Microcystis blooms are generally associated with zooplankton shifts by disturbing interspecific relationships. The influence of Microcystis on competitive dominance by different sized zooplanktons showed species-specific dependence. We evaluated the competitive responses of small Moina micrura and large Daphnia similoides to the presence of Microcystis using mixed diets comprising 0%, 20%, and 35% of toxic M. aeruginosa, and the rest of green alga Chlorella pyrenoidosa. No competitive exclusion occurred for the two species under the tested diet combinations. In the absence of M. aeruginosa, the biomasses of the two cladocerans were decreased by the competition between them. However, the Daphnia was less inhibited with the higher biomass, suggesting the competitive dominance of Daphnia. M. aeruginosa treatment suppressed the population growths of the two cladocerans, with the reduced carrying capacities. Nonetheless, the population inhibition of Daphnia by competition was alleviated by the increased Microcystis proportion in diet. As a result, the competitive advantage of Daphnia became more pronounced, as indicated by the higher Daphnia: Moina biomass ratio with increased Microcystis proportions. These results suggested that M. aeruginosa strengthens the advantage of D. similoides in competition with M. micrura, which contributes to the diversified zooplankton shifts observed in fields during cyanobacteria blooms.

Competition amongst different‐sized cladocerans has attracted the interest of many researchers because of their similar life strategies and overlap in feeding habits. To test the combined effects of food concentration and initial relative abundance on the competition outcomes between small‐ and large‐bodied cladocerans, we grew large Daphnia magna and small Moina micrura with two initial relative abundances under five algal densities. At the lowest food concentration (0.10 mg L−1 carbon), D. magna could not maintain a population in the presence or absence of M. micrura. Under conditions in which both species grew together, food levels as high as 0.30 mg L−1 favoured superior population growth of D. magna, irrespective of inoculation abundance. Under conditions in which both species had equal initial abundance, population growth of M. micrura was inhibited by the presence of Daphnia at ≥0.30 mg L−1 food levels. M. micrura was superior to D. magna at higher food concentrations (≥0.60 mg L−1) and high initial abundance. These results suggest that population growth of large Daphnia was more successful than that of small Moina when the food availability was low, while the small Moina dominated under conditions of high initial abundance and high food supply. Therefore, interspecific competition amongst cladocerans depends not only on the size of competing species, but also on their initial relative abundance and food availability.

To determine the combined effects of temperature and minimal change in food size on small-sized cladocerans, we fed Moina micrura with unicells and small colonies of Scenedesmus obliquus at different temperatures (20, 25, and 30 degrees C) and evaluated the parameters related to development and reproduction. Results showed that development to maturity was not affected by food sizes but significantly promoted by increasing temperature. The number of broods and total offspring significantly increased at 30 degrees C when M. micrura fed on unicellular S. obliquus but was not significantly different among the three temperatures when fed on colonial S. obliquus, i.e. only at 30 degrees C, the reproductive performance of M. micrura fed on unicellular S. obliquus was significantly higher than those fed on colonial S. obliquus, indicating that the effect of food size on the reproductive performances of M. micrura is dependent on temperature. (C) 2015 Elsevier Ltd. All rights reserved.

Superoxide dismutases (SODs) are metalloenzymes that represent one important line of defense against oxidative stress produced by reactive oxygen species in aerobic organisms. Generally, waterborne pollutants caused by irregular anthropogenic activities often result in oxidative damage in aquatic organisms. The aim of this study was to molecularly characterize the manganese superoxide dismutase gene (Dm-MnSOD) in the waterflea, Daphnia magna, and evaluate the mRNA expression patterns quantified by real-time PCR after exposure to three common waterborne pollutants (microcystin-LR, nitrite, and cadmium). The results showed that the full-length Dm-MnSOD sequence consists of 954 bp nucleotides, encoding 215 amino acids, showing well-conserved domains that are required for metal binding and several common characteristics, such as two MnSOD domains. The deduced amino acid sequence of Dm-MnSOD shared over 70% similarity with homologues from Bythograea thermydron, Dromia personata, Cancer pagurus, and Scylla paramamosain. Dm-MnSOD gene expression was up-regulated in response to exposure to the three chemicals tested. The overall results indicated that Dm-MnSOD gene is an inducible gene and potential biomarker indicating these pollutants in the environment.

