingestion rate elsewhere along a spatial gradient. We argue that these aspects of foraging behavior by Daphnia have important implications for how these herbivores manage their elemental composition and our understanding of the dynamics of these herbivore–plant systems in lakes and ponds where spatial variation in food quality is present. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Role of the Predaceous Copepod Parabroteas Sarsi in the Pelagic Food Web of a Large Deep Andean Lake

Parabroteas sarsi is a predaceous calanoid copepod that inhabits both shallow temporary fishless ponds and deep fish lakes of Patagonia and Antarctica. The aim of this study was to analyse the effect of P. sarsi on the plankton structure of a deep Andean lake (>100 m depth) and the zooplankton vertical distribution in order to asses a possible vertical refuge of the predatory copepod against visual fish predation. We tested the extent to which the trophic cascade effect of this predator propagates through the food web. We carried out a vertical sampling in Lake Rivadavia (Patagonia, Argentina) in order to assess zooplankton distribution. P. sarsi showed a vertical distribution towards deeper layers of the water column both at midday and at night, indicating that the copepod had an effective refuge against visual predation. Additionally, we carried out both field and laboratory experiments with the presence of P. sarsi. The predator was observed to affect significantly the survival of the copepod Boeckella michaelseni both in laboratory and field experiments. On the contrary, rotifers and adults of Daphnia cf. commutata were not substantially affected by the predator. B. michaelseni mouthparts revealed an omnivorous diet; therefore a broad phytoplanktonic size spectrum could be affected by this copepod. However, no cascade effect was observed due to the presence of P. sarsi despite the decrease of B. michaelseni abundance.     

Impact of food concentration on diel vertical migration behaviour of <Emphasis Type="Italic">Daphnia pulex</Emphasis> under fish predation risk

Vertical migration of Daphnia represents the best-studied predator-avoidance behaviour known; yet the mechanisms underlying the choice to migrate require further investigation to understand the role of environmental context. To investigate the optimal habitat choice of Daphnia under fish predation pressure, first, we selected the individuals exhibiting strong migration behaviour. The animals collected from the hypolimnion during the daytime were significantly larger, being more conspicuous, and in turn performed stronger diel vertical migration (DVM) when exposed to fish cue. We called them strong migrants. Second, we provided the strong migrant D. pulex with food at high and intermediate (1 and 0.4 mg C l⁻¹, respectively) levels, which were well above the incipient limiting level and of high quality. They traded the benefits of staying in the warm water layer and moved down to the cold water in response to fish cue indicating fish predation. The availability of food allowed the animals to stay in the cold hypolimnion. However, at the low food level (0.1 mg C l⁻¹), which is an additional constraint on fitness, Daphnia moved away from the cold hypolimnion. Poor food condition resulted in strong migrant Daphnia to cease migration and remain in the upper warmer water layer. Although temperature is known to be a more important cost factor of DVM than food, our results clearly show that this is only true as long as food is available. It becomes clear that food availability is controlling the direction of vertical positioning when daphnids experience a dilemma between optimising temperature and food condition while being exposed to fish cue. Then they overlook the predation risk. Thus, the optimal habitat choice of Daphnia appears to be a function of several variables including temperature, food levels and fish predation. Handling editor: S. I. Dodson     

Threshold elemental ratios for carbon versus phosphorus limitation in Daphnia

1. The transition from carbon (C) to phosphorus (P) limited growth in Daphnia depends not only on the C : P ratio in seston, i.e. food quality, but also on food quantity. Carbon is commonly believed to be limiting at low food because of the energetic demands of basal metabolism. The critical C : P ratio in seston (otherwise known as the threshold elemental ratio, TER) above which P is limiting would then be high when food is scarce. 2. A new model that differentiates between the C : P requirements for growth and maintenance is presented that includes terms for both C and P in basal metabolism. At low food the calculated TERs for Daphnia of around 230 are only slightly higher than values of 200 or so at high intake. Seston C : P often exceeds 230, particularly in oligotrophic lakes where phytoplankton concentration is low and detritus dominates the diet, indicating the potential for limitation by P. 3. The analysis highlights the importance of P, as well as C, in maintenance metabolism and the overall metabolic budget, such that food quality is of importance even when intake is low. Further measurements of C and P metabolism at low food, in particular basal respiration and excretion rates, are needed in order to improve our understanding of the interacting roles of food quantity and quality in zooplankton nutrition.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Glacier melting and stoichiometric implications for lake community structure: zooplankton species distributions across a natural light gradient

