Food quality triggers the reproductive mode in the cyclical parthenogen <Emphasis Type="Italic">Daphnia</Emphasis> (Cladocera)

Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Allelopathic effects of the submerged macrophyte <Emphasis Type="Italic">Potamogeton malaianus</Emphasis> on <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis>

Allelopathic effects of the submerged macrophyte Potamogeton malaianus on Scenedesmus obliquus were assessed using a two-phase approach under controlled laboratory conditions. In the co-culture experiment (phase І), the growth of S. obliquus at two different initial cell densities was significantly inhibited by P. malaianus. Moreover, the growth inhibition was dependent on the biomass density of P. malaianus. Antioxidant enzymes (SOD, CAT and POD), MDA, APA, total soluble protein, protein electrophoretic pattern and morphology of S. obliquus were determined after the co-culture experiment was terminated. The activities of SOD, CAT, POD and APA at the low initial cell density were stimulated, the contents of MDA and total soluble protein were increased, and some special protein bands disappeared in P. malaianus treatments. The macrophyte had no effect on the activities of SOD and APA at the high initial cell density, but significantly influenced other physiological parameters of S. obliquus with the increase of biomass density. The morphology of S. obliquus showed no difference in the macrophyte treatments and the controls, and the cultures were dominated by 4-celled coenobia. The results indicated P. malaianus had significant allelopathic effects on the growth and physiological processes of S. obliquus. Moreover, the allelopathic effects depended on initial algal cell density, biomass density of the macrophyte, and their interaction. In the experiment of P. malaianus culture filtrates (phase II), filtrates from combined culture of plant and S. obliquus at the low initial cell density exhibited no apparent growth inhibitory effect on S. obliquus. The result showed that initial addition of growth-inhibiting plant filtrates had no allelopathic effect on S. obliquus. We concluded that the allelopathic effects on S. obliquus were found in the presence of P. malaianus, but not in P. malaianus filtrates. However, the absence of allelopathic effect on S. obliquus might be due to the very low concentrations of allelochemicals in the filtrates. Handling editor: S. M. Thomas     

Grazing on toxic and non-toxic Microcystis aeruginosa PCC7820 by Unio douglasiae and Corbicula fluminea

To explore the potential grazing effects of mussels on Microcystis aeruginosa, a common bloom-forming phytoplankton, Unio douglasiae and Corbicula fluminea were fed with Scenedesmus obliquus, toxic and non-toxic strains of Microcystis aeruginosa as single food and as mixtures in the laboratory. When fed with single foods, U. douglasiae has similar clearance rates on the three algae populations, while C. fluminea has significantly lower clearance rate on toxic M. aeruginosa than those on the other two algae populations. When fed with mixture foods, both the mussels show significantly higher clearance rates than on single foods. The clearance rates of U. douglasiae on the different food mixtures are not significantly different, and C. fluminea has a significantly lower clearance rate on the toxic food mixtures than that on non-toxic food mixtures. Although the relative lower clearance rates of C. fluminea on toxic food, we may still deduce that both the mussels can exert grazing pressure on phytoplankton. The deduction is supported by the composition of the excretion products. The excretion products (faeces and pseudofaeces) of both mussels contained mainly S. obliquus. In both mixed-food treatments, the ratios of S. obliquus to M. aeruginosa in the excrete products are significantly higher than those in the foods. Therefore, it can be concluded that both mussels prefer M. aeruginosa to S. obliquus, and can cause grazing pressure on M. aeruginosa.     

FO-spectra of chlorophyll fluorescence for the determination of zooplankton grazing

