Consumer adaptation mediates top-down regulation across a productivity gradient

Positive and negative impacts of flue gas desulfurization (FGD) gypsum on water quality

Flue gas desulfurization (FGD) gypsum, a by-product of carbon-based energy sources, has typically been incorporated as a component of concrete mixes and wallboard and beneficially used as an agricultural amendment to enhance terrestrial crop production and improve the quality of runoff. These various uses for the by-product aid in reducing the amount that is ultimately landfilled. Limited studies have investigated its benefits when used directly in aquatic settings, such as ponds and lakes, to increase hardness and potentially mitigate eutrophication. A 36-day field mesocosm experiment tested a larger range of FGD gypsum concentrations (500-2000 mg/L) than those previously tested in the literature to investigate its desired and potentially undesired impacts on water quality, including the algal community. High FGD gypsum concentrations, 1000 and 2000 mg/L, were found to have more undesired impacts than the 500 mg/L treatment, including an initial spike in cyanobacteria, a decrease in total zooplankton abundance, and an increase in certain trace metals in the highest treatment. Ultimately, the 500 mg/L FGD gypsum treatment was found to have fewer undesired impacts while still resulting in significant desired effects, including those on hardness and pH, as well as moderate reductions in algal abundance. This experiment provides a better understanding of the effects of FGD gypsum when directly used in an aquatic setting, determines an optimal dose for future field experiments, and helps provide the groundwork for developing an upper threshold on FGD gypsum so as to not have the negative effects outweigh the positive.

Performance of Daphnia simultaneously exposed to nitrite and predation risk: Reduced nitrite tolerance and aggravated predation-induced miniaturization

Nitrogenous pollutants derived from human activities not only pose direct risk on aquatic organisms but may also indirectly endanger the stability of interspecific relations. To date, the effects of the nitrogen-containing pollutants on the induced defense remain unclear. Here, we aim to investigate the induced defense of an aquatic keystone species, Daphnia pulex, which responds to predation risk under nitrite pollution at environmentally relevant concentrations and simultaneously evaluate the effects of their induced defenses on nitrite tolerance. Results showed that increasing nitrite significantly reduced the survival time of D. pulex and posed severe reproductive toxicity, consequently reducing the offspring and broods. In the morphological defensive responses, early nitrite exposure interfered with the spine elongation, but the relative spine length induced by the predation risk was unaffected by the nitrite concentrations with exposure time prolonged, although high-dose nitrite inhibited the spine elongation and the increase of the body size. The integration of biomarker response index analyses further indicated that the reproductive capacity was more seriously impaired than the morphology and the survival. Moreover, the sensitivity analyses of growth and reproduction indicated that predation risk significantly reduced Daphnia's tolerance to nitrite. Conclusively, these findings highlight that long-term nitrite exposure exacerbates the predator-induced miniaturization of zooplanktons, and predation risk also reduces their tolerance to nitrite, which provides new insights into the performance changes of zooplanktons exposed to pollutants under predation risk and the vulnerability of predator-prey interspecific relationships in polluted environments.

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

The decline in freshwater calcium has become a new environmental stressor to Daphnia with high calcium demand, however, the population dynamics and sexual reproduction of Daphnia under low calcium stress are still lack of deep understanding. To evaluate the impact of declined calcium on Daphnia from population level, we respectively exposed two clones of Daphnia pulex (CH and SH) to different calcium concentrations (0.5, 1.0, 1.5, 5.0, 10.0, 25.0 mg L^-1) for 30 days and recorded the population indicators. Results showed that total biomass, average dry weight per individual, total number of ephippia, total number of resting eggs of Daphnia pulex CH clone at 1.0 mg L^-1 calcium decreased by 75.5%, 34.0%, 83.6%, and 77.6% compared with those at 25 mg L^-1 calcium, while SH clone at 1.0 mg L^-1 calcium decreased by 64.6%, 26.1%, 94.5%, and 82.2%, respectively. Importantly, Ca content in dry Daphnia pulex population of CH clone at 1.0 and 1.5 mg L^-1 calcium decreased by 32.7% and 6.7% compared to those at 25 mg L^-1 calcium, and SH clone at 1.0 mg L^-1 and 1.5 mg L^-1 calcium also decreased by 30.9% and 10.5%, respectively. Furthermore, low calcium significantly decreased the perimeter and surface area of ephippia. Interestingly, observation by scanning electron microscope found that low calcium changed the surface of ephippia. The negative impact of calcium decline on Daphnia population and sexual reproduction will inevitably endanger the persistence of species and genes at meta population level.

Local adaptation mediates direct and indirect effects of multiple stressors on consumer fitness

Anthropogenic impacts are expected to increase the co-occurrence of stressors that can fundamentally alter ecosystem structure and function. To cope with stress, many organisms locally adapt, but how such adaptations affect the ability of an organism to manage co-occurring stressors is not well understood. In aquatic ecosystems, elevated temperatures and harmful algal blooms are common co-stressors. To better understand the role and potential trade-offs of local adaptations for mitigating the effects of stressors, Daphnia pulicaria genotypes that varied in their ability to consume toxic cyanobacteria prey (i.e., three tolerant and three sensitive) were exposed to five diets that included combinations of toxic cyanobacteria, Microcystis aeruginosa, and a green alga, Ankistrodesmus falcatus, under two temperatures (20 °C vs. 28 °C). A path analysis was conducted to understand how local adaptations affect energy allocation to intermediate life history traits (i.e., somatic growth, fecundity, survival) that maximize Daphnia fitness (i.e., population growth rate). Results from the 10-day study show that tolerant Daphnia genotypes had higher fitness than sensitive genotypes regardless of diet or temperature treatment, suggesting toxic cyanobacteria tolerance did not cause a decrease in fitness in the absence of cyanobacteria or under elevated temperatures. Results from the path analysis demonstrated that toxic cyanobacteria had a stronger effect on life history traits than temperature and that population growth rate was mainly constrained by reduced fecundity. These findings suggest that local adaptations to toxic cyanobacteria and elevated temperatures are synergistic, leading to higher survivorship of cyanobacteria-tolerant genotypes during summer cyanobacterial bloom events.

Draft genomes for one Microcystis-resistant and one Microcystis-sensitive strain of the water flea, Daphnia pulicaria

Daphnia species are well-suited for studying local adaptation and evolutionary responses to stress(ors) including those caused by algal blooms. Algal blooms, characterized by an overgrowth (bloom) of cyanobacteria, are detrimental to the health of aquatic and terrestrial members of freshwater ecosystems. Some strains of Daphnia pulicaria have demonstrated resistance to toxic algae and the ability to mitigate toxic algal blooms. Understanding the genetic mechanism associated with this toxin resistance requires adequate genomic resources. Using whole-genome sequence data mapped to the Daphnia pulex reference genome (PA42), we present reference-guided draft assemblies from one tolerant and one sensitive strain of D. pulicaria, Wintergreen-6 (WI-6), and Bassett-411 (BA-411), respectively. Assessment of the draft assemblies reveal low contamination levels, and high levels (95%) of genic content. Reference scaffolds had coverage breadths of 98.9-99.4%, and average depths of 33X and 29X for BA-411 and WI-6, respectively. Within, we discuss caveats and suggestions for improving these draft assemblies. These genomic resources are presented with a goal of contributing to the resources necessary to understand the genetic mechanisms and associations of toxic prey resistance observed in this species.