indonesia

Seagrass meadows form a significant coastal habitat throughout the Indonesian Archipelago, extending from intertidal to subtidal, along mangrove coastlines, estuaries, and shallow embayments, as well as coral-reef platforms, inter-reef seabeds and island locations.  Seagrass meadows in Indonesia play a vital role in supporting coastal marine communities and in maintaining diverse flora and fauna. They are an important component of coastal fisheries productivity¹ and they play an important role in maintaining coastal water quality and clarity.  The seagrasses of Indonesia are also important food for marine green turtles and dugongs².

Indonesian seagrasses often form extensive mixed or monospecific meadows. Mixed seagrass communities composed of 8-9 species are common in many coastal areas in Indonesia. Along coastlines dominated by mangrove forests, seagrass communities often provide a functional link and a buffer between the seaward coral reefs and the inshore mangroves. Seagrasses are integral components of Indonesian reefs (i.e., fringing, barrier, patch and atoll), where they occupy a variety of habitats. They are most often found in shallow-water back-reef environments (eg reef flats, moats) and lagoons where they achieve their highest abundances. However, in some locations they dominate the reef crest of barrier reefs and atolls.

It is generally acknowledged that eight genera and 13 species of seagrass inhabit Indonesian coastal waters ³. These include: Cymodocea serrulata, Cymodocea rotundata, Enhalus acoroides, Syringodium isoetifolium, Halodule pinifolia, Halodule uninervis, Halophila spinulosa, Halophila decipiens, Halophila ovalis, Thalassia hemprichii, Halophila minor , Thalassodendron ciliatum and Ruppia maritima. The R. maritima records are based on specimens at Herbarium Bogoriense collected from Ancol-Jakarta Bay and Pasir Putih-East Java, but have never been reported since (the development of Jakarta has destroyed the mangrove forest in Ancol). A 14th species Halophila beccarii, although similarly known from a specimen at the Herbarium Bogoriense, was thought to exist in Indonesian waters, but there is no information on where it was collected and it has not been found in the field⁴. No endemic species have been found⁵.

Seagrass diversity in Indonesia is similar to adjacent countries (eg Philippines=13 species, Papua New Guinea=13 and northern Australia=15^6,7,8 even though habitat diversity is among the highest in the world^9,4.

The distribution of seagrass throughout the Indonesian Archipelago is not completely known with vast areas including the Papua coast unsurveyed¹¹. At least 30,000 km2 are known to occur throughout the Indonesian archipelago⁵. Short et al. ¹¹ identified nine factors that influence the distribution of seagrasses. These include: light, water depth, tide and water movement, salinity, temperature, human impacts, climate change, availability of viable reproductive material (eg seeds and fragments) and competition from other plants. In the tropics, key seagrass habitats occur on shallow fringing reef platforms and sheltered shallow bays where distribution is also driven by the physical micro-topography of the location. Although studies on the geographical distribution of seagrasses throughout the Indonesian Archipelago are scarce, the location of species is undoubtedly a consequence of all or some of these factors.

It has also been suggested that species diversity in Indonesia may be partly a consequence of the oceanic currents that effectively form geographic harriers to seagrass dispersal. High-velocity currents (e.g., >5.0 m.sec-1; e.g., Sape Strait) flowing between the islands through numerous narrow straits (usually in a north-to south direction) may explain some longitudinal differences in species composition of shallow-water benthic communities, especially along the Great Sunda Arc⁹.

Seagrasses in the Indonesian archipelago occur either as monospecific or mixed communities. In monospecific seagrass meadows any one of the 13 known species can dominate the community, which may be a small patch (i.e., <1 m2) on an intertidal reef flat or a dense and extensive subtidal meadow (i.e., > 100 m2). Thalassia hemprichii, Enhalus acoroides, Halophila ovalis, Halodule uninervis, Cymodocea serrulata and Thalassodendron ciliatum usually grow in monospecific meadows and muddy substrates on the seaward edges of mangroves often have meadows of high biomass⁴. Mixed species meadows are typically dominated by pioneering species such as Halophila ovalis, Cymodocea rotundata and Halodule pinifolia. These meadows occur in the lower intertidal and shallow subtidal zones in sandy, stable and low relief sediments⁴.

Among the 13 species, besides R. maritima, T. ciliatum has a distribution limited to only in the south eastern part of Indonesia, and H. spinulosa and H. decipiens have been recorded in only a few locations^9,4.

While seagrasses predominately grow on soft substrata, Thalassodendron ciliatum is often found attached to hard rock and coral limestone at the seaward margin of reefs (i.e., fringing reefs to atolls). With its strong woody rhizomes and roots, it is able to root in a variety of sediment types, including coral rubble. Along the fringing reef of the Nusa Dua and Sanur coast (south Bali) the inner walls of the well-developed grooves of the reef crest, which run almost perpendicular to the incoming swell, are heavily overgrown by T. ciliatum. It appears that in all high energy environments, T. ciliatum has shorter branched stems than in sheltered lagoonal environments (i.e., calm, turbid), where it occurs in mixed communities⁹.

