Scenario analysis is a commonly used method to test various visions and assumptions and the outputs potentially guide spatial decision-making (Sun et al. 2022). In this study, four different scenarios, namely Business-as-Usual, Climate Change Adaptation, Maximum Development and Maximum Conservation Scenarios were developed for this study to reflect the diversity of land-use decisions that determine the conservation-use balance of natural resources (Fig. 3). Each scenario is built on assumptions of LULC change (Suppl. material 1) grounded in perspectives of natural conservation and natural resource protection.

The Business-as-Usual Scenario assumes that the LULC change between 1990 (European Environment Agency 2019a) and 2018 (European Environment Agency 2019b) in Istanbul will continue. It involves expanding urbanised areas, population growth and rising demand for food, water and urbanised land, causing increased pressure on natural areas and resources.

The Climate Change Adaptation Scenario assumes that the population growth is moderate, while the expansion of urbanised areas is minimal. Protecting and rehabilitating natural areas are prioritised actions for climate change adaptation.

The Maximum Development Scenario assumes that the current LULC change trend continues, the mega-projects that have already been included in the Revision Environmental Master Plan of Istanbul have been implemented and the LULC change brought about by this implementation has taken place.

The Maximum Conservation Scenario assumes that Istanbul's population and urbanised areas will stabilise. Protection and rehabilitation of natural land are prioritised actions.

The comprehensive approach “Integrated ES Potential” by Tezer et al. (2018) is proposed to better incorporate the ESs' approach into spatial decision-making and evaluate this integration. It is based on the matrix approach linking LULC classes to ES supply capacities, ranging from 0 for no potential, 1 for very low potential, 2 for low potential, 3 for medium potential, 4 for high potential and 5 for very high potential (Burkhard et al. 2014). In this integrated manner, all ESs are accepted as equal importance and the arithmetic mean value of the sum of the ES supply capacities was calculated to obtain the IES potential for the relevant LULC. The IES potential values calculated for the LULC classes of Istanbul and the classification are given in Table 1. High and very high IES potential areas depict the most functional areas for ensuring ES sustainability, thus, climate risk adaptation and sustainable development.

If the change of high and very high IES potential areas, which represent areas of critical importance for all scenarios, is analysed compared to the current situation, it can be seen that there is a sharp decrease in both high IES potential areas (-6.87%) and very high IES potential areas (-6.14%) in the Maximum Development Scenario. These critical areas are increasing in the Climate Change Adaptation Scenario. However, the sharp increase in these areas is in the Maximum Conservation Scenario, with an areal increase of 2.16% and 9.68% in high and very high IES potential areas, respectively. Very low and low IES potential areas are increasing for the Maximum Development Scenario and the Business-as-Usual Scenario. The areal changes in IES potential classes for each of the scenarios compared to the current situation are given in Table 3.

The IES Potential Maps for the scenarios are shown in Fig. 6. For the second step, the spatial changes related to the IES potentials are analysed using ArcMap 10.6.1. Green areas show that there is an increase in the IES potential, while red areas show that there is a decrease and the beige areas show that there is no change (Fig. 7).

The significant decrease in the high and very high IES potential areas calculated for the Maximum Development Scenario quantitatively is noticeable, especially in the area where the Canal Istanbul is planned to be constructed (Ministry of Environment, Urbanization and Climate Change 2020). For the areas with increases, decreases and no change in the IES potentials, the changes are given in Table 2 as percentages.

The LULC maps modelled with the Scenario Generator Model (Sharp et al. 2020) are visualised in ArcMap 10.6.1. They are given in Fig. 4. Compared to the current situation (CORINE 2018 LC data from European Environment Agency (2019b)), the artificial surfaces have increased (from 26.82% to 30.46%) only in the Maximum Development Scenario, where agricultural areas (from 23.39% to 20.87%), forests and semi-natural areas (from 40.96% to 40.29%) and waterbodies and wetlands (from 8.83% to 8.38%) show decrease. The most significant increase in agricultural areas is in the Maximum Conservation Scenario (23.39% to 27.11%). For forests and semi-natural areas, the most significant increase is also in the Maximum Conservation Scenario (40.96% to 42.61%). In the Climate Change Adaptation Scenario, agricultural areas (23.39% to 26.83%) and forests and semi-natural areas (40.96% to 42.89%) increase like the Maximum Conservation Scenario. The comparison of LULC areas of each scenario is shown in Fig. 5.

