# Zooplankton Community Responses to Oxygen Stress
## 3 Results
The biomass of small Cladocera was also not affected by oxygen concentrations (F = 0.09; \(p\) = 0.74), and high biomass of small Cladocera was observed in different oxygen levels (Figure 7a), except the highest oxygen concentrations (>9 mg L\({}^{-1}\)). Oxygen concentration significantly affect the biomass of Cyclopodia (F = 12.3; \(p\) = 0.001) and Calanoida (F = 4.5; \(p\) = 0.035). Their biomasses generally increased with higher oxygen concentrations (Figure 7b). However, a high biomass of Cyclopodia and Calanoida was observed at low oxygen levels (Figure 7b). The Habitat Preference Index (HPI) revealed that most of the Cladocera species indicated weak preferences for oxic water layers; however, this also indicated their tolerance for anoxic conditions (Figure 8a). Only _Bosmina broiliensis_ had strong preferences for anoxic conditions. In contrast, _Leptodora kindii_ strongly avoided anoxic conditions. Also, _Daphnia cucullata_, _Bosmina crassicornis_ and _Diaphanosoma brachyum_ appeared to prefer oxic water layers (Figure 8a), but in the case of the last species, this could also be its preference for higher temperature in the epilimnion. Copepoda taxa showed no clear preferences for oxygen conditions (Figure 8a). Rotifers seemed to be more tolerant of anoxic conditions (Figure 8b) then crustacean species (Figure 8a). The preference for anoxic conditions was indicated for _Asplanchina priodonta_, _Collothea mutabilis_, _Conchiloides dossuarius_, _Keratella quadrata_, and _Polyarthra vulgaris_. No clear preferences were shown for _Keratella cochlearis_, _Keratella tecta_, _Pompholyx  
Figure 6: The numbers of rotifers and crustacean species in anoxic (grey outline box) and oxic water layers (black outline box) of lakes with oxygen stress (groups G2 to G6). Figure 7: Biomass of Cladocera, Rotifera (**a**), and Copepoda (**b**) along oxygen concentration scales. sulate_, and _Trichocerca similis_. Only _Polyarthra remata_ showed a clear preference for oxic water layers (Figure 8b). The effectiveness of carbon transfer between phytoplankton and zooplankton increased with depth in all lakes, and the oxygen conditions in each layer were an important factor influencing the transfer of matter in the food web (Figure 9). The effectiveness of carbon transfer in the epilimnion ranged from 0.00004% to 0.19%, with an average of 0.044%. There were also significant differences between the groups G1-G3 and G4-G6 in (F = 14.2; \(p\) = 0.001) the epilimnion (Figure 9a). The effectiveness of carbon transfer in the metalimnion ranged from 0.0026% to 5.37%, with an average of 0.64%. There were also significant differences between the groups G1-G4 and G5-G6 (F = 5.5; \(p\) = 0.024) in the metalimnion (Figure 9b). The average effectiveness in the metalimnion of G1-G4 was 0.89%, while it was 0.13% in G5-G6. The highest effectiveness of carbon transfer between phytoplankton and zooplankton was in the hypolimnion, and ranged from 0.007% to 151.84%, with an average of 18.26%. There were also differences between the groups G1-G3 and G4-G5 (F = 4.4; \(p\) = 0.041) in the hypolimnion (Figure 9c). The average effectiveness in the hypolimnion of G1-G3 was 25.4%, while it was 1.28% in G4-G5. Figure 8: Habitat Preference Index (HPI) to the anoxic/oxic water layer for selected Crustacea (**a**) and Rotifera species (**b**). Positive values mean preference for oxic layer, while negative values means preference for anoxic layer. Abbreviations Crustacea (**a**): Chysph.—_Chydorus phahericus_; Bos.ber.—_Bosmina berolinensis_; Bos.cra.—_Bosmina crasiscornis_; Bos.lon.—_Bosmina longispina_; Bos.the.—_Bosmina thersites_; Cer.qua.—_Ceriodaphnia quadrangula_; Dia.bra.—_Diaphanosoma brachyutum_; Dap.cuc.—_Daplnnia cucultata_; Lep.kin.—_Leptodora kindti_; Mes.leu.—_Mesocyclops leuckarti_; The.oit.—_Thermocyclops oithonoides_; cop.Cyc.—copepodites Cyclopoida, nau.Cyc.—nauplii Cyclopoida; Eud.sp.—_Eudiaptomus_ species; cop.Cal.—copepodites Calanoida; nau.Cal.—nauplii Calanoida.