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Оглавление5.3. Seagrass Ecosystems of Papua
LEN MCKENZIE, ROB COLES, AND PAUL ERFTEMEIJER
SEAGRASS MEADOWS form a significant coastal habitat in the Papua coastal region, extending from intertidal to subtidal, along mangrove coastlines, estu-aries, and shallow embayments, as well as coral-reef platforms, inter-reef seabeds and island locations. Seagrasses are a functional grouping of vascular flowering plants that have adapted to the nearshore soft bottom environments of most of the world’s continents. Most are entirely marine although some species cannot reproduce unless emergent at low tide. Seagrass are among the few plants that have migrated back to the seas roughly 100 million years ago during the Cretaceous (den Hartog 1970). Seagrasses probably evolved from a freshwater hydro-phyte (a plant adapted to growing in water or inundated soil) or salt marsh-type primitive stock (den Hartog 1970). A well developed seagrass flora may have existed by the end of the Cretaceous period (Larkum and den Hartog 1989). The earliest fossil records from Malesia (Indonesia, Borneo, and New Guinea) are from well preserved fossils of Cymodocea serrulata described from Miocene deposits northeast of Makassar, South Sulawesi (Larkum and den Hartog 1989).
Seagrasses can survive in a range of conditions including freshwater, estuarine, marine, or hypersaline. There are relatively few species globally (about 60) and these are grouped into just 13 genera and five families. The greatest diversity of seagrasses occurs in the Indo-Pacific region. Global seagrass distribution has been described for most species (den Hartog 1970; Phillips and Menez 1988; Spalding et al. 2003). There is now a broad understanding of the range of species and seagrass habitats although shallow subtidal and intertidal species distributions are better recorded than seagrasses in water greater than ten meters below mean sea level. Surveying deeper water seagrass is time consuming and expensive and it is likely that areas of deep water seagrass are still to be located (Lee Long, Coles and McKenzie 1996).
Short, Coles, and Pergent-Martini (2001) in reviewing the world distribution of seagrasses identified the islands of the southwest Pacific and Indian Ocean, including Papua, as areas where knowledge of seagrass habitats are less well known. Papua is, however, included in the Indo-Pacific Region IX (Short, Coles, and Per-gent-Martini 2001), which has the largest number of seagrass species worldwide and a high species diversity of associated flora and fauna. Short, Coles, and Per-gent-Martini (2001) reported 13 species from Papua New Guinea, 16 species from the Philippines, and 16 species from neighboring northern Australia. Humoto and Moosa (2005) reported that eight genera and 13 species of seagrass inhabit Indonesian coastal waters. These include Cymodocea serrulata, Cymodocea rotun-data, Enhalus acoroides, Syringodium isoetifolium, Halodule pinifolia, Halodule uninervis, Halophila spinulosa, Halophila decipiens, Halophila ovalis, Thalassia hemprichii, Halophila minor, Thalassodendron ciliatum, and Ruppia maritima. Halophila minor was originally reported as H. ovata, but taxonomists now regard H. ovata in the Indo-Western Pacific as only present in the South China Sea and Micronesia (Kuo 2000). The R. maritima record was based on a specimen at Her-barium Bogoriense collected from Jakarta Bay and has never been reported again. A 14th species, Halophila beccarii, although similarly known from a specimen at the Herbarium Bogoriense, was thought to exist in Indonesian waters, but has not been found in the field (Kuriandewa et al. 2003). Among the 13 species, besides
R. maritima, T. ciliatum has a distribution limited to only in the eastern part of Indonesia, and H. spinulosa and H. decipiens have been recorded in only a few locations.
