1 INTRODUCTION

The giant jellyfish Nemopilema nomurai exists widely in the Bohai Sea, the Yellow Sea (YS), the East China Sea (ECS), and the seas around Japan and Korea. N. nomurai is a large scyphozoan with a maximum bell diameter of 1.5–2 m and a weight of 150–200 kg (Zhang et al., 2012; Oh et al., 2022). The outbreaks of N. nomurai in East Asian Seas in summer and autumn have been reported since the end of 1990s (Yan et al., 2004; Li et al., 2007; Uye, 2008; Zhang et al., 2012; Yoon et al., 2014), with estimated biomass of adult N. nomurai up to millions of tons during its bloom (Sun et al., 2015). Global warming and human activities (e.g., urbanization, industrialization, infrastructure growth, aquaculture, overfishing) are compounding pressures for coastal ecosystem and thought to be responsible for the increasing jellyfish blooms. Explosive blooms of N. nomurai might cause direct or indirect damages to marine industries, fisheries, and tourism in coastal areas, and could change the food web and the structure of the marine ecosystem (Kawahara et al., 2006; Yoon et al., 2008).

2 DISTRIBUTION BOUNDARY

In order to reduce and prevent the ecological and economic losses, there was an urgent need to develop a better understanding of the N. nomurai ecology. Over the last decades, substantial attention has been paid to the life cycle, distribution patterns, population dynamics, and possible bloom mechanisms of N. nomurai and the role it plays in marine ecosystems (Kawahara et al., 2006; Purcell et al., 2007; Uye, 2008; Moon et al., 2010; Lucas et al., 2012; Toyokawa et al., 2012; Zhang et al., 2012; Condon et al., 2013; Yoon et al., 2014; Sun et al., 2015; Wei et al., 2015). The life cycle of the N. nomurai consists of both benthic and pelagic stages corresponding to asexually benthic polyps reproduction and sexually pelagic medusa reproduction respectively (Kawahara et al., 2006; Lucas et al., 2012). In YS, benthic polyps of the N. nomurai offshore the Changjiang (Yangtze) River start strobilating in early summer (early May till June) and go through ephyra, metephyrae, and juvenile medusae before developing into adult jellyfish (Zhang et al., 2012; Sun et al., 2015). The medusa bell diameter along with its population abundance keep increasing since the strobilation, reach its peak in August. Then, the population decays from September and ends its annual cycle in October.

Distribution patterns and population dynamics of N. nomurai in pelagic stage are greatly influenced by hydrological conditions in the YS and ECS (hereafter YECS), which determines the jellyfish biomass accumulations and bloom areas (Doyle et al., 2007; Yoon et al., 2008; Wei et al., 2015; Choi et al., 2021). Spatial-temporal distributions of N. nomurai in the YECS were investigated by using various methods including surface visual observation (Wang et al., 2012), bottom trawl surveys (Zhang et al., 2012), drones (Choi et al., 2021), echo counting (Oh et al., 2022), and numerical modeling (Moon et al., 2010; Wei et al., 2015; Choi et al., 2018). Transported by the circulations, medusae of N. nomurai move northward and westward from their original location in the YS and spread progressively to the whole YS, as far north as 37°N–38°N and southward to 30°N (Zhang et al., 2012; Sun et al., 2015). Most of the previous studies have shown the changes in N. nomurai distributions patterns from year to year (Cheng et al., 2004; Zhang et al., 2012; Sun et al., 2015), due to the inter-annual variations of hydrological environments in the YECS. Nevertheless, N. nomurai is rarely observed in the southern part of the ECS except for a few cases caused by typhoon events (Fig. 1). Sun et al. (2015) put forward and examined that pelagic N. nomurai in the YECS is confined to the north of 30°N, which is hence considered as the southern boundary of this species. In order to understand the mechanism driving this distribution pattern of N. nomurai, our previous studies explored the impact of environmental factors in terms of chemical and biological properties and gradients (Wang et al., 2012; Zhang et al., 2012; Feng et al., 2018). Analysis showed that there is no evident relationship between the distribution of the N. nomurai jellyfish and its zooplankton prey, and that nutrients rarely had any contributions. Sun et al. (2015) also concluded that the current systems are thought to be the main driving forces for the distribution patterns, rather than biological processes do. Yet the roles played by the circulations in jellyfish distribution patterns have not been discussed. Therefore, in this study, we examine the possible influences of the physical processes on the distribution patterns of N. nomurai in the southern YS and northern ECS, especially how N. nomurai is restricted to the north of 30° N.

