Ecological risk of coastal ecosystem: A perspective of mangrove ecological sensitivity in small islands, case in Anambas Archipelago District at Natuna Sea

The ecosystem sensitivity in the coastal and small islands is dependent on the vulnerability and resilience level of that ecosystem. Mangroves are one of the primary ecosystems in coastal areas, also high vulnerability and sensitivity due to natural factors such as waves, pollution, and due to the impact of human activities. Based on this reason, the study of vulnerability and sensitivity was needed for mitigation plans and other actions in reducing the risk due to the pressure by a coastal manager. This study was carried out around small islands in Anambas District, especially in Matak Island. The results showed that the level of mangrove vulnerability was relatively high at all locations; the condition of the ecological status was classified as moderate. The status of the social and economic index was classified as medium to high vulnerably. The assessment of the sensitivity level, known as the condition of mangrove ecosystems, is moderate to very sensitive. The areas with the highest sensitivity level are Peninting and Ayerbadong. The high sensitivity in both locations is due to the influence of the economic index, which indicates that the region is economically sensitive and potentially vulnerable.


Introduction
Matak Island and surrounding areas are groups of small tropical islands characterized by small embayments, estuaries, sandy and rocky beaches, mangroves, and coral reefs and considered biodiversity highly in terms of ecosystems and habitats. The Matak Island area is also in the middle of a massive economic development, which makes this area economically sensitive. Then also position in the Natuna Sea and Archipelagic Sea Lanes I potential to increase the impact of vulnerability and sensitivity in the coastal area.
The Environmental Sensitivity Index (ESI) study is an important program which can be further used as input for environmental management. Any activity on coastal and offshore of Matak Island, potential to increase the hazard, vulnerability, sensitivity, and risk to mangrove ecosystem. Overall, at the long time potential to reduce the carrying capacity of the coastal area.
Sensitivity determined as the potential impact on a system from perturbations, including shocks and stress [1]. The issue of sensitivity usually close relation to the topic of sustainable development. The sensitivity concept is one of the sustainability constraints together with, for example, the concept of 2 minimum safety standards, quality standards, carrying capacity, eco-capacity, maximum sustainable yield, critical loads, environmental utilization space, etc. [2]. All such concepts may be useful for policy analysis and operations planning.
The sustainability ecosystem express at least from four attributes [3] such as: (1) it is expressed in one or more measurable parameters; (2) these parameters are linked to sustainability targets; (3) the parameters have proper geographical scale; and (4) these parameters also have a relevant time dimension. It is also stated that ideally, these parameters should be mapped out as quantitative factors, but in reality, it is often confronted with qualitative, fuzzy and incomplete information [2]. In this context, therefore, a better understanding of the habitats and ecosystems and their sensitivity of the Matak Supply Base and surrounding areas in the western and northern parts of the Matak Island environment can be derived through the development of the Environmental Sensitivity Index (ESI). The objectives of the study are to identify the sensitivity level of changes in both ecological and social conditions of coastal ecosystems, particularly mangrove ecosystem and human impact that cause the sensitivity area.

Research area
The research area is in Matak Island, Anambas District, at the Natuna Sea (see figure 2). The sampling sites were selected by the team, along with the coastal areas that are inhabited by mangrove ecosystems. The survey activities cover four main divisions, i.e.: (i) Matak Jetty; (ii) Matak Strait; (iii) Peninting Strait; (iv) Ayerbandong and Mandilang Strait. The field survey and data collection were conducted on June 19 th -26 th in 2014 ago. The kind of data that was collected at that location as follows: 1) Identification of the mangrove species in the study areas and its geographical position; 2) Estimation of density and other ecological function; 3) Sensitivity area map of the mangrove ecosystem. The type of data was collected in the location shown in the Table below. Type of substrate Visual Survey -Data collection was conducted in each habitat of mangrove found in the study area. On each site, the transect grid applied for species identification and density counting. Type of sediment, substrate, and associated fauna living in the mangrove was observed as well directly while sampling process.

