Bohai Bay is a shallow, semi-enclosed bay in the western Bohai Sea, with an average depth of 12.5 m (Sun, 2006). The bay experiences a semi-diurnal tidal regime with an average tidal range of 1.5~2.5 m, which influences the extent and dynamics of intertidal and subtidal habitats. As defined by the ecological zoning scheme of Huang et al. (2023), our study area extends from 117°30′to 119°E and 37°58′to 39°12′N, covering approximately 11,000 km² (Figure 1). The bay is characterized by a gulf accumulation plain geomorphology, featuring a gently sloping seabed and a persistent counterclockwise circulation pattern that governs material transport (Sun, 2006). This circulation is driven in part by the inflow and gradual dilution of high-salinity waters from the northern Bohai Sea. Coastal rivers deliver substantial sediment loads, forming extensive intertidal mudflats that provide critical habitats for benthic organisms and migratory birds (Chan et al., 2009; Gu et al., 2020). Concurrently, the bay supports intensive human activities—including aquaculture, port operations, shipping, sand mining, coastal reclamation, and designated waste dumping—which collectively exert significant pressures on its subtidal habitats.

In this study, the classification of benthic habitats followed a hierarchical, two-step framework adapted for Bohai Bay, which integrates the principles of the European Nature Information System (EUNIS) classification (European Environment Agency, 2022) with local biophysical characteristics (Yang et al., 2025).

First, based on tidal and light regimes, three primary zones were delineated. Littoral zone (intertidal) refers to areas alternated between exposure to air and seawater, supporting amphibious organisms that distinguish them from those found in other zones. Infralittoral zone (subtidal photic zone) refers to areas with sufficient light for photosynthesis (≥ 1% surface irradiance) (Radicioli et al., 2025), supporting photophilic organisms such as seagrass meadows (Sousa, 2001). Circalittoral zone (subtidal aphotic nearshore zone) refers to areas lying below the photic limit yet remaining influenced by wave energy, supporting nutrient cycling and other ecological processes. Second, each of the aforementioned zones is further classified based on substrate types.

The final, high-resolution spatial mapping of these habitat types across Bohai Bay was directly derived from the comprehensive seabed habitat map produced by Yang et al. (2025). From that map, we selected the four most extensive and ecologically representative subtidal soft-sediment habitats for this cumulative impact assessment: infralittoral mud, infralittoral sand, circalittoral mud, and circalittoral sand. The spatial distribution of these four target habitats is shown in Figure 2.

The sensitivity of the four selected benthic habitats (Section 2.3) to the three pressure types was assessed using an expert scoring method. A panel of three experts evaluated a combination of physical habitat attributes (e.g., water depth, substrate stability) and key functional traits of dominant associated fauna (e.g., mobility, feeding mode, trophic level, and physiological tolerance to hypoxia and disturbance) to assign the sensitivity scores. For instance, infralittoral mud habitats are characterized by a benthic community where sedentary, filter-feeding bivalves can constitute approximately 60% of the total biomass, based on local survey data from Bohai Bay (Ministry of Natural Resources of China's National Marine Ecological Early Warning and Monitoring Program, unpublished data). This functional group is known to be highly sensitive to eutrophication-induced hypoxia (Wang et al., 2020). In contrast, the scoring also considered that infralittoral mud communities often include pollution-tolerant, subsurface-deposit feeding polychaetes, which exhibit higher resilience to organic enrichment and physical disturbance (Adesakin et al., 2023; Xue et al., 2025; Wang et al., 2026). Similarly, for circalittoral sandy habitats, the presence of mobile echinoderms indicates a community adapted to more dynamic conditions, resulting in lower sensitivity scores to physical damage. These associations integrate site-specific biological data from Bohai Bay with well-established ecological principles regarding stressor impacts on functional traits. Scores ranged from 0 (least sensitive) to 1 (highly sensitive), with inter-expert consistency verified (CV < 0.15). The final habitat-specific sensitivity scores are provided in Table 3. The detailed scoring records and justifications provided by each expert are documented in Supplementary Material S2.

The cumulative impact on seabed habitats was quantified using a spatially explicit overlay analysis within a Geographic Information System (GIS). All spatial analyses and map production were conducted using ArcGIS Pro (version 10.8). For each pressure type i (physical loss, physical damage, environmental), a pressure-specific impact score I_i was calculated for every raster grid cell as the product of the normalized pressure intensity P_i (0-1) and the habitat sensitivity score S_i (0-1) assigned to the habitat present in that cell:

This calculation was performed across all grid cells using the Raster Calculator tool, integrating the respective pressure intensity and habitat sensitivity data layers. The pressure-specific impact scores. I_i for all pressure types were then summed to generate a single cumulative impact index CI for each cell:

Finally, the continuous values of the cumulative impact index CI were spatially rendered using a graduated color scheme to produce a cumulative impact layer for visualization and further spatial analysis (e.g., identification of impact hotspots, as shown in Figure 3).

