From inducible defences to population dynamics: modelling refuge use and life history changes in Daphnia

We investigated the relative importance of a behavioural defence (refuge use through diel vertical migration) and a life history change (a reduced size at first reproduction) that are used by daphnids to decrease the risk of predation by visually hunting fish. We used an individual based model of a Daphnia population in a stratified lake to quantify the effects of these inducible defences on Daphnia predation-mortality and the resulting Daphnia population dynamics. Our analysis shows that diel vertical migration (DVM) confers a much stronger protection against fish predation than a reduced size at first reproduction (SFR). DVM allows daphnids to withstand a higher predation pressure in the epilimnion and it decelerates a Daphnia population decline more strongly than a reduced SFR. DVM effectively reduces the (P/B) flow of carbon from daphnids to fish.
Many theoretical studies have only considered the fitness benefits of DVM above 'staying up' in the epilimnion of a lake. Our results suggest that 'staying down' in the hypolimnion would confer an even stronger fitness benefit to Daphnia than DVM at times of peak predation risk. Daphnids that remain in the hypolimnion avoid the predation suffered by migrating daphnids around dusk and dawn. Staying down could prevent a Daphnia population decline, while DVM and a reduced SFR can only decelerate the decrease of Daphnia population densities under heavy fish predation. Staying down at high concentrations of fish infochemicals has in fact been observed within a variety of Daphnia clones and species, both in the laboratory and in stratified lakes.     

Trade-offs inDaphnia vertical migration strategies

Summary Planktonic animals performing diel vertical migration (DVM) experience a tradeoff between reduced mortality and reduced reproductive output due to lower food availability in their refuge. Models of DVM as an evolutionarily stable strategy predict that, under certain conditions, strategies of both migration and non-migration can coexist. Vertical profiles of animal abundances during day and night, however, do not allow any discrimination between the behaviour of individuals or subpopulations. We used length-body protein regressions as a measure of the nutritional state ofDaphnia to distinguish possible sub-populations differing in their migration strategy. An overwhelming part of the population migrated downwards during the day. However, the few daphnids in the epilimnion during the day had significantly higher protein content than the animals in the deep water, indicating that these daphnids did not migrate randomly but remained in the surface food-rich water all day. This shows that migrating animals gain no metabolic advantage over non-migrating ones.Supported by a F.P.U. grant (Spanish Goverment)     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g × h — hyalina and the multilocus allozyme genotypes of the hybrid D. g × h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g x h – hyalina and the multilocus allozyme genotypes of the hybrid D. g x h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.     

Depth distribution of Daphnia in response to a deep-water algal maximum: the effect of body size and temperature gradient

1. In the absence of fish predation, Daphnia exploiting a deep‐water algal maximum are faced with a trade‐off. They can either dwell in the epilimnion where development in the warm water is fast, but food shortage causes low egg production, or in the hypolimnion, where food availability is high but development is slow because of low temperatures. 2. We tested the hypotheses that (i) depth distributions of various ontogenetic stages (size classes and egg‐bearing females) differ because daphnids react to light with size‐specific diel vertical migration (DVM) even in the absence of fish (residual predator avoidance hypothesis) and (ii) differently sized daphnids select different depths because the relative importance of temperature and food varies for ontogenetic stages (physiological hypothesis). We used large indoor mesocosms (Plankton Towers) to test these hypotheses experimentally. 3. Temperature was the strongest factor governing the distribution, with larger proportions of the population dwelling in the food‐rich hypolimnion if the temperature gradient was shallow. There were small but significant differences between ontogenetic stages during the day, but not at night. This suggested the existence of a ‘residual’ effect of light on depth distribution in the absence of a fish cue. 4. Although large individuals exhibited greater amplitude of DVM, the physiological hypothesis had to be rejected. A stage‐specific physiological effect is unlikely to be directly triggered by light, hence vertical movement of the individuals should not be synchronised. Rather, being forced into deeper layers by the residual light response during the day, large and egg‐bearing females experience a lower average temperature during day than juveniles. They probably compensate for this by spending longer time periods in warm waters at night.     