Vulnerability index (VI)
The vulnerability value of the mangrove ecosystem is high because it grows in sensitive habitats that are vulnerable to pollution and the influence of rivers and oceans. Mangrove ecosystem was also known as a habitat for fish, crocodiles, migratory birds, fish nursery, and other resources. We have any sensitive ecosystem such as mangroves, brackish swamps, tidal areas, floodplain areas that classified as very high vulnerability areas by [5]. Therefore, the vulnerability index (VI) criteria of the mangrove ecosystem have a score value at 5 (very high).  Figure 1. Site sampling in four divisions in Anambas Island [4] Vulnerability score also refers to the sensitivity level of each species is according to its response against pollution, debris, or other pollutants on the mangrove area. Crude oils and sea waves can pool onto sediment surfaces and are highly persistent. These heavy oils and emulsified oil can be trapped in thickets of aerial roots (pneumatophores) by mangrove, especially on species Avicennia sp. While in a location, we found a susceptible species (Avicennia sp), can state as a highly vulnerable site. Based on the ability to recover from oil spill pollution, mangrove species can be divided onto five-level sensitivity such as Table 2. The first activity of analysis mangrove ecology is identified of mangrove species, through in-situ observation according to the book of mangrove identification [7,8]. Data analysis of mangrove ecology consists of species diversity, density, the status of protection, wildlife habitat, and tidal exposure to mangrove habitat.

Mangrove Species Diversity
Species diversity was calculated on each site of mangrove area based on occurrence mangrove species in each habitat mangrove. The higher number of species found, also increasing mangrove diversity. Value and score of mangrove diversity shown in Table 3.

Mangrove Species Density (Di)
The species density (Di) is the number of a mangrove tree in a specific area: Where, D i is species density of species-i; ni is the total number of the tree of species-i, and A is a sample area where data collected.

Protected of Mangrove area
Protected area mangrove means showing the status of the mangrove area, whether as a protected area or not. If it is a protected area, the area level was classified as a regional or national protected area.

Wildlife habitat
Wildlife habitat means whether the mangrove area is a habitat for wild biota or not, and have functioned as preservation endanger species. Therefor information or recording about wildlife in mangrove forests needed to set a protection program. The increasing number of endanger or endemic species in the mangrove ecosystem show that an increase of sensitivity. The higher number of wildlife criteria shows a critical habitat and sensitivity score value higher too.

Tidal exposure
Tidal exposure defined as the potential impact from the tidal process to coastal or mangrove are and also wave and sea current. While in an extreme wave, the mangrove area potential has an abrasion or rob flood in the terrestrial area. The important thing mangrove function is as coastal protection from wave, rob, and tidal dynamic. The higher tidal frequency or extreme wave on the mangrove area means increasing sensitivity on the mangrove ecosystem and score value high. The next step environmental sensitivity index (ESI) is scoring parameters in each criterion above. The score value range from 1-5 (low-grade to high-grade). [5] Categorized the vulnerability value for mangrove into five scoring criteria. The scoring criteria for the ecological indicator on mangrove parameter as described above shown in Table 3 and 4.  [9,10]; **) Developed from [11]; ***) Developed from [11,12] The criteria for ecological values of mangrove ecosystem is presented in Table 4. The range of value in each description score determined based on the potential record of data and any judgment related to mangrove research. The traceability score in each criterion, there are also remark or a reference by the researcher to expand.  The mangrove area is locally protected (green belt area) The mangrove area is nationally protected (national park) The protected wildlife is not found The protected species were commonly found in Division 4 (onshore barging) 1-2 1 -2 protected wildlife species found 3 Three protected wildlife species found >3 More than three protected wildlife species found Tidal Exposure***) 0,2 Low Exposure Not influenced by the tide Whole mangrove area in the offshore and onshore (Division 4) are commonly highly influenced by the tidal

