The “Two-Dimensional” MU Phase: MU of the sea in China originated in 1989 during pilot studies on Marine Functional Zoning (MFZ), which proposed the establishment of a zoning system that would include both dominant and compatible functions (Wang and Liu, 2011). The People’s Republic of China on Administration of the Use of Sea Areas, released in 2001, mandated that “sea area use must comply with the Marine Functional Zoning.” The ensuing three editions of the national MFZ (1996-2000, 2001-2010, and 2011-2020) consistently adopted this planning framework, which was based on defining dominant functions while also permitting compatible uses. During this phase, sea area use was planar-exclusive, meaning that only one right to the use of sea areas could be established within the same marine space.

The “Three-Dimensional” MU Exploration Phase: With the diversification of marine industries, the conflict between the limited availability of marine spatial resources and the growing demand for sea use has grown considerably. An increasing number of submarine cables and pipelines, cross-sea bridges now need to pass through functional zones such as nursery and mariculture zone or navigation zone, making planar sea use inadequate to meet the rising demand for three-dimensional utilization. Meanwhile, the development of big data technology has created an unprecedented environment and conditions for exploring three-dimensional approaches. In big data scenarios, the wide range of information sources, multi-dimensional content, and efficient data processing capabilities have provided a foundation to prevent potential spatial use conflicts and enable refined management (Gu, 2024). In 2014, China’s approach to MU began to transition from two-dimensional space to three-dimensional space, thus initiating exploration of a new management model: the three-dimensional management of ocean space. A pioneering case involved the concurrent grant of use rights for the same sea area—the thermal discharge area of the Tianwan Nuclear Power Plant in Lianyungang City, Jiangsu Province—to both the plant and the owner of a cross-sea bridge (Li and Liu, 2019). This milestone allowed for the coexistence of two uses, thermal water discharge and a bridge, within a single marine space. In 2016, China’s State Oceanic Administration issued the Opinions of the State Oceanic Administration on Further Regulating the Administration of Sea Area Use for Offshore Wind Power. This document explicitly advocated for “encouraging the stratified and three-dimensional development of sea areas used by offshore wind power projects alongside other development and utilization activities, to maximize the benefits of marine resources” (The State Oceanic Administration, 2016), thereby formally introducing a management framework for a three-dimensional sea area development and use right.

The “Three-Dimensional” MU Development Phase: In 2019, China issued the Guiding Opinions on Coordinating and Advancing the Reform of the Property Right System for Natural Resource Assets (China State Council, 2019), which for the first time proposed “exploring three-dimensional layered rights to the use of sea areas” from the central level. Since then, China’s Hebei, Zhejiang, Guangxi, Hainan, Liaoning and other provinces have begun the practice of three-dimensional layered rights management, issued a series of institutional documents, and used a variety of three-dimensional layered utilization models in the sea area such as “photovoltaic (PV) + mariculture” “ submarine cable conduits + port pool/bridge” and “offshore wind power + marine ranching”.

This momentum continued in 2021 with the issuance of the Overall Plan for the Pilot Reform of the Comprehensive Reform of the Market-Oriented Allocation of Factors (China State Council, 2021). The reform positioned the three-dimensional layered establishment of sea use right as a key regulatory measure for rational and orderly sea use, thereby cementing the transition from a “two-dimensional” to a “three-dimensional” paradigm in marine spatial management. A milestone was reached in 2023 when China’s Ministry of Natural Resources released the Notice on Exploring and Promoting the Work of Three-Dimensional Layered Establishment of Sea Use Right (Ministry of Natural Resources, PRC, 2023c), advancing the sea area use rights system and unified standards for three-dimensional sea use. As the first national-level policy document dedicated to this approach, it provided crucial guidance and standardization for key aspects of three-dimensional sea use, including sea area use argumentation, approval procedures, and the collection of sea area use fees.

Typologies, organized systems of types, can be useful tools when analyzing current developments. The classification of MU in China can be categorized into three types based on the spatial and temporal utilization characteristics of sea-related projects (Table 1).