In the PHYTO-PAM phytoplankton analyzer the minimal fluorescence of dark-adapted samples (F₀) was assessed, which gives direct information on the chlorophyll-a content. Clearance rates (CR) of Daphnia and Brachionus were calculated from a decrease in chlorophyll-a concentration using the PHYTO-PAM fluorometer for non-sacrificial sampling of chlorophyll-a. Clearance rates of Daphnia were measured and compared with those based on the cell-counts method using an electronic particle counter (Coulter counter). Chlorophyll fluorescence-based CR for Daphnia magna were very strongly correlated with Coulter-based CR, signifying the potential suitability of the PHYTO-PAM in grazing experiments. A procedure for determination of rotifer clearance rates was developed and the effects of rotifer density, duration of the grazing period, and food concentration on CR were investigated. Between 10 and 30 rotifers in 2.5 ml food suspension (i.e. 4–12 rotifers per ml) appeared optimal for calculating CR. The application of the deconvolution of F₀-spectra in food selectivity experiments was evaluated using various mixtures of the green alga Scenedesmus obliquus and the cyanobacterium Microcystis aeruginosa fed to Brachionus. CR for Brachionus on M. aeruginosawere lower than on S. obliquusbut this was not caused by toxicity, because no mortality was observed. The higher CR on Scenedesmus than on Microcystis in the mixtures suggested selectivity. The importance of digital suppression of background fluorescence is highlighted in additional experiments with Daphnia feeding on mixtures of Microcystis and Scenedesmus, or on Microcystis alone. Without background correction of filtered samples, negative clearance rates were obtained for the `blue' Microcystis signal. Soluble fluorescing compounds of cyanobacterial origin, phycocyanin, were released from the Daphniaand contributed 40% to the overall-fluorescence. Deconvolution of F₀-spectra for the determination of chlorophyll-a using the PHYTO-PAM appears to be a suitable tool for determination of rotifer CR even at very low food concentrations. A drawback of the method is that rather high rotifer densities are required. The required grazing period, however, is shorter than for cell-count methods, the method is sensitive, clearance rates can be measured at low food concentrations (< 0.1 mg C l^–1) and information on selective feeding can be obtained.     

Food quality effects on life history traits and fitness in the generalist herbivore Daphnia

Summary Life table experiments were conducted on the generalist suspension feeder Daphnia galeata, using as food the two green algae (Chlorophyta) Scenedesmus acutus and Oocystis lacustris. Oocystis was hypothesized to be a lower quality food because it is convered with a thick sheath, believed to reduce digestibility. Results showed that Oocystis is a lower quality food for Daphnia, but only at relatively low food concentrations (0.15 mg C/L) and not at higher concentrations (1.0 mg C/L). At 0.15 mg C/L, Daphnia intrinsic rate of increase (r) when grown on Oocystis was only half that when grown on Scenedesmus. Daphnia r was similar at 0.15 mg C/L Oocystis and 0.075 mg C/L Scenedesmus, indicating that Daphnia requires twice as much Oocystis as Scenedesmus to achieve the same fitness. Intrinsic rate of increase was lower on Oocystis mainly because age at first reproduction was greatly delayed compared to that on Scenedesmus (13.6 vs 7.3 d). In addition, juvenile growth and survivorship were reduced on Oocystis compared with Scenedesmus. Clutch sizes were similar on the two foods, indicating that once individuals reached adulthood, the two foods were similar in quality. In contrast, at high food concentrations (1.0 mg C/L), the two algae were similar in quality for both juveniles and adults, and r was not significantly different on the two foods. Ingestion and assimilation rate experiments whowed that Daphnia consumes the two algae at identical rates, and that adults assimilate the two algae at similar rates. However, juveniles assimilate Oocystis at much lower rates than Scenedesmus, possibly accounting for reduced juvenile growth and delay in age at maturity at low concentrations. Thus, Daphnia exhibits an ontogenetic shift in its ability to utilize Oocystis, and this can result in juvenile bottlenecks in which survival and growth of young age classes are of critical importance in determining population dynamics. Because food quality effects were manifested primarily in juveniles and at low concentrations, food quality effects in nature will depend on phytoplankton abundance and age-structure of Daphnia populations.     

Effects of Microcystis aeruginosa on interference competition between Daphnia pulex and Keratella cochlearis

Laboratory experiments tested the hypothesis that a toxic strain of Microcystis aeruginosa decreases the ability of Daphnia pulex to interfere with Keratella cochlearis. To test a variety of conditions, juvenile and adult Daphnia were exposed to the cyanobacterium for different time periods prior to, and during the experiments. Adult Daphnia not only suppressed rotifers over successive two-day intervals, but also had a significant impact within a 24-hour period. However, the presence of Microcystis (5 × 105 cells ml^–1) decreased the Daphnia effect in both experiments. Although juvenile Daphnia also significantly suppressed Keratella population growth, the presence of Microcystis (10⁵ and 5 × 10⁵ cells ml^–1) caused a significant reduction in daphniid body size and decreased the ability of both nonacclimated and acclimated daphniids to suppress rotifers. Keratella inhalation and mortality are positively correlated with filtering rates and body size of Daphnia. Therefore, the feeding rates and size structure of a Daphnia population will determine its potential to interfere with vulnerable rotifers. In all experiments the presence of Microcystis significantly decreased the ability of Daphnia to interfere with this rotifer despite the fact that Keratella was also inhibited. In the field this effect might be augmented if Microcystis colonies are more easily ingested by cladocerans than by the rotifers.     

