Harmful jellyfish blooms are widely believed to be associated with global warming; however, direct in-situ evidence at an ecosystem level to explain bloom mechanisms is lacking. This study focused on Aurelia coerulea polyps and the local benthic ecosystem and applied field temperature manipulation to explore responses of polyps and benthic ecosystems to global warming, community changes, and polyp survival strategies. In our experiment, local heating (+1.5 °C and +3 °C) led to significant changes in benthic community structure (p < 0.001), a decline in biodiversity, and the proliferation of opportunistic dominant organisms. Aurelia coerulea polyps achieved remarkable expansion, with their coverage percentage reaching 79.73 % in the +3 °C group at the final sampling, significantly higher than the 53.30 % at ambient temperature. During the transition of the benthic ecosystem from stability to instability, jellyfish polyps expanded colonization by virtue of diversified asexual reproduction modes such as budding, longitudinal fission, stolonic budding, which were enhanced under warming, and their rapid responses to environmental change and strong adaptability to warming environments. Our study revealed direct effects on organisms and indirect effects mediated by ecosystems of global warming in the process of jellyfish polyp blooms and emphasized the importance of considering the comprehensive effect of global warming at the local benthic ecosystem level through in-situ experiments when explaining the proliferation of opportunistic and hazardous species with complex life cycles such as jellyfish.

Over the past few decades, the offshore area adjacent to the Changjiang River Estuary (CE), an important fishing ground, has emerged as a hotspot for blooms of large jellyfish Nemopilema nomurai. However, comprehensive and quantitative understanding of the impact of N. nomurai on energy flow and important commercial fisheries within the ecosystem, from an ecosystem-based perspective, remains limited. This study employed six mass-balanced models using Ecopath with Ecosim to investigate the ecological role of N. nomurai as an energy transfer pathway within the food web, treating it as a separate functional group. Nemopilema nomurai was dominant in the combined energy consumption of fishery species and N. nomurai, and consumed a larger proportion of zooplankton production compared to fishery species, serving as an energy loss pathway with minimal production transferred to higher trophic levels. Additionally, scenario analyses revealed that fluctuations in N. nomurai population size had a significant impact on other functional groups owing to direct or indirect competition for food resources. This study provided valuable insights into the ecological role of jellyfish and their impact across various marine ecosystems worldwide.

As the important hotspots of biodiversity, seamounts have an impact on the surrounding marine ecosystems because of their distinct topographic features and environmental parameters. To clarify the influence mechanism of the "seamount effect" of three different types of seamounts, the characteristics of zooplankton communities (i. e., the abundance, diversity, dominant species, and biomass) at the Caroline, Yap, and Kocebu seamounts in the western Pacific Ocean (WPO) were studied. We collected and identified the 310, 197, and 151 zooplankton species/taxa at the Caroline, Yap, and Kocebu seamount areas, respectively. The dominant species/taxa of zooplankton community at the Caroline, Yap, and Kocebu seamount areas were 11, 10, and 8, respectively. The highest values for the biodiversity index (3.71) and abundance (28.85 ind/m3) appeared at the Yap seamount and Caroline seamount, respectively. The high zooplankton diversity was not consistent with the high zooplankton abundance at the three seamounts. The zooplankton dry weight biomass (DW) at the Caroline and Kocebu seamounts was dominated by zooplankton more than 2000 mu m in size. However, the proportion of different sizes in zooplankton DW was relatively uniform at the Yap seamount area. ZooScan results showed that the zooplankton carbon biomass (CB) at the three seamount areas was dominated by copepods (>50%). We observed significant differences between the zooplankton groups/taxa with the second and third most dominant CB proportions (Kruskal-Wallis tests, p < 0.01). Larvae accounted for a relatively high proportion of CB at the Caroline seamount, whereas the proportions of Euphausiid and Chaetognaths at the Yap and Kocebu seamounts were different. Zooplankton gaps were found at all three seamounts. The horizontal distribution patterns of abundance and biomass of the zooplankton community differed among the three seamounts. Correlation analysis showed that the zooplankton abundances at the Caroline and Yap seamount were significantly positively correlated with chlorophyll concentration, whereas the zooplankton abundance and biomass were negatively correlated with seawater temperature at the Kocebu seamount. Combined with the results of the principal components analysis and BIO-ENV, the primary environmental factor affecting the distribution of the zooplankton community at the Kocebu and Caroline seamount was seawater temperature and salinity, respectively. However, the horizontal distribution of the zooplankton community at the Yap seamount was simultaneously affected by multiple environmental factors. We found that the Yap and Kocebu seamount exhibited "seamount effects" on zooplankton communities, but we did not observe any obvious "seamount effect" on zooplankton community at the Caroline seamount area. These findings suggest that the shallow seamounts were not necessarily associated with stronger zooplankton community responses than the intermediate or deep seamounts in the WPO. These results provide valuable insight into the ecological responses of zooplankton community to different types of seamounts, thereby contributing to understanding the mechanism of the "seamount effect".

