Periodicity in the Accumulation of Gelatinous Zooplankton During the Summer Season in the Coastal Area of Iyo-Nada, Japan

Gelatinous zooplankton collected and quantified daily at the sluice gate of the Ikata Nuclear Power Station along the coast of Iyo-Nada in the Seto Inland Sea were analyzed for the period 1998-2004 in an effort to correlate the number of these animals with the physical oceanographic conditions. Sudden periodic and nonperiodic increases of gelatinous zooplankton occur repeatedly from early summer to late autumn in nearshore areas. Periodic increases are synchronous with the spring tidal period. Nearshore tide-induced eddy development may play an important role for the aggregation process of gelatinous zooplankton, and the spring-neap tidal variation of the circulation induces increases of the gelatinous zooplankton population in coastal waters of the Iyo-Nada. Nonperiodic increases are attributable to typhoons and other storms. The strong shoreward currents due to the winds caused by these events transport gelatinous zooplankton to nearshore areas.