In developing recommendations for integrating ESs into spatial planning tools in Istanbul, firstly, the extent to which existing policies, regulations and plans in Turkiye incorporate ESs' approach is analysed.

The ESs' approach is incorporated in several sectoral policies at different utilisation levels in Turkiye. As summarised by Başak et al. (2022), the content of these incorporations involves generating general targets related to the identification, protection, monitoring and evaluation of ESs (Başak et al. 2022). However, it is stated that, without the necessary methodological framework for integrating ESs, putting these targets into practice is unlikely. The National Watershed Management Strategy, like these summarised sectoral policies (Başak et al. 2022), includes the determination of various objectives for ESs. Although it does not constitute a legal obligation, it has the quality of guiding the implementation. In Turkiye, it can be seen that ESs are implemented in two special provision determination projects, namely the Büyükçekmece Dam Lake Watershed and the Melen Dam Lake Watershed Special Provision Determination Project. The legal basis for these projects is the “Regulation on the Protection of Drinking-Irrigation Water Basins”. Even though integrating ESs with spatial plans is not obligatory according to this legal tool, the integration was supported by the related ministries or local-level governmental institutions (TUBITAK-MAM) (Grunewald et al. 2021). In both of these projects, ES information is used in the determination of the protection zone with the mapping of the critical areas in terms of water-related ESs and these areas are shown in the final map regarding the protection areas of the watersheds (Ministry of Agriculture and Forestry 2019, Ministry of Agriculture and Forestry n.d.). In watersheds where watershed protection plans are prepared, the national regulations are invalid and there is a legal obligation to implement the finalised watershed protection plans by the governorship. Therefore, it can be said that ESs have found a place in practice for Melen and Büyükçekmece Dam Lake Watersheds in Turkiye (Ministry of Agriculture and Forestry 2019, Ministry of Agriculture and Forestry n.d.).

In addition to watershed protection plans, the ESs' approach finds its place in the definitions of functional planning as well, in a concrete form and in a way that can be used in practice (Ülgen et al. 2020). “The Principles and Procedures for the Preparation of Ecosystem-based Functional Forest Management Plans” Communique was published in 2017 by the General Directorate of Forestry (General Directorate of Forestry 2017). In the Communique, ESs are expressed as ecosystem functions and only forest ESs are covered and evaluated in three classes: economic, ecologic and socio-cultural. Under these classes, relevant ESs are identified (Quality and speciality wood production, plant products, gene protection, flood prevention, drinking water supply, aesthetic appearance, education, research etc.).

Başak et al. (2022) state that implementing the targets set in sectoral policies also requires political will, institutional rearrangement and capacity building. Therefore, when the environmental master plans are evaluated in terms of ESs incorporation, it was observed that no direct information related to ESs was used. However, the current Environmental Master Plan of Istanbul (2009) indirectly includes ESs in the plan report (Istanbul Metropolitan Municipality 2009). The Plan Systematics Section states that, when making land-use decisions related to new settlements, only minor damage should be given to the natural environment, while providing benefits from nature to these settlements. In the sections related to forestry, forest ESs are referred to as forest functions and the ESs that benefit the public and villagers are defined. In addition, within the scope of Istanbul’s Natural Structure Synthesis, data on Critical Soil Resources for the Sustainability of Water Purification have been produced and mapped (Istanbul Metropolitan Municipality 2009).