Zooplankton play a crucial role in marine ecosystems. Changes in their species composition and population dynamics can lead to structural and functional shifts in the ecosystem. The growth condition and population recruitment rate of zooplankton act as a comprehensive proxy​ for the marine ecosystem. However, research on zooplankton species composition and population dynamics faces considerable challenges due to the complexity of their communities and the difficulties in observation, sampling, and analysis. This study proposes using key growth indicators of selected zooplankton species to assess the health condition of marine ecosystems. Specifically, we examine the seasonal and regional growth variations in Antarctic krill (Euphausia superba), such as the occurrence and extent of shrinking to infer the status of the Southern Ocean ecosystem. Additionally, we employ egg production rates and hatching success of key species of copepods as indicators to evaluate the health of coastal ecosystems. Through these approaches, we aim to establish zooplankton growth metrics as effective proxies for monitoring and indicating marine ecosystem health.

Mesoscale eddies are ubiquitous oceanographic features that play a pivotal role in regulating marine ecosystems by altering water column structure and redistributing biological resources. To examine their ecological effects on zooplankton communities in the South China Sea (SCS), we conducted in situ imaging observations using the PlanktonScope and Underwater Vision Profiler 6 systems within representative cyclonic eddy (CE) and anticyclonic eddy (AE) in April 2023. Integrating satellite remote sensing with in situ environmental measurements, we analyzed eddy-induced variations in zooplankton abundance, community composition, vertical distribution, and diel vertical migration (DVM) behavior. Zooplankton abundance within the upper 300 m was consistently higher in the CE, showing strong aggregation in the eddy core, whereas in the AE, abundance peaked at intermediate depths (50–100 m) near the periphery, forming a characteristic ring-shaped pattern. Seawater temperature and dissolved oxygen were identified as the dominant environmental drivers regulating zooplankton community structure, while nutrient enrichment at 100–150 m in the CE reflected upwelling-driven enhancement of bottom-up control. Zooplankton also exhibited more pronounced DVM amplitudes within the CE than in the AE, indicating that mesoscale hydrodynamic conditions strongly modulate vertical behavioral dynamics and trophic connectivity. Overall, mesoscale eddies exert a profound influence on zooplankton spatial ecology in the SCS by reshaping hydrographic structure, nutrient availability, and behavioral patterns, providing new insights into the coupling between mesoscale physical processes and biological responses in tropical ocean ecosystems.

Dissolved oxygen (DO) concentrations in euphotic waters are governed by combined effects from air-sea exchange, photosynthesis, and aerobic respiration. Bacteria and microeukaryotes both respond to and shape these oxygen dynamics, yet the quantitative contributions of microbial traits versus physicochemical factors to DO variability remain unclear. Moreover, the extent to which DO, and other environmental parameters or inter-domain biotic interactions control microbial diversity and abundance is still unresolved. To fill these gaps, we conducted a two-month summer survey of a shallow coastal area off Shandong Peninsula, a region frequently affected by seasonal hypoxia. Modelling revealed a depth-dependent divergence in DO drivers: physicochemical variables dominated in surface and bottom waters, whereas microbial traits—particularly the abundance of high nucleic acid bacteria—outperformed environmental factors in the mid-depth layer. Under hypoxic conditions, microbial abundances declined and community structures shifted significantly, yet alpha diversity metrics remained unchanged. Inter-domain interactions were the primary force structuring bacterioplankton communities, exerting a stronger influence than environmental variables. Overall, bottom-water deoxygenation in this coastal system was predominantly by physiochemical processes, especially nutrients loading and stratification, rather than by direct microbial feedback. Non-DO environmental variables and inter-domain interactions emerge as the principal forces structuring planktonic communities, underscoring the need for nutrient-management strategies that account for these complex biotic-abiotic linkages.