The two other species with relatively limited distribution are Halophila spinulosa and H. decipiens. H. spinulosa and H. decipiens often form monospecific meadows in subtidal environments. H. spinulosa has been recorded from Riau, the Sunda Suait, the east Java Sea, Lombok and Papua, while H. decipiens has been recorded from locations in Jakarta Bay to the Flores Sea⁹. The location of collection sites suggests that they may be widely distributed, however as these species are adapted for less accessible deep water (>10m) or lower light environments, they are often overlooked during surveys.

The other species of genus Halophila found in the Indonesian archipelago, H. ovalis and H. minor, are in contrast widely distributed. H. ovalis has a wide vertical range and occurs from the intertidal zone down to depths greater than 20m¹³. It is a pioneering species often found growing on recently disturbed sediments⁴. In the high eulittoral, it is often associated with the smaller, more delicate, H. minor, which is frequently buried under the sediment and difficult to observe.

The most abundant seagrass species in Indonesia are Thalassia hemprichii and Enhalus acoroides^14,15. Monospecific meadows of Thalassia hemprichii are the most widespread throughout Indonesia and occur over a variety of habitats and substrate types and large vertical range from the intertidal down to the lower subtidal zone (Brouns 1985). It is frequently the most abundant species of high energy intertidal reef flats with sandy to coarse rubble substrates. T. hemprichii often forms monospecific meadows at the seaward margin of intertidal reef flats where it is subjected to waves and high velocity tidal currents. In these environments the blades are short, but the root/rhizome network is extensive providing a strong anchorage that stabilises and consolidates otherwise loose sediments.

In many sheltered and muddy environments in the Indonesian Archipelago, monospecific stands of Enhalus acoroides occur¹⁵. E. acoroides is the structurally largest seagrass species and is widespread throughout the archipelago. It has been found in a number of environments, ranging from intertidal reef flats to mud-banks adjacent to mangrove forests⁴). Leaf size of E. acoroides is significantly larger in coastal than offshore meadows¹⁷, biomass and shoot densities are higher and epiphyte cover lower, factors attributed to the less severe environments fluctuations of offshore meadows⁴.

Two species of Cymodocea (C. rotundata and C. serrulata) occur in the Indonesian Archipelago. C. rotundata appears more common than C. serrulata, and often forms extensive monospecific meadows in the lower eulittoral. It is highly tolerant to sub-aerial exposure, and is one of the most common seagrasses associated with the intertidal reef flats of fringing reefs, barrier reefs and atolls. C. rotundata is found in a number of reef habitats, but it is most abundant in shallow-water lagoons of wide fringing reefs. For example, it is one of the dominant seagrasses in the shallow lagoon of the extensive Sanur Reef along the southeast coast of Bali. It colonizes a variety of sediments ranging from coarse sands as well as rubble areas where it stabilizes and consolidates the substrate.

Cymodocea serrulata is found mainly in subtidal environments to a depth of ~5m. It is able to grow on a variety of substrates, ranging from silty mud to coarse coral rubble. In relatively sheltered environments with substrates consisting of medium-grained coral sands it was found to form extensive and dense monospecific meadows. In lagoonal mixed seagrass communities, C. serrulata may be the dominant species. C. rotundata and C. serrulata have higher below ground biomass when growing in established mixed meadows than in monospecific pioneering meadows⁴.

Both species of Halodule are found in Indonesia. H. uninervis and H. pinifolia have similar distributions and both are widespread throughout the archipelago. Waycott et al.¹⁸ suggested that H. uninervis and H. pinifolia are conspecific, recognising that the plasticity of blade size is attributed to local conditions. Nevertheless, in Indonesia we retain them as separate entities, as there is no sufficient evidence from Indonesian material to support this merger. Future studies, both ecological and molecular, would help to clarify this. H. uninervis tends to be locally more abundant, but extensive monospecific meadows of H. pinifolia are not uncommon, and occur mainly on muddy or fine-grained calcareous sands². H. uninervis density also depends on the phenotype (wide and narrow leaves². H. uninervis has a wider depth range (lower eulittoral to 8-10 m) than H. pinifolia, which is usually found in a narrow zone between the middle eulittoral to subtidal (1-2 m).

H. uninervis was observed to form monospecific meadows on exposed inner reef flats as well as on steep sediment slopes consisting of silty to coarse-grained sand⁴. H. uninervis is generally considered as a typical pioneering species able to rapidly colonize newly available substrates. H. uninervis has higher below ground biomass when growing in established mixed meadows than in monospecific pioneering meadows¹⁴. H. uninervis is also a preferred food for dugong^20,2.

Syringodium isoetifolium has a wide distribution throughout the archipelago. It is basically a subtidal species, being very sensitive to exposure and desiccation^19,21. The sensitivity to desiccation is most likely related to its leaf morphology. Most seagrasses are tolerant of desiccation for relatively long periods, because their broad leaves overlap one another, thus creating areas where water is trapped and high relative humidity is maintained. The cylindrical leaves of S. isoetifolium do not offer this type of protection, and therefore S. isoetifolium is seldom found above the low spring tide mark.