Dissolved oxygen (DO) is fundamental to maintaining the balance and stability of marine ecosystems. Under the combined influence of climate warming and intensified human activities, hypoxia in coastal waters worldwide is increasing in severity and spatial extent, making deoxygenation a major threat to coastal ecosystem health. As a typical semi-enclosed sea, the Bohai Sea is subject to multiple environmental pressures, including ocean warming, eutrophication, and rapid socio-economic development. In recent years, varying degrees of hypoxia have been reported across multiple subregions of the Bohai Sea, with hypoxic centers primarily located northeast of the Yellow River estuary and southeast of Qinhuangdao (occurring ≥3 times). This study reviews domestic and international research on hypoxia in the Bohai Sea, characterizes the spatial distribution patterns of summer hypoxia over the past two decades, and summarizes the dominant controlling processes. It further outlines the long-term development trends and their influencing factors in the Bohai Sea and proposes key directions for future research on hypoxia in the region. The study aims to provide a scientific basis and decision-making support for ecological health assessment and marine resource management in the region.

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.

Marine microorganisms, characterized by their immense biomass, rapid environmental response, and crucial ecological functions, serve as sensitive indicators for assessing the health of marine ecosystems. This article provides a systematic review of recent advances in marine ecological monitoring and health assessment based on microbial communities. First, we discussed the key characteristics of marine microorganisms as bioindicators, such as their high sensitivity to environmental stress and their functional redundancy in maintaining ecosystem processes. Case studies are presented to highlight the successful use of microbial monitoring to address climate change, pollution events, and ecological disasters. Next, we reviewed the evolution of marine microbial monitoring technologies, spanning from traditional cultivation methods to modern techniques such as high-throughput sequencing and Raman spectroscopy. We also compared the advantages and limitations of these approaches in practical applications. Finally, in response to challenges such as insufficient data standardization and the lack of quantitative assessment metrics, we proposed a systematic framework for future development. This framework emphasizes the need for end-to-end standardization from sampling to data analysis, the creation of intelligent diagnostic models that integrate multi-dimensional “Raman spectroscopy-genetic-environmental” information with the establishment of a national-scale specialized monitoring network. This review aims to provide theoretical supports and technical pathways for the development of a next-generation, high-resolution, real-time microbial-based marine ecological health assessment system.

Under the stresses of climate change and anthropogenic activities, marine ecosystems are undergoing unprecedented changes. Accurately assessing the ecological quality status of marine ecosystems and their spatiotemporal variation characteristics is an urgent scientific challenge. Within marine ecological quality assessment frameworks, macrobenthos serve as critical evaluation criteria due to their distinctive physiological and ecological traits. However, current assessment indicators based on macrobenthic communities are numerous yet singular, lacking a comprehensive integrated evaluation system. To address this gap, the present study proposes a multi-dimensional, multi-indicator evaluation framework constructed from an ecological perspective that integrates both community structure and function. This framework comprises three primary categories and nine secondary indices: (1) biotic indices (M-AMBI and BENTIX index); (2) community structure (Shannon-Wiener diversity index, species richness, taxonomic composition, abundance, and biomass); and (3) functional attributes and stability (functional diversity and stability). Weight assignments were allocated according to indicator importance to calculate the macrobenthic community ecological quality index (MCEQI) for benthic ecological status assessment. Application of MCEQI to analyze the macrobenthic community ecological status in Jiaozhou Bay demonstrated that this index exhibits higher accuracy than single-indicator approaches, providing a more comprehensive reflection of community ecological quality.

The health status assessment of marine benthic ecosystems, as an important basis for maintaining the ecological balance of the oceans, relies on the long-term dynamic monitoring of benthic communities. In this study, we systematically reviewed the history and current applications of ecological assessment indices developed based on benthic organisms, including the perspectives of future fields in marine ecosystem health assessment. Traditional biological indices—including biodiversity indices (Shannon-Wiener index, Pielou index, etc.), functional group analyses (feeding evenness index), and indices (AMBI, M-AMBI, BENTIX index, etc.) based on the proportion of pollution-tolerant/sensitive species have advanced ecosystem health assessment by quantifying the responses of community structure to environmental stressors and facilitating the shift from qualitative to quantitative assessments. Nevertheless, traditional methods are limited by cumbersome procedures for morphological characterization, the limitations of single indicators, and regional differences in applicability. In recent years, environmental DNA (eDNA) technology has made up for the shortcomings of traditional methods by rapidly obtaining biodiversity information through high-throughput sequencing, and has derived novel indices such as gAMBI, which validates its complementarity with morphological methods. Integration of Artificial Intelligence (AI) techniques such as machine learning algorithms (Random Forests, Convolutional Neural Networks, and so on) with statistical analysis has improved the ability of ecosystem assessment models to resolve nonlinear relationships and multiple stressors. Meanwhile, automatic and intelligent image recognition technology offers the possibility of accurate and rapid identification of macrofauna and their monitoring. Future research shall integrate multidimensional data and interdisciplinary techniques to construct a more universal and dynamically responsive assessment system to cope with the potential impacts of global climate change and human activities on marine ecosystems.

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.

As primary producers, phytoplankton constitute a vital component of marine ecosystems, and their community structure largely determines the stability of these ecosystems. Previous studies on phytoplankton community structure in Laizhou Bay have relied primarily on short-term observations or data from one or two sampling cruises. There remains a scarcity of multi-year continuous survey data, and the temporal evolution of phytoplankton community structure has rarely been systematically analyzed. Based on survey data from 2016 to 2023 summer in Laizhou Bay, we investigated the interannual variations of phytoplankton and conducted correlation analyses between phytoplankton communities and environmental factors. Results show that a total of 106 species from 3 phyla were identified over the 8-year period, predominantly diatoms, followed by dinoflagellates. Significant interannual variations in species composition were observed, and the lowest diversity (48 species) occurred in 2019 and the highest (68 species) in 2020. Phytoplankton cell abundance ranged from 39.1×10⁴ cells/m³ to 2 687.3×10⁴ cells/m³, peaked in 2017 and reached the minimum levels in 2021. In the planar distribution, the phytoplankton cell abundance, the nearshore waters of estuaries such as the Yellow River estuary and Xiaoqing River estuary are higher than that of the central part of the bay. Dominant species included Pseudo-nitzschia pungens, Chaetoceros curvisetus, Skeletonema costatum, and Cerataulina pelagica. Correlation analysis revealed that nitrate-nitrogen was the primary environmental factor on phytoplankton abundance. The overall upward trend in the interannual variations of diversity index (H'), richness index (d), and evenness index (J') indicate that phytoplankton community structure tended to improve. This study contributed to clarifying the evolutionary trend and influencing factors of phytoplankton in Laizhou Bay, and provided a scientific foundation for decision-making in marine ecological environment protection of the bay area.

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.

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.

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.