Tracking and understanding the historical variations in phytoplankton community composition in Sanggou Bay hold significant implications for eutrophication management, environmental quality regulation in aquaculture waters, and provides basic materials and scientific basis for carrying capacity. Based on previous investigations of phytoplankton in Sanggou Bay, we summerized four decades of historical data to analyze the long-term variation trends in community structure and identify the key drivers on the current state. Results show that chlorophyll a (chl a) in surface water increased from inner to outer bay in spring but decreased in autumn and winter, showing notable compositional shifts. Nano-phytoplankton dominated chl a in spring (53%) and summer (71%), while micro-phytoplankton prevailed in autumn (52%) and winter (71%). A total of 57 phytoplankton species (including varieties and forms) from 33 genera were identified, mainly diatoms (40 species) and dinoflagellates (14 species), and diatoms remained the dominant group. Cell abundance of phytoplankton was the highest in summer (276×10³ cells/L) and lowest in winter (6.4×10³ cells/L). Species diversity indices had no significant seasonal variation. Over the past four decades, chl a dynamics in Sanggou Bay exhibited two distinct phases: a peak in the 1990s followed by a decline after the 2000s due to the shifts in aquaculture practices, stabilizing post-2000 until 2024. Prolonged large-scale raft aquaculture resulted in the decreases in the number of phytoplankton and diatom species and a notable transition toward smaller-sized diatom dominance. The type and density of culture species and culture modes may be the key factors on the community structure and seasonal variation of phytoplankton.

To investigate the occurrence characteristics and ecological risks of microplastics in zooplankton communities of Jiaozhou Bay, a seasonal survey was conducted in February, May, August, and November that quantified the abundance and characteristics of microplastics within zooplankton. The results showed that the microplastics abundance within individual zooplankton over the four months was 0.30, 0.20, 0.21, and 0.20 items/individual, respectively. The community-level microplastic abundance was 0.19, 5.61, 7.36, and 0.52 items/m³, respectively. The microplastic abundance within zooplankton communities in May and August was significantly higher than that in February and November. The average storage of microplastics by zooplankton communities in Jiaozhou Bay over the four months was 3.19×10¹⁰ items, with the highest value observed in August. Fibers were the predominant shape of microplastics in zooplankton, accounting for 89% of the total, with no significant differences observed among months. Most microplastics were smaller than 500 μm, with an average length of (491±454) μm; microplastics detected in August exhibited significantly greater lengths (P<0.05). The predominant chemical compositions were polyester fibers and cellophane, accounting for 37% and 19% of the total, respectively, with no significant differences observed among months. The mean values of the microplastic pollution load index (PLI) and polymer hazard index (PHI) for zooplankton in Jiaozhou Bay over four months were 3.49±2.94 and 4.33±1.31, respectively, both corresponding to the lowest risk category(PLI<10, PHI<10). The mean bioaccumulation factor (BAF) was 55.59±56.84, indicating no significant bioaccumulation potential of microplastics in zooplankton communities (BAF<1000). Temperature, suspended particulate matter concentration, and chlorophyll a concentration were identified as the primary factors influencing the risk assessment outcomes.

