| In recent years, the ecological role of gelatinous zooplankton especially jellyfish and ctenophore within coastal marine ecosystems with high production has been increasingly concerned globally. This attention is largely a result of forming jellyfish and ctenophore blooms that may intrinsically be linked to over-fishing, eutrophication, climate change, and species invasions. Usually the nuisance blooms cause enormous ecological, economic and societal losses, e.g. diminishing the fish standing stock and commercial harvest due to potential food competitor or predator, hampering fishing activities by clogging and bursting trawl nets, and making swimmer or tourists stung and inconvenient in the beach. So far many cases of jellyfish bloom events affecting negatively to economies or ecosystem have been reported worldwide. Furthermore, it was thought for the North Sea that climate-related increases in jellyfish frequency suggested a more gelatinous future. All about that are suggesting this issue remain to be hotspot. By far various and significant advances on this theme have been made in many sea areas; however, the baseline knowledge of this issue such as ecology and biogeography of large jellyfish except species names was almost ignorant in coastal China Sea among East Asian waters. Because of lack of enough attention to medusas from scientists, basic studies on jellyfish functional group are very weak in our country. Large jellyfishes are usually not studied but directly thrown out to the sea considering as nuisance when they are collected in the trawl. The same status with large ones, not forage, slimy, frangible and hard to process, small medusas are usually ignored for study compared with other forage zooplankton.Based on background mentioned above, this thesis focus on the basic information on seasonal geographical distribution of medusas including large and small size, and qualify ecological indexes such as biomass or abundance in the Yellow Sea and East China Sea. This study, as a fundamental knowledge, may be helpful to describe problems with the causes and effects of jellyfish blooms in the YS and ECS There are some results:Temporal and spatial variation of species composition and abundance of small medusas in the Yellow Sea are studied using samples collected vertically by zooplankton net. Total abundance of small medusas is generally low in the Yellow Sea area. Small medusa mainly distributed the shallower water than 50m Isobath. Seasonal replacement of medusas classes such as hydromedusae, siphonophores and ctenophores, and dominant taxon are extremely apparent. Comparing abundance level and percentage of small medusas occupied total zooplankton abundance among several sea areas, we found that the abundance (0.8 (0.04–1.3) ind. m-3) and percentage (< 0.5 %) of small medusas in the Yellow Sea were all lowest, which indicate that small medusas are not dominant assemblage among zooplankton functional groupsThe difference of COI gene series is small (0.2%) between Stomolophus meleagris L. Agassiz, 1862 collected from China coastal sea and Nemopilema nomurai Kishinouye, 1922 collected from Japan Sea, which is species level. This result preliminarily indicates these two samples are same species. So the taxonomy position of S. meleagris collected from China coastal sea may change into N. nomurai (Scyphozoa:Rhizostomeae:Rhizostomatidae).Semi-qualification on seasonal geographical distribution of biomass/ abundance of large jellyfish in the Yellow Sea and East China Sea are studied, based on swept area method by bottom trawl surveys and surface investigation cruise. 11 taxon are identified, among them N. nomurai, Aequorea spp., Ulmaridae genus sp. are dominant taxon in the Yellow Sea, comparatively, N. nomurai and Cyanea spp., in the East China Sea. Wet weight of these four jellyfish taxon increase exponentially with the bell diameter. The seasonal pattern of total biomass of large jellyfish is: The total biomass is lowest (4.6±9.4 kg km–2) in March when the surface temperature is also lowest during one year; they increase with the increasing temperature. The biomass reach to the highest (22891±25888 kg km–2), and then they will decrease with the drop of temperature. The biomass of Ulmaridae genus sp., species preferring cold temperature, is up to the highest (2780 ind. km–2, 1807 kg km–2) in October, and they mainly distributed in the middle part of the Yellow Sea. N. nomurai and Aequorea spp. usually adapt themselves in cold waters. The biomass of Aequorea spp. is highest in May (8262 kg km–2), and this species mainly distributed northern part of 30°N.Based on the in situ bottom trawl data, seasonal geographical occurrence and distribution of pelagic life stages of N. nomurai and Cyanea spp., bloom forming jellyfish, are studied. Two different patterns of seasonal occurrence and biomass are found: (1) the occurring N. nomurai in the YS (May–December), in May very few young medusas of this species appear at the junction of the YS and ECS; in June they began to expand distribution area in the southern YS; up to August and early September medusas are widespread throughout the YS, and form bloom with overwhelming other gelatinous species (Average biomass is 20446 kg km–2, and averagely account for 86.1 % of the total catch); and then the biomass and distribution area of them gradually shrink from October to December, even decrease to zero in the later month. (2) The appearing Cyanea spp. in ECS (May–October). The biomass of this species reach the maximum in May (Average biomass: 380 kg km–2) and early June; they decrease in August and later month. The high density of these two species distribute in the tidal front or temperature front area. Combining hydrographic condition and fishery data, distributional pattern and life cycle of N. nomurai and Cyanea spp. are discussed and speculated.At last, we acquire the potential feeding pressure of collective N. nomurai on standing stock and production of zooplankton per day, through estimating respiration rate and feeding rate of this species during blooming period. Results indicate that the feeding rate is 8.37 (0.12–37.83) mg C m-2 d-1 in September, 2006, the blooming time, assuming the capture rate of the bottom trawl is 0.1. The feeding pressure of collective N. nomurai on standing stock and production of zooplankton is 11.2% (0.17-50.6%), 134.1% (1.98-605.7%), respectively, assuming food requirement are all zooplankton. So the potential depletion of N. nomurai is very large, even destructive when they are blooming.
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