Glaciers around the globe are melting rapidly, threatening the receiving environments of the world's fresh water reservoirs with significant changes. The meltwater, carried by rivers, contains large amounts of suspended sediment particles, producing longitudinal gradients in the receiving lakes. These gradients may result in changes in the light : nutrient ratio that affect grazer performance by altering elemental food quality. Thus, glacial melting may induce a shift in the phytoplankton carbon : nutrient ratio and hence influence the dominance of herbivorous zooplankton through stoichiometric mechanisms. To test this hypothesis, we combined field and experimental data, taking advantage of a natural light intensity gradient caused by glacial clay input in a deep oligotrophic Patagonian lake. Across this gradient, we evaluated the abundances of two consumer taxa with different phosphorus requirements, the copepod Boeckella gracilipes and the cladoceran Daphnia commutata, using a six‐station transect along the lake. We found significant differences in light : nutrient ratio and stoichiometric food quality of the seston, together with a switch from dominance of P‐rich Daphnia in low carbon : nutrient stations to dominance of low‐P copepods in high carbon : nutrient stations. The laboratory experiments confirmed that the difference in the carbon : nutrient ratio across the gradient is sufficient to impair Daphnia growth. The overall patterns are consistent with our prediction that shifts in the environmental light : nutrient ratio as a result of glacial melting would contribute to shifts in the dominance of stoichiometrically contrasting taxa in consumer guilds.     

Demography of a natural population of Daphnia retrocurva in a lake with low food quality

The hypothesis was tested that Daphnia exhibit low populationrecruitment in a lake with phosphorus (P)-deficient food, whereDaphnia populations failed to increase in response to greatlylowered minnow density. Egg ratio analysis was performed onDaphnia retrocurva from L110 of the Experimental Lakes Area,Ontario, in 1994. L110 has high food abundance (seston carbon(C) 100 µM C), but the seston is P deficient (C:P atomic- 600). Chrysophytes dominated the algal community. The fishcommunity in this lake was manipulated in 1993 and 1994 by theaddition of northern pike, Esox lucius. Zooplanktivorous minnowswere greatly reduced after the piscivore addition, but the expectedincrease in abundance of the Daphnia to this altered predationregime was not observed, If food quality constrains the responseof the Daphnia to changed predation, there should be low recruitment in the relatively sparse Daphnia population in this lake.Birth rates estimated by standard egg ratio analysis were <0.1day^–1 for most of the sample period. In comparison toannual averages from the literature, recruitment in L110 wasmoderately, but not dramatically, low. Daphnia exhibited pronounced‘normal’ vertical migration (up at night, down duringthe day) all year, and a strongly declining size at first reproductionthrough the season. Population density and birth rates seemedto show much less short-term change than previously observedin other lakes. Although the demo graphic predictions for foodquality constraints seemed clear, in situ demography unfortunatelyneither supported nor strongly rejected the overall hypothesisof food quality limitation in this lake with very high C:P ratios.Possibly, direct demographic effects of low food quality willbe difficult to observe in nature.     

Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Field data from seven alpine lakes in Serra da Estrela. Portugal.show that reproduction in Daphnia may be as efficiently controlledby fish predation and copepod predation on eggs in brood cavitiesas it is by food limitation. Body length and clutch size estimatesin Daphnia pulicaria revealed high inter- and intra-populationvariability in maturation size (body size at first reproduction).and in number of eggs per clutch. Daphnia at first maturationin lakes stocked with rainbow trout were half the size of thosefound in fishless lakes (body length of 0.86–0.95 and1.55–1.81 mm. respectively). The mean number of eggs perclutch was reduced to a similar degree by food limitation, predationby fish and copepod predation on eggs in brood cavities, butthe underlying mechanisms of this reduction were different.Food limitation caused smaller clutch sizes in all individuals,so variation remained the same. Fish predation caused the selectiveremoval of individuals with maximum clutches, so variation decreased.Copepod predation caused removal of eggs from brood cavitiesof randomly infested females, so that variation increased, particularlyat a high food level when non-infested females carried largeclutches of eggs.     