Importance of Seagrass
Seagrasses rank as one of the major marine ecosystems in the world. In the last few decades, seagrass meadows have received greater attention with the recognition of their importance in stabilizing coastal sediments, providing food and shelter for diverse organisms, as a nursery ground for fish and invertebrates of commercial and artisanal fisheries importance, as carbon dioxide sinks and oxygen producers, and for nutrient trapping and recycling. Seagrass meadows are rated the third most valuable ecosystem globally (on a per hectare basis, behind estuaries and swamps/floodplains) and the average global value for their nutrient cycling services and the raw product they provide has been estimated at US$19,004 per hayr (in 1994 dollars) (Costanza et al. 1997).
Seagrasses are also food for the endangered Green Sea Turtle (Chelonia mydas) and Dugong (Dugong dugon) (Lanyon, Limpus and Marsh 1989), which are found throughout the seas surrounding Papua, and used by traditional communities for food and ceremonial use. Tropical seagrasses are also important in their interactions with mangroves and coral reefs through fluxes of particulate and dissolved substances, physical interactions, and animal migrations. Along coastlines dominated by mangrove forests, seagrass communities often provide a functional link and a buffer between the seaward reefs and the inshore mangroves. Each of these systems exerts a stabilizing effect on the environment, resulting in important physical and biological support for the other communities. Seagrasses slow water movement, causing suspended sediment to fall out, and thereby benefit corals by reducing sediment loads in the water.
Factors That Influence Seagrass Distribution
The distribution of seagrass in Indonesia is not completely known, with vast areas, including the Papua coast, unsurveyed. At least 30,000 km2 of seagrass meadows are known to occur throughout the Indonesian archipelago (Kuriandewa et al. 2003). Short, Coles, and Pergent-Martini (2001) 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 propagules, 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 microtopography of the location.
Seagrass habitats in Papua are determined by factors that vary among regions and among seasons. It is likely that the distribution of seagrass on the muddy mangrove-lined southern coast of Papua is determined by different factors than that of seagrasses on the reef platforms surrounding coastal islands. Little is known about much of this region but it would be safe to assume that of the nine factors, human impacts, propagule availability, and climate change would have only limited influence, and that determinates of distribution would be suitable bottom type, light availability (depth and turbidity), temperature and exposure to drying, and tide and water movement (including protection from waves). Given the low population density of Papua, human impacts on seagrasses here are probably less severe than on other Indonesian islands, but there have been reports of destruction of seagrass meadows by trawl fishing in Cenderawasih Bay, and mass migration of dugongs into the Torres Strait prompted by a die-off of seagrass meadows in Papua (Marsh, Harris, and Lawler 1997; Putrawidjaja 2000).
Growth and abundance of seagrasses is likely to be higher inshore due to higher nutrient levels rather than in nutrient poorer offshore waters (Kuriandewa et al. 2003). In the high rainfall tropics with a distinct monsoonal wet season, the sea-grass distribution will be influenced by seasonal pulses of sediment laden, nutrient-rich freshwater discharges and run-off (Carruthers et al. 2002). Seasonal freshwater inputs will also determine which seagrass species can survive.
On reef platforms and in lagoons the presence of pooling water at low tide prevents drying out and enables seagrass to survive tropical summer temperatures which would otherwise cause seagrasses to desiccate (Stapel, Manuntun, and Hemminga 1997). The sediments in these locations are often unstable and their depth can be very shallow, restricting seagrass growth and distribution.
A complex set of interactions may impact a single region, including the type of habitat, the time of year, and the species growing. While little is known about long-term natural cycles in the abundance and distribution of seagrasses in Papua, seagrasses nearby in the Torres Strait and northern Queensland, where similar species occur, show abundance trends positively related to nutrient availability, and negatively correlated to sediment input and to years of high storm and freshwater disturbance (Carruthers et al. 2002).
Most Papuan seagrasses are found in water less than ten meters deep and meadows can be monospecific or may consist of multispecies communities, with up to ten species present at a single location. It is generally agreed that of the 13–14 seagrass species in waters around Indonesia itself (Humoto and Moosa 2005), at least 11 are recorded from Papua (S. isoetifolium, C. serrulata, C. rotundata, H. pinifolia, H. uninervis, H. minor, H. spinulosa, H. ovalis, T. hemprichii, E. acoroides, and T. ciliatum); see Table 5.3.1.
