The Mechanism Of Jellyfish Bloom In The Yellow Sea And East China Sea-A Model Study

In recent years, a significant increase in gelatinous zooplankton especially jellyfish has been observed worldwide. Jellyfish bloom has obvious interannual variation. This may intrinsically be linked to over-fishing, eutrophication, climate change, and species invasions. Usually the blooms cause enormous ecological, economic and societal losses. In the worldwide, marine scholars and government departments pay much attention to the jellyfish blooms, and some progress and achievements have been made in the study of ecology and other aspects. However, few studies simulate the gathering of jellyfish using numerical model. Model method is a useful tool to reveal the mechanism of jellyfish gathering and the scyphistoma recruitment which will be helpful to understand the jellyfish bloom and to predict it finally.A particle tracing model, based on output from the Princeton Ocean Model (POM), is established to simulate the accurate spatial and temporal distribution and the migration path of giant jellyfish Nemopilema nomurai in the Yellow Sea and East China Sea. The simulation result is used to study the year-to-year variation of N. nomurai gathering in autumn and its influencing factors. Furthermore, we implement experiment to trace the planula after the jellyfish spawns, and study the recruitment of the scyphistoma to the source areas. The study is helpful to the prediction of jellyfish bloom in the near future.Particles standing for scyphistoma are put initially along the coast from the Hangzhou Bay to Haizhou Bay. Scyphistoma strobilation is triggered by temperature. Subsequently, the particles move with the current, and the vertical migration is considered. The simulated N. nomurai distribution is in good agreement with the observations in August 2009. N. nomurai mainly gather in the tidal front of the Yellow Sea and to the east of Changjiang estuary.The difference between the distribution of N. nomurai in 2008 and 2009 is discussed. Less N. nomurai origin from Subei Shoal, Changjiang estuary and Hangzhou Bay aggregate in the tidal front area in June and July of 2008 than 2009, while more N. nomurai from north of Subei Shoal concentrate there in August and September of 2008 than 2009. In August,2008, there are more N. nomurai gathering in the tidal front in the Yellow Sea and the east of Changjiang estuary.In order to study the role of temperature and current field in N. nomurai gathering of the two years, we carry out a set of sensitivity experiments. The main results are as follows:(1) The interannual variability of circulation is more important than the bottom temperature to the fall gathering of jellyfish. (2) A relative southwestward change of surface current adjacent to Subei Shoal and Changjiang estuary in June could cause the decrease of N. nomurai in tidal front in June and July of 2008; A relative eastward change of surface current in southern Yellow Sea and Esat China Sea in August may respectively induce the increase of N. nomurai in tidal front and the east of Changjiang estuary in August and September of 2008. (3) In Haizhou Bay and Hangzhou Bay, the early strobilation may respectively contribute to the increase of N. nomurai in tidal front and the east of Changjiang estuary in June,2008. However, temperature could not affect the gathering after several months.Tracing planula experiment is carried out to explore how the polyps recruit the possible sources, and the results show that the jellyfish polyps in the east of Changjiang estuary can return to the Changjiang estuary and Hangzhou Bay. Along the coast of Subei Shoal, polyps are locally recruited, no particles return from the gathering areas. In Haizhou Bay, the situation is similar, only one tenth of the polyps are from gathering area.The results of 2008 and 2009 shows similar distribution, but the number of polyps varies. More particles return to Changjiang estuary and Hangzhou Bay in 2009 than in 2008, mainly due to the increase of the particles originating from the east of Changjiang estuary. On one hand, in September 2009, the number of particles aggregating to the east of Changjiang estuary is larger, namely the base number is larger. On the other hand, the west component of wind in late September 2009 is larger than that in 2008, which affects the surface current, and bring more particles to Changjiang estuary and Hangzhou Bay. Therefore, the fundamental reason is attributed to the difference of monsoon and surface current in 2008 and 2009.