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Abstract
In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.
Highlights
Global change is resulting in more variable environmental conditions in both terrestrial and aquatic ecosystems [1,2,3], which may in turn be altering the variability of biotic communities [4,5,6]
Thermal stratification developed at the beginning of May in all four reservoirs in all three years (Figure 1A,C,E,G; Figures A1 and A2), which exhibited similar mean seasonal thermocline depths across all three
Contrary to our Global change is predicted to create more variable environmental conditions [1,2,3], which can predictions, we found that hypolimnetic hypoxia was associated with an increased biomass variability alter community interactions and the variability of ecological communities [4,5,6]
Summary
Global change is resulting in more variable environmental conditions in both terrestrial and aquatic ecosystems [1,2,3], which may in turn be altering the variability of biotic communities [4,5,6]. Understanding the variability of ecological communities is critical to predicting how ecosystems will respond to environmental change [12]. Decreasing dissolved oxygen (DO) concentrations below 2 mg L−1 in the bottom waters of lakes and reservoirs (hypolimnetic hypoxia) may disrupt biotic interactions and change the variability of freshwater ecological communities [13,14]. Water 2019, 11, 2179 reservoirs due to climate change, as the strength of thermal stratification increases due to warmer surface water (epilimnion) temperatures, thereby decreasing the exchange of oxygen between the epilimnion and hypolimnion [15,16,17]. Higher nutrient concentrations due to eutrophication from land use change are driving a decrease in DO concentrations in the hypolimnion of many waterbodies [18,19,20]
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