Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities

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.

Zooplankton Community Responses to Oxygen Stress

“…Many field observations indicated that the maximum zooplankton density during the day is usually in the top part of the metalimnion with a relatively high oxygen concentration [78][79][80][81]. Other field studies revealed that zooplankton could be found in high abundances in anoxic waters [2,23,82,83]. We observed similar patterns where the upper anoxic layers had a higher biomass of zooplankton and also a large proportion of large Cladocera.…”

“…Therefore, inedible algae or cyanobacteria could be the main factor that favors small zooplankton species in these lakes because they are better competitors than large herbivorous filter-feeders in the presence of large inedible algae [62]. However, the dominance of Rotifera and small Cladocera could also be a result of oxygen stress, which may increase the presence and dominance of these taxa in zooplankton communities that are less sensitive to hypoxic conditions [2]. This is consistent with our results, which showed that most rotifers seemed to prefer low oxygen conditions, and the smaller Cladocera were more resistant to oxygen depletion than the larger daphnids.…”

“…Oxygen stress is a common phenomenon in freshwater ecosystems as lakes worldwide are increasingly experiencing the depletion of DO in the water layers below the epilimnion. Dissolved oxygen depletion results from a number of natural factors, but is most often considered a consequence of eutrophication from changes in land use [2]. Increased concentrations of nutrients favor the mass development of phytoplankton, which raises the DO saturation through photosynthesis during the day and reduces the DO saturation through respiration during the night.…”

Data assimilation experiments inform monitoring needs for near‐term ecological forecasts in a eutrophic reservoir

“…During a typical year, BVR is stratified from mid‐March to late October and mixed from November to early March (Hounshell et al, 2021). BVR experiences summer hypolimnetic anoxia and cyanobacterial blooms, both of which are controlled by water temperature and thermal stratification (Doubek et al, 2019; Hamre et al, 2018), making forecasts of water temperature important for water quality management.…”

Unsupervised Machine Learning and Data Mining Procedures Reveal Short Term, Climate Driven Patterns Linking Physico-Chemical Features and Zooplankton Diversity in Small Ponds

“…In our study, the presence of Cladocera decreased between years, but Cyclopoida disappeared with the highest trophic status observed in 2015. Variation in community structure alters ecosystem functioning and biodiversity metrics can indicate how communities influence ecosystems [101][102][103][104]. Seasonal and interannual increases in Daphnia abundance have been associated with P limitation due to higher requirements in Daphnia than in other taxa [105].…”