The percentage of live coral reefs in each observation station is presented in Fig. 2. Coral reef condition is determined based on the percentage of live coral cover consisting of hard corals (Acropora and non-Acropora). Criteria for assessing coral reef ecosystems' condition based on the percentage of coral cover [18].
Figure 2 shows that the average percentage of coral reef cover from 2014–2018 is in the fair category (< 50%). The average percentage of coral reef cover from 2014 had a declining trend until 2017 and increased in 2018. Observations show that in 2014, coral reef cover in the good category (> 50%) occurred at stations 2 and 3, and coral reef cover the smallest occurred in 2017 at station 1 with a poor category (< 25%).
In the East Bintan Regional Marine Protected Area (KKLD), coral reefs are well developed and cover a very large area that can be found starting from Malang to Kijang Village. The width of the coral reef ranges from 100 m to 1000 m. The total area of coral reefs located on the coast of East Bintan, including Manipur Island and the small islands around it, is 6,066.76 ha [19]. It was found that 35 genera of stony coral with relatively different coral reef conditions [20].
Table 1
The land-use area from 2014–2018
Land use
|
2014
|
2015
|
2016
|
2017
|
2018
|
|
|
(ha)
|
|
|
Plantation
|
1.656
|
3.047
|
3.106
|
2.905
|
2.766
|
Settlement
|
635
|
635
|
358
|
501
|
501
|
Bare Land
|
114
|
-
|
-
|
-
|
205
|
Secondary Mangrove Forest
|
-
|
-
|
46
|
42
|
42
|
Dryland Farming
|
1.424
|
139
|
293
|
133
|
133
|
Shrubs
|
-
|
-
|
-
|
-
|
139
|
Mixed Garden
|
1.476
|
1.456
|
1.475
|
1.697
|
1.492
|
The land use pattern for five years from 2014–2018 is presented in Table 1 and Fig. 3. In 2014, the widest land use pattern was for plantation (31%), and the smallest was open land (2%). Plantations' expansion occurred in a quite significant trend in 2015, reaching 57% of the total area. However, the trend of plantation increased by 2% in 2016 (59%) and decreased in 2017 (55%) and 2018 (52%). The settlement land-use trends are fairly flat and do not experience a significant increase. The area of settlement in 2014 and 2015 was 12%, decreasing in 2016 (7%) and expanding in 2017 with the same area in 2018 (9%). Area of dryland farming experienced a significant reduction from 2014. In 2014, the area for dryland farming was 27% and decreased to 3% in 2018. Land use for mixed gardens was evenly distributed every year (2014–2018) with insignificant fluctuation.
The modeling results using SWAT resulted in the highest soil loss in 2017, and the lowest was in 2015. The increasing trend occurred between 2015 to 2017 and decreased in 2018 (Table 2).
Table 2
Surface run-off, soil erosion dan sediment from 2014–2018
Soil Loss
|
2014
|
2015
|
2016
|
2017
|
2018
|
Surface Runoff (mm/years)
|
88.2
|
51.4
|
82.9
|
108.9
|
70.0
|
Soil Erosion (t/ha/year)
|
4.3
|
3.5
|
5.1
|
6.8
|
3.8
|
Sediment (t/year)
|
1,376.1
|
1,111.7
|
1,598.7
|
2,115.3
|
1,192.7
|
Rainfall, land use, and soil type greatly affect soil erosion on Bintan island's east coast. Based on Fig. 4, even though annual average rainfall variability appears to be the main driving force for the trend in surface run-off and soil erosion, other factors such as land use and management changes are also important. The soil loss is influenced by rainfall patterns, soil erodibility, slope factors, cropping systems, and management practices [21].
The analysis results of the effect of surface run-off, soil erosion, and sediment on hard coral reef cover over the five years (2014–2018) appear to have the same pattern. In general, the higher the surface run-off value, soil erosion, and sediment yields, the smaller the hard coral reef cover, as happened in 2015–2018. Surface run-off, soil erosion, and sediment were negatively correlated with hard coral reef cover. The correlation coefficient (r) of each parameter from high to low is soil erosion > soil sediment > surface run-off (Table 3).
Table 3
Correlation analysis results
|
Hard Coral Reef Cover
|
Surface Runoff
|
Soil Erosion
|
Soil Sediment
|
Surface Runoff
|
1
|
|
|
|
Direct Runoff
|
-0.47
|
1
|
|
|
Soil Erosion
|
-0.72
|
0.91
|
1
|
|
Soil Sediment
|
-0.71
|
0.91
|
1.00
|
1
|
The factor contribution translated into the coefficient of determination (R2) shows that the soil erosion factor is the most contributing factor to hard coral reef cover (52%) (Fig. 5f), further soil sediment (50%) and surface run-off (22%). Other factors that affect coral reefs damage, including bleaching and low salinity of sea water [22].
