Habitat suitability model of future coral reef region in the Halmahera Waters under the projection of environmental pressures from future climate change and coastal pollution

The coral reef region of the Halmahera waters situated within the tropical area of the coral triangle, covers approximately 193,126 km2, with coral reef habitats occupying roughly 958.47 km2. The coral reefs in Halmahera waters are essential for ecosystem services and local fisheries. However, they are at risk from climate change and coastal pollution. This study aimed to develop habitat suitability models for coral reefs in the Halmahera waters under future climate change scenarios. MaxEnt modeling was employed to project suitability for the coral Acropora spp. using occurrence data. Suitability projections were made for 2050 under IPCC RCP 2.6, 6.0, and 8.5 scenarios, assuming no measures are taken to mitigate coastal water pollution. Environmental factors considered in the modeling included temperature, salinity, current velocity, pH, sunlight penetration, and chlorophyll levels. Results demonstrated a significant decline in habitat suitability, with an 85% loss of highly suitable areas under RCP 2.6 – 8.5 compared to current conditions (741 km2). Chlorophyll levels, a marker of coastal pollution, were predicted to be the most influential factor, followed by temperature and salinity. The findings highlight the need to reduce global carbon emissions and implement conservation measures to mitigate coastal water pollution in preserving coral reef habitats.


Introduction
The Halmahera waters are located within the tropical region of the coral triangle.This region has been recognized as a center for the mega-biodiversity of tropical coral reef organisms [1].Coral reefs provide direct benefits such as capture fisheries, aquaculture, and tourism, and indirect benefits such as erosion control, carbon sequestration, and serving as spawning, nursery, and feeding grounds for fisheries species [2].Data from a specific area on Halmahera Island, specifically the Kayoa District, indicates that the estimated economic value of coral reefs, both direct and indirect, amounts to 74 billion IDR per year [3].Therefore, coral reefs are an ecosystem whose presence is crucial to support the livelihoods of communities in the waters of Halmahera.
However, the existence of this ecosystem is threatened due to anthropogenic pressures and climate change.Coastal land conversion, terrestrial pollution from trash, household and industrial waste, as well as runoff from maritime transportation, along with coral extraction for construction material, have been identified as anthropogenic factors that have reduced the extent of live coral cover in the waters of Halmahera [4].Data shows that environmental pressures suspected of being anthropogenic have led to low live coral cover on small islands in the Oba district, Northern Halmahera, which is only around 14.33-39.13%[5].On a broader scale, out of 12 coral reef stations in the waters of Halmahera, data reveals that the average extent of dead coral cover in shallow waters (0.5-2.0 m) exceeds that of live coral cover [6].Recent predictions indicate that the sustainability of coral reefs in the Maluku Waters is threatened by future coastal pollution and climate change [7].These environmental pressures are further compounded by predicted changes in water dynamics within the coral reef habitat due to upcoming climate changes.The accumulating anthropogenic environmental pressures and climate change significantly reduce the habitat's capacity to support optimal coral reef life [8].
Therefore, optimal conservation is necessary to preserve the marine resource habitats in Halmahera waters in future climate change.This optimization can be developed from an accurate baseline study regarding the current condition of coral reefs and their environmental pressures, as well as predictions of the changes in the future.Thus, this study aimed to provide a baseline ecological predictions study related to the dynamics of coral reef habitats under the pressures of climate change in the Halmahera waters.The future prediction of these dynamics will be developed using the Habitat Suitability Model -Maximum Entropy (HSM -MaxEnt) ecological algorithm, based on climate change scenarios derived from the IPCC AR5 (Intergovernmental Panel on Climate Change -Assessment Report 5), namely RCP (Representative Concentration Pathway) 2.6, 6.0, and 8.5 scenarios for the year 2050.
The Representative Concentration Pathways (RCPs) are a set of possible developments in emissions and land use scenarios, based on consistent scenarios representative of current literature.They were chosen to represent a broad range of climate outcomes, based on a literature review, and are not forecasts or policy recommendations.The RCPs are designed to be compatible with the full range of emissions scenarios available in the current literature, with and without climate policy [9].
The three Representative Concentration Pathways (RCPs) delineate distinct trajectories of radiative forcing, each shaping future climate scenarios.RCP 2.6 delineates a stabilization path characterized by a radiative forcing peak around 2040, followed by a subsequent decline, ultimately reaching 2.6 W/m² by 2100.RCP 6.0 outlines a stabilization trajectory without overshoot, resulting in a radiative forcing of 6 W/m² after 2100.Conversely, RCP 8.5 illustrates a trajectory featuring an increase in radiative forcing, reaching 8.5 W/m² by the year 2100.These paths play a critical role as essential tools for evaluating potential future climate variations and guiding strategies for mitigation and adaptation [10].

Study area
The research area encompasses the coral reef region of the Halmahera waters, including the coastal region of Halmahera Island, Obi Island, Bacan, Tidore, Ternate, and the northern side of Morotai.This region, situated within the tropical area of the coral triangle, covers approximately 193,126 km 2 , with coral reef habitats occupying roughly 958.47 km 2 (Figure 1).The density of human settlements, serving as latent sources of anthropogenic pressure, is unevenly distributed throughout the coastal waters, being concentrated mainly on the western side of this area, specifically around the cluster of Ternate and Tidore Islands, extending to the southern side of Bacan Island.Meanwhile, on Halmahera Island, population settlements are more prevalent in the northern part.

