# Zooplankton Community Responses to Oxygen Stress
## 1 Introduction  
The dissolved oxygen (DO) concentration in the water column is a crucial parameter for the successful development of many pelagic organisms [1]. 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. When the algae die, they sink towards the bottom and are decomposed by bacteria, which further reduces the DO in the water column [3]. Recent changes in the climate also contribute to hypolimnetic oxygen depletion due to higher surface water temperatures, which strengthen thermal stratification, thereby decreasing the exchange of oxygen between the epilimnion and hypolimnion [4; 5; 6]. A similar process has been observed for marine ecosystems, whereeutrophication induced oxygen depletion was responsible for approximately half of the known dead zones [7]. Dissolved oxygen concentrations below 2 mg L\({}^{-1}\) are referred to as hypoxic conditions, and the near-complete absence of DO (<0.5 mg L\({}^{-1}\)) is referred to as anoxic conditions [7; 8; 9]. Oxygen stress also depends on the water temperature, which controls both oxygen solubility and the metabolic demand of aquatic ectotherms [9]. Oxygen depletion disrupts biotic interactions and causes significant changes in freshwater food webs [10; 11]. However, the oxygen tolerance and threshold values can vary considerably for different organisms [12]. Generally, fish seem to be more vulnerable to hypoxia than crustaceans and mollusks [13]. With regard to zooplankton communities, it is generally assumed that oxygen stress plays an important role in determining the community structure and vertical distribution [14; 15; 16]. Anoxic water layers may disrupt zooplankton diel vertical migrations, and, during the day, zooplankton may experience a trade-off between (i) increased metabolic stress and mortality due to low DO in the hypolimnion and (ii) mortality due to increased fish predation and increased cellular damage due to UV radiation exposure in the epilimnion [9; 17]. Our previous research in humic lakes revealed that anoxic conditions 2-3 m below the surface were the main factor limiting the biomass and diversity of zooplankton [15]. Other field studies suggest that decreases in metalimnetic and hypolimnetic oxygen concentrations resulted in a shift in the size of zooplankton communities from large to smaller species [14; 18; 19]. However, other studies showed that metalimnetic hypoxia could be a refuge for large zooplankton species as they are more tolerant to lower oxygen concentrations than most pelagic fish [20; 21; 22; 23; 24]. Deep chlorophyll maxima, which are often followed by metalimnetic hypoxia, are also an additional refuge for zooplankton [25; 26]. The results from field studies revealed that even glacial relicts, such as _Eurytemora lacustris_, were the most numerous in hypoxic metalimnia, suggesting that this species could tolerate the warmer water and lower oxygen concentrations due to better food resources [27]. Other experimental research indicated high physiological and demographic costs for zooplankton in an environment with a low oxygen concentration [28; 29; 30; 31]. The impact of oxygen stress on pelagic organisms in lakes remains largely enigmatic, and there is a lack of evidence showing the direct impacts of oxygen stress on zooplankton communities [23; 32]. This is especially important as zooplankton are a key component of freshwater food webs and biogeochemical cycles, serving as the major trophic link between primary producers (phytoplankton) and fish.