Spatial patterns of zooplankton abundance, biovolume, and size structure in response to environmental variables: a case study in the Yellow Sea and East China Sea*

Song SUN, Haochen XIAN, Xiaoxia SUN, Mingliang ZHU, Mengtan LIU,**

1 Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences,Qingdao 266071, China

2 Jiaozhou Bay National Marine Ecosystem Research Station, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

3 University of Chinese Academy of Sciences, Beijing 100049, China

4 Laboratory for Marine Ecology and Environmental Science, Laoshan Laboratory, Qingdao 266237, China

Abstract The Yellow Sea (YS) and East China Sea (ECS) are highly dynamic marginal seas of the northwestern Pacific Ocean.To gain an in-depth understanding of zooplankton community structure,zooplankton abundance, biovolume, and size structure in summer 2017 in the YS and ECS were assessed using ZooScan imaging analysis.Zooplankton abundance and biovolume ranged 2.94-1 187.14 inds./m3 and 3.13-3 438.51 mm3/m3, respectively.Based on the biovolume data of the categorized size classes of 26 identified taxonomic groups, the zooplankton community was classified into five groups, and each group was coupled with distinctive oceanographic features.Under the influence of the Yellow Sea Cold Water Mass, the Yellow Sea offshore group featured the lowest bottom temperature (10.84±3.42 ℃) and the most abundant Calanoids (mainly in the 2-3 mm size class).In the Yellow Sea inshore group, Hydrozoans showed the largest biovolume and dominated in the 3-4-mm and ＞5-mm size classes.The East China Sea offshore group, which was affected by the Kuroshio Branch Current, featured high temperature and salinity, and the lowest bottom dissolved oxygen (2.58±0.5 mg/L).The lowest values of zooplankton abundance and biovolume in the East China Sea offshore group might be attributed to the bottom dissolved oxygen contents.The East China Sea inshore group, which was mainly influenced by the Zhejiang-Fujian Coastal Current and Changjiang Diluted Water, was characterized by high chlorophyll a and the largest biovolume of carnivorous Siphonophores (280.82±303.37 mm3/m3).The Changjiang River estuary offshore group showed the most abundant Cyclopoids,which might be associated with the less turbid water mass in this region.Seawater temperature was considered the most important factor in shaping the size compositions of Calanoids in different groups.

Keyword: zooplankton; size structure; ZooScan; Yellow Sea; East China Sea

1 INTRODUCTION

The Yellow Sea (YS) and East China Sea (ECS)are highly productive marginal seas on the northwest shelf of the Pacific Ocean.The YS is surrounded by Chinese mainland in the west and north and by the Korean Peninsula in the east (Yu et al., 2022).On the south, the YS borders the ECS along the line from the mouth of the Changjiang (Yangtze) River to Cheju Island off South Korea.The most prominent oceanographic features of the YS are the Yellow Sea Cold Water Mass (YSCWS) in summer and the Yellow Sea Warm Current (YSWC) in winter (Liu et al., 2021b; Yu et al., 2022).In the ECS, the Yellow Sea Coastal Current (YSCC) in the north converges with the Kuroshio Current (KC), Taiwan Warm Current (TWC), and Zhejiang-Fujian Coastal Current(ZFCC) in the south, and it is also affected by Changjiang Diluted Water (CDW) (Son et al., 2005;Yuan et al., 2008; Zhu et al., 2022).Interactions between interlaced currents and water masses result in significant exchanges of both energy and materials(Chen et al., 1995), which support and structure the productive plankton community in this region.

Zooplankton are key components of the marine food chain, and they support the major biological production processes of the entire ecosystem by transferring primary production to higher trophic levels (Sun et al., 2010; Robert et al., 2014).Therefore, it is important to study the characteristics of zooplankton communities to obtain an overall understanding of the potential elemental circulation and energy transfers in marine ecosystems.The YS and ECS are areas of high ecological sensitivity and have abundant plankton productivity.Because zooplankton are the major consumers of primary production and the main food source for many fish and other higher trophic organisms, they have been the focus of many studies in the YS and ECS (Sun et al., 2010; Chen and Liu, 2015).However, due to global change and increasing anthropogenic activities, the ecosystems in the YS and ECS have undergone dramatic changes in recent decades,including changes in physical and chemical properties (temperature, pH, dissolved oxygen, and nutrients) (Lin et al., 2005; Chen et al., 2007),increasing ecological disasters (giant jellyfish,green tide and hazardous algae bloom) (Zhang et al., 2012; Sun et al., 2015; Liu et al., 2016; Yan et al., 2022) and decreases in fishery stocks (Iversen et al., 1993; Zhao et al., 2003), and these changes in turn have impacted the zooplankton community.Therefore, it is necessary to conduct a detailed study of zooplankton communities when ecosystems change.