Data collection
The data collected in the study consists of two parameters: (a) the distribution parameter of coral reefs and (b) the coastal water parameter, which serves as the habitat for the coral reefs.Acropora spp. is chosen as a bioindicator for the coral reef distribution parameter and obtained from the Global Biodiversity Information Facility [11].The selection was based on the occurrence of Acropora spp. as a common reef builder in tropical regions [12].These spatial occurrence GPS points were subsequently filtered to remove spatial autocorrelation using the method of Brown et al. [13].In the context of the study, the environmental parameter was acquired through the method outlined by Simon-Nutbrown et al. [14].This method combines projected environmental data for future climate changes, encompassing variables such as sea surface temperature, salinity, and ocean current velocity under the RCP AR5 2.6, 6.0, and 8.5 scenarios, along with spatial distribution data for environmental variables associated with coastal water pollution, including chlorophyll, pH, and sunlight penetration.
The model was developed assuming that coastal water pollution will persist until 2050 without significant mitigation.Environmental data was obtained from Bio-ORACLE 2.2 and NASA MODIS OCSMI.

Model development
The Habitat Suitability Model (HSM) was developed using the MaxEnt algorithm and QGIS software to predict the suitability of coral reef habitats amid dynamic changes in environmental variables due to climate change and coastal water pollutants.The model was created using a 70:30 data occurrence ratio for training and testing, based on the methods of Freeman et al. and Tittensor et al. [15,16].Moreover, the model's accuracy underwent evaluation through the calculation of the area under the curve (AUC).This metric represents the average area under the receiver operating characteristic (ROC) curve and serves as a means to assess the model's performance.

Results and Discussion
The calculated AUC showed a value of 0.9, indicating the validity of the developed model.Three water variables were detected to significantly contribute to the suitability model compared to other variables (Figure 2).The highest contribution was chlorophyll, followed by light penetration.The contribution of other factors was detected below 10%.The finding of chlorophyll as the major contributing factor (over 63.1%) indicates that the level of this variable plays significantly in creating optimal environmental conditions to support coral reef life.Chlorophyll level is an indicator of eutrophication in coastal waters [17].A high level of dissolved chlorophyll indicates a high density of algae, typically resulting from increased nutrient levels in coastal waters or eutrophication.The physiology of coral reef species can be disrupted, leading to decreased calcification rates, weakened immunity, and disruptions to the reproductive and recruitment systems [18].The presence of algae also leads to reduced sunlight penetration and competes for habitat space with coral symbiotic photo-pigments [19].Light penetration is also a main contributing factor (15.8%) to the habitat suitability model.
Furthermore, although other variables have been detected with low impact on habitat suitability, their accumulation alongside other factors may significantly impact coral reefs.The dynamics of environmental conditions due to climate change and water pollution will synergistically constrain the optimal habitat space for coastal marine organisms [20].For example, even individual sea surface temperature is not a major factor in the model.However, the increase of temperature in future climate change can interact with coastal water pollution to restrict the habitat space of coral reefs.Disruptions to reproduction, decreased growth rates, and weakened immunity to diseases will occur more prominently with rising sea surface temperatures and increased nutrient levels in seawater [21,22].
The significant impact on coral reef habitat suitability is detected on the projection map in the Halmahera Waters for the year 2050 (Figure 3).While currently, almost all coastal areas of the landmasses in Halmahera waters provide optimal habitat for coral reefs, the 2050 projection shows a highly significant decrease in habitat suitability, especially under the highest emission projection, RCP 8.5.
Spatially, the remaining area suitable for healthy coral reef life is mainly located on the western side of the waters, among the cluster of islands between Tidore and Ternate.Calculations from the projection map reveal that areas categorized as "very suitable," with habitat suitability probability (P) higher than 0.75, have significantly decreased by approximately 85% across RCP 2.6 -8.5 (Table 1).The analysis shows that significant changes in habitat suitability in the year 2050 will depend on chlorophyll levels or anthropogenic runoff, which is predicted to reduce the environmental support for optimal coral reef life.Studies comparing the spatial pattern of habitat suitability and the distribution of terrestrial-coastal ecosystems from the Global Mangrove Watch revealed that the coral reef habitat pattern aligns with the presence of mangrove forests [23,24].The "very suitable" habitat is concentrated on the western side of Halmahera waters, as the coast of these small islands hosts mangrove forests.Mangrove forests can naturally alleviate environmental pressures such as water pollution, especially sediment, erosion, and nutrient runoff [25].Therefore, the presence of these natural coastal ecosystems will play a crucial role in the coral reef's resilience in facing upcoming climate changes.

Conclusion
This finding model demonstrated that a significant impact on the habitat suitability of coral reefs in Halmahera waters may occur due to the dynamics of environmental parameters from water pollution and climate change in 2050.The extent of the decrease in habitat suitability is estimated to be more than 85% under IPCC AR5 RCP 2.6 -8.5, leaving areas on the western side of the waters that are relatively still favorable to the coral reef community.The western region is believed to have higher resilience due to the presence of mangrove forests, which naturally act as a barrier filter to reduce terrestrial pollutant runoff levels.Therefore, conservation optimization is necessary to mitigate terrestrial pollutant runoff, especially from densely populated areas, and to preserve overall coastal ecosystems like mangroves and seagrasses to enhance coral reef resilience in future climate changes.

Figure 1 .
Figure 1.Map of the region and population density in the Halmahera Waters.

Figure 2 .
Figure 2. Contribution (%) of aquatic variables to the habitat suitability prediction model of coral reefs in the Halmahera Waters in 2050