Cladoceran communities in offshore Suruga Bay, Japan: How are they formed?

Although marine cladocerans are known as coastal zooplankton, they sometimes appear in offshore waters in large numbers. In Suruga Bay, located in the center of Japan and opens to the Pacific Ocean, cladocerans occur abundantly in offshore waters during the spring and summer seasons. However, the mechanisms by which these offshore populations form in the bay are still unknown. The aim of this study was to clarify the factors that explain these offshore cladoceran populations by combining field sampling and numerical experiments, using particle-tracking methods. Cladocerans showed different occurrence patterns in coastal and offshore areas. Abundance of Penilia avirostris , the dominant offshore species, was much higher in offshore than in coastal areas. The unique filter feeding of Pe. avirostris may enable it to survive in offshore Suruga Bay with less abundant food conditions. On the other hand, while Pleopis polyphemoides and Podon leuckartii occurred abundantly in coastal areas, they appeared in very small numbers in offshore areas. This suggests that offshore environments are unfavorable for them to maintain populations. Particle-tracking experiments demonstrated that particles released from the coast of Suruga Bay were transported to and accumulated at the inner offshore region of the Bay from March to August. Those patterns were not observed in other months. Our results suggest that a combination of transport of coastal populations by surface circulation currents and biological characteristics of the species to survive in oligotrophic and high-salinity offshore environments contribute to form periodic mass occurrences of cladocerans in offshore Suruga Bay.