1. Jellyfish blooms are increasing globally in frequency and intensity, introducing complex ecological interactions, yet the mechanisms by which they alter ecosystem structure remain poorly characterized due to a lack of sustained field observations. 2. We conducted targeted time-series observations in Jiaozhou Bay, a representative coastal ecosystem experiencing an Aurelia coerulea bloom, tracking temporal responses of plankton communities and trophic interactions to fluctuations in the jellyfish population. Combining empirical modelling with field data, we further quantified the dual roles of jellyfish as top-down predators and bottom-up nutrient regenerators, linking bloom dynamics to changes in plankton structure and biogeochemical cycling. 3. During high A. coerulea aggregation, the structure of the plankton changed markedly, characterized by a sharp decline in zooplankton coupled with the proliferation of phytoplankton. The top-down feeding effect triggered potential cascading impacts, decoupling trophic pathways by weakening zooplankton-mediated energy transfer and releasing phytoplankton from grazing pressure. Concurrently, jellyfish contributed to biogeochemical processes by releasing bioavailable ammonium and phosphate (low N/P ratio < 16), with phosphate regeneration playing a particularly critical role in stimulating primary production. 4. Overall, this study demonstrates that jellyfish blooms act as ecological disturbances, restructuring plankton dynamics and trophic linkages and ultimately destabilizing local ecosystem functioning. Given the broad distribution and increasing prevalence of jellyfish populations, the findings underscore the need to incorporate jellyfish dynamics into coastal ecosystem-based management, especially for predicting and indicating planktonic regime shifts.

Annual large-scale green tides in the Yellow Sea initially appeared in the Subei Shoal, Jiangsu Province, with Ulva prolifera identified as the dominant species. Molecular identification of Ulva species has been a key focus during green tide monitoring. In this study, we used the chloroplast gene tufA (encoding elongation factor Tu) as a molecular marker to assess species composition and frequency across 11 Ulva populations. These were sampled from three coastal sites in Jiangsu and Shandong Provinces at six time points, totaling 420 biologically identified Ulva individuals. Additionally, the sporulation features of Ulva spp., which are directly linked to biomass variation, were examined to understand why U. prolifera dominates these blooms. (1) TufA proved to be a highly accurate and unambiguous molecular marker for Ulva species identification. We found that propagules in the coastal waters of Yancheng, Jiangsu, consisted mainly of Ulva aragoensis and U. prolifera, whereas floating biomass sampled during green tides in Shandong was predominantly U. prolifera. (2) A lower sporulation ratio in U. prolifera compared to U. aragoensis affects their relative proportions in floating biomass. This contributes to U. prolifera becoming the dominant floating species during the annual large-scale green tides in the Yellow Sea.

The Yellow Sea Cold Water Mass (YSCWM) is a major hydrographic feature of the Yellow Sea, but seasonal changes in zooplankton community composition and trophic-transfer-related patterns associated with the YSCWM remain insufficiently resolved. We conducted field surveys in April and July 2024 and used ZooScan imaging to quantify zooplankton taxonomic composition, biomass, size structure, and size-based functional proxies, including the normalized biovolume size spectrum (NBSS) slope, size diversity, mean body size, and trophic ratios. Zooplankton abundance, biovolume, and carbon biomass were significantly higher in July than in April, particularly in the cold water mass (CWM) region, and copepods dominated these metrics across seasons and regions. In the CWM region, zooplankton communities in July were characterized by steeper NBSS slopes, lower size diversity, smaller mean body size, and higher proportions of taxa classified as carnivorous than in April, suggesting a shift toward a smaller-bodied trophic structure with lower potential for efficient energy transfer. Specifically, the NBSS slope became steeper from -1.16 ± 0.16 in April to -1.19 ± 0.24 in July, while size diversity declined from 2.05 ± 0.21 to 1.73 ± 0.20; both indicators were positively correlated with the zooplankton-to-phytoplankton biomass ratio (Z/P) in July. Redundancy analysis showed clear seasonal differences in community-environment relationships: zooplankton community structure was associated with both surface and bottom hydrographic variables in April, but closely related to bottom-water temperature, salinity, and dissolved oxygen in July. These findings suggest a seasonal reorganization of zooplankton size structure and trophic-transfer-related proxies in the YSCWM region.