Interactive effects of higher temperature and dissolved organic carbon on planktonic communities in fishless mountain lakes

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Effects of stoichiometric dietary mixing on<Emphasis Type="Italic"> Daphnia</Emphasis> growth and reproduction

Herbivores often encounter nutritional deficiencies in their diets because of low nutrient content of plant biomass. Consumption of various diet items with different nutrient contents can potentially alleviate these nutritional deficiencies. However, most laboratory studies and modeling of herbivorous animals have been done with diets in which all food has uniform nutrient content. It is not clear whether heterogeneous versus uniform food of equal overall nutrient content is of equivalent nutritional value. We tested the effects of dietary mixing on performance of a model organism, Daphnia. We fed two species of Daphnia ( D. galeata, D. pulicaria) with diets of equivalent bulk stoichiometric food quality (C:P) and studied whether they would produce equivalent performance when C:P was uniform among cells or when the diet involved a mixture of high C:P and low C:P cells. Daphnia were fed saturating and limiting concentrations of a uniform food of moderate C:P (UNI) or mixtures (MIX) of high C:P (LOP) and low C:P (HIP) algae prepared to match C:P in UNI. Daphnia were also fed HIP and LOP algae separately. Juvenile growth rate and adult fecundity were measured. D. galeata performance in UNI and MIX treatments did not differ, indicating that partitioning of C and P among particles did not affect dietary quality. Similarly, D. pulicarias performance was similar in the MIX and UNI treatments but only at low food abundance. In the high food treatment, both growth and reproduction were higher in the MIX treatment, indicating some benefit of a more heterogeneous diet. The mechanisms for this improvement are unclear. Also, food quality affected growth and reproduction even at low food levels for both D. pulicaria and D. galeata. Our results indicate that some species of zooplankton can benefit from stoichiometric heterogeneity on diet.     

Antibiotics affect the growth responses of <Emphasis Type="Italic">Daphnia magna</Emphasis> to poor food quality

We tested the hypothesis that exposure to antibiotics alters the growth and reproductive responses of Daphnia magna to changing stoichiometric food quality. To do so, we measured growth and reproduction of differentially P-nourished Daphnia in the presence and absence of sublethal concentrations of antibiotics. We found that exposure to an antibiotic cocktail significantly reduced an index of the microbial load of Daphnia and altered its growth responses to changing dietary P-content. Growth rates of Daphnia consuming the most P-rich and P-poor food increased with antibiotic exposure but were negatively or not affected in animals eating mildly to moderately P-limiting food. Similar effects were found in a subsequent experiment where daphnid neonates were exposed to natural bacterial communities prior to receiving antibiotics and being fed different food C:P ratios. In contrast, antibiotic effects on Daphnia reproduction were either not detected (number and size of broods) or were relatively minor (day of first reproduction). We also found no evidence that gut flora provides defense against pathogenic bacterial infection; instead, infection rates in Daphnia by a bacterial microparasite, Pasteuria ramosa, decreased in animals that had experienced prior antibiotic exposure. Our results demonstrate that antibiotic exposure reduced the microbial load and altered growth rates of an important zooplankton herbivore. Given the mediating role of animal’s food C:P ratio, our results show that interactions between Daphnia and its microbial symbionts vary in strength and nature partly with the host’s nutritional state.     

The interaction of Chaoborus size and vertical distribution determines predation effects on Daphnia

1. Larvae of Chaoborus, the phantom midge, are important pelagic planktivores in many freshwater lakes and ponds. The effect of Chaoborus on its prey depends on its size, especially mouth gape diameter, and vertical migration pattern, which affects predator–prey spatial overlap. These two features vary considerably in different Chaoborus species and instars. In this study, the interacting effects of both Chaoborus size and vertical distribution on population growth of Daphnia pulex was analysed with a field enclosure experiment and a matrix population model. 2. In the field experiment, Daphnia were grown in four replicated treatments that included a control (no Chaoborus) and three combinations of instar III and IV Chaoborus of two species (C. trivittatus and C. americanus). Parameters of the matrix model were based on differences between Chaoborus species and instars in capture and ingestion of Daphnia of differing sizes (prey vulnerability) and in vertical overlap with Daphnia in each treatment (density risk). 3. In comparison with the control, the two treatments containing the smaller, migratory C. americanus showed a significant effect on Daphnia population growth rate, while the treatment containing only the larger, non‐migratory C. trivittatus did not. The model accurately simulated these effects. 4. A Daphnia predation risk model, which uses prey vulnerability and density risk parameters, illustrated the individual and combined effect of the different Chaoborus types on Daphnia. Daphnia have a high prey vulnerability to the large C. trivittatus, but overall predation risk was low because of very little overlap. On the contrary, the smaller C. americanus affects only a small range of Daphnia instars, each with a low vulnerability, yet those instars that were vulnerable had a very high density risk because of an increased overlap. 5. This analysis of Daphnia predation risk parameters with coexisting Chaoborus species strongly supports an integrated approach using both size and vertical distribution to determine the ultimate predation effect on Daphnia.     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

