Assessment of Blue Carbon Stock of Mangrove Ecosystem in Telok Bangko Bontang

Mangroves have high primary productivity so they are known as good carbon storage and absorbers (carbon sequestrators). This ability plays a very important role in the regulation of climate mitigation because the ability of mangrove sequestrators can be utilized to absorb CO2 anthropogenic emissions. Telok Bangko, Loktuan Village, North Bontang, East Kalimantan is an area that has a mangrove ecosystem area, but is also surrounded by industrial areas and settlements. For this reason, this study aimed to assess the mangrove tree species, information on biomass, potential carbon stocks, and CO2 absorption values at that location. Sampling was carried out using the crossed quadrant transect method. At each location the transect was carried out four times. Each transect has four crossed quadrants measuring 10 m x 10 m. In each quadrant, species identification was carried out and then the circumference of the trunk at breast height (Diamter at Breast Height) for mangrove trees (diameter > 4 cm) was measured using a tape measure. Calculation of estimated stored biomass in mangroves using allometric equations using DBH data. Next, estimates of carbon stocks and carbon uptake are calculated. These results will be presented quantitatively descriptive. From this study, 11 species were found, namely Rhizophora mucronata, Avicennia marina, Sonneratia alba, Sonneratia ovata, Derris trifoliata, Ceriops tagal, Lumnitzera racemosa, Xylocarpus granatum, Terminalia catappa, Thespesia populnea, and Wedelia biflora. The results of estimating the carbon stock of the mangrove ecosystem in Bangko Bay were 2236.36 tonnes/ha with a biomass value of 4819.74 tonnes/ha. While in the HGB area, the results of carbon stocks were 2980.48 tonnes/ha with a biomass value of 6423.44 tonnes/ha. Estimated mangrove CO2 absorption in Bangko Bay was 8207.44 tons/ha. Meanwhile, mangrove CO2 absorption in HGB land was 10938.35 tons/ha.


Background
Mangroves are vegetation that grows along the intertidal zone in tropical and subtropical countries [17] [6].Mangrove primary productivity is high so it is known as a good carbon store (carbon sequestrator) [16].This ability plays an important role in the regulation of climate mitigation because the ability of mangrove sequestrations can be utilized to absorb CO2 anthropogenic emissions [15] [10].
The carbon stored in the mangrove ecosystem is called "blue carbon".This term actually refers to carbon in aquatic ecosystems, then the carbon is stored in the form of biomass in coastal swamp ecosystems (saltmarsh), seagrass beds (seagrass), and mangroves.So that this coastal ecosystem has an important role in maintaining the composition of atmospheric gases that affect climate [1] [14].
Wahyudi et al., (2018) reported that mangrove carbon stocks in Indonesia reached 891.70 tons C/ha with a total national mangrove carbon stock of 2.89 Tt C. Indonesia has the largest mangrove forest area in the world, namely 3.49 million ha, but 52% (1 .82 million ha) of which have been damaged.The rate of global mangrove damage is estimated at 0.7 -3% per year [13].
One of the existing mangrove ecosystems in Indonesia is in Telok Bangko, Loktuan Village, North Bontang, East Kalimantan.This area covers 6 hectares.The mangrove ecosystem in this area is used as a conservation area and an educational tourism area.Based on a survey report by BIKAL, BEBSiC, and the Bontang City Environmental Agency (2003) the mangrove vegetation that dominates the mangrove forests and beaches of Bontang City are mangroves (Rhizophora sp.), Api-Api (Avicenia sp.), tancang (Brugueira sp. ), Sea Creepers (Sonneratia, sp.), Cemara (Casuarina sp.), and Teruntum (Luminitzera littorea).Therefore, the mangrove ecosystem in this area has the potential to store carbon.
However, the area around Telok Bangko has experienced many developments so that the area has been used as an industrial area, ponds, construction of public facilities (ports, recreation) and settlements.The effect of development is the resulting carbon emissions.For this reason, this study aimed to assess the mangrove tree species, information on biomass, potential carbon stocks, and CO2 absorption values at that location.

Methodology
The research was conducted on October 18 2022 in Telok Bangko, Loktuan Village, North Bontang, East Kalimantan.Sampling was taken in 2 (two) places representing the condition of the mangrove ecosystem, the first is the Telok Bangko Mangrove (near the mangrove planting location) (00°09'52.95"N and 117°28'45.18"E) which has an area of 6 Ha.The second is in Mangrove HGB land (near the mangrove planting location) (00°09'43.45"N and 117°28'37.24"E) with an area of 10 Ha.Sampling was carried out using a non-destructive method.
Aboveground and belowground biomass carbon stocks can be calculated from the habitus of mangrove trees with a circumference of ≥ 10 cm measured at breast height (GBH, 1.37 m) and identified down to the species level.The diameter of the tree is calculated by dividing the thickness by π [19].Allometric equations developed by [9] for mangrove species in Southeast Asia were used for estimation of above-ground biomass (Wtop) and underground biomass (WR).Wtop = 0.251ρD 2.46   WR = 0.199ρ 0.899 D 2.22   Information: Wtop: Aboveground biomass (g) WR: Biomass below ground level (g) ρ: specific gravity of wood (g/cm3).Data on the ρ wood of each tree species were obtained from Komiyama (2005) and from the World Agroforestry Database (www.worldagroforestry.org)D: tree diameter (cm) Analysis of carbon stocks is calculated by the formula: C= B x %Corganic where: C: carbon content of biomass (kg) B: total biomass (Wtop + WR) (kg) %Corganic: the percentage value of carbon content, is 0.464 [8].The estimated value of CO2 absorption is known by using a comparison of the relative molecular mass of O2 (44) and the relative atomic mass of C ( 12) [11] so that CO2 absorption = 3.67 x carbon stock is obtained.
Sampling was carried out by drawing a 100 m transect line and then making a 10x10 m square (4 quadrants).Identification was carried out on each plot, counting the number of trees stands, saplings and seedlings [2].Mangrove biomass data is obtained from measuring the DBH (Diameter Breast Height) of trees.Analysis of estimating mangrove biomass and carbon stock using allometric equations [9].