3 POSSIBLE INFLUENCES OF CIRCULATION SYSTEMS IN YECS ON N. NOMURAI DISTRIBUTION

The hydrodynamics of the YS and ECS are complex and highly variable, influenced by four main coexisting forcings in the area: wind stress, the Kuroshio Current and its invasion branches, tides and the Changjiang River input (Su, 1998). The East Asian Monsoon system produces strong prevailing north- northwesterly winds in winter, and moderate and less persistent south-southeasterly winds in summer. The seasonal reversal winds create nearly opposite Ekman surface currents during winter and summer, and strongly influence shelf current systems. In response to the seasonality of monsoon winds, the circulation in the YECS has a strong seasonal variability.

From the initial occurrence in late May until its disappearance in October, the population dynamics of N. nomurai is strongly influenced by estival hydrological conditions in the YECS during summer seasons. The seasonal circulation during summer season (May–October) in the YECS is characterized by the Changjiang River Plume, the coastal currents on the inner shelves, Taiwan Warm Current (TWC) and the Kuroshio Current on the outer shelf and its invasion branches (Su, 1998). Possible influences of the above processes are discussed separately to get a better understanding of the distribution of N. nomurai in the YECS.

3.1 Changjiang River plume

Changjiang River has the most important freshwater discharge with seasonal variations in the YECS with 70% of its annual flux occurring from May to October. The intense river discharge in summer spreads offshore with a residual current velocity much higher than in winter (Li et al., 2017) and controls the expansion of the river plume (Changjiang Diluted Water/(CDW)) (Beardsley et al., 1985). The CDW extends northeastward from the Changjiang estuary by the Ekman transport induced by the southerly monsoon and spreads across the shelf (Beardsley et al., 1985; Lie et al., 2003). Besides, the strengthened TWC is thought to enforce the northeast turning of the CDW (Naimie et al., 2001).

Toyokawa et al. (2012) first observed wild ephyrae of N. nomurai offshore of the Changjiang River estuary. Sun et al. (2015) confirmed that it is off the Changjiang River where the pelagic stages of N. nomurai first appear, and is therefore one of its principal breeding places. High correlation between the dispersion of CDW and the appearance of N. nomurai has also proved by Yoon et al. (2008). In another word, the CDW is the major factor controlling the northward movement of the juvenile N. nomurai.

Furthermore, in the Changjiang River plume area, buoyancy fronts formed due to the distinct density between the CDW and the surface ECS water. The plume fronts act like as upper-layer transport barriers and the materials within the frontal regions could hardly cross the boundaries despite the tidal influences (Li et al., 2022), and thus play an important role in biomass aggregation and blooms (Wang et al., 2019; Zhang et al., 2020; Li et al., 2021). Laboratory experiment showed that low salinity gave better survival of scyphistoma and medusa and low salinity in June may be helpful for the jellyfish bloom (Sun et al., 2015). Therefore, the frontal processes act as transport barriers preventing N. nomurai from moving southward, and provide enough residence and growth time for juvenile N. nomurai in the frontal region and thus favor jellyfish blooming.

3.2 Seasonal surface circulation pattern

Figure 2 presents the schematic seasonal surface circulation patterns during summer. Moderate southerly winds prevail in the YECS, and surface circulations turn to northwestward from late May.

