Population Ingestion Rate of Aurelia coerulea on Mesozooplankton in Masan Bay, Korea

Aurelia coerulea is a bloom-forming scyphozoan that recurs in Masan Bay, a semi-enclosed embayment on the southern coast of Korea. To quantify its population-level predation impact on mesozooplankton, medusae and zooplankton were sampled monthly at six stations from May to September 2013, except for August. Individual ingestion rates were estimated from gut-content analysis combined with temperature-dependent digestion time, and population ingestion rates were evaluated relative to mesozooplankton biomass and production. Medusa abundance peaked in May (2.11 medusae m−3) and declined sharply thereafter, whereas the individual body size and the total population biomass increased. Oithona similis was the dominant prey, indicating positive selectivity from June to September despite its low biomass in the ambient zooplankton community. Mesozooplankton abundance, biomass, and production ranged from 4300 to 26,900 ind. m−3, 6.9 to 20.6 mg C m−3, and 2.26 to 5.82 mg C m−3 d−1, respectively. Individual ingestion rates of the jellyfish on the mesozooplankton ranged from 0.31 to 5.31 mg C medusa−1 d−1. Population ingestion rates ranged from 0.06 to 0.66 mg C m−3 d−1, equivalent to 0.3–6.6% and 1.1–11.8% of the biomass and production of the mesozooplankton, respectively. Predation impact of A. coerulea on mesozooplankton in Masan Bay varied seasonally and depended primarily on medusa abundance, because population ingestion rates decreased after May despite increasing individual ingestion rates associated with medusa growth. A modified calculation of the population ingestion rate, considering jellyfish distribution only in the upper layer under water-stratification conditions in summer, was also discussed.