Quantitative effects of pycnocline and dissolved oxygen on vertical distribution of moon jellyfish Aurelia aurita s.l.: a case study of Mikawa Bay, Japan

Blooms of moon jellyfish Aurelia aurita s.l. occur in various vertical distribution patterns within the water column. Reasons for these distribution patterns have remained obscure. To quantify the influence of pycnocline and low dissolved oxygen (DO) on the vertical distribution of A. aurita aggregations, we investigated temperature, salinity, DO, and observed densities of A. aurita at 1–2 m depth intervals via video camera in a eutrophicated, enclosed bay, Mikawa Bay, Japan, for 3 years. During the observed period, stratification and hypoxic status of the bay varied seasonally and interannually due to climatic events, such as rainy season and typhoon passage. Both sharp pycnocline and low DO limited A. aurita vertical distribution. The more strongly stratified the water column, the more the upper boundary of A. aurita distribution was restricted. Bottom hypoxic water limited the lower boundary of A. aurita distribution. The DO threshold for in situ distribution was estimated to be ~2.5 mg l−1, which is much higher than the experimentally obtained, sublethal values identified in previous studies. Our results show that climatic events affect A. aurita vertical distribution through changes in the physical characteristics of the water column.