The first category is “temporary” sea area use, in which different marine projects are spatially connected but do not overlap temporally. This category primarily allows for suitable, alternative sea use activities to be conducted before the dominant function is utilized. For example, within marine protected areas, low-impact tourism and fishing activities that do not disrupt the ecological functions of the reserve may be permitted during seasons when the protected species are not migrating, spawning, or inhabiting the area. Similarly, open mariculture can be conducted in the undeveloped oil and gas zone.

The second category is “three-dimensional” sea area use, in which different marine projects are spatially independent, but overlap temporally. In this model, projects respectively utilize distinct vertical strata—water surface, water body, seabed, and subsoil—within the same maritime area. Representative examples include the colocation of submarine tunnels and cross-sea bridges, or cross-sea bridges and navigation channels. This method of multi-layered utilization of marine space is being discussed more frequently in China both in theory and practice.

The third category is “co-existence” sea area use, in which different marine projects are interconnected both spatially and temporally. These activities are conducted simultaneously within the same marine space. Examples include developing recreational tourism within marine fishery zones, where facilities like deep-water cages and mariculture platforms are leveraged for activities such as recreational fishing and catering services. Another example is the deployment of artificial reefs within fishing grounds, which enhance fish catching and promote reproduction.

Before 2019, China managed sea-use projects primarily through the MFZ system. Following a significant policy integration in 2019, the MFZ was consolidated with other spatial plans, such as land-use planning, into a unified National Territory Spatial Planning (NTSP) framework. This reform integrated management of both terrestrial and marine spaces under a single system. Despite this evolution from MFZ to the comprehensive NTSP, the core mechanism for marine management has remained centered on marine use control. Both systems employ a detailed descriptive approach for sea-use projects, characterized by the model of “Functional Zone + Type of Sea Area Use.” This method has ensuresd that each marine area is designated with a specific function and that any project within it is precisely categorized according to a standardized typology of sea-use activities.

The primary feature of MU is to determine whether a project and the dominant function of a functional zone can “coexist”, which requires a functional conformity analysis of the project. Nonconformity mainly manifests in two scenarios. The first scenario is functional exclusion, in which case the project conflicts with the dominant function of the functional zone. For example, when attempting to implement tourism-oriented projects in a port zone, the noise and air pollution generated by freight transportation could hinder the development of the tourism industry. The second scenario is environmental exclusion, in which case environmental impacts induced by the project would render the dominant function of the functional zone inoperable, or the exploited dominant function would exert adverse effects on the project. For example, conducting open mariculture in a developed and utilized oil and gas zone may result in fish mortality as a result of water pollution. Conformity mainly manifests in two scenarios: the project is consistent with the dominant function of the functional zone, or the project is inconsistent with the dominant function of the functional zone, but aligns with a compatible type out of the three aforementioned categories (temporary, three-dimensional, and co-existence sea area uses).

Compared with “temporary” and “co-existence” sea area uses, which marine space as a two-dimensional space, “three-dimensional” marine utilization is distinct and independent. It vertically stratifies the marine space into four parts: water surface, water body, seabed, and subsoil (Figure 2). The water surface layer is the space at the sea level and a certain height above it, which is used for ship navigation and cross-sea bridges. The water body layer is the space filled with seawater between the sea level and the seabed, which is used for mariculture and marine capturing. The seabed layer is the surface of the seabed, which is used for submarine cable conduits laying and sea sand mining. The subsoil layer is the space below the seabed, which is used for oil and gas exploitation and submarine tunnel construction.

China has categorized its projects into three types based on factors such as the exclusivity of marine space, the safety of projects, and the quality of the environment: The first category encourages three-dimensional and layered utilization of sea areas, including cross-sea bridges, mariculture, thermal water discharge, submarine cable conduits, and submarine tunnels. The second category prohibits three-dimensional and layered utilization of sea areas, including reclamation and sea sand mining. These projects completely change the natural attributes of sea areas, or the development and utilization of the sea areas demonstrates strong exclusivity, making it impossible to use the sea in layers with other activities. The third category allows for the three-dimensional and layered utilization of sea areas. These projects have been rigorously demonstrated to support conditions for three-dimensional utilization.