Results and Discussion
So far, at least 202 species of mangroves have been recorded in Indonesia, including 89 species of trees, 5 species of palms, 19 species of climbers, 44 species of herbaceous soil, 44 species of epiphytes and 1 species of ferns.Of the 202 species, 43 species (including 33 species of trees and several species of shrubs) were found to be true mangroves, while other species were found around mangroves and were known as associated mangrove species [20].
Based on table 2, the types of mangroves found in Telok Bangko found five mangrove species, all of which belong to true mangroves, namely Rhizophora mucronata, Avicennia marina, Sonneratia ovata, Ceriops tagal, and Xylocarpus granatum.While the mangroves found in the HGB land are nine species which are divided into five true mangroves namely Rhizophora mucronate, Avicennia marina, Sonneratia alba, Lumnitzera racemosa, and Xylocarpus granatum and four associated mangroves namely Derris trifoliata, Terminalia catappa, Thespesia populnea, and Wedelia biflora.While the distribution of habitus in the two lands found habitus of trees, saplings and seedlings.In Telok Bangko, 314 trees were found, while in the HGB land, 554 trees were found.Tree habitus makes the biggest contribution to the process of carbon sequestration, especially in the trunk.The largest organic matter in plants is stored in stems, especially in the first and lowest stem segments in flowers and fruit [12].So that stems can be used to estimate the carbon potential of a stand and are widely used as the basis for calculating the potential for estimating carbon in a stand.This is closely related to the dimensions of the tree, namely diameter at breast height (Dbh) as an indicator in measurement activities [7].The results of the estimation of the carbon stock of the mangrove ecosystem in Telok Bangko were 2236.36 tons/ha with a biomass value of 4819.74 tons/ha.While in the HGB area, carbon stocks yielded 2980.48 tons/ha with a biomass value of 6423.44 tons/ha.

Figure 1. Graph of Biomass and Carbon Stock Estimation
From these results, it can be seen that the carbon stocks in the HGB area are greater than those in the Telok Bangko area.This is presumably because it is influenced by the number of individual trees found.Habitus of trees in HGB land was found to be more abundant than in Telok Bangko.So, this can affect the amount of biomass in the two regions.
The high or low value of biomass and carbon will also affect carbon sequestration activities.From the calculation of the ability to absorb carbon, it shows that the ability of mangroves in HGB land is Mangroves as the main constituent of forests, have an important interaction with carbon dioxide (CO2), especially in leaves.This is because the process of photosynthesis requires CO2 as a source.The process of photosynthesis of plants will absorb CO2 in the atmosphere and convert it into organic compounds.The results of photosynthesis are used by plants to carry out growth in horizontal and vertical directions, which are funded by increasing diameter and height.So that it can be said that plant biomass will increase along with plant growth [4].The addition of plant biomass will be followed by the addition of carbon stock content [3].Empirical studies state that an increase in the content of bound carbon stocks above the soil surface is affected by an increase in biomass.The pattern of relationship between biomass and carbon stock has a maximum positive correlation (R) value [18].
The existence of trees in large numbers in forest areas makes the function of forests as carbon sinks indispensable.This will be sustainable in the long term as long as the forest is still dominated by trees.The contribution of forests that consume carbon must always be considered by all parties in managing it, so that carbon estimates can continue to be balanced, in addition to the need for studies on carbon dioxide to support it [12].

Conclusion
The conclusion of this study is that from this study 10 species were found, namely Rhizophora mucronata, Avicennia marina, Sonneratia alba, Derris trifoliata, Ceriops tagal, Lumnitzera racemosa, Xylocarpus granatum, Terminalia catappa, Thespesia populnea, and Wedelia biflora.The results of estimating the carbon stock of the mangrove ecosystem in Bangko Bay were 2236.36 tons/ha with a biomass value of 4819.74 tons/ha.While in the HGB area, the results of carbon stocks were 2980.48tons/ha with a biomass value of 6423.44 tons/ha.Estimated mangrove CO2 absorption in Bangko Bay was 8207.44 tons/ha.Meanwhile, mangrove CO2 absorption in HGB land was 10938.35tons/ha.