Diel horizontal migration of zooplankton: costs and benefits of inhabiting the littoral

1. In some shallow lakes, Daphnia and other important pelagic consumers of phytoplankton undergo diel horizontal migration (DHM) into macrophytes or other structures in the littoral zone. Some authors have suggested that DHM reduces predation by fishes on Daphnia and other cladocerans, resulting in a lower phytoplankton biomass in shallow lakes than would occur without DHM. The costs and benefits of DHM, and its potential implications in biomanipulation, are relatively unknown, however. 2. In this review, we compare studies on diel vertical migration (DVM) to assess factors potentially influencing DHM (e.g. predators, food, light, temperature, dissolved oxygen, pH). We first provide examples of DHM and examine avoidance by Daphnia of both planktivorous (PL) fishes and predacious invertebrates. 3. We argue that DHM should be favoured when the abundance of macrophytes is high (which reduces planktivory) and the abundance of piscivores in the littoral is sufficient to reduce planktivores. Food in the littoral zone may favour DHM by daphnids, but the quality of these resources relative to pelagic phytoplankton is largely unknown. 4. We suggest that abiotic conditions, such as light, temperature, dissolved oxygen and pH, are less likely to influence DHM than DVM because weaker gradients of these conditions occur horizontally in shallow lakes relative to vertical gradients in deep lakes. 5. Because our understanding of DHM is rudimentary, we highlight potentially important research areas: studying a variety of systems, comparing temporal and spatial scales of DHM in relation to DVM, quantifying positive and negative influences of macrophytes, focusing on the role of invertebrate predation, testing the performance of cladocerans on littoral versus pelagic foods (quantity and quality), investigating the potential influence of temperature, and constructing comprehensive models that can predict the likelihood of DHM. Our ability to biomanipulate shallow lakes to create or maintain the desired clear water state will increase as we learn more about the factors initiating and influencing DHM.     

Causes of Daphnia midsummer decline in two deep meromictic subalpine lakes

1. Daphnia are key organisms in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamic follows regular seasonal patterns, with spring peaks followed by summer population declines (midsummer declines, MSDs). Midsummer declines show high inter-annual variation, which has been attributed to different causes. However, the mechanisms controlling the MSD remain poorly understood, especially in deep stratified lakes.
2. We tried to disentangle the factors causing Daphnia MSDs in Lake Lugano and Lake Iseo (in Switzerland and Italy), two deep peri-alpine lakes with similar trophic status and vertical mixing dynamics, characterised by phosphorus accumulation in the hypolimnion and variable mixing during late-winter turnovers.
3. Specifically, we assessed the effects of three different hypothetical pathways according to which: (1) winter air temperature controls MSDs by influencing mixing depth during turnovers and epilimnetic phosphorus replenishment; (2) vernal air temperature influences MSD by accelerating the timing of spring population peak; and (3) summer temperature influences MSDs by increasing fish predation. We assessed the relative strength of these pathways using structural equation modelling on long-term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo).
4. Between the hypothesised pathways, the one driven by winter air temperature (through P replenishment) influenced Daphnia abundance in spring in both lakes, but the effects propagated to summer Daphnia abundance only in Lake Lugano. Additionally, summer Daphnia abundance was influenced by the summer air temperature through a positive (although weak) effect. By comparison, vernal air temperature had no detectable effects on summer Daphnia abundance.
5. The results revealed marked differences between the meromictic study lakes and the shallow hypertrophic water bodies that were the focus of previous research on Daphnia MSD, and also between the two study lakes. The influence of epilimnetic P replenishment on the summer Daphnia abundance in Lake Lugano, which was recovering from past eutrophication, may have reflected the greater susceptibility of deep, stratified lakes to P depletion after spring compared to shallow hypertrophic lakes or reservoirs. This effect might not have been detected in Lake Iseo because P was more consistently depleted during the study period (i.e. variance in the predictor was too low to detect an effect).
6. This study highlighted the complexity of the effects of climate variability on Daphnia MSD in deep lakes, showing that the responses can differ even between two neighbouring lakes with similar vertical mixing dynamics and trophic status. At the same time, the results suggest that future increases in winter air temperature, caused by global warming, may cause critically low densities of Daphnia during spring and summer and compromise the ability of zooplankton to control phytoplankton biomass.

