# Zooplankton Community Responses to Oxygen Stress
## 4 Discussion
This, together with the high UV radiation in surface waters, limits the distribution of zooplankton to a very narrow water layer [86]. The other limiting factor for zooplankton is the humic stress related to the high content of dissolved organic carbon, humic substances, and low pH [36; 87; 88]. However, we found a few species that were present only in dystrophic conditions. One of them was _Holog medium gibberum_, which is widely distributed in water bodies of the boreal zone [89]. This species is rare for the Polish fauna and was noted in softwarelobelia lakes [90]. There is a relatively long list of rotifer species limited in their occurrence to dystrophic lakes, e.g., _Kerattella paludosa, Lecane acus, Lecane galeata, Lecane elasma, Polyarthna minor, and Microcodon clavus_. However, _Trichocerca simoneae_ only recently inhabited Polish dystrophic lakes, and now is among those species achieving the highest densities [91]. This successful invasion is, however, possible only in habitats with a low number of zooplankton species of low density, and dystrophic lakes seem to offer such conditions. The oxygen deficits in harmonic lakes did not reduce the diversity of Rotifera and Crustacea communities. We found a similar number of species in lakes with different trophic conditions (oligotrophic, mesotrophic, eutrophic), as well as in lakes with different oxygen conditions (G1-G4). Moreover, our results suggest that oxygen deficits in low trophic conditions could even increase the crustacean diversity as they created niches for small-bodied Cladocera. This is an interesting phenomenon as most often the opposite is true, where a well-oxygenated hypolimnion zone promotes a large diversity of crustacean zooplankton due to vertical niche segregation [16; 81; 92]. Our results also confirm this idea, as most of the lakes without oxygen depletion supported planktonic glacial relicts that have high habitat requirements. We found glacial relicts in 11 lakes, and among them were _Daphnia longiremis_, _Bythotrephes brevimanus_, _Cyclops lacustris_, _Eurytemora lacustris_, and _Heterocope appendicitisa_. Our study extends our knowledge about the distribution of planktonic relict species in central Europe. Until now, we did not know much about glacial relicts in Poland as data only existed from a few lakes that were studied often [16] and a few single records [81; 90; 93; 94] that were available. Consequently, our research has identified many new records, which suggest that glacial relicts are more common in the deep lakes of northern Poland. Another important finding is a revision of the distribution of _Bythotrephes_ in Poland as most previous research from Poland indicated the occurrence of _B. longimanus_[16; 41; 81; 94; 95]. We found that in NE Poland _B. brevimanus_ is the most common species. The geographical distribution of the genus _Bythotrephes_ presented by [PERSON] and [PERSON] (2016) and [PERSON] (2018) confirms our finding [96; 97]. The distribution of _B. longimanus_ is more to the north in the boreal zone and also from lakes in the Alps, while the prevalence of _B. brevimanus_ is clear in the north of the European mainland, from the Netherlands to Belarus and the Baltic republics [97; 98]. However, a detailed description of this species will be presented in a separate paper. The results of our study indicate that oxygen stress is an important factor that reduces the effectiveness of matter transfer between phytoplankton and zooplankton. The primary producers convert inorganic carbon into organic carbon, which is incorporated into new zooplankton biomass. Therefore, the productivity and effectiveness of matter transfer are frequently measured in carbon mass units [99; 100]. We observed large differences between the primary and secondary production in the water depth layers.