Influences of the Jellyfish Bloom of Aurelia aurita upon Plankton Production including Fish Larvae

本研究的目的是量化水母（Aurelia aurita）的大量出现对浮游生物群落包括鱼卵和幼虫的影响。东京湾是日本富营养化程度最高的海湾之一，平均auita丰度为0.32 ind./m3。估计aurita种群的摄食率在0.7 ~ 7 mg C/m3/d之间。aurita种群的平均丰度对浮游动物群落的摄食影响并不显著，因为据估计，最占优势的桡足动物Oithona davisae的产量约为100 mg C/m3/d。但是，当aurita丰度超过1 ind./m3时，该密度在春夏季经常出现，则计算种群摄食量对应于浮游动物日产量的10 ~ 10%。虽然浮游鱼类与水母争夺浮游动物，但水母可能会超越浮游鱼类，因为这些鱼类是白天进食的，而水母则是从早到晚进食的。最具代表性的浮游动物，石竹不摄取硅藻，而硅藻通常是最主要的浮游植物。死硅藻向海水中释放大量DOC，导致细菌数量增加，从而形成微生物食物链，然后将原本用于鱼类生产的能量通过细菌- hnf -纤毛虫-微型桡足动物（Oithona davisae）转移到水母的生产中。随着水母食物链的发展，丝状鱼和收获鱼等中食性鱼类的数量有望增加，但微生物和水母自身的大量营养物质的再生将加速海湾的富营养化。

Aim of the present study is to quantify the impact of massive occurrence of a jellyfish, Aurelia aurita on plankton community including fish eggs and larvae. In the case of Tokyo Bay, one of the most eutrophicated embayment in Japan, average abundance of A.aurita was 0.32 ind./m3. Estimated feeding rates of the A.aurita population ranged between 0.7 and 7 mg C/m3/day. The feeding impact of the average abundance of A.aurita population on the zooplankton assenmblages is not so significant because the production rates of a copepod, Oithona davisae which was the most dominant zooplankton were estimated to be ca. 100 mg C/m3/day. However, when the abundance of A.aurita exceeds 1 ind./m3, which density was frequently found during spring and summer, the population feeding is calculated to correspond to >10% of the daily production of zooplankton.Although planktivorous fishes compete with jellyfish for zooplankton, jellyfish may surpass the planktivourous fishes considering that these fishes are day light feeder in a contrast to jellyfish that feeds from morning to night.The most representative zooplankton, Oithona davisae ingest no diatoms which are usually the most dominant phytoplankton. Microbial food chain is also developed by the increase of bacteria which was caused by abundant DOC released into seawater from dead diatoms, and then the energy that previously went into production of fishes may be switched over to the production of jellyfish via bacteria-HNF-ciliates-micro copepod (Oithona davisae).When the jellyfish food chain is developed, medusivorous fishes such as filefish and harvest fish are expected to increase, however, eutrophication of embayment will be accelerated by huge regeneration of nutrients via microbes and jellyfish itself.