Impact of Improved Bottom Hypoxia on Zooplankton Community in Shallow Eutrophic Lake

We followed changes in the abundance of meso- and microzooplankton after an existing bottom hypoxia improved by the introduction of water with high oxygen content into the bottom. Mesozooplankton, calanoid copepod Acartia hudsonica showed ten times higher abundance under high oxygen condition at the bottom than under control where there was less oxygen at the bottom. On the other hand, during summer when microzooplankton such as rotifers and small cyclopoid species Oithona spp. were dominant, no difference in their distribution pattern and abundance was observed. Results have suggested that oxygen concentration at the bottom is one important factor determining the abundance and distribution of mesozooplankton even in local-scale probably by providing bottom refuge, but not for microzoooplankton. Since spring season is important for recruitment of many commercial fish juveniles, the distribution of oxygen in the lake seems very important for the fish stock in the lake.