Estimation of seagrass blue carbon stocks in Ahmad Rhang Manyang and Ujung Pancu Waters, Aceh Besar District

. The blue carbon was concepted by the United Nations Environment Program (UNEP) in 2009, refers to the ability of marine and coastal ecosystems such as mangrove, seagrass, and tidal marshes to capture and store carbon dioxide through photosynthesis. This study aims to analyze the estimation of seagrass blue carbon stock and sequestration in Ahmad Rhang Manyang and Ujung Pancu. Data were collected using the plot transect method to assess cover, density, biomass and use model calculations called Seagrass Carbon Converter (SCC) to assess seagrass carbon stocks. Three species of seagrass were found at four research stations are Halodule pinifolia, Halophila ovalis, Halophila minor . The result showed that seagrass cover in Ahmad Rhang Manyang waters was 27.95% in the poor category and in Ujung Pancu waters was 39.4% in the medium category.. The seagrass carbon stock in Ahmad Rhang Manyang 123.93 gram C/m 2 and in Ujung Pancu was 231.43 gram C/m 2 . The seagrass carbon absorption value in Ahmad Rhang Manyang was 4.16 TC/Ha/year (tons of carbon per hectare per year) and Ujung Pancu is 15.5 TC/Ha/year.


Introduction
The blue carbon was concepted by the United Nations Environment Program (UNEP) in 2009, refers to the ability of marine and coastal ecosystems such as mangrove, seagrass, and tidal marshes to capture and store carbon dioxide in their biomass and sediment through photosynthesis [1].These ecosystems can take in carbon faster than the terrestrial ecosystems [2].
Seagrass captures and stores carbon dioxide from the atmosphere through diffusion system or the process of transferring CO2 from the atmosphere to the sea which is then used for the photosynthesis process [3,4].The great carbon sequestration potential by seagrasses plays the important role in reducing carbon dioxide levels in the atmosphere, thereby offering a viable way for climate change mitigation (5,6).
Research on seagrass blue carbon has been carried out in several regions of Indonesia, where the carbon storage ranges from 0.52 -65.98 tons C/ha [7][8][9].However, there has been limited research on this topic within Aceh waters.Therefore, conducting this research is important to collect data inventory, which can serve as a foundational resources to take the action to conserve and preserve seagrass ecosytem.
Several place in Aceh Waters such as Pulo Aceh Island, Matahari Island, Lamteng Bay, Ujung Pancu and Ahmad Rhang Manyang Bay covered with seagrass beds.The research locations in Ahmad Rhang Manyang waters and Ujung Pancu waters were chosen based on consideration of the closest access that could be reached by researchers.This study aims to analyze the estimation of seegrass blue carbon stock and sequestration in those areas.

Seagrass Coverage and Density
Seagrass data collection was carried out using the Seagrass Net parallel transect plot method (Fig 2) [10].The transect was used based on the consideration of the seagrass meadows area at the research location where the seagrass grows horizontally along the coastline, with the length of the transect lines adjusted.The sampling point was taken at 5 meters after the first vegetation was found from the shoreline [11].Visual observations of seagrass cover were carried out on 50cm x 50cm plots beside the 50m transect line.

Seagrass Biomass and Carbon
Seagrass samplesfor biomass (3 -5 shoots) were taken to the laboratory and separated into above ground parts (leaf blades and leaf midribs) and below ground parts (roots and rhizomes).The samples that have been separated were dried using an oven at 60 o C for 48 hours and weighed until reached constant dry weight [12].After that, the biomass that has been obtained is used to find the carbon value through the equations.

Seagrass Coverage
Seagrass coverage is the percentage of seagrass canopy that covers the subtrate area.The calculation for percentage used the equation [13]: After the seagrass cover for each location is obtained, the seagrass health status can be determined based on Decree of Minister of Environment No. 200 of 2004 which is classified into 3 categorizes (Table 1).

Seagrass Density
The density of seagrass for each species was calculated by dividing the number of seagrass shoots by the plot quadrant area of 0.25 m 2 using the following equation [14]: where D is density (shoots/m2), N = number of shoot, A = plot area

Seagrass Biomass
Biomass sampling was carried out on seagrass shoots which were then separated into two parts consisting of above-ground biomass (AGB) and below-ground biomass (BGB).The calculation for biomass used the equation [12]: =  (5)

Carbon Stock and Sequestration
Seagrass carbon stock is the amount of carbon that can be absorbed and stored in seagrass body tissues.In this research, carbon storage estimation were carried out using the Seagrass Carbon Converter (SCC) developed by LIPI using an approach to biomass content, cover and seagrass density in one area, the formula used is as follows [15]: The assessment of seagrass condition is determined through the canopy percent cover and its classification is aligned with the categories stipulated in Decree of Minister of Environment no.200 of 2004.Seagrass condition in Ahmad Rhang Manyang was categorized as damaged/poor with a value of 27.95%, while in Ujung Pancu was categorized as moderate/unhealthy with a value of 39.40% (Fig 3).

