The giant jellyfish Nemopilema nomurai is a large scyphozoan which mainly inhabits the Yellow and East China Seas (YECS) and the Japan Sea. Since the late 1990’s, massive blooms of N. nomurai have been frequently reported in the YECS.
The increase of jellyfish bloom has been a worldwide problem and the possible causes for this phenomenon have been reviewed by Dong et al.
Generally speaking, global warming and human activity (e.g., overfishing, eutrophication, aquaculture and coastal construction) are thought to be the mechanism of the increasing jellyfish blooms.
However, although the YECS has been at a warm phase since 1990’s and anthropogenic influence becomes more significant year after year, bloom events of N. nomurai do not happen every year even in recent years.
To explain this incontinuity of N. nomurai bloom, put forward a hypothesis: jellyfish live on the sea bottom as polyps in stable environment; when bottom temperature changes unusually, it will trigger the polyps asexual reproduction to produce medusa, and jellyfish bloom is observed.
Temperature change event instead of temperature itself deserves more attention under this hypothesis. Adult N. nomurai was found moving with current in summer and gathering in September to form a large biomass in the Yellow Sea.
The jellyfish biomass and concentrating areas are different from year to year (Cheng et al., 2004). Inter-annual variation of the physical environment is essential for that of jellyfish growth and behavior including gathering.
The growth of N. nomurai at each life stage has its own optimal temperature. For example, bottom water temperature plays an important role in the strobilation of scyphistoma which adheres to the sea bed. Circulation determines the transport of medusas.
The hydrological environment of the YECS is influenced by the East Asian Monsoon, Kuroshio and river plumes. The monsoon wind changes its direction from northerly dominant in winter to southerly in summer.
A tidal front locates between the well mixed coastal water and a cold dome named the Yellow Sea Cold Water Mass in the central Yellow Sea during stratified season.
Water temperature of the Yellow- and East China Sea were observed to change abruptly in different years with a warming trend in recent several decades.
Circulation of the Yellow- and East China seas is complicated. In winter, the coastal currents run southward along the coast of Shandong and Zhejiang provinces.
Northward warm currents named the Taiwan Warm Current and Yellow Sea Warm Current flow on the mid-shelf.
In summer, coastal currents turn to northward and a river plume the Changjiang Diluted Water extends northeast-ward from the Changjiang estuary to the southern area of the Yellow Sea.
A cyclonic circulation forms in the central Yellow Sea while the YSWC vanishes. The TWC is enhanced by the southerly wind; and it could confine the eastward spread and drive the plume to run northward into the Yellow Sea.
These circulation branches compete with each other and vary significantly in extension and intensity from year to year and may carry medusas to different areas.
Particle tracking models are widely used to simulate the transport and distribution of zooplankton and fish larvae. Moon et al (2010) simulated the distribution of N. nomurai in the YECS based on hydrodynamic model ROMS (regional ocean model system) excluding tidal processes.
Their results show that about 70% of N. nomurai released near the Changjiang estuary in spring enter the Japan Sea in autumn. Luo et al. (2012) compared the simulation based on POM including tides with that of NEMO excluding tides.
The simulation with NEMO excluding tides shows similar distribution of particles in fall with the result of Moon et al. (2010), while the simulation with POM including tides is totally different.
The aggregation of N. nomurai near tidal front is simulated by Luo’s particle tracking model with inputs from POM. According to this model, few (about 2% of) particles originated in the Yellow Estuary Current Stream enter the Japan Sea.
What is the role played by changes in temperature and circulation in the inter-annual variation of jellyfish gathering? This question is difficult to answer as the life cycle of N. nomurai is complex and the temperature quantity functions are unclear for many life stages.
We have to simplify this problem by just considering the temperature’s impact on the strobilation in together with the effect of transport by circulation.
Thus we deploy a particle tracking model with the movement of particles triggered by temperature to simulate the N. nomurai aggregation in the years of 2008 and 2009.
Distribution of particles from June to September is analyzed to understand the difference in jellyfish gathering between the 2 years. The causes of variation are discussed through carrying out model sensitivity experiments.
Science Direct / ABC Flash Point News 2023.