# Zooplankton Community Responses to Oxygen Stress
## 4 Discussion
For example, _Polyarthra vulgaris, P. remata, P. major, Asplanchna priodonta,_ and _Kellicottia longispina_ can tolerate oxygen conditions close to 0 mg L\({}^{-1}\), although their maximum abundance occurred at high oxygen concentrations [68; 69]. This contrasts with our study where the maximum biomass of Rotifera was found in low oxygen conditions. The information about oxygen preferences for rotifer species is often inconclusive. [PERSON] (1977) showed that for _P. vulgaris_ the preferable oxygen range was significantly lower than for _K. cochlearis_[70], which is supported by our results. However, this contrasts with the results from Latvian lakes, where _P. vulgaris_ abundance fluctuated before or after low oxygen conditions [64]. Additional studies on _K. cochlearis_ suggest that this species can tolerate hypoxic waters, but oxygen stress resulted in a smaller body size [71]. Many authors suggest that _Asplanchna_ sp. is sensitive to oxygen limitation [64; 70; 72], while in our study _Asplanchna priodonta_ had a strong preference for low oxygen conditions. The results of our study clearly indicated that most of the rotifer species could tolerate low oxygen conditions better than microc crustaceans and many of them may reach higher density in hypoxic and anoxic conditions. Many crustacean species can tolerate DO between 1 and 2 mg L\({}^{-1}\), with lethal oxygen concentrations for daphnids and copepods between 0.3 and 1.0 mg L\({}^{-1}\)[73; 20; 72]. Even glacial relicts like the copep _Eurytemora lacustris_, which is sensitive to oxygen depletion requires a minimum of 1.0 mg O\({}_{2}\) L\({}^{-1}\)[74; 75]. Laboratory experiments for _Daphnia_ species established a threshold oxygen level at which animals succumbed to oxygen stress at 0.5-1.0 mg L\({}^{-1}\)[20]. Crustaceans experience increased metabolic stress in anoxic waters, however, andlow-oxygen tolerance experiments suggest that _Daphnia_ are able to regulate oxygen metabolism across a wide range of DO and produce clones with different amounts of hemoglobin [20]. Other laboratory experiments have shown that crustaceans exhibit reduced survival in anoxic conditions, and mortality can occur within a few hours [17; 76; 77]. The results from field studies are more ambiguous. 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; 82; 83; 23]. We observed similar patterns where the upper anoxic layers had a higher biomass of zooplankton and also a large proportion of large Cladocera. Our findings indicate that the upper anoxic layer could be a very important refuge from predation for large species during the day. Studies in marine and freshwater systems have shown that fish are more sensitive to low DO than zooplankton. Most of the freshwater fishes avoid areas where the DO is below 2-4 mg L\({}^{-1}\), while the DO below 2 mg L\({}^{-1}\) is lethal for most species [11; 21; 22; 84; 85]. Although some species of fish developed behavior to dwell for short time periods in the oxygen-deficient meta- and hypolimnion and forage there even in highly hypoxic conditions [32], the results of our study revealed that even large _Daphnia_ could tolerate much lower oxygen concentrations and were often found in the upper anoxic layer. Additionally, our results indicated that the large predatory cladocerans, _Leptodora kindii_ strongly avoid anoxic conditions. We also showed that zooplankton prefers to experience metabolic stress in the anoxic layer during the day to avoid predation. Zooplankton diversity differed between harmonic and dystrophic lakes. The lowest diversity of crustaceans and rotifers were found in dystrophic lakes. Our previous studies indicated that humic lakes do not offer many niches for zooplankton because of the sharp thermal gradient that results in a shallow layer of oxygenated water [15].