In recent decades, harmful algal blooms (HABs), particularly red tide events caused by dinoflagellates, have occurred frequently in China’s coastal waters, threatening marine ecological security, economic and social development, and human health. Considering the diversity and abundance of dinoflagellate cysts in sediments from 267 stations in China’s four major seas during 2014-2018, this study integrated historical HAB records and literature on the distribution and abundance of vegetative cells in water bodies. By applying the typical red tide risk assessment method, risk maps for red tide outbreaks of 26 target dinoflagellate species in China’s seas were compiled, and the assessment results were interpreted and discussed. The target dinoflagellates exhibited varying degrees of red tide risk values. Among them, 11 species (Scrippsiella acuminata, Scrippsiella donghaiensis, Gonyaulax spinifera, Alexandrium catenella, Levanderina fissa, Pseudocochlodinium profundisulcus, Alexandrium pacificum, Alexandrium minutum, Gonyaulax polygramma, Akashiwo sanguinea,and Prorocentrum donghaiense) showed high-risk characteristics at certain monitoring stations, whereas 21 species displayed moderately high-risk characteristics. Additionally, the risk values of different dinoflagellates varied across China’s four major seas and presented distinct regional features. For example, the red tide risk values of Pr. donghaiense and Ps. profundisulcus in the East and South China Seas, respectively, were significantly higher than those in other sea areas. Correlation analysis showed that 37 pairs and 5 pairs of dinoflagellate species showed significantly positive and negative correlations in their risk values, respectively, reflecting certain synergistic or competitive relationships in their ecological niches. This study provides a scientific basis for preventing and controlling red tides in China’s coastal waters, and a reference for similar studies in other marine areas worldwide, and for the prediction and forecasting of red tides.

It is essential for ecosystem-based fisheries management to investigate the spatiotemporal distribution characteristics and environmental driving mechanisms of the key zooplankton species (Acetes chinensis) during its total allowable catch project period. We applied DBSCAN (density-based spatial clustering of application with noise) algorithm to process summer 2021 Beidou VMS (vessel monitoring system) data to extract information of the fishing locations associated with A. chinensis catches. We then combined these locations with remotely sensed marine environmental data and investigated the distribution patterns and key environmental drivers of A. chinensis habitats in the Haizhou Bay during summer in MaxEnt model using geographic detector method. Results indicate that suitable habitats for A. chinensis were concentrated in mainly three regions: 120°01′~120°17′E and 34°43′~34°47′N; 119°48′~119°58′E and 34°34′~34°42′N; and 119°58′~120°12′E and 34°25′~34°33′N. Although A. chinensis is a small, planktonic shrimp, its spatiotemporal distribution was not solely determined by surface residual currents, and the dominant factors influencing its distribution varied in different fishing periods. In the entire fishing season, the water depth, phytoplankton abundance, sea surface temperature, and sea surface salinity emerged as critical drivers of the habitat distribution. The optimal conditions in Haizhou Bay were determined as: water depth of 3.5~11.0 m, phytoplankton abundance of 9.5~17.0 mmol/m³ or 27.5~30.0 mmol/m³, sea surface temperature of 24.2~25.5 ℃, and sea surface salinity of 21.0‰~23.6‰. During the peak catch period, surface residual currents became an additional important factor on the distribution of the shrimp habitats. Future research shall focus on developing a comprehensive observation indicator system for this key species and conducting further assessments to ensure the sustainable production of A. chinensis resources.

The Yellow Sea Cold Water Mass (YSCWM), acting as a distinct physical barrier, forms a stable, low-temperature, high-salinity environment; however, the mechanisms by which it influences the distribution patterns of benthic microeukaryotes and the associated environmental drivers remain poorly understood. In this study, we systematically characterized microeukaryotic communities in sediment samples collected from two stations in the Bohai Sea and fifteen stations located within and surrounding the YSCWM, using environmental DNA (eDNA) sequencing with the 18S rDNA gene as the molecular marker. The results show that Dinoflagellata constituted the dominant taxonomic group both within and outside the YSCWM; however, the microeukaryotic community structures of the two regions differed significantly (ANOSIM: R=0.576 3, P=0.001). Compared with regions outside the YSCWM, communities within the YSCWM exhibited lower species richness (median: 1 529 vs. 1 672) but displayed a more complex co-occurrence network structure (4 062 vs. 3 138 edges), suggesting greater potential ecosystem stability. Deterministic processes, particularly heterogeneous selection, predominantly govern community assembly, in which water depth exerts a stronger influence on communities outside the OYSCWM communities, whereas temperature and median sediment grain size play more critical roles in shaping communities within the YSCWM. These findings demonstrate that the YSCWM plays a critical role in structuring sedimentary microeukaryote communities and highlights environmental DNA (eDNA) technology as an effective approach for elucidating marine microbial biogeographic patterns and evaluating ecosystem structure.