To understand the horizontal features of the zooplankton community, zooplankton samples from the YS and ECS were analyzed in terms of abundance and biovolume using ZooScan imaging analysis.The ZooScan Integrated System is a semiautomated system for indoor zooplankton sample analysis that can quickly and accurately obtain the numerical and size data of zooplankton samples(Gorsky et al., 2010; Chang et al., 2012;Vandromme et al., 2012).Based on biovolume data,we analyzed the spatial patterns of simplified zooplankton taxonomic composition and zooplankton size structures of five identified zooplankton groups in response to environmental variables.The results of this study provide basic data for the study of longterm variations in zooplankton communities.

2 MATERIAL AND METHOD

2.1 Field sampling

The research sampling was conducted on board the R/VKexue3in the Yellow Sea and East China Sea from August 24 to September 27, 2017.Environmental profiles and plankton samples were collected from 73 stations (Fig.1).At each station,abiotic parameters (temperature, salinity, and dissolved oxygen) were collected via the deployment of a Sea-Bird 917 plus CTD instrument mounted on a carousel sampler with Niskin bottles.Seawater samples for chlorophyll-a(Chl-a) determination were collected from depths of 0, 5, 10, 20, 30, and 50 m and near the sea floor.The water samples were filtered through Whatman GF/F filters (25-mm diameter, 0.7-μm pore size), and the filters were immediately stored at -20 °C.In the laboratory, the filters were extracted in 5 mL of 90% aqueous acetone (v/v) for 24 h, and Chl-aconcentration was measured using a Turner Designs Model 7200 fluorometer.

Zooplankton were sampled using a conical plankton net (0.8-m mouth diameter, 500-μm mesh size), which was towed vertically from 4 m above the sea floor to the surface at ～0.5 m/s.After collection, the samples at the cod-end were immediately fixed in 5% (v/v) buffered formalin and seawater solution.

2.2 ZooScan analysis

Zooplankton samples were used to quantitatively measure the abundance and biovolume of mesozooplankton using a semiautomatic ZooScan system (HYDROPTIC).Before scanning, each sample was split into 1-256 aliquots with a Motoda box splitter to obtain aliquots containing approximately 1 500-2 000 zooplankton objects.

Fig.1 Map of sampling stations (a) and circulation system (b) in the Yellow Sea (YS) and the East China Sea (ECS)

Subsamples were poured onto the scanning cell(15 cm×24 cm), and zooplankton individuals were separated manually.Subsamples were then scanned,and the raw images were processed with ZooProcess and digitized at a resolution of 2 400 dpi.Detected objects were digitally separated, and segmented images were automatically classified into taxonomic groups according to a learning set (Gorsky et al.,2010; Chang et al., 2012; Vandromme et al., 2012).The automated classification results were then manually checked and corrected.The organisms detected were classified into 26 zooplankton taxa or categories: Chaetognaths, Appendicularians,Thaliaceans, Cladocerans, Calanoids, Cyclopoids,Harpacticoids, Eumalacostracans (mainly Mysids and Euphausiids), Amphipods, Luciferids, Ostracods,Alima larvae, Brachyuran larvae, Nauplii, Hydrozoans(except Siphonophores), Siphonophores, Noctiluca,Bivalve larvae, Gastropod larvae, Echinoderm larvae, Polychaetes, Pteropods, Radiolarians, Fish larvae, Eggs, and Others.Images, identified as“Phytoplankton”, “Detritus”, “Fiber” and “Unknown items”, were removed for further analyses.

To obtain the biovolume of zooplankton individuals, each zooplankton object was assumed to be an ellipsoid, and the volume was calculated as (4/3) π (Major/2) (Minor/2)2, where the Major and Minor axes (mm) were directly provided by ZooScan.The equivalent sphere diameter (ESD)was calculated for each zooplankton object based on the biovolume data.The abundance (inds./m3) and biovolume (mm3/m3) of each taxonomic group were then calculated based on the splitting ratio and the volume of seawater filtered.The volume of filtered seawater was calculated as the towed depth multiplied by the net mouth area.