In addition to these, in Turkiye, The National Spatial Strategy Plan is still being prepared and the projects related to the development of this plan involve an ESs-based conservation approach (Ministry of Environment, Urbanization and Climate Change 2020, Ministry of Environment and Urbanization 2021a, Ministry of Environment and Urbanization 2021b). The legal basis for The National Spatial Strategy Plan is the “Regulation on Spatial Plan Making” and The National Spatial Strategy Plan hierarchically precedes the environmental master plans. Three reports are published within the scope of The National Spatial Strategy Plan, namely The Strategic Environmental Assessment Scoping Report, the Draft Strategic Environmental Assessment Report and the Draft Strategic Environmental Assessment Report Annex. Examining these reports shows that ESs are directly covered in all three reports. It is observed that the reports include information on ESs in many dimensions. ESs are used strategically as a component in determining the conservation approach. In addition, ESs are used practically and the critical areas in terms of IESs are mapped nationwide. The areas critical for climate change adaptation are evaluated in light of the IESs approach. In addition, indicators, such as critical areas in terms of IESs per capita and critical areas in terms of climate change adaptation per capita, are also identified and calculated. The National Spatial Strategy Plan is being prepared, so Turkiye has not yet planned to guide the environmental master plans.

ESs approach provides a framework to define and conceptualise the interconnections of nature and human well-being. It is also considered an essential common language amongst stakeholders, decision-makers from different sectors or actors from different spatial units with conflicts of interests (Hauck et al. 2013). The ES research community recognised three modes of ES knowledge use to be employed in different planning stages, namely conceptual, instrumental and strategic use (McKenzie et al. 2014, Grunewald et al. 2021, Wei and Zhan 2023). Conceptual use involves conveying the importance of nature to society from the perspective of ESs. In comparison, strategic use involves advocating for specific actions, policies or resolutions to address stakeholder issues and conflicts. Instrumental use refers to scientists using technical tools to assist decision-makers in selecting the best policy options and intervention ways (Wei and Zhan 2023) (Fig. 8).

ES information produced through these defined three modes (conceptual, strategic and instrumental) can be linked to many stages of spatial planning (Fig. 9). ESs can be used conceptually in all the stages of the spatial planning process involving stakeholder participation by providing a way to communicate the importance of nature to people, the global common risks/local hazards and risks and providing a common language in stakeholder dialogue where the stakeholders have conflicts of interests. It can provide new ways of thinking and facilitate the generation of new ideas (Wei and Zhan 2023). Strategies, goals and objectives involving environmental conservation, rehabilitation, human well-being and sustainable development can be set and shaped, based on ESs so that ESs can be used strategically in spatial planning. The instrumental use of ESs is closely related to more phases of spatial planning. ES information can guide the development of plan alternatives and ESs-based information can be produced afterwards. An approach, such as IES, as used in this study, can enhance the spatial and quantitative ES information mentioned above. It also can help to reveal the potential consequences of the alternative plans in terms of ESs, for which ESs trade-off/synergy analysis or the evaluations based on the IES potentials can be used in this way. This qualitative and spatial information can also facilitate the communication of potential consequences to decision-makers. The generated ES information can be utilised in conceptual, strategic and instrumental modes and can be linked to many spatial planning stages (Fig. 9). Thus, ESs-based spatial decision-making can sustain ESs and ensure sustainable development with improved human well-being.

In light of the Istanbul case study findings and the theoretical information related to integrating ESs into spatial decision-making, recommendations are developed for integrating ESs into spatial planning tools in Istanbul. These recommendations include high-level plans (Regional Plan and Environmental Master Plan).

The Regional Plan of Istanbul is “the setter of the socio-economic development trends, development potential, priority intervention areas and sectoral targets of the Istanbul Region. It aims to ensure sustainability and effective and appropriate use of resources. The Plan will determine the relationship between the plans, policies and strategies produced at the national level to ensure the Istanbul Region's socio-economic development”. “The Regional Plan is a top-scale plan that also guides the strategic plans to be prepared by public institutions, especially local governments” (Istanbul Development Agency In press). ESs can be incorporated into the plan since “the sustainability, effective and appropriate use of resources" is defined as an aim in the general definition of the plan. ESs can also be used as a facilitator within the scope of plan preparation studies.

Within the scope of the Regional Plan preparation studies:

• Preliminary policy and strategy studies can involve ESs regarding the setting of the policies and strategies.
• Evaluation of the previous regional plan can include ESs-based evaluations to assess progress. ES indicators can be set to be monitored. The change in IES potential can be monitored.
• Current situation analysis can involve ESs-based assessments for the evaluation of Istanbul’s current environmental situation. The IESs' approach can be used in this analysis.
• ESs can provide a common language amongst the stakeholders in the thematic meetings, meetings with institutions and provincial meetings. It can facilitate the communication of ESs to stakeholders regarding environment-related strategies.