Nemopilema nomurai Kishinouye, 1922, has emerged as a dominant large jellyfish species in the East Asian Marginal Seas over recent decades, posing increasing ecological and management challenges. To elucidate its ecological role, two Ecopath models representing N. nomurai bloom (2012-2013) and non-bloom (2014) periods in the northern East China Sea and southern Yellow Sea were developed. Ecological network analysis was employed to assess variations in ecosystem trophic status between these two periods. Results from keystone index and mixed trophic impact analysis revealed that N. nomurai occupied a more pivotal ecological role during bloom period, exerting widespread negative effects on multiple functional groups and thereby disrupting energy transfer within the food web. During bloom period, there were notable increases in total system throughput, total biomass, total primary production, and ascendency, indicating an expansion in ecosystem scale. Conversely, declines in Finn's cycling index and average path length suggested weakened recycling processes and shortened trophic linkages. Furthermore, decreased average mutual information, along with modified constraint efficiency, reflected a less organized and more structurally constrained energy pathway. Generally, these shifts depicted an ecosystem status characterized by high scale but reduced efficiency, disrupted trophic organization, and diminished ecosystem maturity during N. nomurai bloom period. These findings enhance our understanding of the ecological impacts of N. nomurai within marine food webs and emphasize the importance of integrating jellyfish dynamics into ecosystem-based management strategies and ecosystem health assessments.

The active carbon flux mediated by the diel vertical migration (DVM) of zooplankton is a crucial component of the marine biological carbon pump. This study investigated how mesoscale eddies influence the active carbon flux driven by zooplankton DVM in the South China Sea. On 13 and 16 April 2023, a comprehensive survey was conducted to compare zooplankton dynamics in cold and warm core eddies. Using ZooScan imaging technology, we compared zooplankton taxonomic composition, abundance, carbon biomass, DVM patterns, and the resultant active carbon flux across these eddies. The results demonstrated notable differences in the vertical distribution of zooplankton biomass and taxonomic composition between day and night. Zooplankton in the cold eddy (CE) exhibited a greater DVM amplitude (ΔDVM = 16.8 m) than those in the warm edddy (WE) (ΔDVM = 9.7 m). Consequently, the active carbon flux mediated by zooplankton DVM in the 0–100 m water layer of the CE (4.59 mg C/(m2·d)) was higher than in the WE (1.47 mg C/(m2·d)). These findings enhance our understanding of zooplankton-driven ecological processes in mesoscale eddies and their contribution to the carbon cycle, providing valuable insights for regional carbon flux assessments and improvements in global carbon pump models.

Jellyfish play crucial roles in marine food webs, and accurately understanding their trophic dynamics is essential. Stable isotope analysis (SIA) has emerged as a valuable tool for unravelling these complexities. However, establishing clear protocols for jellyfish sample preparation is vital to avoid potential biases in isotopic data interpretation. In this study, we focused on Nemopilema nomurai, Cyanea spp. and Aequorea spp. medusae collected in the East Asian water, and measured their SI δ13C and δ15N values. Our results highlighted two principal sources that may influence the determination and interpretation of SIA for jellyfish: tissue-specific variation in SI compositions and the lipid effect on δ13C. Our data indicated significant tissue-specific differences in isotopic compositions, which could introduce substantial bias in SIA. The findings highlight the necessity of clearly removing jellyfish tentacles during tissue sampling for SIA, as their inclusion may lead to an overestimation of δ15N values. Furthermore, our results demonstrated a significant increase in both δ13C and δ15N values after lipid-extraction, emphasizing the importance of lipid correction for SIA. Notably, the δ13C values for three jellyfish after chemical lipid removal differed distinctly from the values predicted by generalized and species-specific (Aurelia) lipid corrections reported in previous studies. We thus highlight jellyfish species-specific of mathematical lipid normalization, which might due to the heterogeneity in lipid content among jellyfish species as proved by the variation in C:N ratios. Based on our findings, we proposed species-specific lipid corrected equations for N. nomurai, Cyanea spp. and Aequorea spp. Given the importance of accurately determining jellyfish trophic ecology in marine ecosystems, this study provides a refined methodology aimed at enhancing the precision of SIA for the jellyfish populations in the ecosystems of East Asian waters.

Nutrients play a crucial role in sustaining marine ecosystems and supporting mariculture, especially in seaweed aquaculture. Currently, seaweed farming, such as kelp cultivation, is entirely dependent on the natural supply of nutrients. Sanggou Bay in Shandong Peninsula, Yellow Sea, is renowned for its 60-year history of kelp cultivation; however, it is recently facing an increasing demand for nitrogen and phosphorus due to the expansion in aquaculture scale and production. There is no doubt that nutrient addition can enhance and sustain the production, but it is crucial to understand its effect on kelp growth under current nutrients condition and the potential ecological risks. Our in-field nutrient enrichment experiments show that nitrogen and phosphorus additions promoted the kelp biomass during the early growth stages, and have no adverse effects on phytoplankton or seawater nutrient levels throughout the experiment. From a long-term perspective, increasing nutrient supply appears to be an essential strategy for sustaining the aquaculture of kelp.