Multiple defence strategies of <Emphasis Type="Italic">Daphnia</Emphasis><Emphasis Type="Italic">galeata</Emphasis> against predation in a weakly stratified reservoir

In the presence of size-selective fish daphnids were shown to exhibit two alternative inducible defence strategies: They may either escape predation by active migration or adopt a life history strategy, e.g., reproduce earlier and at a smaller size. Depending on the type of habitat, migration may either be vertically (in deep stratified lakes) or horizontally (in shallow lakes with macrophytes) oriented. Concerning behavioural defence strategies, daphnids living in medium-deep, weakly stratified water bodies with a poorly developed littoral face a dilemma, since the littoral provides no shelter and the availability of a deep-water refuge is unpredictable. We studied the population dynamics, life history changes (size at maturity) and daytime vertical distribution of Daphnia galeata in a weakly stratified reservoir in relation to predation by juvenile fish during 6 years. While temperature gradients were usually small, oxygen concentrations suggest that a low-oxygen refuge for daphnids was available in every year to some extent. Our results indicate that, depending on predation intensity and stratification patterns, daphnids exhibit both, behavioural and life history defences. In years with a high biomass of young-of-the-year (YOY) perch Daphnia abundance declined rapidly at the end of the clear water stage while at the same time the vertical distribution at daytime shifted to deep strata providing a low-oxygen refuge and the size at maturity decreased. However, while the life history response in some years lasted throughout most of the summer period, a shift in daytime vertical distribution was exhibited for much shorter periods. Both traits were much less expressed in years with low YOY fish densities and no negative correlation between them could be verified. We suggest that under high predation pressure in this relatively shallow reservoir no strictly alternative (either behavioural or life history) strategies exist, but that daphnids make use of the full range of possible anti-predator defences available, at least during short periods when predation is most intense. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Effects of aquatic insect predators on zooplankton in fishless ponds

We removed the surface-orienting aquatic insects from a fishless pond to determine their predation effects on zooplankton behavior and size structure. A second fishless pond served as the unmanipulated reference system in this two year study. In the reference pond and the treatment pond prior to manipulation, daphnids exhibited pronounced diel vertical migrations. Following the removal of surface-orienting aquatic insects from the treatment pond, daphnid migration changed to a reverse migration strategy that was significantly different from that observed in the reference system. Average daphnid body size increased significantly following predator removal in the treatment system. Our data indicate that predation by aquatic insect predators, such as notonectids and dytiscids, may affect daphnid migration behavior in fishless systems. Vertical migration by daphnids may allow coexistence with surface-orienting insects in ponds that are deep enough to provide a spatial refuge from these predators.     

The effect of different zooplankton grazing patterns resulting from diel vertical migration on phytoplankton growth and composition: a laboratory experiment

Diel vertical migration (DVM) of herbivorous zooplankton is a widespread behavioural phenomenon in freshwater ecosystems. So far only little attention has been paid to the impact of DVM on the phytoplankton community in the epilimnion. Some theoretical models predict that algal population growth in the epilimnion should depend on the herbivores migration and grazing patterns: even if migrating zooplankton consume the same total amount of algae per day in the epilimnion as non-migrating zooplankton, nocturnal grazing should result in enhanced algal growth and favour algal species with high intrinsic growth rates over species with lower intrinsic growth rates. To test these hypotheses we performed experiments in which several algal species were confronted with different feeding regimes of Daphnia. In the experiments algal growth did not only depend on the absolute time of grazing but was comparatively higher when grazing took place only during the night, even when the grazing pressure was the same. Furthermore, algal species with higher intrinsic growth rates had higher advantages when being grazed upon only discontinuously during the night than algal species with a smaller intrinsic growth rate. The grazing pattern itself was an important factor for relative algal performance.     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

Absence of typical diel vertical migration in Daphnia: varying role of water clarity, food, and dissolved oxygen in Lake Eymir, Turkey

Diel vertical migration (DVM) is a complex and dynamic behaviour against predation because the reaction of migrating organisms to light intensity plays a primary role, but is modified by other factors. In the relatively shallow but thermally stratified Lake Eymir, Daphnia pulex de Geers utilized vertical refugia afforded by the hypolimnion during both day and night. Differences in general vulnerability to fish predation determined the differences in their mean residence depths (MRDs) of different population categories such as most conspicuous and vulnerable individuals of adult with eggs inhabited the deepest depth, whereas juveniles stayed close the thermocline. In late spring, profoundly high amplitude of displacement within the hypolimnion, probably due to the hypolimnion being well-lit and relatively well-oxygenated for the fish and rather unsafe for the large-sized daphnids, was recorded. Therefore, the large-sized daphnids daytime refuge was close to the bottom whereas at night they moved upward to benefit from warmer water temperature along with food availability in the presence of fish predation but still remained below the thermocline. In summer, the insignificant amplitude of the hypolimnetic, which later became epilimnetic, displacements were probably due to the near-anoxic condition found below the thermocline. This might have deterred the fish, thus providing a safer refuge for daphnids in the below thermocline, which afterwards became the above thermocline. Low oxygen availability was regarded as the summer proximate factor. The abundant food and warmer water conditions found in the below/above thermocline also accounted for absence of DVM in summer. Consequently, this study suggests that DVM by Daphnia is an adaptation that is plastic to changing environmental conditions.     