Following these specific principles and rules, in this study, we developed a functional conformity evaluation matrix for projects (Figure 3). The evaluation principles include the following: (1) enhancing adaptability and flexibility, and avoiding overly stringent assessment to provide greater possibilities for subsequent MU. (2) improving the efficiency of marine space utilization to prevent the waste of spatial resources caused by the long-term idleness of unused marine areas. (3) maintaining the stable execution of the dominant function while integrating the requirements of “three-dimensional” sea area use.

On the basis of these evaluation principles, we established the following specific evaluation rules: (1) Functional zones with significant changes in natural marine attributes are permitted to conduct marine utilization projects that make minor alterations to natural attributes. These projects represent low-intensity development and will not exceed the ecological baseline or development framework of the functional zone. (2) Functional zones with large marine areas may conduct small-scale development activities such as point-like utilization. Functional zones with large marine areas often require substantial investment, making it difficult to fully develop or effectively utilize all areas in the short term. Point-like utilization enables precise utilization of relatively underused marine areas, allowing fragmented sea spaces to generate benefits rapidly. (3) For functional zones that may remain undeveloped for a long time (e.g., solid mineral, oil and gas, and renewable energy zones), projects without fixed infrastructure construction are allowed before the dominant function is utilized. This approach facilitates rapid restoration of the marine environment to its original state during subsequent development of the dominant functions. (4) The dominant function of the marine reservation zone, as defined by ICZP, remains unclear or difficult to develop. To enhance the utilization efficiency of marine spatial resources while facilitating the restoration of marine areas to their original state, the marine reservation zone permits short-term occupied projects without fixed infrastructure construction or unavoidable linear projects. (5) As mentioned earlier, encouragement of three-dimensional sea use projects, such as cross-sea bridges, mariculture, thermal water discharge, submarine cables and pipelines, and submarine tunnels may utilize marine space through vertical stratification. (6) Other special purpose sea area uses, as defined by the Guidelines for Classifying Land and Sea Use for the National Land and Space Survey, Planning and Zoning, refer to special sea areas excluding those designated for military purposes, scientific research and education, marine conservation and restoration, coastal protection, pollution discharge and waste disposal, and protecting cultural heritage. Because of their diverse functional characteristics, an analysis based on actual conditions is required. Therefore, we initially determined these areas to be potentially suitable for MU.

Note: The specific utilization purposes of “other sea areas” remain unclarified. Among secondary marine utilization types, “Agriculture, forestry and animal husbandry islands” mainly targets uninhabited islands. “Military sea area use” is of particularity—these types are excluded from the scope of this discussion.

By using this evaluation matrix, we identified suitable types of sea-use projects within a specific functional zone. For example, in the nursery and mariculture zone, industrial projects, salt production, and oil and gas extraction would be incompatible, whereas capture, scientific research, and coastal protection could be permitted. Additionally, fishery infrastructure, renewable energy projects, and submarine cable and pipeline laying may be allowed under specific conditions, provided that they do not disrupt mariculture operations and involve low-intensity construction. In Dalian, Liaoning Province, an innovative approach has been successfully implemented in sea cucumber farming zones. By using techniques such as “installing piles without clearing the breeding area” and “installing piles with sea cucumbers present,” offshore solar projects have been integrated without affecting sea cucumber cultivation (The People’s Government of Liaoning Province, 2025a).

In MU scenarios, the ecological impact assessment of a project on its functional zone requires comprehensive consideration of various ecological and environmental factors. This approach requires ensuring that projects do not cause irreversible harm to biodiversity, and avoiding long-term adverse effects on the marine ecosystem as a result changes in hydrological conditions. The most critical aspect is that compatible sea-use projects must comply with the environmental quality standards of the marine functional zones—including seawater quality, marine sediment quality, and marine biological quality—and must not cause irreversible changes to the environmental quality.

Regarding the environmental quality standards for functional zones, China has not yet issued updated national standards. Therefore, the standards outlined in the National Marine Functional Zoning (2011–2020) (Table 3) can still be applied. For the environmental quality requirements of various sea-use projects, we developed an evaluation matrix (Table 4) based on the definition of sea area use types, characteristics of sea use impacts, expert experience, and comparative analysis of seawater, sediment, and biological quality standards. We determined seawater quality according the Chinese national standard GB3097-1997, marine sediment quality according GB18668-2002, and marine biological quality according GB18421-2001.