Accumulation and breakdown of large algal blooms often causes serious hypoxia in lakes, representing a major potential threat to zooplankton grazers such as Daphnia. Generally, populations comprise different age groups, whose molecular responses to environmental stressors are different. The aim of our study was to investigate the age-dependent effects of oxygen depletion on D. magna survival and selected gene expressions of energy-relevance and stress-defense. Four age groups of D. magna (1, 4, 7 and 14 days) were exposed to 0.0625, 0.125, 0.25 mM dissolved oxygen (DO) for 48 h. Results showed that survival rates of 14-day-old individuals in 0.0625 mM DO and 0.125 mM DO were much lower than those in 0.25 mM DO, while no significant difference was found in remaining age groups. Low DO up-regulated hemoglobin expression compared with the controls in all age groups; the similar pattern in a-esterase was observed when 1-day-old individuals were exposed to 0.0625 mM, 4and 7-day-old individuals at 0.125 mM, respectively. HSP70 and CAT expression were significantly upregulated in 1and 7-day-old individuals exposed to 0.125 mM. The findings indicate that age structure should be taken into account when interpreting dynamics of Daphnia populations affected by oxygen depletion. Age may modify energy reallocation and alter susceptibility of the organisms.

The aim of this study was to assess the inter-specific differences in survival and reproduction traits of two species of cladocerans to nitrite gradient. Daphnia similis and Daphnia magna were exposed to different nitrite levels for 21 days. Results showed that survival time, number of moults, clutches per female, offspring per clutch, and total offspring per female decreased significantly with increasing nitrite concentration. The negative effects of nitrite on life-history traits of the two Daphnia species were dose-dependent and can be fitted by three-parameter logistic model. According to the model, the EC(50)s of survival time, number of moults, clutches per female, offspring per clutch, and total offspring per female for D. similis were 8.1, 7.1, 5.0, 4.4, 3.1 mg L-1, respectively, whereas those for D. magna were 3.9, 3.7, 2.3, 1.2, and 1.1 mg L-1, respectively, suggesting the tolerances of the two Daphnia species to nitrite are inter-specific. (C) 2012 Elsevier Ltd. All rights reserved.

The degradation of cyanobacterial blooms often causes hypoxia and elevated concentrations of ammonia, which can aggravate the adverse effects of blooms on aquatic organisms. However, it is not clear how one stressor would work in the presence of other coexistent stressors. We studied the toxic effects of elevated ammonia under hypoxia using a common yet important cladoceran species Daphnia similis isolated from heavily eutrophicated Lake Taihu. A 3 × 2 factorial experimental design was conducted with animals exposed to three un-ionized ammonia levels under two dissolved oxygen levels. Experiments lasted for 14 days and we recorded the life-history traits such as survival, molt, maturation, and fecundity. Results showed that hypoxia significantly decreased survival time and the number of molts of D. similis, whereas ammonia had no effect on them. Elevated ammonia significantly delayed development to maturity in tested animals and decreased their body sizes at maturity. Both ammonia and hypoxia were significantly detrimental to the number of broods, the number of offspring per female, and the number of total offspring per female, and significantly synergistic interactions were detected. Our data clearly demonstrate that elevated ammonia and hypoxia derived from cyanobacterial blooms synergistically affect the cladoceran D. similis.

Daphnia spp. feeds on microalgae and can potentially be used to screen microalgae, which are grown in ammonia-containing wastewater to scavenge available nutrients. A key step for this to work is that Daphnia spp. can still feed in the presence of elevated levels of un-ionized ammonia. To test that, we studied the life-history of three species of Daphnia (Daphnia obtusa, Daphnia similoides, and Daphnia similis) exposed to ammonia-containing water for 21 days. Results showed that survival, development, and fecundity of three tested Daphnia species were negatively affected by un-ionized ammonia. Although there were some differences in details among three species, the three-parameter logistic model fitted the data of three species well and showed similar patterns of response to un-ionized ammonia toxicity. After the 21-day exposure, the increasing NH3N concentrations slightly decreased the survival time by about 1 or 2 days in D. similoides and D. similis, but by 10 days in D. obtusa. The time to maturity was delayed in all three species after being exposed to ammonia. With increasing un-ionized ammonia concentrations, the number of moults decreased from 14.50 to 8.75 in D. obtusa, 13.60 to 11.20 in D. similoides, and 14.25 to 12.25 in D. similis. The overall reproduction per female was reduced by 81.4% in D. obtusa, 82.1% in D. similoides, and 64.6% in D. similis under the NH3N concentration of 0.58mg/L. The EC(50)s for total offspring per female in D. obtusa, D. similoides, and D. similis were 0.27, 0.55, and 0.54mg/L, respectively. Based on the experimental results, D. similoides and D. similis, showing stronger ammonia-resistance than D. obtusa, were more promising for screening microalgae cultivated in ammonia-containing wastewater.