In the autumn of 2022, a red tide species Eucampia zodiacus bloomed in the nearshore water of Rushan, northern Yellow Sea, lasting for more than 40 days. This nearshore area is the main aquaculture domains for oyster Crassostrea gigas in China, and the important bait for oysters is phytoplankton. It has been shown that there are large seasonal variations of phytoplankton in this area, but there are fewer studies related to phytoplankton feeding by oysters, especially the effect of red tide outbreaks on their feeding selectivity. By analyzing the phytoplankton in water samples and oyster stomach contents, two complete blooming and recession periods of red tide were identified, with the second one having the longest duration (about 18 days) and the highest abundance (2.23×10⁶ cells/L). At the peak of the red tide, the algae abundance in the stomach contents of oysters (2.11×10⁴ cells/ind.) was significantly higher than that during the recession period of the red tide (1.44×10³ cells/ind.). During the blooming period of E. zodiacus, oysters tended to randomly feed on this dominant species, and preferred to feed on 5 diatom genera such as Nitzschia and Coscinodiscus, etc., during the recession period of the red tide, the oyster's feeding selectivity index decreased for most of phytoplankton species, preferring only Pleurosigma sp. and shifting to avoid its previously preferred genera Navicula and Paralia sulcata. The study indicated that the oyster may have limited control over the outbreaks of the E. zodiacus because it is not a preferred bait for oysters, and the oysters’ feeding selectivity changed significantly before and after the outbreaks of E. zodiacus, which suggests that its outbreaks may have affected the subsequent feeding behavior of oysters.

Marine ecosystem health is under severe pressure. Therefore, scientific and effective evaluation indicators for its assessment and early warning are urgently required. Plankton is a sensitive indicator of ecosystem changes and is critical in the marine food web, supporting important ecosystem services such as carbon sequestration and fishery production. Based on survey data of Jiaozhou Bay from 2003 to 2013, this study screened two types of zooplankton functional group indicators closely related to environmental changes: the dry weight biomass of traditional forage zooplankton (large crustaceans, large copepods, and small copepods), and the dry weight biomass proportion of gelatinous zooplankton (jellyfish, chaetognaths, and tunicates). These indicators represent the productivity level and stability of the functional group structure, respectively. According to the range and thresholds of the historical data, a graded evaluation standard was established. By constructing a two-dimensional coordinate plane based on zooplankton functional group indicators, according to the graded evaluation standard, the historical health status of the ecosystems in Jiaozhou Bay was classified into ecosystems with benign and risky development. These were further subdivided into seven types of health status, namely, rich-product, healthy-sustainable, benign-development, gelatinous-development, gelatinization, high-risk, and resource-scarce. This study provides a typical case using zooplankton functional groups as assessment indicators to establish an assessment system for offshore ecosystem health.

To investigate the characteristics of the macrobenthic community and its benthic ecological quality in Geligang, a typical intertidal flat in northern Liaodong Bay, 12 stations (including non-harvesting areas, Mactra veneriformis harvesting areas, and Solen grandis harvesting areas) were deployed in the area in June 2024, and macrobenthic fauna and environmental factors were surveyed. Results show that (1) a total of 35 species of macrobenthos were identified, mainly Crustacea (14 species), Mollusca (10 species), and Polychaeta (9 species), of which five species dominated, including Potamocorbula laevis and M. veneriformis; and the community mean abundance and biomass are 163.67 ind./m² and 284.47 g/m² respectively. (2) The cluster analysis and non-metric multidimensional scaling (NMDS) showed that the community similarity between non-harvesting area and M. veneriformis harvesting area was high, while that in the S. grandis harvesting area sits as a separate unit. (3) The Abundance-Biomass Curve (ABC) shows that the macrobenthic community structure in the intertidal flat was stable. The Mantel’s test showed that chlorophyll a and water temperature were important factors on the changes in local macrobenthic community. (4) The benthic ecological quality assessment based on AMBI and M-AMBI indices showed that only a few sites (e.g., No.3) were in moderate or severe disturbance status, and the overall benthic habitat quality was in good condition. The M-AMBI index was better adapted than the single index under the disturbance of mechanized harvesting. This study revealed the benthic ecological status in the intertidal flat, provided a scientific basis for balancing shellfish resource development and habitat protection, and suggested to prioritize the use of the M-AMBI index for long-term monitoring and management assessment.

