The potential of seagrass ecosystems in Waai and Liang coastal waters on Ambon Island as carbon storage sites

Seagrass ecosystem is one of the coastal ecosystems that can absorb carbon and subsequently store it at a high capacity. This research aimed to study carbon stocks in biomass based on seagrass type and density, differences in carbon stocks between below-ground and above-ground, and the potential for carbon storage of the seagrass ecosystem on the coast of Waai and Liang, Ambon Island. Five seagrass species species were found, including Enhalus acoroides, Halophila ovalis, Halodule pinifolia, Thalassia hemprichii and Cymodocea rotundata,. Based on the results, halassia hemprichii was predominant in Liang waters and Halodule pinifolia was predominant in Waai waters. The highest seagrass biomass in both the below ground and above ground was found for Cymodocea rotundata in Waai waters and Thalassia hemprichii in Liang waters. The total carbon stock in Liang waters was 39.75 gC/m2, whereas that in Waai waters was 56.46 gC/m2. The results showed that the total carbon stock below ground was much greater than above ground, suggesting that more carbon is stored below seagrass substrate than above it in both Waai and Liang coastal waters.


Introduction
The concentration of carbon dioxide (CO2) in the atmosphere has been increased since the Industrial Revolution, along with the development of human civilization, causing global warming.According to Global Monitoring Laboratory (GML) National Oceanic and Atmospheric Administration (NOAA) data, the global CO2 concentration increased to 418.39 ppm in April 2022, from 415.82 ppm in April 2021, or to an annual increase of 2.57 ppm.Moreover, the rate of increase in CO2 in Indonesia was 0.1988 ppm/month for the period 2012-2018, and statistical tests showed that the rate of increase in CO2 accelerated over time [1].This can exacerbate global warming and cause serious impacts, one of which is making coastal areas more vulnerable to beach erosion and rising sea levels.Moreover, adaptation in coastal areas is more difficult due to the limited availability of data on the adaptive capacity of coastal ecosystems [2].
Approximately 93% (40 Teratons) of CO2 on Earth is stored in the ocean, so the ocean plays an essential role in the carbon cycle through photosynthesis [3].Carbon sequestration by marine ecosystems occurs through biological processes such as photosynthesis.Plankton, mangroves, and seagrass meadows are among the marine and coastal ecosystems that function as carbon absorbers.1329 (2024) 012006 IOP Publishing doi:10.1088/1755-1315/1329/1/012006 2 Seagrass meadows can store 2.52 ± 0.48 Megagrams of carbon per hectare (MgC/ha) of living seagrass biomass globally [4].This absorbed carbon is then stored and transported by seagrass to various components, such as the sediment where the seagrass grows and the biomass.
Seagrass communities are found in several coastal areas around Ambon Island, including Tantui, Tanjung Tiram, Hative Besar, Passo, Suli, Liang, and Waai.Five species of seagrass have been found in those areas: Cymodocea rotundata, Halodule pinifolia, Halophila ovalis, Enhalus acoroides and Thalassia hemprichii.However, information on the potential carbon storage on Ambon Island is still limited.Research on carbon storage on Ambon Island has been conducted in Waai village by [5], who focused on two seagrass species, E. acoroides and T. hemprichii, with carbon storage values ranging from 112.38-126.34gC/m 2 for E. acoroides and from 9.31-11.38gC/m 2 for T. hemprichii, indicating the potential for carbon sequestration in that location.However, conducting research in other areas is important to estimate Ambon Island's carbon sequestration potential.This study aimed to study the carbon storage potential of seagrass ecosystems on biomass and substrate based on the species and density levels and to examine the distribution of carbon storage in Waai and Liang coastal waters.The results of this research are expected to provide further information on the potential of seagrasses on Ambon Island and to assist in coastal management.

Methods
Sample collection was taken from two stations located in seagrass ecosystem in Waai and Liang coastal zones (Figure 1) on October 28-29, 2023, using two sampling stations.Each station was divided into 5 line transects with 5 different sampling points.Line transects are positioned perpendicular to the coastline, each transect line is spaced 50 meters apart and placed a quadratic transect with a dimension of 50 x 50 cm with the distance between plots is 20 m from the edge of the beach to the shore.
Laboratory analysis, including seagrass and sediment sample preparation, was carried out at the Geology Laboratory, Center for Deep-Sea Research, The National Research and Innovation Agency (PRLD BRIN) in Ambon, while sediment fraction analysis and organic carbon analysis of seagrass and sediment were conducted at the Productivity and Aquatic Environment Laboratory (ProLing) IPB University.

Figure 1. Research location map 2.1. Seagrass density
The formula in [6] was used to calculate seagrass density, by calculating the number of seagrasses stands in the unit area.Seagrass species density was measured by installing a 100 m transect line parallel to the shoreline and then calculating 0.5 × 0.5 m quadratic transects at points 0, 20, 40, 60, 80, and 100 m.

Seagrass biomass
To calculate seagrass biomass, the formula from Duarte [7] was used:  =  ×  B is the seagrass biomass (grams/m 2 ), W is the dry weight of the seagrass (grams), and D is the species density of the seagrass (inds/m 2 ).

Carbon content
The carbon content of tissue samples, including leaves, rhizomes, roots, and sediment, was determined using the Walkley and Black method [9].
The carbon content in seagrass tissue was determined using the method described in [8] and [9] as follows: where B is the ml titration blank, A is the ml titration sample, and 12/4000= the milliequivalent weight of C in grams.
Then, to calculate carbon storage, the following formula was used: SC (gC/ 2 ) =   (%) x B (g.  −2 ) where SC is the seagrass carbon stock (gC/m 2 ), Corg is the organic carbon content of the sample (%), and B is the biomass (gram dry weight).

Environmental parameters support seagrass life
In the coastal waters of Ambon Island, various types of typical ecosystems can be found, such as mangrove ecosystems, brackish swamps, estuaries, seagrasses, and seaweed [10].The characteristics of the aquatic environment are important factors for supporting the survival of biota or organisms in a marine environment.
A previous study estimated that the seagrass area in the waters of Waai Village reached 58.538 m 2 (± 5.85 Ha) [11].In this seagrass area, seagrass density was found to be diverse and ranged from 49 to 203 stands/m 2 .E. acoroides and T. hemprichii had high densities in dense and sparse seagrass areas, but H. ovalis was not found in dense seagrass areas.H. ovalis and H. uninervis are the main diets of Dugong, so the presence of these two species indicates that the seagrass ecosystems of Waai coastal waters are the feeding grounds for Dugong.In addition, 34 fish species from 31 genera and 26 families were found to be diverse in said seagrass ecosystem.However, in the seagrass ecosystem in Liang coastal waters, 5 seagrass species were found with the most dominant species being C. rotundata [12].  1 shows the standard values of temperatures, sanility and pH for seagrass life.The temperature parameters at the research location ranged from 30 to 32°C (Table 1).Based on the Quality Standard of PP RI No. 22 of 2021, the optimum temperature for seagrass is 28 -30°C, while the temperature tolerance value for seagrass growth ranges from 26 -36°C [13]; therefore, the temperature at the research site is favorable for seagrass growth.The salinity at the research site ranged from 29 -30‰.
Variations in salinity values at each transect of the research site did not significantly differ.The pH at the research site ranged from 7.1 -8.1.The pH value at this location supports seagrass life.The results found were different from a prior study [11], where the water temperature in seagrass areas of Waai coastal area was ranged from 24 to 28°C and the salinity was ranged from 24 to 30 ‰; these results are also different from another study [14], where the temperature of the coastal waters of Waai station ranged from 28.30°C and the salinity was 28.63‰.Additionally, it is also different from a previous report (temperature 29.5 -29.9°C, salinity 33‰, and pH 8.1) [15].This difference could be explained by the sampling period when measurements took place: May-August 2014 [11], March 2016 [14], and March and April 2016 [15].The period of data collection in previous studies covered the end of the northwest monsoon to the beginning of the southeast monsoon period, where water characteristics, especially temperature were lower than those of the data collected in this study, which took place in transitional season 2.
The coastal areas of Waai have different sediment profiles, dominated by fine (muddy and sandy) and coarse (sandy gravel, gravelly sand, and gravel) textures [16].Liang coastal area has a substrate type that consists of dead coral fragments and sand [12].Substrate type is a crucial component of seagrass ecosystems because it provides habitat and supplies nutrients.The fractionation results for the seagrass substrate revealed different sediment distributions in each layer at both research locations.At Waai station, the sediment fraction was dominated by silt, with an average of 45.25-78.70%,followed by clay and sand fractions at 2.00-12.74%and 13.25-52.75%,respectively (Figure 2).Moreover, at Liang station, the sediment fraction was dominated by seagrass, with an average of 51.76-85.81%,followed by clay at 1.59-17.37%and sand at 12.54-31.96%

Halodule pinifolia was predominant in Waai and Thalassia hemprichii in Liang coastal areas
This study found five species of seagrasses (Table 2) from the Hydrocharitaceae family, consisting of T. hemprichii, E. acoroides and H. ovalis, and from the Cymodoceaceae family, consisting of H. pinifolia and C. rotundata species.At both research locations, the seagrass meadows had a mixed composition or multispecific bed consisting of 2 to 5 species of seagrasses.The seagrass species with the highest distribution was Cymodocea rotundata.Each seagrass species has different characteristics in influencing productivity rates and the quantity of carbon stored in seagrass ecosystems, making seagrass species one of the variables affecting the amount of stored carbon reserves.Research conducted by [17] in the waters of Mandiri village, Morotai Island Regency, obtained similar results; C. rotundata was the species with the highest composition.Accordingly, this was likely due to the habitat that was suitable for the life of this species, especially the type of substrate.It was previously observed that C. rotundata is found in shallow waters on mud and sand substrates [18].This species of seagrass can grow on various types of substrates ranging from muddy to coarse coral fragments and forms wide and dense monospecific species.C. rotundata also has high adaptability to open water or not too submerged in water so that it can grow abundantly [19].In Waai coastal waters, H. pinifolia had the highest species density, ranging between 237.33 and 1,357.55ind/m 2 (Figure 3), while E. acoroides had the lowest, with a value of 2-10 ind/m 2 .H. pinifolia is a pioneer seagrass species that grows quickly and is commonly found in muddy or sandy areas.The characteristics of the Waai coastal area allow for a high population of this species at that location.This species can be found in 85 out of 423 locations in Indonesia [20].
Moreover, the highest species density was found in T. hemprichii, with a value of 229,33-496 ind/m 2, and the lowest was found in E. acoroides, with a value of 0,67-4 ind/m 2 .T. hemprichii seagrass is most common in Liang coastal waters because this species can live on all substrates types, varying from coral soft substrates, and can be found on mud but will become dominant on sandy substrates [21].T. hemprichii also can survive waves because this species has thick and wide leaves and a rhizome that is strong enough to withstand waves [22].According to [20], the seagrass species T. hemprichii is the most prevalent seagrass species discovered in 423 locations in Indonesia.

Seagrass biomass influences its carbon storage
Figure 4 shows that the above-ground (AG) biomass of C. rotundata (189.04 gDW/m 2 ) was greater than that of the other species found at the Waai station (transect W2).Similarly, the highest belowground (BG) biomass (190,5 gDW/m 2 ) was found for the same species at W2.Moreover, the lowest seagrass biomasses, both in the AG and BG, were found in Halodule ovalis.In general, the total seagrass biomass (AG+BG) (Figure 5) obtained in Waai coastal area ranged between 115.77 and 621.37 gDW/m 2 , with an average value of 299.668 gDW/m 2 .The results show that seagrass biomass is higher in the BG than in the AG, with a ratio of 64.30:35.70%.The fact that C. rotundata had the highest biomass may also be influenced by its high density in Waai coastal waters.Seagrass density and morphology also affect biomass [23].A greater number of rhizomes and roots that penetrate the substrate will produce pore space in the substrate that can aid in nutrient absorption in rhizomes and substrates so that the biomass of a seagrass species will also increase [24].
In the Liang coastal waters, T. hemprichii biomass had the highest value compared to the other species found in L1, namely 165.6 gDW/m 2 in the AG and 236.7 gDW/m 2 in the BG.The seagrass species with the lowest biomass value was H. ovalis.The total seagrass biomass obtained in Liang coastal waters ranged from 24.85 -461.52 gDW/m 2 , with an average value of 202.652 gDW/m 2 .Seagrass biomass tended to be greater in the BG than in the AG, with a ratio of 61.77:38.23%.

Liang coastal waters
The value of seagrass biomass below ground (BG) is relatively high since it is not disturbed by environmental changes or by marine biota, while the lower value of biomass above the substrate (AG) occurs because this part is food for some marine biota [25].The seagrass biomass value when compared to the carbon stock value should have a directly proportional value, where if the seagrass biomass value of a species is high, the seagrass carbon stock value of that species will also be high.According to [26], this occurs because the values of biomass and carbon stock are determined by the same factors, namely, the density value and morphological size of the species.The results show C. rotundata had the highest biomass and carbon stock values in Waai station, whereas T. hemprichii had the highest biomass and carbon stock values in Liang station.This result is different from that of [27] for Pamegaran Island, where although C. rotundata has the highest biomass value in that location, T. hemprichii had the highest carbon stock, presumably due to the influence of significant morphological size.