Medium Exposure
Moderately influenced by the tide High Exposure Highly influenced by the tide Sources: *) Developed from [9], [10]; : **) Developed from [11]; : ***) Developed from [11], [12] 2.2.8. Socio-economic index Socio economy data that relate to mangrove are cover demographic structure and its parameters, sensitive area (in the perspective of socio-economy), and mapping. Socio economy data analyses are comprised of, i.e., age structure, population income and expenditure, socio-economic valuation, and development of environmental sensitivity index of the local communities socio-economic at the Matak Supply Base and surrounding areas in the western and northern parts of Matak Island. As mentioned, socio-economic value is similar to the Social Values Index (SI), which comprises the economic value index (ESV) and the social value of resource uses (SV). The formula of the Social Values Index (SI) as follows [13]. Where, SI is the social value index; SV is the social value of the resources component, and ESV = economic value component of the resources.

Social value
Social value for the mangrove ecosystem is calculated based on criteria such as (i) tourism development potential area, (ii) fishing ground, and (iii) other mangrove utilization such as for construction/housing materials. Source: Adopted and modified from [14] The social value is calculated through a geometrical average as described in the following formula:   Where; SVj is the social value of j site; Sji is score value criteria of i-social value at j-site.

Economic value
The economic value of mangrove resource is calculated based on the geometrical average of social value and ecological value multiplied by the standardized economic value of mangrove resource /ha/year issued by (Ministry of Environment KLH 1999) , i.e. USD 15,877.42/ha/year. This value was taken from similar research in Barelang and Bintan Island, which is an assumption to be identical to the recent study [13]. EcVi is the economic value of mangrove resource at i-site; EcVji is shown the economic value of mangrove of 1....n; EcVji-max is the maximum economic value of mangrove resource observed.

Social Economic Index (SI)
The economic value of mangrove resource calculated through the following formula [13]: Where, SIj = socio-economic value of j-site, SV = social value, and EcV = economic value.

Environmental sensitivity index analysis (ESI)
An Environmental Sensitivity Index (ESI) value describes the relative environmental sensitivity of each area was calculated and displayed on the spatial profile. The area to be outlined on the sensitivity maps represents an integration of three main components of the ESI, namely Vulnerability Index (VI), Ecological Index (EI), and Social Index (SI). The integration of these values can be represented in a composite equation, as described in Equation [5]. Where, CESI i is the composite ESI of area 1…I ; VI is the vulnerability index; EI is the ecological index; and SI is social values index (SI), which comprises economic value index (EcV) and social value of resources use (SV) with formulas as below. Where, VC is vulnerability components (shoreline values); EC is ecological components (ecosystem value and species value), and SC is social components (economic values and social-uses values). Each component, i.e., vulnerability, ecological and social, has a value between 1 (minimum = the least sensitive) to 5 (maximum = the most sensitive). Since the composite ESI (CESI) is a multiplication among these components, hence the CESI values range from 1 (the least sensitive) to 125 (the most sensitive). This formula is developed by [15] which is inspired by ESI guidelines [11]. Therefore, the composite environmental sensitivity index (CESI) has a value between 1 -125 and is categorized as follows (Table 6).

Results and Discussion
The reduction of mangrove coverage is a common cause of human settlements or other conversions such as at Antang and Butun. However, the mangrove ecosystem in Payalaman relatively stable, and no significant changes was observed, while Mangrove in Batu Ampar and Mandilang naturally grow and slightly expand in height and width of the tree.

Figure 2. Map of Mangrove Distribution in Matak Island
Mangrove ecosystems in the western and northern part of Matak Island and the surroundings have a significant ecological function, environment and socio-economic functions which include: 1) to Maintain coastal water quality; 2) to reduce the severity of impact from the coastal storm, wave, and flood damage; 3) as Nursery areas and feeding grounds for commercial fisheries; and 4) as important habitat and feeding grounds for benthic and birds. The mangrove ecosystem found in the study area has unique characteristics since it is located on rocky islands with a minimum supply of freshwater, indicated by a lack of major estuary. A similar condition is also observed in the western and northern coastline of Matak Island and other surrounding small islands.