Additionally, the habitat sensitivity scores (Table 3) were spatially combined with the habitat distribution map (Figure 2) using the “Reclassify” and “Combine” tools of ArcGIS Pro to generate the stressor-specific habitat sensitivity maps presented in Figure 4.

Four types of subtidal seabed habitats were identified in the Bohai Bay ecoregion: infralittoral mud, infralittoral sand, circalittoral mud, and circalittoral sand, as shown in Figure 2.

The infralittoral zone covers approximately 60% of the bay’s total area and falls within the sunlit layer of the water column, allowing enough light penetration to sustain photosynthesis and primary productivity. This zone contains diverse substrate types, such as mud and sand. Muddy sediments are the most widespread, while sandy substrates are primarily found in the southern and northeastern regions, where stronger water movement affects sediment distribution.

The circalittoral zone, where minimal sunlight penetrates to the seafloor, is mainly situated in the eastern offshore regions of the bay. Muddy sediments dominate this zone, accounting for roughly 96% of its total area. The remaining 4% consists of sandy substrates, primarily concentrated in the eastern part of Bohai Bay. These sandy patches support dense populations of Amphioplus japonicus (an echinoderm).

Different types of subtidal seabed habitats exhibited distinct sensitivities to various stressors, as quantified by the expert-derived sensitivity scores in Table 3. The spatial distribution of these sensitivity values, in response to (a) physical loss, (b) physical damage, and (c) eutrophication, are visualized in the respective habitat sensitivity maps (Figure 4). These maps highlight the geographic variation in potential vulnerability associated with each pressure type across the study area.

The application of the spatial overlay framework (Section 2.5) yielded the cumulative impact intensity on subtidal habitats across the Bohai Bay ecoregion. The resulting cumulative impact intensity is illustrated in Figure 3. The area and proportion of each habitat type affected by different pressures are summarized in Table 4 and Table 5.

The pressure intensity on habitats in Bohai Bay ranges from 0 to 1.45 (Figure 3), with high-value areas concentrated in three core regions: seabed surrounding Caofeidian Port (northeast), adjacent to Tianjin Port (northwest), and coastal seabed near Huanghua Port and Shandong (south). These regions have undergone land reclamation since 2000 and feature diverse sea uses, including impermeable/permeable structures, shipping channels, submarine cables/pipelines, and aquaculture.

The distribution of pressure intensity across different habitat types shows marked variation (Table 4). Circalittoral mud is primarily affected by low pressure (0-0.35), with other pressure levels near zero. Infralittoral mud distributes across multiple pressure ranges: 95.68% (0-0.35), 4.13% (0.35-0.6), and 0.17% (0.9-1.45). Infralittoral sand accounts for the largest high-pressure (0.9-1.45) area (29.83 km²), representing 76.9% of the total high-pressure area. Low pressure (0-0.35) dominates all habitat types, while high pressure (0.9-1.45) accounts for only ~0.5% of the total pressed area.

As detailed in Table 5, infralittoral mud is most affected by environmental pressure (31.96%), followed by physical damage (4.72%) and physical loss (0.17%). Infralittoral sand is impacted by all three pressure types, with physical damage accounting for 7.71% of its area. Circalittoral mud is affected by physical damage (0.47%) and environmental pressure (9.18%); circalittoral sand only by environmental pressure (3.63%).

Our spatially explicit assessment reveals pronounced spatial heterogeneity in cumulative impacts on Bohai Bay subtidal habitats, with the infralittoral zone—especially muddy substrates—emerging as a primary impact hotspot. This pattern stems from the synergistic effects of the zone’s intrinsic ecological attributes (e.g., high organic matter content, low permeability) and close proximity to anthropogenic disturbances (e.g., land-based pollution, reclamation) (Liu et al., 2024; Zhang et al., 2025). Eutrophication dominates as the prevailing stressor in these shallow, productive areas: our data quantifies that environmental pressure (primarily eutrophication) affects an area 54 times larger than physical habitat loss across all studied habitats. This finding is consistent with long-term observations in the Bohai Sea and its sub-basins, where nutrient enrichment has driven trophic status regime shifts and sustained elevated chlorophyll-a levels. For instance, assessments in the Bohai Sea and Laizhou Bay report chlorophyll-a concentrations up to 0.015 mg/L under nutrient-enriched conditions, with a critical dissolved inorganic nitrogen (DIN) threshold of 0.37 mg/L marking a trophic regime shift and reflecting the system’s vulnerability to nutrient loading (Xin et al., 2019; Liu et al., 2019).

Eutrophication induces excessive algal blooms, and subsequent decomposition of algal biomass triggers hypoxia (dissolved oxygen < 2 mg/L)—a stressor particularly harmful to infralittoral mud habitats, where benthic organisms (e.g., filter-feeding bivalves) depend on adequate oxygen for respiration (Diaz and Rosenberg, 2008; Shi et al., 2023). Global research confirms the strong link between nutrient enrichment, water column stratification, and coastal hypoxia expansion. A 2024 summer investigation in the Bohai Sea illustrated this: temperature stratification and weak hydrodynamics facilitated widespread hypoxia, exacerbated by high phytoplankton biomass and subsequent organic matter remineralization (Shi et al., 2023). Comparative analyses of benthic fauna tolerance show interspecific variability, with many species exhibiting sublethal or lethal effects at dissolved oxygen levels ≤ 2 mg/L—indicating that conventional hypoxic thresholds may underestimate ecological risk in shallow, organic-rich systems (Vaquer-Sunyer and Duarte, 2008).