2.3 Statistics

Canonical redundancy analysis (RDA) was performed to explore the influence of environmental drivers on zooplankton size structure.Before analysis, the biovolume data for each taxonomic group were categorized into 7 size classes (＜0.5 mm,0.5-1 mm, 1-2 mm, 2-3 mm, 3-4 mm, 4-5 mm,and ＞5 mm) based on ESD, which were then log transformed (lg(x+1)) to reduce the overall skewness in the data distribution.Taxonomic classes with biovolumes less than 1% were removed to reduce the influence of rare components.Original data for environmental variables were normalized to a common scale.Statistical comparisons of zooplankton and environmental parameters between groups were performed using one-way analysis of variance (ANOVA) or the nonparametric Kruskal-Wallis test.RDA was conducted using software Canoco 5.ANOVA (parametric and nonparametric)and Pearson’s correlation analysis were conducted using IBM SPSS Statistics 22.The significance levels for the tests were set atP＜0.05 orP＜0.01.

3 RESULT

3.1 Hydrography

Both surface and bottom temperatures exhibited an increasing trend from north to south in the study area (Fig.2a-b).The surface temperature ranged between 21.75 and 28.02 °C, with elevated levels recorded in the southern ECS and the 3600 transect.In the southern YS, strong stratification was indicated in the bottom temperature, and the lowtemperature YSCWM extended to part of the northern YS.In the ECS, a lower bottom temperature was recorded at the offshore stations.

The surface salinity varied from 21.63 to 33.98(Fig.2c).The minimum value of surface salinity occurred in the Changjiang River estuary, where CDW mixed with inshore seawater and a lowsalinity diluted water jet was observed.The bottom salinity was clearly lower in the YS than in the ECS(Fig.2d).A strong intrusion of KBC was indicated by the high salinity in the offshore regions of the ECS, where the maximum salinity peaked at 34.48 at the DH7 transect.A strong salinity front was observed along the coast of Zhejiang and Fujian provinces, where high-salinity oceanic water mixed with low-salinity coastal waters.

Compared with the YS, the ECS was characterized by a low level of bottom dissolved oxygen (Fig.2e).Areas with low levels of bottom dissolved oxygen extended northward to the inshore regions of the Changjiang River estuary and reached their minimum at the 3300 transect.In the study area, the average Chlaranged from 0.15 to 57.51 μg/L, with maximum values observed at the offshore stations along the CJ transect (Fig.2f).In addition, a high level of Chlawas also recorded in the coastal waters of Zhejiang and Fujian provinces.

3.2 Zooplankton abundance and biovolume

Zooplankton abundance ranged from 2.94 to 1 187.14 inds./m3(mean 264.42±245.55 inds./m3)(Fig.3a).In the YS, higher abundance was observed in the northern YS and the coastal stations of the 3300 transect.In the ECS, a low abundance of zooplankton was distributed mainly in the offshore regions of the Za and Zb transects, and the adjacent DH transect.Zooplankton biovolume ranged from 3.13 to 3 438.51 mm3/m3(mean 452.05±570.11 mm3/m3) (Fig.3b).Zooplankton biovolume was also higher in the northern YS and the southern 3300 transect in the YS.In the ECS, biovolume was higher in the inshore regions than in the offshore regions.The maximum zooplankton abundance occurred at 3100-03, while the biovolume was comparatively low at this station.

Zooplankton were classified into 26 diverse taxonomic groups in this study (Table 1).In terms of abundance, the most abundant zooplankton group were Calanoids (95.83±114.36 inds./m3), which accounted for a proportion of 36.24% of the total zooplankton community.Chaetognaths and Siphonophores, with average abundances of 27.81±28.14 inds./m3and 24.01±45.36 inds./m3, were ranked second and third, respectively.For biovolume,Siphonophores, Eumalacostracans, Chaetognaths,Calanoids, and Hydrozoans exceeded 10% in total zooplankton biovolume.Siphonophores showed the largest biovolume (92.23±199.54 mm3/m3), and its proportion reached 20.40%.Eumalacostracans ranked second, accounting for 19.79% and averaging 89.45±219.48 mm3/m3.Although the abundance of Calanoids was significantly higher than that of the other groups (Kruskal-Wallis test,P＜0.001), the biovolume of which was only ranked fourth(15.23%) in the total zooplankton.Compared with Siphonophores and Eumalacostracans, the biovolumes of Calanoids and Chaetognaths were significantly lower (Kruskal-Wallis test,P＜0.001).The minimum values of abundance and biovolume were 0.30±1.00 inds./m3(Pteropods, 0.11%) and 0.01±0.05 mm3/m3(Nauplii, 0.003%), respectively.

