| Summary: | 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.
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