In the matrix, “√” indicates that the sea-use project is permitted under that specific quality standard, while “×” indicates it is not permitted. For example, if a fishery infrastructure project requires seawater quality no worse than Class II, it would be represented in the matrix as: Class I “√”, Class II “√”, Class III “×”, Class IV “×”.

By referencing both Table 3 and Table 4, a preliminary assessment can be made to determine whether a sea-use project meets the environmental protection requirements of the functional zone.If the environmental quality requirements of the sea-use project are not lower than the current environmental quality standards of the functional zone, the project is unlikely to have a significant impact on the local ecological environment. In such cases, the project can generally maintain the existing environmental quality standards of the functional zone. Therefore, in MU scenarios, from an ecological impact perspective, compatibility is achievable when the environmental quality requirements of the sea-use project are equal to or higher than the standards of the functional zone.

Marine utilization demonstration refers to a comprehensive assessment process that can be used to determine the feasibility of a proposed project and formulate relevant countermeasures. This determination is based on surveying, investigating and analytically evaluating of multiple factors, including: The natural geographical conditions and resource-environmental status of the subject sea area; the area’s locational characteristics and regional socio-economic conditions; existing regional productivity layout and the historical evolution of sea use; marine functions and projected gains and losses from the sea use; disaster prevention and mitigation, and national defense security.

The key focus of the marine utilization demonstration may vary across different types of MU projects. For “temporary” and “co-existence” sea area uses, the emphasis is on analyzing and evaluating the degree of conformity between the project and the functional zone of the marine area where it is located, as well as the scope and extent of its impacts on marine resources and ecology in the functional zone. The focus of “Three-dimensional” sea use is on demonstrating the necessity and feasibility of the three-dimensional development and utilization of the sea area. This type of use requires an analysis of the coordination feasibility among different marine users and surrounding stakeholders, compatibility among different sea-use projects, the rationality of the sea use duration, and the spatial scope of sea use. Taking “cross-sea bridge + mariculture” as an example, its spatial scope is typically clarified through a diagram (Figure 4).

The Three-dimensional layered right to the use of sea areas is the process of confirming the right to use the specific sea area occupied by each type of sea-use activity when two or more types of sea use activities simultaneously utilize different depths of space in the same area. The connotation is that more than one subject of sea use may appear in the same area, and that sea-use activities of different natures or uses can be compatible.

Legal Basis: Article 2 of the Law of the People’s Republic of China on Administration of the Use of Sea Areas (Law of Sea Areas), released in 2001, stipulated that “ the sea areas refer to the water surface, water body, seabed and subsoil of the inland waters and territorial seas of the People’s Republic of China” (China Standing Committee of the National People’s Congress, 2001). This legislation clarified that sea areas are three-dimensional space resources, and implied that human beings could utilize marine space more extensively, including not only the water surface, but also water body of different as well as the seabed and subsoil. Article 23 of the Law of Sea Areas stipulated that “ pwners of the right to the use of sea areas may not impede the non-exclusive use of sea areas that does not interfere with their use of the sea areas in accordance with law “. Under reasonable arrangements, the use of different layers of space in the same sea area is allowed to conduct sea use activities. Specification for Sea Area Use Register Investigation issued in 2009 stipulated that “in case of special needs, the vertical use range of the sea shall be defined in accordance with the actual situation of the water surface, water body, seabed and subsoil occupied by the sea for project purposes” (Standards Press of China, 2009), which defined the scope of three-dimensional and layered utilization of sea areas from an institutional level.

Management Feasibility: In the administration of the use of sea areas, China has adopted a system of separation of ownership and rights of use. Article 3 of the Law of Sea Areas has stipulated that “the sea areas are owned by the State. Any entity or individual that intends to use the sea areas is required to obtain the right to their use in accordance with law.” The Ministry of Natural Resources of China, as the state administration of marine space resources, has the responsibility to establish the right of the sea areas (water surface, water body, seabed, and subsoil) by layers. For example, within the same marine space, aquaculture rights can be established in the water body, submarine tunnel rights can be granted for the seabed, and surface crossing right for bridges can be allocated on the water surface. Those applying for sea area use may submit an application to the national administrative department for marine spatial resources in accordance with the law. Upon approval, they may obtain a sea area use right certificate and formally become a holder of the sea area use right.