The TWC, with high temperatures, salinity, and density, flows northeastward between 50- and 100-m isobaths offshore of Zhejiang and Fujian Provinces (Su et al., 1994). The direction and velocity of the TWC are relatively stable, flowing northeastward all year around, even during strong northerly winds in winter (Zhu et al., 2004; Guan and Fang, 2006). During summer, the TWC is strengthened under the effects of southerly monsoons and its inshore branch intrudes into the Changjiang River estuary, forming a barrier to the southward movement of the YECS. In addition, the TWC water accumulated off the Changjiang River estuary creates northward baroclinic pressure gradients, causing blocking effect to the N. nomurai. Along with the TWC, the ECS coastal current (ECSCC) off the Fujian-Zhejiang coast flows northward as well in summer (Guan and Fang, 2006).

The Kuroshio, known as the western boundary current of the North Pacific subtropical gyre, flows northeastward along the continental slope of the ECS. The Kuroshio plays a crucial role in the YECS not only by regulating the circulation dynamics of the shelf, but also by carrying heat and nutrients into the area (Lie and Cho, 2016). A portion of Kuroshio subsurface waters (KSSWs) can intrude into the ECS shelf from the shelf off the northeast of Taiwan, China (Zhou et al., 2018). These intruded waters may move further northeastward into the coastal seas off the Zhejiang coast (Yang et al., 2013) and reach the south of the Changjiang River mouth (Beardsley et al., 1985; Chen et al., 1994; Zhu et al., 2004). Wei et al. (2015) also observed that the KSW intruded into the Changjiang River estuary through a narrow path along the inner shelf, as north as 31°N. The Kuroshio intrusion water brings water with favorable temperature, nutrients, algae constituents, and zooplankton, which would benefit the survival rate of N. nomurai ephyrae (Sun et al., 2015). Moreover, the strong fronts between the KSW and upper-layer water controlled by the CDW also have restricting effects in southward spread of the N. nomurai.

4 DISCUSSION AND CONCLUSION

In late May, when the N. nomurai polyps started strobilation, the East Asian Monsoon system switches into summer mode as well and produces south-southwesterly winds. The circulation regime in the YECS changes into summer pattern. The Changjiang River steps into its flood season with 70% of its runoff flux spreading offshore in the next three months (Dai et al., 2008). Along with the increasing runoff flux, the offshore plume intensified and turns northwestward. The stable and persistent TWC south of the Changjiang River estuary is intensified as well in summer. Strong front is formed between the two distinct water mass around Latitude of 30°N and acts as transport barrier, preventing any of the released ephyrae, metaphase, or juvenile medusa of N. nomurai from escaping into the south.

Even though the physical process is thought to be the driving factor of N. nomurai distribution in the YECS, we still cannot draw the conclusion upon our current example that physical environments are more important than other factors. Generally, the spatial distribution of jellyfish is influenced not only by the physical environment but also by the chemical, biological environments, the biology and behavior of the jellyfish. In our case, zooplankton prey distribution had no significant differences on both sides of 30°N and showed no obvious impact on the N. nomurai distribution. This phenomenon can be explained from the different origins of N. nomurai and other zooplankton. Unlike N. nomurai, most of the other zooplankton exists widely in the ocean and the physical processes could only influence their patchy distributions at local scales. Meanwhile, offshore of the Changjiang River is one of the N. nomurai in the YECS and its distribution pattern is strongly influenced by the physical processes since the very beginning of its pelagic stage.

In conclusion, location of the initial breeding places of the N. nomurai, combined with seasonal horizontal transport characteristics associated with the Changjiang River plume and northward circulation patterns in the area under influences of monsoon system, are considered the main factors determining the spatial distribution of N. nomurai north of 30°N. The relationship between the inter-annual circulation characteristics in the YECS and the N. nomurai population dynamics and the distribution patterns will be further investigated in our future work.

5 DATA AVAILABILITY STATEMENT

All data analyzed during the current study areavailable within the article (Zhang et al., 2015).