Diel vertical migration by males,females, copepodids and nauplii in a limnetic population of diaptomus (Copepoda)

Diel vertical migration (DVM) by the sexes and life history stages of the copepod Diaptomus novamexicanus (Herrick) was investigated during two summers in a subalpine lake (Castle Lake, California, USA). Migratory behavior was quantified repeatedly by 1300 and 21–2200 hr sampling in the epilimnion. Population density and vertical distribution were estimated routinely at 1300 hr and at 2200 hr on selected dates. Males had greater night to day biomass differentials and ratios than females. The sexes showed comparable DVM participation (% of total biomass migrating) and amplitudes of migration (distance of ascent). Copepodids displayed higher participation, night/day ratios and amplitudes than younger copepods, but adult participation differed only from that of nauplii. Active participation in high amplitude ascents thus increased with age and was greatest in adult males. Analysis suggests that such relative behaviors probably occur in most limnetic Diaptomus, and that the magnitude of ascent into the epilimnion is associated with daytime vertical distribution.     

Habitat partitioning of native and exotic Daphnia in gradients of temperature and food: mesocosm experiments

1. Invasion of tropical zooplankton into temperate lakes provides an interesting opportunity to explore habitat segregation in a thermal gradient. 2. We explored differing vertical positioning of native and exotic Daphnia (Daphnia mendotae and Daphnia lumholtzi) in a large indoor mesocosm system (Plön plankton towers) during 2 month‐long experiments. The two towers were manipulated to provide gradients of both temperature (15–29 °C) and algal food (0.05–0.58 mg C L^?1) and a day–night cycle. 3. Both juvenile and adult D. lumholtzi showed a ‘typical’ vertical migration pattern, with higher densities in the epilimnion at night than during the day. They avoided the food‐poor middle layer. In contrast, D. mendotae adults showed little tendency to migrate into the epilimnion at night, remaining in the cooler hypolimnion while juveniles migrated. The vertical distribution of D. mendotae appeared unaffected by the presence of D. lumholtzi. 4. The strong migration behaviour of D. lumholtzi in the absence of fish cues suggests that this behaviour may be a constitutive trait. Habitat partitioning of the two species is probably the result of different thermal tolerances, with D. mendotae constrained to remaining in deeper water by high temperatures in the epilimnion and the tropical D. lumholtzi able to use the warm epilimnion at night.     

Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina

The vertical distributions of bacteria and algae in a steeply stratified, highly humic lake were studied during three 24 h periods in summer. The highest bacterial and algal densities and biomasses were recorded in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobium) which occurred at very low light intensity (< 1.5 μmol m⁻² s⁻¹). The numbers and biomasses of bacteria, both in the epilimnion and hypolimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Because of vertical migration, the diurnal vertical distribution of algae was more variable than that of bacteria. Particularly in July and August, when cryptomonads were abundant, the biomass of algae was much higher in the epilimnion during the day than at night. The flagellated chlorophytes, Chlamydomonas spp. and Scourfieldia cordiformis, stayed mainly in the upper hypolimnion close to the oxic-anoxic boundary zone where only a small proportion of Daphnia longispina was continuously present. Unpalatable Mallomonas chrysophytes with silicified plates and bristles, and small, presumably heterotrophic, flagellates stayed in the oxic epilimnion together with a dense (up to 300 ind l⁻¹) population of D. longispina. The results indicated that, besides the physical and chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorganisms in this polyhumic lake.     