     

Sensitivity of Daphnia species to phosphorus-deficient diets

The life history of freshwater cladocerans such as Daphnia spp. is strongly affected by their environment. Factors such as temperature, food quantity and even the presence or absence of predators influence growth, reproduction and morphology of individuals. Recently, it has also become clear that the quality of the food can affect various life history traits of Daphnia. More specifically, the effect of the elemental composition of algae, expressed as the C:P ratio, has been studied intensively. Daphnia species differ in their response to differences in the C:P ratio of their food. Until now, it has been unclear whether these species differences are driven by phylogenetic constraints or by adaptation to particular environmental conditions. Here we present laboratory experiments with 12 Daphnia species from three different subgenera originating from a broad range of habitats. We compared somatic growth rates and sensitivity to variation in the nutrient stoichiometry of the food with habitat parameters, taking into account the phylogenetic history of the species. No associations between fitness and habitat parameters were detected. However, we found a trade-off between sensitivity to P-deficient diets and the maximum growth rate on a P-sufficient diet. In several cases, this trade-off helps to explain the association between species distribution and habitat parameters. We observed no correlation of the sensitivity to P limitation with the phylogenetic history of the genus Daphnia. Thus, we conclude that the differential responses among Daphnia species to variation in P content in food were driven mainly by adaptations to their local habitats, and are not constrained by deep evolutionary patterns.     

Intraspecific variation in diet, growth, and morphology of landlocked Galaxias maculatus during its larval period: the role of food availability and predation risk

Food availability and predation risk have been shown to affect phenotypes during early life history of fishes. Galaxias maculatus, a small fish widely distributed around the southern hemisphere, clearly exhibits a complex trade-off between feeding and predation avoidance during growth over the larval period. We studied the effect of different environmental variables on diet, growth, mortality, and morphology through field surveys and data revision in the literature for limnetic G. maculatus larvae in five oligotrophic lakes of Patagonia. Both number of food categories and prey ingested by larvae were directly related to zooplankton density. Larval growth rate was related with zooplankton density and temperature. Lakes with high zooplankton densities and low predation risk had larvae with deeper bodies and shorter caudal peduncles, while in lakes with less food and high predation risk larvae were slender with shallower bodies and longer peduncles. Food availability and predation risk seem to operate on the swimming performance of G. maculatus larvae through the slenderness of the body and the length of the caudal peduncle. The observed phenotypic variation in growth and morphology could be a key feature that has allowed this species to successfully colonize a wide variety of environments in the southern hemisphere.     

Sediments,not plants,offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large‐bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes. 2. We performed ‘habitat choice’ experiments to test the response of Daphnia magna to predation cues at both the horizontal and vertical level by mimicking a ‘shallow littoral’ zone with plants and a ‘deeper pelagic’ zone with sediments. 3. Initial separate response experiments showed that natural plants, artificial plants and predation cues all repelled D. magna in the absence of other stimuli, while sediments alone did not trigger any significant response by D. magna. 4. The habitat choice experiments showed that, in the presence of predation cues and absence of plants, Daphnia moved towards areas with sediment. In the presence of both plants and sediments, Daphnia moved away from the plants towards the sediments under both shallow and deep water treatment conditions. 5. Based on these results, we suggest that Daphnia in Mediterranean shallow lakes avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, as also observed in subtropical shallow lakes. This pattern is not likely to change with water level alterations, a common feature of lakes in the region, even if the effectiveness of the refuge may be reduced.     

Phosphorus acquisition and competitive abilities of two herbivorous zooplankton, <Emphasis Type="Italic">Daphnia pulex</Emphasis> and <Emphasis Type="Italic">Ceriodaphnia quadrangula</Emphasis>

Contrary to an expectation from the size-efficiency hypothesis, small herbivore zooplankton such as Ceriodaphnia often competitively predominate against large species such as Daphnia. However, little is known about critical feeding conditions favoring Ceriodaphnia over Daphnia. To elucidate these conditions, a series of growth experiments was performed with various types of foods in terms of phosphorus (P) contents and composition (algae and bacteria). An experiment with P-rich algae showed that the threshold food level, at which an individual’s growth rate equals zero, was not significantly different between the two species. However, the food P:C ratio, at which the growth rate becomes zero, was lower for Daphnia than for Ceriodaphnia, suggesting that the latter species is rather disfavored by P-poor algae. Ceriodaphnia showed a higher growth rate than Daphnia only when a substantial amount of bacteria was supplied together with a low amount of P-poor algae as food. These results suggest that an abundance of bacteria relative to algae plays a crucial role in favoring Ceriodaphnia over Daphnia because these are an important food resource for the former species but not for the latter.