Effects of water mass structure on vertical distribution of moon jellyfish Aurelia aurita s.l. within aggregations in four Japanese coastal areas

Abstract

A sharp pycnocline and low dissolved oxygen (DO) both restrict the vertical distribution of moon jellyfish Aurelia aurita s.l. in Mikawa Bay, Japan. To determine if the distribution of this species is elsewhere similarly limited, we conducted field observations in Tokyo Bay, and compiled published data reporting vertical profiles of Aurelia, seawater density and DO for Tokyo Bay, Mikawa Bay, Urazoko Bay and Lake Nakaumi. Vertical profiles of Aurelia abundance and environmental variables from these coastal areas revealed the more strongly stratified the water column, the more the upper boundary of Aurelia distribution was restricted. Moreover, a threshold value of strength of stratification (∆σ_{t max}: 0.7–1.1) was apparent that effectively restricted the vertical distribution of this species. Relative differences in the strength of stratification and threshold value are suggested to cause variations of vertical distribution pattern of Aurelia among coastal areas, as distribution was frequently associated with the pycnocline in Tokyo and Mikawa bays, but not in Urazoko Bay. The DO threshold for in situ Aurelia distribution was estimated to be ~2.5 mg L^-1.