Size distribution of Daphnia longispina in the vertical profile

D. longispina of the meromictic lake El Tobar is a round-headed form. It never has a helmet, but in summer a small proportion of immature individuals (0.9–1.2 mm females and males) have one or two neck teeth. The size structure of this Daphnia population, as well as the vertical distribution and migration of different size-classes, were studied in September and November of 1991 and April of 1992. The large variation in mean size and size at first reproduction, as well as the occurrence of different patterns of vertical migration are interpreted as responses to different predator situations. At the end of April, when Daphnia mortality by visually hunting predators is dominating, a typical nocturnal migration is adopted and size distribution is biased to smaller size classes. In November, when mortality is mainly attributed to the nocturnally migrating Chaoborus, Daphnia shows a reversed migration pattern. In September, when the population of Daphnia is responding to both visual (fish) and non-visual predators (Chaoborus), it adopts a pattern of twilight migration. The presence of neck teeth in vulnerable size classes in September might be an additional adaptative response to Chaoborus predation. In September, the size structure of the Daphnia population is shifted to larger classes and the vertical distribution of size classes shows a pronounced segregation between juveniles and adults. Juveniles are found closer to the surface, while adults dwell predominantly in the rich, deep waters near the oxicline. This suggests that an additional advantage of the ascent of the adult Daphnia exploiting those deep resources is the release of young in more oxygenated and warmer waters. The Daphnia population of lake El Tobar is known to be clonally diverse, and the changing frequency of genotypes could play an important part in the observed seasonal differences in behaviour and size.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Diurnal changes in the vertical distribution of cladocerans in a biomanipulated lake

The diel vertical migration of cladoceran zooplankton in LakeVesijärvi was studied after a distinct improvement of the waterquality following mass removal of fish. Four out of seven speciesshowed diurnal changes in vertical distribution. The two speciesdominating at 0–10 m, Bosmina crassicornis and Daphniacucullata, migrated reverse, while hypolimnetic Bosminalongispina and Ceriodaphnia quadrangula ascended at night.The migration did not cross the thermocline, suggesting thatzooplankton had a minor role in restricting the availability ofphosphorus for phytoplankton. The reverse migration in theepilimnion was likely connected to the feeding behaviour of thedominating planktivores. During night, smelt (Osmeruseperlanus (L.)) migrated from the hypolimnion to the surface tofeed on daphnids, while roach (Rutilus rutilus (L.)) thatinhabited the 5–15 m depth, was day-active and fed mainly on Bosmina. In the hypolimnion, the cladocerans used the low oxygenconcentration as a refuge and at night migrated to layers moreprofitable for feeding. This suggests that the predation by fishwas the ultimate factor of the vertical migration, but otherenvironmental factors determined its magnitude.     

Distribution of Daphnia in a trade-off between food and temperature: individual habitat choice and time allocation

1. In a thermally stratified water column with a deep‐water algal maximum, Daphnia face a trade‐off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known. 2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10 °C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep‐water algal maximum. 3. The migration behaviour of individual neonate and egg‐bearing Daphnia hyalina × galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion. 4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg‐bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg‐bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions.     

Enhancement of the phototactic reaction in Daphnia hyalina by a chemical mediated by juvenile perch (Perca fluviatilis)

Experiments with Daphnia hyalina demonstrated that the phototacticreaction to changes in light intensity is strongly enhancedin the presence of a chemical substance mediated by juvenileperch. In constant light intensity, the daphnids were foundclose to the bottom of the experimental aquarium. The phenomenonmay result in a diel vertical migration. This would decreasepredation pressure near the surface.     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

Effects of a fluctuating temperature regime experienced by Daphnia during diel vertical migration on Daphnia life history parameters

Many freshwater zooplankton species perform a diel vertical migration (DVM) and spend the day within the lower, colder hypolimnion of stratified lakes. Trade-offs that arise from this migration have already attracted much attention and the cold temperature in the hypolimnion is thought to be the main cost of this behaviour. In this study we additionally looked at the extra costs daphnids have from being exposed to a fluctuating temperature regime (cold during the day and warm during the night) which is less well studied until today. In our experiment Daphnia hyalina Leydig and Daphnia magna Straus either spent 24 h in constant warm water (19 °C), 24 h in constant cold water (12 °C), or spent 12 h in warm and 12 h in cold water in an alternating way (fluctuating temperature regime). We expected the values of the life history parameters of Daphnia in the fluctuating temperature regime to be exactly halfway between the values of the life history parameters in the warm and cold treatments because the daphnids spent exactly half of the time in warm water, and half of the time in cold water. Concordant with earlier studies our results showed that age at first reproduction and egg development time were reduced at higher temperatures. In the fluctuating temperature regime the values of both parameters were exactly halfway between the values at permanently warm and cold temperature regimes. In contrast, somatic growth was higher at higher temperatures but was lower in the fluctuating temperature regime than expected from the mean somatic growth rate. This suggests that a fluctuating temperature regime experienced by migrating daphnids in stratified lakes involves additional costs for the daphnids.