Within the Regional Plan:

• The strategies related to the environment can involve ESs. One of the four visions of the current Regional Plan of Istanbul 2024-2028 (Istanbul Development Agency 2022) is Green Transformation and Target 2.2 is set as “The city’s natural resources will be protected and sustainable resource management will be ensured for their efficient use”. This target can be improved with ESs, as ESs can be used as a layer to identify sensitive areas that need to be protected. It can also be used to raise public awareness about conservation.
• The generation of the Istanbul Spatial Development Map can involve ESs as a layer to indicate critically significant areas.
• The evaluation of the plan can involve ESs in the related strategies and targets. Within Target 2.2 of the Regional Plan of Istanbul 2024-2028 (Istanbul Development Agency 2022), one of the performance indicators can be set as ESs-based, for example, the Ratio of Critically Significant ES Areas (The ratio of the sum of the high and very high IES capacity areas to the whole area of Istanbul, in percentage). Another indicator can be set as the Critically Significant ES Area, based on the IES potential per capita (the Ratio of the sum of the high and very high IES potential areas to the number of inhabitants in m²/person).

Istanbul’s Environmental Master Plan can cover ESs in several sections. In the current Environmental Master Plan of Istanbul (Istanbul Metropolitan Municipality 2009), the central conflict is defined between economy and ecology and the central philosophy of the plan is “resolving the conflicts between the natural and built-up environment”. In this context, as ESs provide a framework for the relationship between humans and nature, they can be used at many stages of the plan to help address these conflicts.

Within the Plan Systematics:

• ESs can be used to define conflicts. The ESs framework involves concepts like the potential of ES and the demand for ES (Burkhard et al. 2014). These concepts can help to understand and identify these conflicts. This work may require an ESs' assessment specific to Istanbul made at the provincial scale. However, matrix-based approaches can be used as a first step (Burkhard et al. 2014, Jacobs et al. 2015, Burkhard and Maes 2017, Campagne and Roche 2018) as they are easy to understand and implement.

Within the Natural Structure Section:

• ESs can be used within the scope of the Ecological and Biological Structure Sub-Section. Modelling and mapping approaches can also be used here to assess and map the ESs of Istanbul. Critical ESs in terms of vulnerability, sustaining water quality or adaptation to climate change can be defined and mapped.
• The Section of Forest Areas can involve ESs, as forest areas can provide multi-layers, even for the most diverse ESs.
• The Section Agricultural Areas can also incorporate ESs as many provisioning ESs related to agriculture are classified (Crops, fodder, fibre, biomass for energy etc.).
• An analysis of urbanised areas intersecting with critical ES areas can enhance the section on the Residential Areas and Quality of Life.

Within the SWOT Analysis Section:

• ESs can enhance the SWOT Analysis because Istanbul's ESs-based strengths, weaknesses, opportunities and threats can be defined in light of the definitions, quantification, analyses and assessments made in the previous sections.

Within the Synthesis Section:

• ESs can be used as a layer of spatial information in the Natural Threshold Synthesis. They can be used to identify the areas of protection and to define the degree of protection.
• A map of Istanbul’s green-blue network can be created with ES information for the Spatial Sustainability Synthesis.
• The spatial information related to the critical ESs can enhance the Natural Structure Tolerance Rating Map.
• By incorporating IESs in Natural Threshold Synthesis, Spatial Sustainability Synthesis and the Natural Structure Tolerance Rating, ESs-based Natural Structure Synthesis can be obtained.

Within the Planning Approach Section:

• ESs can enhance the Targets and Strategies Section. ESs-based strategies can be set within the scope of environment-related targets (e.g. protection of critical ES areas).

Within the Plan Decisions Section:

• ESs-based identification of protection areas can be made in the Protection Areas for Environmental Sustainability Section as in(?) the proposed IESs.

In this way, the decision-making process related to Istanbul's Environmental Master Plan can be enhanced by integrating ESs to improve well-being and achieve sustainable development.