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 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.

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.

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.

The simulation and prediction of dynamic changes in zooplankton community play a critical role in evaluating the health of bay ecosystems. There remains significant uncertainty in the quantitative prediction of dynamic changes within zooplankton community using classical plankton ecosystem dynamics models. Jiaozhou Bay was used as a representative bay in China’s coastal regions facing multiple pressures from human activities and climate change. Utilizing long-term observation data on zooplankton abundance and environmental factors in Jiaozhou Bay such as temperature, salinity, nutrients, and chlorophyll a concentration from 1997 to 2010, we developed a technical and methodological system for predicting zooplankton abundance in Jiaozhou Bay by applying Empirical Dynamic Modeling approach. Our model achieved an effective prediction with a coefficient of determination (R²) of 0.64, root mean square error (RMSE) of 108.6 and a relative error of 42.5%. After analyzing 15 380 937 potential combinations, we pinpointed temperature and silicate as critical factors influencing the change of zooplankton abundance in Jiaozhou Bay. A deeper look into the research data suggested that the change of zooplankton abundance in Jiaozhou Bay from 1997 to 2010 was a result of interaction between bottom-up control and top-down control in the food web. The quantitative prediction method for changes in zooplankton abundance and the causal identification of driving factors developed in this study exhibited spatial transferability, highlighting their substantial potential as a foundational approach for the future health assessment model of coastal bay ecosystems in China.

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.

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 Tianjin coastal zone is an important ecological security barrier for the Beijing-Tianjin-Hebei urban agglomeration. To scientifically assess the evolution of its ecological health, an ecological health evaluation system was constructed for the coastal zone based on the Driving forces-Pressure-State-Impact-Response (DPSIR) model, and environmental monitoring or census data covering 2018~2023, in which 30 indicators were applied. The Analytic Hierarchy Process (AHP) and Entropy Weight Method were used to determine the weight of each indicator, and the health level of the coastal zone was comprehensively evaluated. In addition, the degree of coupling coordination among the sub-systems of DPSIR was ascertained, and the reliability of outcome was determined through a weight sensitivity experiment. Results show that the ecological health level of the coastal zone showed a fluctuating upward trend in general, and the health grade was increased from “relatively unhealthy” to “moderately healthy”. All the sub-systems were in moderately coordinated state, indicating that the health condition of the coastal zone of Tianjin has been improving year by year under the policy of promotion by the government. The region has initially formed a “green growth” model for the marine economy. However, a certain lag existed in ecosystem restoration relative to countermeasures, posing potential ecological disaster risks. The weight sensitivity analysis showed that the evaluation system constructed in this study has high robustness and can objectively reflect the evolution of the ecological health of the coastal zone in Tianjin, providing a scientific basis for refined governance in the coastal zone of Tianjin.

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 elucidate recent changes in nutrient structure and potential phosphorus limitation in China’s coastal seas, this study analyzed the spatial distribution and interannual variability of dissolved inorganic nitrogen (DIN), phosphate (PO₄^3-), and their molar ratio (N/P) in surface waters of the central Bohai Sea, Yellow Sea, East China Sea, and the Changjiang River Estuary, using cruise data from National Natural Science Foundation of China shared surveys, and compared these with historical observations. The results indicate that in 2015 and 2017, most areas of the central Bohai Sea and Yellow Sea exhibited elevated N/P ratios, generally exceeding the Redfield ratio (16 : 1), reflecting pronounced nutrient imbalance and a tendency toward phosphorus limitation. In most years, the East China Sea also exhibited elevated N/P ratios; however, lower values were observed in parts of the outer shelf and in regions influenced by Kuroshio intrusion, suggesting possible nitrogen limitation in these areas. The Changjiang River Estuary maintained persistently high N/P ratios (>64 : 1), indicating stronger phosphorus limitation compared with other regions. Comparison with historical data revealed that N/P ratios in the central Bohai and Yellow Seas exhibited a long-term pattern of “initial increase followed by decline”, with peak values occurring around 2008 and 2005, respectively, suggesting that both seas have remained in a persistently high N/P state. Overall, nutrient imbalance is widespread in China’s coastal seas, although pronounced interregional and intraregional differences exist, reflecting the combined influence of land-based inputs and regional oceanographic processes. These findings provide a scientific basis for understanding the evolution of nutrient structure and the associated risk of nutrient limitation in China’s coastal seas.