Population changes of Daphnia caused by declined calcium concentration: Evidences from population dynamics and sexual reproduction

Persistent and sex-independent effects of decreased calcium concentration inhibiting morphological defense of Daphnia: Evidences from morphological traits and expression of the associated genes

The continuous decline in calcium concentration in freshwater as a widespread environmental stress can have complex effects on the interspecific relationships of organisms, such as interference with the anti-predation defenses of Daphnia with high calcium demand. The natural population of Daphnia includes individuals with different developmental stages and sexes. Here, we measured the effects of decreased calcium concentration on morphological defense of Daphnia after different numbers of molts or under various sexes and the expression of genes related to signal recognition, carapace formation, reproductive allocation, and stress defense. Results showed that decreased Ca concentration resulted in the altered effects of fish kairomone on the change rates of body size, change rates of spine length, and change rates of relative spine length disappear. Furthermore, male Daphnia also developed morphological defense under fish predation risk, which was also inhibited by decreased Ca concentration, but no significant difference was observed in the intensity of induced defense between male and female Daphnia at low Ca concentrations. Importantly, decreased Ca concentrations did not alter the increase in expressions of genes related to neural signaling by fish kairomone. Fish kairomone promoted the expression of reproduction-related genes, whereas decreased Ca concentration inhibited their expression. Fish kairomone altered the expression of carapace-related genes, but most were disturbed by decreased Ca concentration. Decreased Ca concentration inhibited the increased expression of stress defense-related genes by fish kairomone. This study contributes to a more comprehensive understanding of the effects of environmental changes on interspecific relationships among aquatic organisms of different developmental stages and different sexes.

Impact of atrazine on the dynamic response of Daphnia pulex populations to fish predation risk

Herbicide pollution is persistent, which not only has a negative impact on individual organisms, but also may alter population dynamics and stability of interspecific relationships. Cladocerans, an important part of zooplankton, are often simultaneously exposed to environmental pollutants and predation risk in the aquatic environment. To evaluate the combined effects of atrazine and fish predation risk on the population traits of cladocerans, we exposed Daphnia pulex to different concentrations of atrazine (0, 0.05, 0.10, and 1.0 mg L−1) with or without fish (Rhodeus ocellatus) kairomone, recorded the key population traits, and fitted Gaussian model to population dynamics. Results showed that fish kairomone increased the population density at the end of the experiment and resting eggs production, and tended to decrease the total biomass and the average dry weight per individual of D. pulex. Atrazine reduced the total biomass, the average dry weight per individual, and resting eggs production of D. pulex populations. Atrazine also decreased the population density at the end of the experiment of D. pulex in fish kairomone treatment, and attenuated the promoting effect of fish kairomone on resting eggs production and the reduction of the total biomass. The findings highlighted the importance of considering the combined impact of environmental pollutants and predation risks on zooplankton populations.

A Non-Stressful Temperature Rise and Greater Food Availability Could Increase Tolerance to Calcium Limitation of Daphnia cf. pulex (Sensu Hebert, 1995) Populations in Cold Soft-Water Lakes

Calcium (Ca) is an important driver of community structure in freshwaters. We examined the combined effects of increased temperatures and variations in food quantity on the tolerance to low Ca of Daphnia pulex. The aim was to predict the impact of climate warming on this keystone zooplanktonic species in cold-climate lakes. We conducted a factorial life-history experiment in a clone of North American Daphnia cf. pulex to analyse the interaction effects of a temperature increase (17.5 °C−21 °C) within their physiological preferred range and expected by climate warming over the next few decades and a narrow Ca gradient (0.25−1.74 mg Ca L−1) under stressful vs. abundant food conditions. We found a striking positive synergistic effect of Ca and temperature on D. pulex reproduction at high food conditions. Although the increase in temperature to 21 °C greatly reduced survival, high energy allocation to reproduction at high food levels allowed the population to succeed in poor Ca (<0.25 mg Ca L−1). Results suggest that climate warming and higher food availability will make the populations of many cold and Ca-limited lakes more tolerant to low Ca levels with higher growth population rates, thereby altering zooplanktonic community structures and inducing potential cascading effects on the food web.