Fig.2 Spatial patterns of surface temperature (°C) (a), bottom temperature (°C) (b), surface salinity (c), bottom salinity(d), bottom dissolved oxygen (mg/L) (e), and weighted average chlorophyll a (μg/L) (f) of the entire water column in the Yellow Sea and the East China Sea

3.3 Zooplankton community group

Based on environmental variables and the biovolume data for the size classes of each taxonomic group, the zooplankton communities were classified into five groups according to RDA (Fig.4).The environmental features of the five groups are summarized in Table 2.The YS stations were mainly distributed in the upper left of the RDA plot,while the ECS stations were mainly distributed in the lower right of the plot, and stations from the Changjiang River estuary (CRE) were distributed in the middle.The Yellow Sea inshore group (YSI),which was characterized by low levels of bottom salinity (BS) (Fig.2; Table 2), mainly included coastal stations in the Yellow Sea.The Yellow Sea offshore group (YSO) was distributed in the deeper regions of the Yellow Sea.Under the influence of the YSCWM, the YSO showed significantly lower levels of bottom temperature (BT) (10.84±3.42 °C)(Kruskal-Wallis test,P＜0.01).In addition, the YSO also had the highest bottom dissolved oxygen (BDO)and lowest Chla.The East China Sea inshore group(ECSI) mainly included the coastal regions of the ECS (including the CRE) and the southwestern part of the sampling area in the ECS.Due to the influence of CDW, the ECSI group showed the lowest surface salinity (SS) (29.83±3.42) and high levels of BT and Chla.Under the influence of both TWC and KBC, the East China Sea offshore group (ECSO) featured the highest levels of surface temperature (ST) (27.21±0.37 °C), SS(33.75±0.11), and BS (34.39±0.07) (Kruskal-Wallis test,P＜0.01) and the lowest level of BDO (2.58±0.5 mg/L).The Changjiang River estuary offshore group (CREO) mainly included stations from the offshore regions of CRE.Relative to the ECSO group, the CREO group showed lower values of ST,SS, and BS.

Fig.3 Horizontal distribution of zooplankton abundance (inds./m3) (a) and biovolume (mm3/m3) (b) in the Yellow Sea and the East China Sea

The YSO group showed the highest value of average abundance (362.41±248.28 inds./m3) among all groups, and the value was significantly higher than that of the ECSO group (Kruskal-Wallis test,P＜0.05).Calanoids were numerically dominant in all groups (Fig.5).In the ECSO group, Calanoids reached its highest proportion of 49.68% in total abundance.In the ECSI group, Siphonophores ranked second, with a proportion of 26.84%.The YSI group showed the highest abundance of Eumalacostracans (mainly Mysids and Euphausiids)among all groups, with an average value reaching 55.98 inds./m3and accounting for a portion of 21.09% of the total abundance.The compositions of the average biovolume were different from those of the average abundance.The largest biovolume was detected in the ECSI group (717.11±578.87 mm3/m3),which was due to the predominance of Siphonophores(39.16%) and Eumalacostracans (19.59%).The YSI and ECSO groups had significantly lower average biovolumes than the ECSI group (Kruskal-Wallis test,P＜0.05).The YSI group was characterized by an extremely high proportion of Hydrozoans(69.28%).Calanoids were the most abundant taxon in the YSO (41.04%) and ECSO groups (32.52%),while in the CREO group, Eumalacostracans ranked first with a proportion of 34.42%.