Infralittoral mud habitats are also highly sensitive to physical disturbances; bottom trawling, for example, causes severe damage to soft-sediment communities, further amplifying ecological risk (Kaiser et al., 2002). In high-impact areas, the spatial patterns of cumulative pressure align with documented shifts toward smaller, pollution-tolerant benthic taxa and reduced species diversity (e.g., lower Shannon-Wiener index). This indicates that the combined stresses of eutrophication and physical disturbance are driving a simplification of community structure (Cai et al., 2014), which may signal a decline in ecosystem functions and services.

Our integrated pressure-sensitivity analysis reveals that the spatial pattern of cumulative impacts is governed by two overarching factors: the dominance of eutrophication as the most widespread stressor and the pronounced vulnerability of the infralittoral zone. The infralittoral zone, covering ~60% of the bay, bears the brunt of combined pressures, with its muddy substrates most affected by environmental stress (31.96% area impacted) and sandy substrates notably impacted by physical disturbances (e.g., 2.90% by physical loss). In contrast, the circalittoral zone is primarily influenced by eutrophication (9.18% of mud habitats), with minimal direct physical impact, underscoring depth as a buffer against localized disturbances but not against basin-scale pollution.

Several limitations of this study should be acknowledged when interpreting these results. First, our assessment focused on subtidal habitats and excluded the ecologically critical intertidal zone, thereby offering an incomplete picture of whole-system pressures—a common constraint in seabed-focused assessments (Korpinen and Andersen, 2016). Second, the habitat sensitivity scoring, while based on expert synthesis of local data and established ecological principles, inherently involves subjectivity. Third, our framework considered three primary local stressors but omitted other potentially important pressures such as climate change, hazardous substances, and anthropogenic noise, whose long-term and interactive effects could further modulate the cumulative impacts described here.

This study developed and applied a spatially explicit framework to assess the cumulative impacts of multiple stressors on subtidal benthic habitats in Bohai Bay. The key findings are:

Based on the findings of this study, the following targeted management measures are proposed:

1. Implement spatially differentiated management strategies. The infralittoral zone is a pressure hotspot, characterized by sufficient light and high ecological value, and requires priority conservation efforts. Within this zone, protection priorities should be further differentiated:

For infralittoral muddy substrates, where environmental pressure affects over 30% of the area, focused prevention and control of long-term stressors from land-based pollution and marine development is crucial. It is recommended to enhance the control of pollution sources, including aquaculture wastewater and ship discharges, with a long-term target for key indicators such as C_DIN < 0.5 mg/L.

For infralittoral sandy substrates, which are more susceptible to physical damage (the proportion of physical damage is 1.6 times that in muddy substrates), strengthened supervision of engineering activities like sand mining and dredging is necessary (Van Dover, 2011).

For the circalittoral zone, which is predominantly influenced by environmental pressure, the primary focus should be on water quality improvement initiatives and ecological restoration.

2. Implement differentiated management strategies for different pressure types. In areas dominated by physical loss, strict control should be imposed on new reclamation activities, and an ecological compensation mechanism based on offsetting habitat loss with restoration should be promoted. In regions primarily affected by environmental pressure, efforts should focus on strengthening land-based pollution control and establishing a coordinated management system linking river basins, estuaries, and the bay.

3. Establish a hierarchical habitat protection system. Infralittoral mud substrates, where environmental pressure affects over 30% of the area, require prioritized prevention and control of long-term stress from land-based pollution and marine development. For infralittoral sand substrates, enhanced supervision of activities such as sand mining is necessary. In circalittoral zones, emphasis should be placed on improving water quality and implementing ecological restoration.

4. Enhance dynamic monitoring and early warning systems. It is recommended to combine remote sensing technologies (e.g., monitoring habitat changes through image inversion) with field surveys to refine protection strategies for different habitats. This approach will help balance marine development with ecological conservation and safeguard the service functions of marine ecosystems.

Future research could be expanded in the following directions:

Climate change-related pressures, such as habitat inundation due to sea-level rise and the effects of seawater warming on benthic metabolic processes (Levin and Dayton, 2009);

Hazardous substance pressures, including the cumulative effects of heavy metals and persistent organic pollutants in sediments (Ramirez-Llodra et al., 2011);

Anthropogenic noise pressures, such as behavioral disturbances to sound-sensitive species (e.g., Sciaenidae) caused by shipping and underwater construction (Stocker, 2002).

Additionally, interactions among multiple pressures—e.g., the synergistic effect of eutrophication and warming on the expansion of hypoxic zones—should be investigated. The incorporation of interaction coefficients or machine learning algorithms could enhance the mechanistic interpretability of cumulative impact assessments.