Unlike neighboring Australia, where structurally small species (e.g., members of the genera Halodule and Halophila) comprise the majority of the coastal nearshore seagrass meadows, Papuan seagrass are dominated by structurally large seagrasses (e.g., the genera Thalassia, Enhalus, and Cymodocea). Seagrasses have the ability to act as a biosink for nutrients, sometimes containing high levels of tissue nitrogen and phosphorous. Macro-grazers—Dugongs (Dugong dugon) and Green Sea Turtles (Chelonia mydas)—may also be an important feature in structuring seagrass communities in Papua.
Seagrass habitats along the coastline of Papua and associated reefs can be generally categorized into four main habitats (Table 5.3.2), similar to those in tropical northern Australia (see Carruthers et al. 2002). These four broad groups of sea-grass habitats are river estuary, coastal, reef, and deep water. In their natural state, these habitats are characterized by low nutrient concentrations, are primarily nitrogen limited, and are influenced by seasonal and episodic coastal runoff. All seagrass habitats in Papua are influenced by high disturbance and are both spatially and temporally variable. However, the spatial and temporal dynamics of the different types of seagrass habitat are poorly understood. Each of these four habitat types has a number of dominant processes that influence seagrass growth, survival, and community biodiversity.
River estuary habitats can be subtidal or intertidal, contain many seagrass species, and are often highly productive. In Papua, these habitats are closely associated with mangrove forests, characterized by fine sediments, and prone to high sedimentation and anoxic conditions. The dominant influence of river estuary habitats is terrigenous (from the land) runoff from wet-season rains. Increased river flow results in higher sediment loads that combine with reduced atmospheric light to create potential light limitation for seagrass (McKenzie 1994). Associated salinity fluctuations and scouring make river and inlet habitats a seasonally extreme environment for seagrass growth. Catchments to river estuary habitats often support a large range of land uses, including agricultural, mining, and forestry (logging). These land use practices result in increased sediment inputs (Spalding et al. 2003).
In river estuary systems, differences in the life history strategies of seagrasses results in varying species assemblages. E. acoroides is a slow turnover, persistent species with low resistance to perturbation (Bridges, Phillips, and Young 1981; Walker, Dennison, and Edgar 1999), suggesting that there are some coastal habitats that are quite stable over time. However, E. acoroides is susceptible to disturbance and it is predicted that removal of a 1 m2 area from a meadow would take more than 10 years for full recovery (Rollon et al. 1998). In contrast, C. serrulata, H. uninervis, and H. ovalis are more ephemeral (Birch and Birch 1984). H. uninervis and H. ovalis are considered pioneer species growing rapidly and surviving well in unstable or depositional environments (Bridges, Phillips, and Young 1981; Birch and Birch 1984). C. serrulata grows in deeper sediments, and has been linked to increased sediment accretion (Birch and Birch 1984).
Coastal habitats are both subtidal and intertidal and support the most diverse seagrass assemblage of all habitat types. Physical disturbance from waves and swell, associated sediment movement, and macro-grazers primarily control seagrass growing in coastal habitats. Episodic events such as cyclones or storms can have severe impacts at local scales, making this a dynamic and variable habitat. Sediment movement due to prevalent wave exposure creates an unstable environment where it is difficult for seagrass seedlings to establish or persist. Areas of seagrass that have been physically removed by a cyclone can take many years to regrow (Preen, Lee Long, and Coles 1995). Succession or recolonization after extreme loss has been suggested to be directional and modified by small-scale perturbations, resulting in patchiness in seagrass distributions (Birch and Birch 1984). Cymodocea and Syringodium are seen as intermediate genera that can survive a moderate level of disturbance, while Halophila and Halodule are described as ephemeral species with rapid turnover and high seed set, well adapted to high disturbance and high rates of grazing (Walker, Dennison, and Edgar 1999). The end result of this successional process, however, varies with geographic location.
