Plankton samples, including both phytoplankton and zooplankton, were collected in an inner bay of the Bohai Sea ( Figure 1 ) aboard the fishing boat “Jintangyu02066” on November 14, 2023 (autumn), and April 17, 2024 (spring), respectively. The offshore distances of all stations range from 2.34 Km (station 7) to 15.39 Km (station 4). The seafloor of all stations located at neritic area of the Bohai Sea were shallower than 5.0 m. Phytoplankton samples were gathered using a standard net III (diameter 37.0 cm, mesh size 76 μm), trawled from a depth of 0.5 m off the bottom to the surface at each site, and preserved in acid Lugol’s solution (1.5% final concentration). Zooplankton samples were collected using a standard net II (diameter 31.6 cm, mesh size 160 μm), also trawled from bottom to surface. After each tow, zooplankton specimens were fixed in a formaldehyde solution (2% final concentration) for subsequent analyses. Collectively, a total of 48 samples were collected and preserved in darkness at 4°C. In the laboratory, both phytoplankton and zooplankton samples were identified to the lowest taxonomic level using a binocular dissecting microscope (Olympus SZX16), referencing Guo (2004); Sun et al. (2015); Wang and Song (2017), and Zhang et al. (2019).

Seawater temperature (°C) and salinity were measured using a WTW Cond 3210 SET 1 portable water quality analyzer (Xylem, Munich, Germany). Chlorophyll a (Chl a) concentration was determined by filtering 1 L of seawater through a Whatman GF/F glass fiber filter and stored at -20°C. Plankton retained on the filter was extracted in 90% (vv⁻¹) acetone, and fluorescence was measured following the JGOFS protocol (Knap et al., 1996) using a Turner Trilogy fluorometer Model 10 (Turner Designs, US). A PHSJ-3F pH analyzer was used for the pH measurement. For dissolved oxygen (DO), samples were collected in an iodine flask, treated with alkaline potassium iodide and manganese sulfate, and titrated with a standard sodium thiosulfate solution. Additionally, 100 mL water samples for nutrient analysis (ammonium-NH⁴⁺, nitrate-NO₃ ⁻, nitrite-NO₂ ⁻ and phosphate-PO₄ ³⁻) were filtered through a Whatman GF/F glass fiber membrane (0.7 μm), fixed with chloroform, and stored at -20°C. Nutrient concentrations were analyzed using a SEAL QuAAtro nutrient analyzer (Germany) (Ma et al., 2019, 2023). Zinc ions (Zn²⁺) were measured by inductively coupled plasma mass spectrometry, with concentrations determined using an atomic absorption spectrophotometer (SpectrAA FS220, Australia).

The dominance index (Y) of species in plankton (including phytoplankton or zooplankton) was calculated using the following formula (Xu and Chen, 1989):

where n_i is the number of individuals of species i in all samples, f_i is the occurrence frequency of species i in all samples and N is the total number of all taxa. Species with Y ≥ 0.02 represented as the dominant species in plankton assemblage. Furthermore, all stations located at neritic area of the Bohai Sea with seafloor shallower than 5.0 m, thus we treat all environmental variables obtained from surface layer (1 m) can represent whole water column in our results for environment-plankton analysis. Moreover, the average value of each parameter was represented as mean ± SD in the following text.

Distributional data, including sampling maps, phytoplankton, zooplankton, and environmental variables, were visualized using ODV (Ocean Data View, Version 4.7), Surfer (Version 13.0), Grapher (Version 12.0), and OriginPro 2021 (Version 9.6). In addition, the Biota-Environment analysis was conducted based on Spearman’s correlation between log-transformed abiotic parameters and square root-transformed abundance data (t-test), utilizing both PRIMER (Version 5.0) and OriginPro 2021 (Version 9.6). Furthermore, the slope of the phytoplankton-zooplankton (Δ_K ) was carried out to quantize their ecological interaction.