Table 1 Summary of the average±standard deviation (SD)and percentage of abundance (inds./m3) and biovolume (mm3/m3) for the 26 zooplankton groups identified

3.4 Zooplankton size structure

Zooplankton size properties are summarized by abundance and biovolume for the five groups in Figs.6-7.For abundance, the ＜0.5-mm, 0.5-1-mm and 1-2-mm size classes dominated the five groups;the lowest proportion of 86.13% was observed in the ECSI group and the highest proportion of 97.78% was found in the YSO group (Fig.6a).The 0.5-1-mm and ＜0.5-mm size classes ranked as the first two size classes in the YSI group.Calanoids and Eumalacostracans dominated the 0.5-1-mm size class with average abundances of 57.86 inds./m3and 39.39 inds./m3, respectively.In the YSO group,the average abundance increased gradually in the ＜0.5-mm, 0.5-1-mm, and 1-2-mm size classes and peaked in the 1-2-mm size class at 181.45 inds./m3.Calanoids contributed the largest proportion of 68.17% in the 1-2-mm size class,while in the 0.5-1-mm size class, Thaliaceans were the most dominant taxonomic group (35.33%).In the CREO group, the average abundance decreased gradually in the ＜0.5-mm, 0.5-1-mm, and 1-2-mm size classes.Calanoids were the most abundant taxa in the ＜0.5-mm and 0.5-1-mm size classes.The ECSI and ECSO groups showed similar size properties.In addition to Calanoids, Siphonophores were important contributors in the ＜0.5-mm, 0.5-1-mm, 1-2-mm, and 2-3-mm size classes in the ECSI group.

In terms of biovolume, the five groups exhibited distinctive features in their taxonomic compositions in each size class (Figs.6b, 7b, d, f, h, & j).In the YSI group, zooplankton were dominated by the 3-4-mm and ＞5-mm size classes, which was due to the large biovolume of Hydrozoans.The YSO group featured a large biovolume of the 1-2-mm size class, which was mainly composed of Calanoids.The 3-4-mm and ＞5-mm size classes accounted for 64.05% of the CREO, and Eumalacostracans were the most important contributor (86.84%).In the ECSI group, Siphonophores were the most important contributors to the 1-2-mm, 2-3-mm, 3-4-mm,and 4-5-mm size classes.The biovolume of Eumalacostracans increased gradually with size class.The taxonomic composition in each size class of the ECSO group was different from that of the ECSI group.In the ECSO group, Calanoids and Chaetognaths were the dominant taxa in the 1-2-mm and 2-3-mm size classes, respectively.

4 DISCUSSION

4.1 Environmental variable

Fig.4 Results of RDA based on zooplankton biovolume size classes and selected environmental variables

The distribution patterns of environmental variables are primarily determined by the circulation system and climatic conditions.As expected, the significantly low values of BT (Fig.2b) demonstrated the prevalence of the YSCWM in the offshore regions of the YS, and most of the stations in the YSO group were included in this region (Fig.4).In the upper layer of the YSCWM area, nutrients are depleted after spring phytoplankton blooms (Fu et al., 2016).With markedly low temperatures and high salinity below the strong pycnocline and thermocline, the YSCWM hinders vertical mixing and thus curtails primary productivity (Park et al.,2011).This may explain the significantly low Chl-aconcentrations in the YSO group (Fig.2f & Table 2).

The Kuroshio and its branch currents are believed to be the most influential factors affecting the ECS and the adjacent YS oceanographic environments (Wang et al., 2018; Liu et al., 2021a).In the offshore regions of the southern ECS (the ECSO group), KBC intrusion could be observed at the bottom of the offshore stations based on BT, BS and BDO (Fig.2b, d, & e) (Yang et al., 2012; Liu et al., 2021a).The intrusion of warm KBC transports heat and nutrient-laden deep seawaters to the coastal regions of the ECS, where a coastal upwelling area form (Yang et al., 2012, 2013; Wang et al., 2018).The formation of a pycnocline results in the accumulation of nutrients in the nearshore regions of Zhejiang and Fujian Provinces, which supports the high levels of phytoplankton biomass in this region (Fig.2f & Table 2).

Table 2 Summary of environmental variables for different groups (YSI, YSO, CREO, ECSI, and ECSO) in the Yellow Sea and the East China Sea

Fig.5 Zooplankton taxonomic composition in different groups (YSI, YSO, CREO, ECSI, and ECSO) of the Yellow Sea and the East China Sea

Fig.6 Relative abundance (%) (a) and relative biovolume (%)(b) of seven size classes for each group (YSI, YSO,CREO, ECSI, and ECSO) in the Yellow Sea and the East China Sea

The Changjiang River estuary has an extremely dynamic hydrographic environment due to mixing and interactions between runoff and various continental shelf currents (YSCC, SSCC, TWC,ZFCC and KC) from the YS and ECS (Lü et al.,2006; Zhu et al., 2019).In this study, the low values of ST, SS, and BS at the nearshore stations in the CRE could be attributed to the enhanced discharge of the Changjiang River in summer (Ning et al.,1998; Gong et al., 2003).In the offshore stations in the CREO group, higher values of BT, SS, BS, and BDO were observed, which might be attributed to the ECS offshore waters (Qi et al., 2014).