Carbon stocks in below ground (BG) were greater than those above ground (AG)
The seagrass carbon in the study site was stored both in above ground (AG) and below ground (BG).In Waai coastal waters, C. rotundata had the highest AG carbon stock value, which was especially common in W2, with a value of 6.84 gC/m 2 .The highest BG carbon stock value was found for the seagrass species H. pinifolia in W2, at a value of 6.88 gC/m 2 (Figure 6).The results show that the total carbon stock (AG + BG) in Waai station varied among the transect measurements.The highest value was found in W2, at 23.99 gC/m 2 , while the lowest value was found in W4, at 3.54 gC/m 2 , with an average total carbon stock value of 11.29 gC/m 2 .
In Liang coastal waters, T. hemprichii seagrass species found in L1 had the highest carbon stock values for both AG (5.13 gC/m 2) and BG (6.49 gC/m 2 ).The high carbon stock value of this seagrass species is determined by the high density in Liang station that allows this species to store carbon.In contrast to that in Waai, C. rotundata had the lowest carbon stock value in this location.Overall, the highest total carbon stock value was found in L1, with a value of 13.60 gC/m 2 , while the lowest was found in L2, with a value of 0.88 gC/m 2 and an average of 7.83 gC/m 2 .
The results showed that carbon stocks were mostly stored at the bottom of the substrate (BG).This result confirms previous report [28], in which the BG carbon stock contributed 60% and the AG carbon stock 40%.Figure 7 shows that the highest carbon stock was in the BG.The BG carbon stock value is determined by factors such as the size of seagrass rhizomes and roots, while the AG carbon stock is influenced by the size of leaves and stems.These results are like those of previous studies in different locations, such as [27] in Pamegaran Islandand and [29] in Moreton Bay, Australia, where carbon stocks in the BG were greater than those in the AG.

Carbon storage potential of Waai and Liang seagrass ecosystems
The amount of carbon stock indicates the amount of CO2 that can be absorbed by an ecosystem.The greater the carbon stock value of an ecosystem is, the greater the role of the ecosystem in absorbing and storing it.The total carbon stock value (AG + BG) of Waai coastal waters varies across measurement transects, with a total carbon stock value of 56.46 gC/m 2 (Figure 8), while in Liang, the average total carbon stock is 39.33 gC/m 2 .The results of this study revealed a carbon stock value of 259.32 gC/m 2 in T. hemprichii and E. acoroides seagrasses in Waai station [5].This carbon stock value is also lower than that reported in the results of research on two other waters on Ambon Island, namely, 301.10 gC/m 2 in Tanjung Tiram Waters, Poka [30] and 447.28 gC/m 2 in Suli Waters [31].
In several locations in Ambon Island waters, seagrass ecosystems have experienced changes in both seagrass species composition and cover [32].This is due to the influence of human activities and natural factors (currents and waves).In the waters of Galala, seagrass ecosystems are under pressure due to the construction of the Merah Putih Bridge, which has caused damage to the ecosystem, and seagrass cover has decreased due to excavation, sedimentation, and changes in currents.In Tanjung Tiram waters, the 'bameti' activity or the activity of collecting shellfish and shrimp at low tide damages seagrasses because they are trampled or stripped.However, in Waai station, the seagrass area is close to the river mouth, and market activities, community housing, and sea transportation, which are also common in Liang.This can cause a decrease in seagrass area.A reduced seagrass area will affect the carbon stock value.

Conclusion
The results of this study indicate the potential of seagrasses on the coast of Waai and Liang to store carbon to mitigate climate change.This can be seen from the seagrass density on the Waai coast dominated by H. pinifolia, which ranged from 237.33−1357.55ind/m 2 , and on the Liang coast dominated by T. hemprichii (229.33-496ind/m 2 ).The total carbon stock (AG+BG) in Waai Waters was 56.46 gC/m 2 , and that in Liang Waters was 39.75 gC/m 2 , with higher carbon stock values found below the substrate (BG) than above the substrate (AG).The results showed a smaller carbon stock value than did previous research conducted at the same location in Waai and other places on Ambon Island; thus, efforts are needed to maintain and preserve seagrass ecosystems in Waai and Liang.

Acknowledgment
We thank the Indonesia Endowment Fund for Education (LPDP) for funding this research as a part of JVTH Thesis scholarship.

Figure 2 .
Figure 2. Distribution of sediment texture in Waai and Liang coastal waters

Figure 3 .
Figure 3. Seagrass density in a. Waai and b.Liang coastal waters

Figure 7 .
Figure 7.Total carbon stock percentage of each transect above ground (AG) and below ground (BG) in a. Waai and b.Liang coastal waters

Figure 8 .
Figure 8.Total carbon stocks in a. Waai and b.Liang coastal waters

Table 1 .
Temperature, salinity, and pH measurements in each transect at the Waai and Liang stations