Combined effects of four Microcystis aeruginosa strains and Scenedesmus obliquus concentrations on population dynamics and resting egg formation of two Daphnia species

We examined the hypothesis that toxic effects of Microcystis aeruginosa to population dynamics, ephippial production, and resting egg formation of Daphnia were restricted by food quality levels. Four unicellular, toxic M. aeruginosa strains with contrasting concentration of microcystin and two Daphnia species were used in this study. The number of offspring at first reproduction, population densities, and maximum population growth rate of two Daphnia species were lower at the highest toxic M. aeruginosa 7820 treatment compared with other treatments under two concentrations of Scenedesmus obliquus. The maximum number of offspring at first reproduction of two Daphnia species appeared in the higher toxic M. aeruginosa 526 strain at the high concentration of S. obliquus. The effects of M. aeruginosa strains, S. obliquus, and their combination on the number of offspring at first reproduction and the maximum population growth rate of two Daphnia species were significant (P < 0.001). Two Daphnia species could not reproduce in the highest toxic M7820 treatment under two concentrations of S. obliquus. They had lower population size and maximum population growth rate in the higher toxic M526 treatment at the low concentration of S. obliquus, but they were higher at the high concentration of S. obliquus. This result suggests that high S. obliquus concentration could relieve the toxicity of M. aeruginosa to Daphnia, and Daphnia could utilize the lower toxic Microcystis as food. The cumulative ephippia numbers of two Daphnia species were more at the high concentration of S. obliquus than those at the low concentration. The percentage of ephippia containing no resting eggs of two Daphnia species was evidently higher at the low concentration of S. obliquus but was lower at the high concentration of S. obliquus. Our results indicated that the cumulative ephippia numbers of Daphnia were population density dependent at the high-level food, and the productions of ephippia in Daphnia were significantly controlled by microcystin concentration at the low-level food.     

Effects of CO2 concentrations on the freshwater microalgae, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus (Chlorophyta)

In order to investigate the possible impacts of increased atmospheric CO₂ levels on algal growth and photosynthesis, the influence of CO₂ concentration was tested on three planktonic algae (Chlamydomonas reinhardtii, Chlorella pyrenoidosa, and Scenedesmus obliquus). Increased CO₂ concentration enhanced significantly the growth rate of all three species. Specific growth rates reached maximal values at 30, 100, and 60 M CO₂ in C. reinhardtii, C. pyrenoidosa, and S. obliquus, respectively. Such significant enhancement of growth rate with enriched CO₂ was also confirmed at different levels of inorganic N and P, being more profound at limiting levels of N inC. pyrenoidosa and P in S. obliquus. The maximal rates of net photosynthesis, photosynthetic efficiency and light-saturating point increased significantly (p < 0.05) in high-CO₂-grown cells. Elevation of the CO₂ levels in cultures enhanced the photoinhibition of C. reinhardtii, but reduced that of C. pyrenoidosa and S. obliquus when exposed to high photon flux density. The photoinhibited cells recovered to some extent (from 71% to 99%) when placed under dim light or in darkness, with better recovery in high-CO₂-grownC. pyrenoidosa and S. obliquus. Although pH and pCO₂ effects cannot be distinguished from this study, it can be concluded that increased CO₂ concentrations with decreased pH could affect the growth rate and photosynthetic physiology of C. reinhardtii, C. pyrenoidosa, and S. obliquus.     

Carbon dioxide fixation by Chlorella kessleri, C. vulgaris, Scenedesmus obliquus and Spirulina sp. cultivated in flasks and vertical tubular photobioreactors

CO₂ at different concentrations were added to cultures of the eukaryotic microalgae, Chlorella kessleri, C. vulgaris and Scenedesmus obliquus, and the prokaryotic cyanobacterium, Spirulina sp., growing in flasks and in a photobioreactor. In each case, the best kinetics and carbon fixation rate were with a vertical tubular photobioreactor. Overall, Spirulina sp. had the highest rates. Spirulina sp., Sc. obliquus and C. vulgaris could grow with up to 18% CO₂.     