The high average rainfall in 2017 (Fig. 4) caused a high incidence of soil loss and resulted in low hard coral reef cover in its year (Fig. 5a-c). The high rainfall and scanty vegetation cover lead to soil structure ravage and cause further the soil to be unable to store and absorb water during the rainy season, upsurge surface run-off, and impact increasing erosion rates. The erosion affects the environment, not limited to the on-site area, but can also extend to the off-site location. Soil erosion processes include the destruction of soil aggregates and the transport of sediments.
The Sediments generally settle at the bottom of the foothills, flooded areas, waterways, rivers, or coastal areas. In many regions, soil erosion affects soil quality, reducing soil nutrient content, with a consequent increase in food production costs [23], [24]. The heavy rains increased sediment time on reefs [25]. On the south Moloka‘i coastal in Hawaii, 1–3 large rain events per year drove overland runoff and contributed 50% of the sediment on reefs [26, 27].
High values of surface run-off and soil erosion correlated with Orthic Acrisols soil type and cultivated land use. In the USDA soil taxonomy, Acrisols correspond to the Humult, Udult, and Ustult suborders of the Ultisols and Oxisols with a Kandic horizon. This soil has very low resilience to degradation and moderate sensitivity to yield decline. This soil is acid, base status < 50%, and strong leaching. It's one of the most inherently infertile soils of the tropics and easily degraded chemically and organically very quickly when utilized. The estimated soil erosion rates were generally higher in cultivated areas of Orthic Acrisols than Vertic Cambisols and Gleyic Podzols soil types at slope steepness of over 35%. Cambisols and Podzols soil types have high resilience to degradation, which corresponds to the soil erodibility value, which is smaller than Acrisols. Soils with low infiltration exhibited K-factors higher than 0.04 and were generally susceptible to soil erosion [21, 28]. The decline in water quality and health of coral reefs will be accelerated by the process of land-use change on islands, which pedologically comes from erosive soil parent material with small and steep watershed systems [7].
Soil loss generation was high from cultivated lands due to the lack of conservation techniques. In east Bintan island, mixed garden, plantation, and bare land area with dense canopy cover contributed increasing of soil loss. Land-use changes from plantations to dryland farming and mixed gardens, as happened in the 2015–2016 period, also contributed to increased soil loss and decreased hard coral reef cover. Pollutants from agricultural land contribute at least 25% to coral reefs worldwide [29, 30]. An intensive agricultural system on erotic land will directly transmit sediment, inorganic, and organic nutrients to sedimentation in the estuary through aquifer systems, which eventually reach vulnerable reef ecosystems [31]. Forest conversion through extensive logging in the Solomon Islands resulted in the loss of 1587 ha of coral reef habitat and reduced the population of coastal Acroporid corals [32].
Sugarcane and pineapple plantations on the island of Maui, Hawaii, were the most significant drivers of coral reefs cover from coastal nitrogen flow from groundwater than other land uses [33]. The conversion of wetland (paddy) agriculture on Ishigaki Island, Japan, to sugarcane plantations in the 1980s led to a 2 ha decline in coral reefs cover over ten years [34]. Wetland farming (rice fields) can function as a sediment reservoir.
In the east coast, Bintan island, land-use changes from dry land farming to shrubs in the 2016–2018 period can reduce soil loss and increased hard coral reef cover. On the other hand, on Kuroshima island, Japan, a 10% reduction in coral reefs cover was due to an increase in pasture land for livestock grazing during 1977–2005 [35]. Soil erosion increases as indicated by the decrease in vegetation cover, incompatibility between land use and its capability [36]. Land use for mining and settlement areas is very small on the east coast of Bintan island and may not affect increasing soil loss. Ex-mining land, such as that occurred in Papua New Guinea, causes a decrease in coral density, expansion, calcification of corals due to high levels of heavy metals (Zn and Pb) on the reef even after mining has stopped [37]. The high population in the coastal area will increase feces or pathogenic fungi on the reef [38]. The existence of mangrove forests on the east coast of Bintan Island is very small. Even though mangrove populations have received considerable attention at the interface of coral reefs, if land-use systems on land encourage excessive erosion, the mangrove ecosystem will not function as a barrier to coral sedimentation [31].
Land cover plays a critical role in the soil erosion process because estimated soil erosion rates were low in areas covered with vegetation canopy regardless of slope gradients. The land vegetation landscape greatly influences the condition of the coral ecosystem on the coast. Land covered by dense vegetation will: (1) absorb rainwater and reduce freshwater input to coastal areas, (2) stabilize the soil to reduce surface run-off, riverbank erosion, and coastal sedimentation, (3) absorb and improve soil nutrients and heavy metal mineralization in soil and groundwater, and (4) providing a growing environment for habitats or other species that connect land and sea [39]. A land-based land conservation concept must build by inviting a series of socio-economic interests and stakeholders into the coastal area planning process [7].