4.2 Zooplankton abundance, biovolume, and taxonomic composition

In this study, we calculated the integrated zooplankton abundance and biovolume in the YS and ECS based on ZooScan imaging analysis.The abundance of zooplankton in this study (both the YS and ECS, August-September) was in the range of 2.94-1 187.14 inds./m3, with an average abundance of 264.42±245.55 inds./m3.According to studies by Chen et al.(2011) and Chen and Liu (2015),zooplankton abundance averaged 1 118.8 inds./m3(93.6-13 552.2 inds./m3) and 970.2 inds./m3(105.6-5 169.7 inds./m3) in the YS and ECS in summer(June-July) and autumn (November), respectively,and these values were much higher than those in our study.To perform detailed comparisons, we separated the YS and ECS samples along a line from the mouth of the Changjiang River to Cheju Island,Korea.The average abundances of the YS and ECS were 319.90±271.11 inds./m3and 221.12±217.23 inds./m3, respectively.Compared with this study, Wang et al.(2013) reported a lower average abundance (165.6±13.6 inds./m3) in the YS (July-August).In the ECS, Xu et al.(2003) reported an annual abundance of 43 inds./m3(with 43 inds./m3in summer and 79 inds./m3in winter), which is also lower than that in this study.Because of the mesh size of the sampling net, the average biovolume(452.05±570.11 mm3/m3) (with 397.21±622.51 mm3/m3in the YS and 494.85±529.55 mm3/m3in the ECS) in this study was significantly lower than those reported by García-Comas et al.(2014) and Dai (2016).

In terms of abundance, the most dominant taxon in this study were Calanoids (36.24%), followed by Chaetognaths (10.52%) and Siphonophores (9.08%).The proportion of Copepods (Calanoids, Cyclopoids,and Harpacticoids) reached 41.1% of the total zooplankton abundance.The proportions of Copepods and Chaetognaths were similar to those in previous studies (Chen et al., 2011; Chen and Liu, 2015), but the proportion of Siphonophores (mainly distributed in the coastal regions of the ECS) was markedly higher in our study.

4.3 Spatial variability in zooplankton community structure

Based on the RDA results, zooplankton were classified into five groups, and the horizontal distribution of each group was coupled with distinctive oceanographic features.

Fig.7 Zooplankton taxonomic composition in each size class for each group (YSI, YSO, CREO, ECSI, and ECSO) in the Yellow Sea and the East China Sea

Different from the studies of Dai et al.(2016)and García-Comas et al.(2014), zooplankton sampling was conducted using a 500 μm-mesh net in this study, which might increase the collection of larger organisms.The 0.5-1-mm size class was the most abundant size class in the YSI, ECSI and ECSO groups.While in the CREO and YSO groups,the most abundant size class was ＜0.5 mm and 1-2 mm, respectively.The highest zooplankton abundance and the lowest Chlawere observed in the YSO group.Zooplankton respond to elevated phytoplankton biomass to generate higher abundance and larger biovolume (Dai et al., 2016).However, as the YSCWM hinders the development of primary production, the low levels of phytoplankton biomass may restrict the growth and expansion of zooplankton (Jang et al., 2021).In the YSO group,Calanoids (142.83 inds./m3) contributed 39.41% of the total zooplankton, and a large portion of Calanoids consisted ofCalanussinicus(1-2-mm size class in this study; Table 1 & Fig.7c), which is consistent with previous reports (Wang et al.,2021; Zhao et al., 2022).The prevalence of YSCWM in summer shelters cold-water species(such asCalanussinicusandEuphausiapacifica)from damage associated with elevated temperatures in the upper layer (Wang et al., 2003; Pu et al.,2004; Yoon et al., 2006; Liu and Sun, 2010).To cope with the food scarcity in the YSCWM, these species maintain a low metabolism status (Li et al.,2004; Tao et al., 2015) and access a wider spectrum of food, including detritus, eggs, ciliates, and microzooplankton (Huo et al., 2008; Tao et al.,2015; Kim et al., 2019).In addition, the YSO group also featured the highest amount of Thaliaceans,which might be attributed to the typical temperate warm water speciesSalpafusiformis(Liu et al., 2012).In terms of abundance, the YSI group mainly comprised the ＜0.5-mm, 0.5-1-mm, and 1-2-mm size classes (96.08%).However, it showed the highest biovolume among all groups, which was attributed to the large-sized Hydrozoans (especially in the southern transect 3300).In contrast with the YSO group, 0.5-1 mm was the most abundant size class in YSI, which was attributed to small Calanoids(such asParacalanusparvus) and Eumalacostracans.As the body size of copepods is inversely correlated with ambient temperature (Corkett and McLaren,1979), the dominance of Calanoids in 0.5-1 mm in the YSO group may be due to the relatively high temperature in this region.