Effects of the novel allelochemical ethyl 2-methylacetoacetate from the reed (<Emphasis Type="Italic">Phragmitis australis</Emphasis> Trin) on the growth of several common species of green algae

Bioassays were performed to investigate the effects of the novel allelochemical, ethyl 2-methylacetoacetate (EMA), isolated from the reed (Phragmitis australis) on the growth of three common species of algae; Scenedesmus obliquus, Selenastrum capricornutum and Chlamydomonas reinhardtii. The results demonstrated that EMA has three quite different types of effect on these three species of algae. The growth of S. capricornutum was significantly inhibited by EMA during the whole cultivation period. The EC₅₀ values of EMA on S. capricornutum was 0.6 mg L⁻¹(7 days). However, the inhibitory effect of EMA on S. obliquus was apparent during the first 4 days of batch cultivation and then the inhibitory effect disappeared, and a stimulating effect was observed instead. The EC₅₀ value of EMA on S. obliquus was 0.43 mg L⁻¹(4 days). In addition, following the addition of EMA, the cells of S. obliquus and S. capricornutum became significantly larger than the normal untreated one and the algal cells changed morphologically. The microstructure of the algal cells was disrupted by the addition of EMA. There was no significant inhibition of the growth of C. reinhardtii by EMA, but cell motility was affected.     

Daphnia vertical distribution and the presence of toxic cyanobacteria

Daphnia can alter its vertical position in the water column in response to chemical cues from predators. In this study we tested the hypothesis that a Daphnia pulex clone with little evolutionary exposure to cyanobacteria would move away from patches of cyanobacteria (Anabaena affinis and A. flos-aquae) which contain potent endotoxins. Daphnia was censused at 2 h intervals for 6 h in laboratory columns in which there was a steep vertical gradient of cyanobacteria. Data were analyzed by repeated-measures ANOVA. Control (no Anabaena) and experimental columns showed no significant differences in Daphnia distributions. Daphnia in experimental columns frequently moved into areas with dense concentrations of Anabaena and stayed there for long periods of time. Our results show that this D. pulex clone does not exhibit a rapid (within 6 h) avoidance response to toxic Anabaena.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Relative effects of Daphnia and Ceriodaphnia on phosphorus-chlorophyll relationships in small urban lakes

Empirical models based on zooplankton biomass were used to predict mean summer chlorophyll a (Chl a) and to examine how zooplankton influenced the total phosphorus (TP) - Chl a relationship. Four years of data were analyzed for three lakes having similar TP concentrations but varied abundances of Daphnia and Ceriodaphnia. Mean TP did not correlate significantly with mean Chl a during the study period, although mean Daphnia density was a good predictor of Chl a concentration (p > 0.001). Both residuals from the TP - Chl a relationship (p > 0.001) and Secchi depth (p > 0.007) were negatively correlated with Daphnia abundance. Ceriodaphnia abundance was positively correlated with Chl a (p > 0.002) and Secchi depth (p > 0.001). Mean size of Daphnia during spring was the best predictor of the Daphnia-Ceriodaphnia shift in mid-summer. Early establishment of a large-sized Daphnia cohort may prevent their summer elimination by Chaoborus and intensify competition with Ceriodaphnia. These results imply an important link between Daphnia and Ceriodaphnia thereby limiting the utility of Chl a - TP model predictions in these small, urban lakes. This linkage and the differential effect of these two zooplankton species on planktonic algae deserve further consideration in similar lakes where phytoplankton and zooplankton tend to be tightly coupled.     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

The ecological role of chemical stimuli for the zooplankton: predator-induced morphology in Daphnia

Summary Numerous adaptive predator-induced responses occurred when eight clones representing seven Daphnia (Crustacea: Cladocera) species were tested against three common predators: fourth instar larval phantom midge Chaoborus americanus, adult backswimmer Notonecta undulata, and small sunfish Lepomis macrochirus. The predators were confined within small mesh bags, suggesting that the signal for induction is chemical. The induced responses included longer tail spines, longer heads, smaller bodies, increased egg clutches, and decreased lipid reserves. Each Daphnia species responded to each of the three predators in a unique manner. Induced responses in the above characters showed no significant association. The induced morphological changes are generally consistent with current theories of what is an adaptive response for the various sizes of Daphnia exposed to tactile and visual predators. The abundance of induced responses in these experiments suggests that predator-induced responses are a widespread and ecologically important phenomenon of the freshwater zooplankton.     