To understand the impact of rainfall on the functional group characteristics of macrobenthic communities in the shellfish farming area of Xiaoqing River estuary in Laizhou Bay, Bohai Sea, survey data of rainfall were collected in July (pre-rainfall) and August (post-rainfall) 2019 and analyzed. By integrating Shannon-Wiener niche breadth, Pianka niche overlap, and canonical correspondence analysis (CCA), the variation in dominant microbenthic species and their environmental drivers were examined. Results are as follows. (1) Eight dominant species were identified, of which Moerella iridescens and Nephthys oligobranchia were consistently dominant in the two months. (2) Significant shifts in life-form and feeding functional groups were observed: July was dominated by endobenthic-planktivores (mean density 265 ind./m², 71%), while August shifted to carnivores (mean density 103 ind./m², 58%), with the GS/GSB ratio increasing from 0 to 2/3. (3) Dominant species exhibited marked niche breadth variation (0.494~2.945), featuring coexistence of broad-niche species (e.g., Nephthys oligobranchia) and narrow-niche specialists (e.g., Potamocorbula laevis), with weak interspecific competition (60% of species pairs showed a narrow niche overlap value (<0.3). (4) CCA revealed that the salinity (32.7% explained variance, P<0.01) was the primary driver of functional group succession (GS/GSB ratio) and niche differentiation. The stenohaline bivalve Potamocorbula laevis (optimal salinity 22~34) served as a bioindicator for salinity thresholds. This study shows that rainfall-induced environmental changes (salinity, dissolved oxygen, pH, chlorophyll a) critically drive functional group succession and niche partitioning in macrobenthos, providing a scientific basis for conservation and sustainable utilization of benthic resources in the Xiaoqing River estuary.

The Yellow River Estuary as a vital ecological bridge linking freshwater and marine ecosystems, with its ecological health significantly impacted by the Yellow River water and sediment regulation project. To assess the environmental ecological quality of the estuary during this regulation period, data on macrobenthos and bottom water environmental factors were collected on three occasions throughout the 2023 water and sediment regulation phase. Four indices—AMBI, M-AMBI, BOPA, and Shannon-Wiener diversity—were employed to gauge the ecological status of the estuary. To reconcile the discrepancies in evaluation standards among these biological indices, an endeavor was made to establish a comprehensive biological index (CBI) using the entropy weight method. This approach aimed to unify the evaluation criteria across the four indices. Furthermore, the Nemerow Comprehensive Water Quality Evaluation Index (P) was introduced to validate the results. The survey revealed a total of 133 macrobenthos species belonging to five taxa, with polychaetes and mollusks emerging as the dominant groups. Notable shifts were observed in the macrobenthos community structure across the three periods: before, during, and after the water and sediment regulation. During the regulation period, the benthic ecological quality of the Yellow River Estuary was generally classified as “moderate” or “mildly polluted”. Pollution levels intensified at stations nearer to the estuary, indicating poorer ecological quality in these areas. The CBI exhibited a strong correlation with bottom water environmental factors and aligned well with the findings of the comprehensive water quality evaluation index. The evaluation grades for the ecological status at each monitoring station were distinctly stratified, demonstrating that the CBI method, constructed using the entropy weight method for various biological indices, is a viable tool for assessing the environmental ecological quality of the Yellow River Estuary. These research findings offer fresh perspectives for evaluating the environmental ecological quality of this crucial estuary.