Most environmental parameters, except for NO₂ ⁻ and PO₄ ³⁻, exhibited distinct seasonal variations in the neritic area of the Bohai Sea ( Figure 2 ). In spring 2024, average values for temperature (16.6 ± 1.0°C), pH (8.1 ± 0.1) and NO₃ ⁻ (0.2 ± 0.1 mg/L) were higher by 7.3°C, 0.2, and 0.1 mg/L, respectively, compared to autumn 2023. Additionally, Chl a concentrations were significantly higher at stations 1−5 in spring 2024 than in autumn 2023, while they were nearly equal at other stations ( Figure 2 ). Furthermore, salinity (average 28.4 ± 0.2), DO (average 10.2 ± 0.2 mg/L), NH⁴⁺ (average 0.1 ± 0.0 mg/L), and Zn²⁺ (average 17.8 ± 7.3 μg/L) were lower in most stations during spring 2024 compared to autumn 2023, where values were 27.9 ± 0.2, 8.0 ± 0.2 mg/L, 0.0 ± 0.0 mg/L, and 9.6 ± 4.5 μg/L, respectively ( Figure 2 ).

Total phytoplankton abundance at each station ranged from 6.6 to 40.2 × 10⁶ cells/m³ (average 18.4 ± 9.8 × 10⁶ cells/m³) in autumn 2023, compared to 0.3–3.0 × 10⁶ cells/m³ (average 1.0 ± 0.8 × 10⁶ cells/m³) in spring 2024, indicating an average value 18.4 times higher in autumn. Similarly, zooplankton showed higher abundance in autumn 2023, with average value (852.1 ± 421.1 ind/m³) being 3.4 folds higher than that in spring 2024 (253.4 ± 180.5 ind/m³) ( Figure 2 ). The trend of increased plankton abundance in autumn 2023 coincided with variations in salinity, dissolved oxygen, NH₄ ⁺, and Zn²⁺ ( Figure 2 ). In terms of trophic interactions, average abundance ratio of phytoplankton: zooplankton was 2.2× 10⁴: 1 and 0.4× 10⁴: 1 in autumn 2023 and spring 2024, respectively ( Figure 3 ). Additionally, the phytoplankton-zooplankton slope in autumn 2023 (Δ_K = -9.96) was steeper than in spring 2024 (Δ_K = -8.27) ( Figure 3 ).

Overall, a total of 64 phytoplankton species (32 genera) from 3 phyla, and 26 zooplankton species (20 genera) from 7 phyla were recorded in the neritic area of the Bohai Sea during autumn 2023 and spring 2024 ( Tables 1 , 2 ). Among them, phytoplankton species richness was higher in autumn 2023 (57 species) compared to spring 2024 (33 species) ( Table 1 ), while zooplankton showed little variation between the two seasons ( Table 2 ). Besides, in autumn 2023, there were 49 species in the phylum Bacillariophyta (86.0%), 1 species in Chrysophyta (1.8%), and 7 species in Pyrrophyta (12.2%), whereas spring 2024 featured only Bacillariophyta species ( Table 2 ). Based on the dominance index (Y ≥ 0.02), phytoplankton species and 7 zooplankton species were identified in both seasons ( Figure 4 ; Tables 1 , 2 ), with 9 phytoplankton species noted in autumn 2023 and 4 phytoplankton species in spring 2024 ( Table 1 ). For zooplankton, 6 species were dominant species in autumn 2023 and 5 in spring 2024, respectively ( Table 2 ).

Dominant species of both phytoplankton and zooplankton exhibited clear seasonal variation ( Figure 4 ; Tables 1 , 2 ). Among the targeted co-occurring phytoplankton, only Chaetoceros curvisetus and Pseudo-nitzschia pungens were dominant in both seasons. In autumn 2023, dominant species included Eucampia zodiacus (Y = 0.18), C. lorenzianus (Y = 0.07), Coscinodiscus subtilis (Y = 0.04) and C. asteromphalus (Y = 0.02), while Rhizosolenia setigera (Y = 0.40) and Skeletonema costatum (Y = 0.17) were dominant species at spring 2024 ( Figure 4 ; Table 1 ). For zooplankton, four species (Acartia pacifica, Calanus sinicus, Aidanosagitta crassa, and Noctiluca scintillans) were identified in both seasons ( Table 2 ). Copepodite (Y = 0.04) and Nauplius (Y = 0.03) were the dominant species in autumn 2023, whereas Polychaete larva (Y = 0.04) was the dominant taxon in spring 2024 ( Figure 4 ; Table 1 ).

Additionally, the abundance proportions (AP) of all dominant phytoplankton were ≥ 87.8% (average 91.6 ± 1.9%) in autumn 2023 and ≥ 56.2% (average 83.6 ± 10.5%) in spring 2024, respectively. Similarly, dominant zooplankton species exhibited AP values of ≥ 87.9% (average 94.0 ± 3.6%) and ≥ 42.8% (average 88.3 ± 14.7%) during these seasons ( Figure 4 ). In autumn 2023, the top three dominant phytoplankton species were C. castracanei (AP = 34.3%), E. zodiacus (AP = 17.8%) and Ceratium macroceros (AP = 9.1%). In contrast, those species shift to R. setigena (AP = 40.3%), C. curvisetus (AP = 22.7%) and S. costatum (AP = 16.7%) in spring 2024 ( Figure 4A ). Similarly, the dominant zooplankton species in autumn 2023 were Noctiluca scintillans (AP = 54.4%) and A. crassa (AP = 16.6%), which changed to C. sinicus (AP = 34.3%) and N. scintillans (AP = 22.2%) in spring 2024 ( Figure 4B . Overall, compared to phytoplankton, zooplankton exhibited less variation in dominant species across seasons, indicating a stronger community stability ( Figure 4 ; Tables 1 , 2 ).

Dominant phytoplankton and zooplankton species showed varying correlations in both interspecific and trophic level relationships during autumn 2023 and spring 2024 ( Figure 5 ). In terms of interspecific relationships, most dominant phytoplankton species displayed significant positive correlations, indicating mutually beneficial coexistence, except for S. costatum-C. curvisetus, which exhibited a significant negative correlation, indicating competitive interactions in autumn 2023 ( Figure 5A ). In spring 2024, significant positive correlations were observed among C. asteromphalus-P. pungens, C. curvisetus-E. zoodiacus, S. costatum- P. pungens/R. setigena/C. asteromphalus, suggesting mutually beneficial coexistence ( Figure 5B ). Targeted zooplankton, only A. crassa-C. sinicus showed a significant positive correlation, indicating mutual coexistence in autumn 2023 ( Figure 5A ). However, in spring 2024, the interspecific relationships among C. sinicus/Brachyura zoea larva-A. pacifica, A. crassa-N. scintillans, Polychaeta larva-C. sinicus were mutually beneficial, all exhibiting significant positive correlations. Conversely, the relationship between A. crassa-A. pacifica was competitive, showing a significant negative correlation ( Figure 5B ).

As for trophic level relationships in phytoplankton (prey)-zooplankton (predator), Spearman’s rank correlation indicated significant positive correlations for R. setigera-N. scintillans, C. compressus-C. sinicus, E. zodiacus-A. crassa in autumn 2023, while C. macroceros-N. scintillans, C. castracanei-A. pacifica exhibited negative correlations ( Figure 5A ). In spring 2024, significant positive correlations were observed between R. setigena and A. pacifica, as well as P. pungens/C. asteromphalus/S. costatum with Polychaeta larvae, while R. setigena-A. crassa, P. pungens-Nauplius, E. zoodiacus-Brachyura zoea larva exhibited negative correlations ( Figure 5B ). Specifically, these patterns suggest that each zooplankton species has unique preferred food items in both autumn 2023 and spring 2024, with R. setigera being a favored food source for both N. scintillans and A. pacifica.

Plankton dominant species displayed varied responses to environmental parameters during autumn 2023 and spring 2024 ( Figures 6 , 7 ; Tables 3, 4). Regarding phytoplankton-abiotic interactions, only E. zoodiacus, P. pungens and C. asteromphalus exhibited similar trends with environmental variables, suggesting they may prefer overlapping niches and face strong competition ( Figure 6 ; Tables 3). Additionally, C. castracanei, R. setigena, C. compressus and S. costatum thrived in high-temperature and high-pH conditions, contrasting with E. zoodiacus, P. pungens, C. lorenzianus, C. asteromphalus, C. subtilis, and C. curvisetus ( Figure 6 ). Meanwhile, each species exhibited distinct trends in response to nutrient availability, reflecting their varying utilization efficiencies. Notably, all species, except R. setigena and S. costatum, showed an increasing trend with Zn²⁺ ( Figure 6 ).

Regarding zooplankton-abiotic interplay, our results revealed that N. scintillans, Nauplius and Copepodite exhibited similar trends in response to the complex environmental variables of temperature, salinity, Chl a, dissolved oxygen (DO), and pH, indicating strong competition among these species ( Figure 7 ). Except above-mentioned three species, A. crassa, A. pacifica and Polychaete larva also benefited from low temperature and high DO conditions. Notably, only C. sinicus and Brachyura zoea larva displayed an increasing trend with rising temperature ( Figure 7 ; Tables 4). Furthermore, with the exception of A. pacifica, A. crassa, and Nauplius, other species displayed unique responses to nutrient variables ( Figure 7 ; Table 4). Specifically, all species, except C. sinicus, demonstrated an increasing trend with Zn²⁺ ( Figure 7 ).

To further quantify the physical-biological interplay in the neritic area of the Bohai Sea during autumn 2023 and spring 2024, we conducted principal component analysis (PCA) using abundance of phytoplankton, zooplankton and their dominant species to assess abiotic influences ( Figure 8 ). The PCA revealed that two principal components effectively distinguished the environmental conditions across the two seasons, accounting for a substantial proportion of biotic variation ≥ 71.0% in autumn 2023 and ≥ 71.1% in spring 2024 ( Figure 8 ). Furthermore, each dominant species (both phytoplankton and zooplankton) exhibited unique correlations with specific environmental parameters throughout the seasonal variations ( Figure 8 ). For instance, A. crassa showed a significant positive correlation with PO₄ ^3-. However, at spring 2024, its significant positive correlation shifted to with both temperature and DO ( Figure 8 ). This phenomenon demonstrates that plankton can effectively leverage external environmental factors to survive seasonal changes.

Marine eukaryotic plankton, including both phytoplankton and zooplankton, represents a vast diversity of organisms that serve as essential food sources for commercial fish through fundamental trophic level transfers (prey-predator interactions) (Cordier et al., 2022; Omstedt, 2024). Consequently, the bioindex reflecting the abundance ratio of phytoplankton to zooplankton is crucial for exploring and understanding plankton trophic interactions. Previous studies reported abundance ratios in pico-, nano-, and microplankton of approximately 10⁶: 10³: 1 in the Mediterranean Sea (Tanaka and Rassoulzadegan, 2002) and the Tropical North/West Pacific Ocean (Sohrin et al., 2010; Wang et al., 2023c), forming a pyramid shape from low to high trophic levels (Trebilco et al., 2013). Our findings regarding the phytoplankton-to-zooplankton ratio align with this pattern. Furthermore, the abundance ratio in autumn 2023 was 5.5 times higher than in spring 2024 ( Figure 3 ), and the steeper phytoplankton-zooplankton slope collectively indicates that the plankton community in the former season experienced lower feeding pressure on zooplankton and stronger environmental resistance than in the latter season (De Vargas et al., 2015; Cordier et al., 2022).

Plankton species diversity plays a vital role in regulating ecosystem processes and resource utilization efficiency, thereby influencing marine ecosystem functioning and biogeochemical cycling (Chapin III et al., 1997). Similarly, a higher diversity of functionally similar species enhances the stability of resistance and resilience in marine ecosystem processes (Ibarbalz et al., 2019; Benedetti et al., 2021; Chust et al., 2024). Consistent with observational studies using both optical microscopy (Marić et al., 2012; Godrijan et al., 2013) and metabarcoding (Piredda et al., 2017; Armeli et al., 2019), species diversity of both phytoplankton and zooplankton was higher in autumn 2023 compared to spring 2024 ( Table 2 ), showing clear seasonal variations. Furthermore, considering the significantly higher plankton abundance ( Figure 3 ), it can be logically concluded that the plankton community in autumn 2023 exhibited greater resistance and resilience to harsh environmental conditions than in spring 2024.

As for prey-predator interactions, the fatty acid composition of phytoplankton is recognized as a crucial factor influencing food quality for higher trophic levels (Becker and Boersma, 2003; Boersma et al., 2009; Chen et al., 2010; Peng et al., 2024). Among various phytoplankton species, diatoms are particularly noted for their high levels of unsaturated fatty acids, which are essential for the cell differentiation, growth, reproduction, immune function, and other biological processes of zooplankton (Wichard et al., 2008; Yeung et al., 2020; Peng et al., 2024). Thus, it is reasonable to suggest that higher diatom abundance contributes to increased zooplankton populations, as observed in autumn 2023 ( Figure 3 ). Additionally, our findings indicate that all phytoplankton species in spring 2024 belonged to the phylum Bacillariophyta ( Table 2 ), aligning with Murphy et al. (2020), which found that warming significantly enhances the ecological importance of diatoms. Moreover, the minimal variation in both total and dominant zooplankton species between autumn 2023 and spring 2024 ( Figure 3 ; Table 2 ) may be attributed to their strong selective feeding abilities (Serandour et al., 2023).

Physicochemical factors, e.g., temperature, salinity, Chl a, nutrients, pH, dissolved oxygen, heavy metal, are crucial in reshaping complex plankton compositions through bottom-up control (resource limitation) (Power, 1992; Pörtner and Farrell, 2008; Wang et al., 2023c, 2024b; Lennartz et al., 2024). Specifically, temperature enhances species biodiversity by modulating temperature-dependent metabolic processes (Vázquez-Domínguez et al., 2007; Archibald et al., 2022). However, our results indicate that only a few phytoplankton and zooplankton species showed a positive correlation with temperature ( Figure 8 ). We speculate that in specific locations, the surrounding environmental conditions may exceed the temperature thresholds for these species (Holding et al., 2013; Stuart-Smith et al., 2015), potentially explaining the observed loss of both biodiversity and abundance in spring 2024. Moreover, previous studies suggest that higher trophic levels are generally more vulnerable to elevated temperatures, as the metabolic demands of consumers are more sensitive to warming, leading to decreased consumer fitness (Lopez-Urrutia et al., 2006; Rall et al., 2010). However, our findings do not align with aforementioned viewpoint, as both species composition and total abundance of zooplankton showed little variation between autumn 2023 and spring 2024 ( Figure 3 ), despite an average temperature increase of 7.3°C from autumn 2023 to spring 2024 ( Figure 2 ).

Nutritional availability is crucial for influencing phytoplankton concentrations, as it is closely linked to increased primary productivity in terms of both quantity (abundance) and quality (lipid unsaturation) (Premakumari et al., 2024). This productivity then transfers to higher trophic levels through essential prey-predator interactions (Šolić et al., 2010; Våge and Thingstad, 2015; Holm et al., 2022). Our results indicate that dominant phytoplankton species exhibited distinctly different trends in response to nutrients in autumn 2023 and spring 2024 ( Figure 6 ). This variability can be explained by two factors: 1) each species has a unique nutrient affinity (Strom and Fredrickson, 2008), and 2) avoid harmful competition (Sommer, 1989; Litchman et al., 2004; Kenitz et al., 2013). Regarding zooplankton, DO is a vital limiting factor for survival and growth, as heterotrophs must oxidize large compounds from their environment to release energy for biological processes (Fenchel, 2014; Qian et al., 2023). Our findings support this perspective ( Figure 7 ).

In recent decades, anthropogenic CO₂ emissions have induced global warming, triggering unprecedented and lasting impacts on marine ecosystems worldwide (Yasumiishi et al., 2020; Carvalho et al., 2021; Wang and Wu, 2022). This poses threats to biodiversity and ecological functions, particularly through poleward dispersal (Ershova et al., 2015; Hastings et al., 2020; Møller and Nielsen, 2020; Wang et al., 2022), changes in phenology and adaptation (Poloczanska et al., 2013; Atkinson et al., 2015), and mean body size miniaturization (Li et al., 2009; Daufresne et al., 2009; Qian et al., 2023). For surface-dwelling species in large marine environments, poleward dispersal is a prominent aspect of plankton’s response to global warming (Hastings et al., 2020). Whereas at a specific location, plankton face two options: 1) enhance their temperature tolerance through long-term adaptive evolution (Ward et al., 2019) or 2) extirpation. In this perspective, our study on plankton seasonal variations provides a fundamental benchmark for understanding the adaptive strategies of phytoplankton and zooplankton to rapid warming. Meanwhile, our results indicate that several dominant plankton species exhibited a positive correlation with temperature during the warmer spring of 2024 ( Figures 6 , 8 ). Thus, we deduce that these “winner” plankton species, with strong adaptation abilities (Casoli et al., 2020; Boutin et al., 2023), are likely to dominate the neritic area of the Bohai Sea in the future.