Spatial avoidance of littoral and pelagic invertebrate predators by <Emphasis Type="Italic">Daphnia</Emphasis>

Studies on spatial avoidance behaviour of predators by prey often ignored the fact that prey typically face multiple predators which themselves interact and show a spatial pattern in abundance and predation rates (PRs). In a series of laboratory experiments, we investigated predation risk (PRI) and horizontal migration of the cladoceran Daphnia magna between open water and vegetation in response to two important invertebrate predators with a contrasting spatial distribution: pelagic Choaborus and vegetation-associated Ischnura. As expected, PRI by Chaoborus was higher in open water due to higher numbers and higher PRs of Chaoborus, while for Ischnura, PRI was highest in the vegetation due to higher densities, despite lower PRs of Ischnura. In accordance with this, Daphnia moved into the vegetation in the presence of the pelagic Chaoborus alone. In the presence of Ischnura alone, however, Daphnia showed no response. We hypothesize this may be the result of a constitutive behaviour of Daphnia to avoid pelagic fish, which impedes a response to the open water. In the combined predator treatment, Daphnia migrated to the open water zone. The increased risk of predation in the vegetation, due to a facilitating effect of Chaoborus on Ischnura PRs is believed to have caused this migration of the Daphnia. This response of Daphnia declined through time and Daphnia moved toward the vegetation. A decline in the activity of the Ischnura larvae through time may have switched the risk balance in favour of the vegetation environment.     

<Emphasis Type="Italic">Daphnia</Emphasis>species diversity in Kenya,and a key to the identification of their ephippia

The distribution of Daphniaspecies in tropical Africa is poorly known and understood. Daphniaare assumed rare in tropical regions, but systematic studies covering large areas are sparse. We sampled the active community (live zooplankton) and/or the dormant community (diapausing egg banks in the sediment) of 41 standing water bodies in Kenya in search for Daphnia.Overall the dormant communities yielded 11 species of Daphnia, a species richness more than twice the species richness found in the active communities. Dormant community species diversity better reflects the spatial, and particularly the temporal (multi-annual) variation in environmental conditions available to Daphniain these tropical standing waters. Hence, we suggest that the dormant community be taken into account when assessing local zooplankton diversity, especially in fluctuating tropical lake ecosystems, where the presence of each local Daphniaspecies in the active community may be strongly seasonal or erratic. Geographic distribution data from this study are supplemented with previous records of Daphniain East Africa to provide an overview of the known distribution of Daphniain Kenya and neighbouring countries. We also present a detailed key for morphological identification of the ephippia of the 11 Daphniaspecies encountered, complemented with photographs and drawings of diagnostic characters.     

Contrasting ‘Top-Down’ Effects of Crustacean Zooplankton Grazing on Bacteria and Phytoflagellates

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.     

Distribution and ecology of Demospongiae from the circalittoral of the islands of the Aegean Sea (Eastern Mediterranean)

Size-selective predation by fish is often considered to be a primary driver of seasonal declines in large-bodied Daphnia populations. However, large Daphnia commonly exhibit midsummer extinctions in ponds lacking planktivorous fish. A number of empirical and theoretical studies suggest that resource competition and its interaction with nutrient enrichment may determine variable dominance by large Daphnia. Low resource levels may favor competitive dominance by small-bodied taxa while large Daphnia may be favored under high resource conditions or following a nutrient/productivity pulse. Nutrient enrichment may also influence the strength of invertebrate predation on Daphnia by affecting how long vulnerable juveniles are exposed to predation. We investigated these hypotheses using an in situ mesocosm experiment in a permanent fishless pond that exhibited seasonal losses of Daphnia pulex. To explore the effects of nutrient enrichment, Daphnia plus a diverse assemblage of small-bodied zooplankton were exposed to three levels of enrichment (low, medium, and high). To explore the interaction between nutrient enrichment and invertebrate predation, we crossed the presence/absence of Notonecta undulata with low and high nutrient manipulations. We found no evidence of competitive reversals or shifts in dominance among nutrient levels, Daphnia performed poorly regardless of enrichment. This may have been due to shifts in algal composition to dominance by large filamentous green algae. Notonecta had significant negative effects on Daphnia alone, but no interaction with nutrient enrichment was detected. These results suggest that Daphnia are not invariably superior resource competitors compared to small taxa. Though predators can have negative effects, their presence is not necessary to explain poor Daphnia performance. Rather, abiotic conditions and/or resource-based effects are probably of greater importance.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera