# Zooplankton Community Responses to Oxygen Stress
## 4 Discussion  
Our study provides valuable insight into the role that oxygen stress plays in altering zooplankton communities and carbon transfer in planktonic food webs under different trophic conditions. Freshwater zooplankton in the study lakes were relatively tolerant of anoxic conditions, and large changes in community structure were only found in lakes with the greatest oxygen deficits. The first response of zooplankton to oxygen depletion that we observed was reduced biomass in lower water layers, but generally not in the layer where oxygen deficits began, i.e., in the anoxic metalimnion. We observed even higher biomass of zooplankton in anoxic metalimnia, although warmer waters where the oxygen solubility is lower and the oxygen demand (respiration) is higher [9]. The second response of zooplankton that we observed was a switch in dominance from large to small species. Large Cladocera were the dominant component of zooplankton in lakes with oxic conditions (G1-G3), while in lakes with large oxygen depletion (G4-G6) Rotifera and small Cladocera were the dominant component of the zooplankton communities. The switch from large to small Cladocera could be connected with a higher trophic status and with the dominance of inedible algae or cyanobacteria. Large-bodied cladocerans dominated in low trophic level lakes and oxic conditions because they are better competitors for food than small-bodied species [53; 54; 55]. However, larger cladocerans experienced difficulty feeding in the presence of cyanobacteria because they do not feed selectively, and longer algae filaments clog their filtering apparatuses [56; 57; 58; 59]. The mass development of inedible algae _Gonyostomum semen_ is a common phenomenon in humic lakes in NE Poland [15; 60; 61], and the lakes from G4 were eutrophic and dominated by cyanobacteria. 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. The results from field studies also suggest that decreased meta- and hypolimmetic oxygen concentrations resulted in a shift in zooplankton from large to smaller species [14; 18; 19]. We observed a similar pattern of changes in zooplankton community structure as a result of oxygen stress, regardless of trophic status, which indicated that oxygen stress is one of the most important factors altering zooplankton structures. Rotifers are generally more resistant to oxygen deficits than crustaceans [63; 64], and laboratory experiments have shown that hypoxia extends the lifespan of rotifers [65]. The results of our study confirm that rotifers are more tolerant of anoxic conditions. Additionally, we found that the greatest biomass of rotifers was observed in anoxic conditions, which suggests that they may have a competitive  
Figure 9: The effectiveness of carbon transfer (%) between the phytoplankton (primary production) and zooplankton (secondary production) in epilimnion (**a**), metalimnion (**b**), and hypolimnion (**c**) in lakes with different oxygen conditions. Note: the differences in scales between the y-axes in the figures. advantage over crustaceans. Rotifers are often suppressed by large _Daphnia_ both through exploitative competition for shared and limited food resources, as well as through mechanical interference [66; 67]. The results from more than 500 freshwater Swedish localities also confirmed that many rotifer species can tolerate hypoxic and anoxic conditions.