Phosphorus (P) is a vital macronutrient essential for phytoplankton growth. However, in certain oceanic regions, P is often scarce, and numerous studies have demonstrated that dinoflagellates exhibit a robust adaptive capacity, including the utilization of dissolved organic phosphorus (DOP), to cope with P deficiency. In this study, we investigated the physiological and molecular responses of the dinoflagellate Gymnodinium impudicum, a species frequently associated with coastal blooms, to P-deficient conditions, with a focus on its DOP utilization strategies. Our findings reveal that P deficiency significantly inhibits the growth of G. impudicum, and the algal cells likely adapt to P deficiency through phospholipid degradation mechanisms. Among the tested DOP sources, glucose-6-phosphate (G6P), adenosine triphosphate (ATP), and sodium tripolyphosphate (TPP) were able to support the growth of G. impudicum, whereas glyphosate (Gly) was not efficiently utilized. Notably, an increase in alkaline phosphatase activity was observed during cultivation under P-deficient conditions. Furthermore, ATP was assimilated following extracellular degradation, while G6P appeared to be directly absorbed via endocytosis. This study provides critical insights into the adaptive strategies of G. impudicum under P-deficient conditions, offering a theoretical foundation for assessing the potential risks of this species bloom in both P-deficient and eutrophic environments.

Studies on the formation process and physio-ecological characteristics of primary polyps would help understand the causes of giant jellyfish Nemopilema nomurai blooms in the coastal sea of East Asia. A new mode of settlement and metamorphosis from planulae into primary polyps was observed in this study by artificially breeding N. nomurai polyps in a large tank. N. nomurai planulae could successfully metamorphose into primary polyps with ≤4 tentacles in the seawater, even though they did not initially colonize the hard substrates as previously reported. The developed primary polyps were then able to hang upside down on detritus via the mucus secreted by planulae and float on the seawater surface. Their resettlement on the large, hard substrates showed significant preference to plastic materials (e.g., polyethylene plates); however, the resettlement density was significantly reduced owing to the increase of age. The resettlement of primary polyps was also affected by the combination of salinity and age. The survival of primary polyps increased, but their resettlement percentage significantly decreased at hyposalinity of 10–23 and older age. This study also found that primary polyps on the detritus could normally develop into individuals with 16 tentacles. There was no significant difference in their survival, development of 14–16 tentacles, and calyx growth at salinities of 15–33, indicating their euryhalinity adaptability. This study suggested that floating in the seawater by attaching to detritus was a possible living mode of N. nomurai polyps inhabiting the estuaries’ surroundings, which may favor polyp population recruitment and maintenance.

Meroplankton significantly influences both benthic and pelagic ecosystems. This study investigates interannual variations of meroplankton in the Southern Yellow Sea (SYS), China, during summertime from 2012 to 2023 (noncontinuous), focusing on the interannual variations of key groups and their environmental factors under climate change. Findings revealed that meroplankton abundance was lower in June than in July, with peak abundances recorded in 2013 for June and 2012 for July, respectively. Spatial distribution patterns showed higher abundance in coastal shallow waters (<50 m) and lower in the Yellow Sea Cold Water Mass (YSCWM) (>50 m). Gastropoda and Bivalvia larvae were the most predominant and widely distributed groups, while body length of Bivalvia larvae varied across the surveyed periods; Gastropoda and Polychaeta larvae exhibited minimal differences. Brachyura and Echinodermata larvae showed no significant body length variations. Key environmental factors affecting meroplankton included sea bottom temperature (SBT), sea bottom salinity (SBS), and depth. Notably, SBT correlated positively with increased abundance of most groups, whereas SBS and depth were associated with decreased abundance. This study reveals the broad influence of the Pacific Decadal Oscillation (PDO) on the meroplankton abundance in the Yellow Sea, to which echinoderm larvae show significant sensitivity. This research highlights the significant interactions between environmental factors and meroplankton dynamics, providing valuable insights into their ecological roles in response to climate change in the SYS. By elucidating these relationships, the study enhances our understanding of meroplankton ecology and benthic-pelagic coupling, positioning meroplankton as important ecological indicators for assessing the impacts of climate change on marine ecosystems.