Fig. 3. Seagrass Cover in Rhang Manyang and Ujung Pancu
The seagrass density can be seen in table 3. Based on this data, the lowest seagrass density was obtained at station 2 with 368.44 individuals/m 2 in the Ahmad Rhang Manyang.The highest seagrass density was obtained at station 3 with 846.11 individuals/m 2 in Ujung Pancu waters.Both waters are dominated by Halodule pinifolia.

Table 3. Seagrass Density
Ujung Pancu waters exhibited higher seagrass cover and density than in Ahmad Rhang Manyang waters.This discrepancy can be attributed to various factors including fishing activities, seagrass extent, and substrate composition.The waters of Ahmad Rhang Manyang possess a more limited expanse of seagrass due to their proximity to the coral reef system, consequently influencing the average values of cover and density.In contract, Ujung Pancung predominantly features a muddy subtrate type, impacting nutrient absorption and subsequently influencing segrass growth.Generally, sediments that contain mud are generally richer in organic material than sandy sediments [16].The results of biomass analysis show that seagrass in Ujung Pancu exhibits higher below ground biomass at 423.13 grams C/m 2 and above ground biomass with a value of 321.28 grams C/m 2 , in contrast to the seagrass in Ahmad Rhang Manyang where the below ground biomass at of 227.44 grams C/m 2 and 159.7 grams C/m 2 for above ground biomass (Fig 4).

Seagrass
The largest seagrass carbon stock were obtained from seagrass beds in Ujung Pancu waters at 231.43 grams C/m 2 and the smallest seagrass carbon stocks were obtained from seagrass beds in Ahmad Rhang Manyang at 123.93 grams C/m 2 (Fig. 5).

Fig. 5. Seagrass Carbon Stock
Carbon sequestration is the ability of seagrass to store carbon over a long period of time.Observation results show that carbon stock in tonnes per hectare each year in Ahmad Rhang Manyang waters at 4.16 tonnes of carbon per hectare per year (TC/ha/year) and in waters Ujung Pancu at 5.15 tons of carbon per hectare per year (TC/ha/year) (Fig 6).These values show overall that seagrass in Ujung Pancu waters store more carbon each year than Rhang Manyang waters.Seagrass in Ujung Pancu exhibits higher carbon stock and sequestration than segrass in Ahmad Rhang Manyang.A large biomass value influences the amount of carbon stock and sequestraion in seagrass stands.Seagrass with a large biomass will store large carbon stocks [17].The related topic has also been studied at Megiat Beach, Nusa Dua Bali where the total above ground carbon were lower than the below ground carbon [18].The higher carbon stock in the above ground part likely attributed to its placement within the substrate, rendering it less susceptible to direct impacts from fluctuations in water conditions such as temperature and sunlight exposure [19].Also, the smaller substrate size has a positive effect on the system for absorbing nutrients and organic materials.

Conclusion
There are two seagrass species found in Ahmad Rhang Manyang waters, namely Halodule pinifolia and Halophila ovalis and also two species in Ujung Pancu waters namely Halodule pinifolia and Halophila minor.Seagrass condition in Ahmad Rhang Manyang waters is categorized as damaged/poor with a value of 27.95%, while in Ujung Pancu waters is categorized as moderate/unhealthy with a value of 39.40%.The largest seagrass carbon stock were obtained from seagrass beds in Ujung Pancu waters at 231.43 grams C/m 2

2. 1
Study Area This research was conducted in May and June 2023 at Ahmad Rhang Manyang waters and Ujung Pancu waters, both are located in Aceh Besar Regency (Fig 1).The laboratory work was carried out at the Marine Chemistry and Fisheries Biotechnology Laboratory, Faculty of Marine Affairs and Fisheries, Syiah Kuala University.

Table 2 .
Seagrass Distribution There were two seagrass species found in Ahmad Rhang Manyang, namely Halodule pinifolia and Halophila ovalis and two species in Ujung Pancu namely Halodule pinifolia and Halophila minor (Table2).The two research locations are categorized as associated seagrass meadows.