Species preference for seagrass restoration using vertical distribution model in Tidung Island, Indonesia

Tidung Island is one of the small inhabited islands and a tourist destination that allows the degradation of seagrass meadow and requires physical restoration using transplantation techniques. The seagrass species’ preference suitability needs to assess with this island for this case. This study aims to provide information on species that are possible to choose in seagrass transplantation based on their growth zones. The number of seagrass species, depths, and types of substrates was taken using seagrasswatch guideline by line transects along the coast to the reef slope with 10% plot intervals from the total length of the transect. Seagrass growth is divided into three zones: near the coast (back), middle, and near the reef slope (front). Enhalus acoroides, Cymodocea rotundata, and Thalassia hemprichii were found in almost all zones. Halophila ovalis, Halophila minor, and Halodule uninervis were distributed in the middle to front zone, while Syringodium isoetifolium was only in the middle zone. These findings suggest that transplant areas with high anthropogenic disturbances can use Enhalus acoroides and Thalassia hemprichii because they have the persistent trait. Meanwhile, the areas with low anthropogenic disturbance and low nutrients can use Halophila spp. and Halodule spp., because they can grow shoots and recover quickly.


Introduction
Seagrass meadows provides ecosystem services such as provisioning (fisheries and biodiversity enrichment), regulating (coastal protection, climate regulation, ocean acidification buffer, disease control, improved water quality, and stabilisation of sediment), supporting (primary production), and cultural (tourism and recreational opportunities) [1][2][3][4].Seagrasses' condition and area have recently decreased due to disturbances such as anthropogenic activities and climate changes [5].The disturbances occur continuously, so the seagrass does not have a chance to recover [6].Mass tourism, coastal development, port activities, destructive fishing gears, and marine debris influence seagrass conditions [7,8].
Katwijk et al. [12] represent that the replantation technic failed nearly 60%.One of the failure factors is choosing the seagrass species.Species selection is an essential step for direct seagrass restoration.Seagrass species have persistent, opportunistic, and colonizing traits influenced by vertical seagrass distribution.Thus, assessing seagrass species' preference for transplantation is vital to support the goals of seagrass restoration by direct restoration.

Study area
Tidung Island is located in Kepulauan Seribu Selatan District, Thousand Islands District Administration (Figure 1), which consists of two islands: Tidung Besar Island and Tidung Kecil Island.Tidung Besar is an inhabited island, while Tidung Kecil is a conservation island with an area of approximately 84.53 ha, with 5,157 people in 2020.Tidung Island can access via speed boat from Marina Ancol port for 1.5 hours.Hayati et al. [13] reported that Tidung Island is one of the tourist destinations in the Thousand Islands, with the number of tourists to 149,691 in 2017 and decreasing in 2018 to 141,038 people.Tourism has changed the land use in Tidung Besar for homestays and other tourism facilities.

Seagrass species diversity and distribution
Three replications for each of the eight sampling sites were observed in July 2018, namely L1, L2, L3, L4, L5, L6, L7, and L8 (Figure 1).A total of 3 zones are divided based on the beach-to-reef slope length into three classes: near the coast (back), middle, and near the reef slope (front).Each station has a different length from the beach to the reef slope, ranging from 60 to 172 meters.The number of seagrasses was counted on the shoot of each seagrass species, while species sightings were totaled of seagrass species found in a plot transect from the beach to reef slope with a 10% line interval (Figure 2).Kuo and den Hartog [14] was used to identifying the seagrass species in the field.

Figure 2.
The sampling illustration method using line transect.

Statistical analysis
We analysed the diversity of seagrass species using the Shannon Diversity and Evenness Index among site samplings with replications.Analysis of Variance was performed to compare seagrass species among zones.We proposed a post-hoc pairwise t-test when the p-value was significant (<0.05).We analysed data using PAST 4.03 software [15].The rank of sightings was conducted by the percentage of sightings when the p-value was insignificant.Furthermore, this rank developed to indicate a seagrass species' preference for restoration or transplantation.

Seagrass species diversity
Eight species of seagrass were detected on Tidung Island (Figure 3).Enhalus acoroides, Cymodocea rotundata, Halophila ovalis, and Thalassia hemprichii were the most common seagrass species in all sites sampling.Syringodium isoetifolium, Halodule uninervis, and Halophila minor were observed in three sites sampling, while Oceana serrulata (current name) or Cymodocea rotundata were not noticed in L1, L4, and L8 sites sampling.
The diversity index shows that L1 has the highest species diversity, with as many as six seagrass species (Figure 3).Meanwhile, the lowest diversity was observed in L3, five seagrass species.The diversity index rises with the number of species and the evenness of their abundance.The higher the index, the more diverse the species in the habitat.One species is only present in the community, that is equal to 0 for the diversity index.A habitat's diversity will increase when the evenness index becomes closer to 1.Even though Tidung Island has a high diversity of seagrass species, four are dominant in each sampling location.The dominant seagrass species in all locations were persistent such as Enhalus acoroides, Thalassia hemprichii, opportunistic Cymodocea rotundata, and colonizing Halophila ovalis.4).Syringodium isoetifolium was not detected in the front zone.The type of substrate in the front zone is sand with coral.Syringodium isoetifolium is an opportunistic species that needs many nutrients to produce seeds and seedlings, recover and persist from disturbances.The sand substrate with rubble has high porosity, which cannot trap nutrients well.The persistent species also require many nutrients for growth, although the recovery rate is low to disturbances.Based on the assessment, all zones contained Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, and Halophila ovalis.In addition, E. acoroides was most dominantly found in all zones in each sampling location.It is the most adaptive species to environmental characteristics with muddy sand substrate, sand with rubble, and sand with coral (Figure 4).This species can still survive in the highest and lowest tides that are exposed to the sun without shade.The long leaves provide an advantage for penetration of sunlight for Enhalus acoroides.