To evaluate the effects of the thermal effluent from the Hongyanhe Nuclear Power Plant on zooplankton communities in the surrounding sea area, the distribution and structure of zooplankton communities in Liaodong Bay were investigated in the summer from 2017 to 2020. Zooplankton community diversity, abundance, and structure were evaluated at 11 stations near the Hongyanhe Nuclear Power Plant alongside zooplankton-environment relationships on the basis of measurements of seawater temperature, salinity, and chlorophyll a concentration. Significant interannual changes in zooplankton communities associated with the thermal effluent from the power plant were observed. Variation in species dominance, particularly that of Calanus sinicus, Acartia bifilosa, and the blooming Noctiluca scintillans, altered the zooplankton community structure across the four years. These results suggest that the thermal effluent from the nuclear power plant could influence and alter zooplankton community structure to some extent. However, global climate change also more strongly influenced zooplankton communities at a broader spatial scale during the summers of 2017-2020.

The frequency of jellyfish blooms in marine ecosystems has been rising globally, attracting significant attention from the scientific community and the general public. Low-altitude remote sensing with Unmanned Aerial Vehicles (UAVs) offers a promising approach for rapid, large-scale, and automated image acquisition, making it an effective tool for jellyfish population monitoring. This study employed UAVs for extensive sea surface surveys, achieving quantitative monitoring of the spatial distribution of jellyfish and optimizing flight altitude through gradient experiments. We developed a “bell diameter measurement model” for estimating jellyfish bell diameters from aerial images and used the Mask R-CNN algorithm to identify and count jellyfish automatically. This method was tested in Qinglan Port, where we monitored Acromitus flagellatus populations from mid-April to mid-May 2021 and late May 2023. Our results show that the UAVs can monitor jellyfish with bell diameters of 5 cm or more, and the optimal flight height is 100–150 m. The bell diameter measurement model, defined as L = 0.0103 × H × N + 0.1409, showed no significant deviation from field measurements. Compared to visual identification by human experts, the automated method achieved high accuracy while reducing labor and time costs. Case analysis revealed that the abundance of A. flagellatus in Qinglan Port initially increased and then decreased from mid-April to mid-May 2021, displaying a distinct patchy distribution. During this period, the average bell diameter gradually increased from 15.0 ± 3.4 cm to 15.5 ± 4.3 cm, with observed sizes ranging from 8.2 to 24.5 cm. This study introduces a novel, efficient, and cost-effective UAV-based method for quantitative monitoring of large jellyfish populations in surface waters, with broad applicability.

Mesoscale eddies are key oceanographic features influencing zooplankton community structure and ecosystem function. However, the vertical impacts of cyclonic and anticyclonic eddies on zooplankton energy transfer efficiency remain unclear in the northern South China Sea (SCS). We conducted a field survey in April 2023, collecting zooplankton samples with a multi-net system and analyzing them via ZooScan imaging technology. Size-based and trophic indicators-including the normalized biovolume size spectrum (NBSS), size diversity, and average equivalent spherical diameter (ESD)-were used to assess energy transfer efficiency across depth layers and eddy types. Results indicated significantly higher zooplankton total abundance, biovolume, and carbon biomass within cyclonic eddies (mean +/- SD: 93.2 +/- 25.7 ind./m3, 45.4 +/- 20.9 mm3/m3, 2.9 +/- 1.5 mg C/m3) compared to anticyclonic eddies (mean +/- SD: 82.2 +/- 23.0 ind./m3, 37.8 +/- 14.0 mm3/m3, 2.4 +/- 0.9 mg C/m3) in the upper 300 m. Small copepods dominated all depth layers in both eddy types, comprising over 70% of the total abundance. Functional indicators, including the NBSS slope, size diversity, and average ESD, indicated higher energy transfer efficiency in cyclonic eddies within the upper 300 m. However, at the 0-25 m depth layers, anticyclonic eddies exhibited flatter NBSS slopes and higher size diversity than cyclonic eddies. Zooplankton productivity declined consistently with depth, while energy transfer efficiency to higher trophic levels showed a fluctuating vertical pattern and tended to rebound in deeper layers. Our findings highlight the crucial role of mesoscale eddy dynamics in structuring zooplankton communities and regulating energy flow in pelagic ecosystems of the northern SCS.