The ECSI group was characterized by the highest total biovolume and biovolume of the taxon Siphonophores, which was mainly attributed to the 1-2-mm, 2-3-mm, 3-4-mm, and 4-5-mm size classes.The prevalence of Siphonophores might be explained by the high temperature and coastal upwelling in this region, as coastal upwelling brings abundant nutrients and supports primary production,offering sufficient food for Siphonophores (Li et al.,2012).However, because Siphonophores are highly carnivorous predators, the prevalence of Siphonophores may restrict the growth and expansion of other zooplankton taxonomic groups(Lo et al., 2012; Ezhilarasan et al., 2018), which might be the reason for the low abundances of Calanoids and other zooplankton taxa in the ECSI group.In addition, the distribution of Calanoids in the 0.5-1-mm size class may also be attributed to the higher ST and BT (Corkett and McLaren, 1979).Although the coastal CRE was severely influenced by the CDW, it was classified into the ECSI group according to the RDA results.The ZFCC flows northwestwards into the CRE, where it encounters the CDW and turns right (Yuan et al., 2008; Zhu et al., 2022).Therefore, the zooplankton community of the coastal CRE was influenced by both the CDW and ZFCC.Although the CDW changed the hydrographic conditions greatly in the coastal CRE,the ZFCC might play a more important role in shaping the zooplankton community.Among all groups, ECSO showed the lowest abundance and biovolume (Fig.5).Under the influence of KBC,ECSO exhibited higher values of ST, BS, and SS and a lower value of BDO.It has been demonstrated that hypoxia may negatively affect zooplankton through a decreased growth rate and increased mortality (Richmond et al., 2006; Elliott et al.,2012).This might explain the low abundance and biovolume in this region.The CREO group featured higher levels of BT, SS, and BS.Compared with the ECSO group, the CREO group had higher BDO and BT values.Among all groups, the CREO group showed the highest values of Calanoids, Cyclopoids and total abundance in the ＜0.5-mm size class.Although the mesh size of the plankton net reached 500 μm, zooplankton was most abundant in the ＜0.5-mm size class in the CREO group, which could be attributed to the high abundances of small sized Calanoids and Cyclopoids.In the ＜0.5-mm size class, Calanoids and Cyclopoids (mainlyOithonasp.) contributed 69.44% of the total abundance.According to a study by Ezhilarasan et al.(2020), the peak values ofOithonasp.in the offshore waters are consistently associated with lower-turbidity water masses compared with the coastal areas, which might be the reason for the abundant Cyclopoids in this region.The predominance of small copepods in this group may also be attributable to the high ST and BT.

5 CONCLUSION

Based on the categorized size classes of biovolume data, the zooplankton community was classified into five groups, and each group was coupled with distinctive oceanographic features.In the central Yellow Sea, the zooplankton community was mainly influenced by the Yellow Sea Cold Water Mass, with the Yellow Sea Coastal Current and Subei Shoal Coastal Current mainly affecting the coastal zooplankton community.In the East China Sea,the zooplankton community was under integrated influences from Kuroshio Branch Current, Taiwan Warm Current, Zhejiang-Fujian Coastal Current,and Changjiang Diluted Water.However, as the Yellow Sea and East China Sea are highly dynamic,further studies should be carried out to investigate the real-time status of this region.

6 DATA AVAILABILITY STATEMENT

The datasets generated and analyzed during the current study are available in the Science Data Bank repository, https://www.scidb.cn/anonymous/ZUVaN2Zh.

7 ACKNOWLEDGMENT

We thank Shujin GUO, Qingjie LI, and Tao LIU(Institute of Oceanology, Chinese Academy of Sciences) for zooplankton sampling in the field.We are also grateful to the crew of the R/VKexue3for their assistance in field survey.