Species selection for restoration
The species selection for seagrass restoration is related to the frequency of sightings for the different species in each zone (Table 2).Thalassia hemprichii was sighted in all zones (back, middle, front).Enhalus acoroides is the second species found in all zones.Oceana serrulata, Halophila minor, and Syringodium isoetifolium were unique species because the sighting frequency is low.Environmental gradient impact on substrate type, organic matter content, and water quality that influences the sighting frequency of seagrass species [16].Moreover, environmental gradient affects seagrass morphological plasticity [17].
According to the statistical analysis, there was no significant difference between seagrass species with growth zones (p-value > 0.05).The potential for species restoration lies within all species of seagrass.However, the implementation of seagrass transplantation must pay attention to the source of nutrients.Cause of several species of seagrass is intolerant to high metric organics.Furthermore, sediment type is highly considered for seagrass species with short roots on seagrass transplantation.The growth zone is insignificant for classifying seagrass species, so seagrass sightings are ranked.We selected three seagrass species that are found in all zones.The first is Thalassia hemprichii (37.59%),Enhalus acoroides (27.11%), and Halophila ovalis (16.40%) (Figure 5).Table 2 already shows the number of occurrences of the dominant species in all zones of Tidung Island.These species are a reference for restoring seagrass meadows on Tidung Island.Enhalus acoroides and Thalassia hemprichii are two seagrass species with the greatest adaptability and persistence.At the same time, Halophila ovalis is a pioneer in seagrass meadows and suitable for species' preference for seagrass transplantation.

Discussion
Species selection for seagrass restoration using transplantation needs to be done carefully [18].Seagrass has different levels of adaptation to environmental stressors and disturbances [19].Habitat characteristics are fundamental because they positively correlate with the seagrass species found.Tidung Island is a tourist and residential island that puts pressure on seagrass meadows.The pressure is the nutrient enrichment that impacts the seagrass community's structure.Therefore, the results showed that persistent seagrasses were more dominant than colonizing and opportunistic seagrasses.Based on nutrient requirements, Enhalus acoroides and Thalassia hemprichii need more nutrients so it prefers growth in nutrient-rich habitats.The evidence of nutrient-rich habitat is a muddy sand substrate.This substrate is found in almost all sampling sites.
The relationship between habitat and species selection for seagrass restoration is to determine the species with high adaptive skill with the environment after planting.Seagrass species have persistent, opportunistic, and colonizing traits [20].Enhalus acoroides and Thalassia hemprichii are persistent species.In contrast, Halophila ovalis is colonizing species.These three seagrass species appear more than others in all growth zones (Figure 4).Tidung Island has a robust anthropogenic pressure input on seagrass meadows from residential and tourist activities.In addition, breakwater influences water flushing and affects nutrient enrichment.
The use of seagrass species for restoration needs other topic research to support this result in Tidung Island.Short et al. [21] used the Transplant Suitability Index (TSI) model with some parameters and an experiment improved to select seagrass species.The growth and survival rate of seagrass species are parameters in that model.Thus, the TSI model is used not only for site selection but for species that have already been adaptive at the restoration site.The ecological, economic, and social integration of restoration activities with the transplant method is urgently needed for the sustainability of the restoration program.

Conclusion
Thalassia hemprichii, Enhalus acoroides, and Halophila ovalis are seagrass species' preferences for restoration or transplantation in each zone.These species have highly tolerant and adaptable to environmental changes.Nutrient sources, sediment type, and organic metrics content are considered for the implementation of seagrass transplantation.These findings suggest that transplant areas with high anthropogenic disturbances can use Enhalus acoroides and Thalassia hemprichii because they have the persistent trait.Meanwhile, the areas with low anthropogenic disturbance and low nutrients could use Halophila ovalis because they can grow the shoots and recover quickly.

6 Figure 5 .
Figure 5.The species preferences ranking for seagrass restoration on Tidung Island.

Table 1 .
Diversity and evenness index on Tidung Island.Seagrass species vertical distribution Almost all seagrass species are in the back, middle, and front zones (Figure