Vulnerability and ecological index
The ESI value of the mangrove ecosystem is calculated based on vulnerability value, ecological value, and social value. Based on CESI criteria, the vulnerability index of mangrove towards oil spills is the highest (the most vulnerable) while it was located in the intertidal area with a muddy substrate.  It means that the mangrove ecosystem is easily exposed to the oil spill and absorbs oil at the event of an oil spill. Mangrove is vulnerable since this plant is biologically sensitive to oil exposure considering its habitat for many aquatic biotas such as fish, crustaceans, echinoderms, mollusks, benthic animals, plankton and terrestrial wildlife. Besides, the mangrove ecosystem also plays a significant role in supporting local fisheries, as they are functioned as a nursery ground, feeding ground, and spawning ground [16]. Mangrove ecosystem as the most sensitive area to oil spill since this ecosystem has a complicated physical configuration and muddy substrate, which is difficult to clean up while spilling event [11]. The maximum number of mangrove species that have been observing in the study area are 4 (four) species, with density ranges from 200 -1,000 ind/ha. Mangrove areas were not explicitly categorized as a protected area (i.e., Nature Reserve and Nature Conservation Areas). However, based on Indonesian law, mangrove is categorized as a protected area as stipulated in Presidential Decree No. 32/1990 regarding Protected Area Management. As a small island ecosystem, the most of mangrove ecosystems in the study area were exposed to tidal wave except at Dusun Sedak. The mangrove ecosystem in Dusun Sedak relatively protects by rubble and stone. This research also found some changes in mangrove coverage were observed and mostly causes by conversion into human uses mainly as settlements. However, it is also found increasing of mangrove width and tree height at Ayerbandong. An ecological variable of mangrove ecosystem in the study area is described in Table 7. In addition, the ecological value for the 2014 survey in each component then determined by the above data and presented in Table 8. Each criterion of each mangrove site is then weighted to know its ecological index, as shown in the Table 9.

Socio-economic index
The economic value of a mangrove ecosystem is defined by the function of a mangrove ecosystem in supporting economic activities and the daily life of the local people living nearby. The social benefits of the mangrove ecosystem can be classified as direct and indirect benefits. The direct benefit is defined by the use of mangrove trees for firewood (Tancang or Rhizhophora sp.), traditional rooftop or thatching (Nypa fruticans), and construction materials (Sonneratia sp. and Rhizhophora sp.). The indirect benefit defined as protection and conservation function of mangrove area as well as a nursery ground for larvae and juveniles of fishes and crustaceans and feeding ground for some marine biota. Mangrove functions also as a spawning ground for some marine biota. The Social Index of Mangrove in the study area ranges from 1.82 to 4.15. The highest index was found in the north of Peninting, and the lowest index at the estuary of Antang river. The detail of the socio-economic parameters was shown in Error! Not a valid bookmark self-reference.10.   Table 11 shows the values ESI and sensitivity levels of each unit of analysis in the study area. It shows that based on the 2012 study, the mangrove ecosystem in the study area have a sensitivity level ranges from moderate to very sensitive. Compare to the distribution of sensitivity levels in the 2009 study, and it can be seen that there are changes in sensitivity level at Batu Ampar A, Telayan, Antang B and Mandilang B. Significant difference of ESI in 2014 is observed at Ayerbandong B due to increasing of mangrove width and tree diameter species of Rhizopora sp. and Bruquiera sp.

ESI for mangrove ecosystem
The sensitivity level in Matak Jetty division ranges from less sensitive to very sensitive. The less sensitive is found along the rocky shoreline with low economic activities. In contrast, the sensitivity level is located in the mangrove area, which is highly essential both from an ecological and economic point of view. The Sensitive level is the mangrove ecosystem in Batu Ampar. The waters area of the bay is classified as a moderate level to sensitivity, while local people utilized the area as a fishing