Use Feasibility: The distribution of marine resources has three-dimensional characteristics, For example, marine biological resources and marine chemical resources are generally distributed in the water body layer, renewable energy (e.g., tidal energy and wave energy) are generally distributed in the water surface or water body layer, and exploitable mineral resources (e.g., oil and gas and sea sand) are generally distributed in the seabed or subsoil layers. The realization of the value of a variety of marine resources depends on all kinds of marine activities. According to three-dimensional distribution characteristics of maritime resources, sea-use projects may not necessarily utilize all vertical layers. Therefore, through reasonable arrangements, a portion of these sea activities may be vertically compatible and would not interfere with each other. For example, the space above the water surface could be used to build a cross-sea bridge, the water surface layer could be used to carry out sea tourism or open up sea lanes, the water body layer could be used to develop mariculture, the seabed layer could be used to lay submarine cable conduits, and the subsoil layer could be used to dig submarine tunnels and then be mined for mineral resources.

Technical Feasibility: With the application of new technologies to the utilization and management of marine resources, the three-dimensional and layered utilization of sea areas and rights management have established a technical foundation. For example, satellite remote sensing and global positioning systems can achieve sub-meter and centimeter-level positioning and measurement of sea areas, further reducing the cost and time required to determine the scope of sea areas; acoustic doppler current profiler and side-scanners can achieve accurate measurements of ocean depth and seabed topography, thus helping to determine the three-dimensional extent of the sea area. The Time and Space Geographic Information System has increased the management of water depth information and data, making the approval and registration of the three-dimensional layered right to the use of sea areas more scientific. The Dynamic Surveillance and Monitoring System of Sea Area Use provides information about changes in sea area use, which is helpful for the supervision and management of three-dimensional sea area uses. These technical means can be used to achieve an accurate survey and positioning of the location, boundary, and scope of the sea area and to effectively meet the needs of the three-dimensional layered right to the use of sea areas.

The ocean is characterized by fluidity and openness, and the location of the water surface and the depth of the water body are always in a dynamic state of flux. Therefore, it is difficult to accurately define the boundary of marine space. To establish three-dimensional layered rights, it is necessary to clearly define the users and the specific layers and scope of their usage as well as to identify any temporarily occupied spaces (i.e., short-term spaces utilized by maritime projects). This process is crucial to prevent conflicts from arising about ambiguous maritime boundaries among different users during their activities.

In November 2023, China issued the Guidelines for Defining the Scope of the Sea for Three-Dimensional Layered Construction of Sea Areas (Trial) (Ministry of Natural Resources, PRC, 2023b). These guidelines clarified that the scope of three-dimensional space of the sea area is generally defined according to the marine space occupied by the primary sea-use project or the main sea area space utilized in the sea-use activities. They also established that the space for the use of the sea area is defined according to the layers of the water surface, water body, seabed, and subsoil and that the scope of the heights and depths are determined by numerical quantification or textual qualification. For example, offshore PV, cross-sea bridges, and other projects involve the use of multiple layers, which then define the scope of the right to the use of these sea areas according to the space occupied by the main project. Similarly, mariculture, submarine cable conduits, warm (cold) water discharge, and other projects involving the use of a particular sea layer can be used in the textual description of the scope of the elevation. (Table 5).

The approval of the three-dimensional layered right to the use of sea areas applies to two situations (Figure 5). The first situation is applied to a three-dimensional layered right in a sea area where the right to the use of the sea areas has already been established. In this case, the project applicant should reach an agreement with the original owner of the right to use the sea area, and then may complete the procedures for the use of the sea in accordance with the law. This process prevents conflicts between the newly established right to use and the original right to use. The second situation is applied to a three-dimensional layered right in the sea areas where the right to the use of sea areas has not been established. In this case, after reviewing the feasibility of three-dimensional and layered utilization of sea areas, the approval process can be conducted in accordance with the law, and information of the spatial scope of the sea area can be clarified.