Reef habitats support seagrass communities of high biodiversity and can be highly productive. Fringing reef platforms are almost always intertidal. Shallow unstable sediment, fluctuating temperature, and variable salinity in intertidal regions characterize these habitats. Nutrient concentrations are generally low in reef habitats, however intermittent sources of nutrients are added by seasonal runoff and seabirds. The primary limiting nutrient for seagrass growth (either phosphate or nitrogen) in carbonate sediments can vary between geographic locations around the world (Short, Dennison, and Capone 1990; Fourqurean, Zieman and Powell 1992; Erftemeijer and Middelburg 1993; Udy et al. 1999). Tight nutrient recycling strategies of T. hemprichii (e.g., the location of nitrogen in the rhizomes), aids in survival in the nutrient-poor reef habitat when leaves are shed due to desiccation stress (Stapel, Manuntun, and Hemminga 1997). Reef seagrass communities also have unique faunal interactions. Bioturbation by shrimps can be so prevalent in some reef environments as to prevent seagrass growth (Ogden and Ogden 1982; Tomascik et al. 1997). A region of bare sand often separates coral heads from seagrass meadows; previous research suggests this is maintained by parrotfish and surgeonfish associated with the coral (Randall 1965).
Deep water seagrasses occur at subtidal depths greater than 10 m, and are restricted to where high water clarity allows sufficient light penetration for photo-synthesis (Lee Long, Mellors, and Coles 1993). Deep water seagrass areas can be extensive and dominated by Halophila species (Lee Long, Mellors, and Coles 1993; Lee Long, Coles, and McKenzie 1996). Large monospecific meadows of seagrass occur in this habitat (e.g., Halophila decipiens), which contrasts with coastal and reef habitats where the seagrass meadows are generally diverse and mixed (Coles et al. 1987). Halophila species display morphological, physiological, and life history adaptations to survive low light conditions. Halophila species have rapid growth rates and are considered opportunistic species (Birch and Birch 1984). H. decipiens has an open canopy structure with relatively little below ground biomass and high leaf turnover and rhizome elongation rates (Josselyn et al. 1986; Kenworthy et al. 1989). Halophila species also have high seed production. For example, Kuo and Kirkman (1995) reported H. decipiens seed banks of 176,880 seeds per m2. The distribution of deep water seagrasses, while mainly influenced by water clarity, is also modified by seed dispersal, nutrient supply, and current stress. Although the ecological role of deep water seagrasses is poorly understood, some deep water meadows are important dugong feeding habitat (Lee Long, Coles, and McKenzie 1996; Marsh and Saalfeld 1989; Anderson 1994). Unfortunately, deep water systems are the least understood seagrass community.
The four broad groups of seagrass habitats in Papua contain a large range of life history strategies, which provides some insight into the dynamic but variable physical nature of Papuan seagrass habitats. The species present in the different habitats reflect the observed physical and biological impacts, suggesting that reef, deep water, and coastal environments are particularly variable and dynamic, while estuarine habitats have stable areas but are extremely harsh. Of these seagrass habitat types in Papua, both estuarine (including large shallow lagoons) and coastal seagrass habitats are of primary concern with respect to water quality due to their location immediately adjacent to catchment inputs.
Papuan Seagrasses
Papua includes the most eastern province of Indonesia (formerly known as Irian Jaya) and extends west from the northeast border of Papua New Guinea to Halmahera (north Maluku Province). It encompasses the overall north and south coasts and northern offshore oceanic islands. Overall this is a region separated from the main Indonesian archipelago by relatively complex bathymetry, where waters are very deep, and even islands only a few tens of kilometers apart might be separated by depths of over 1,000 meters (Spalding, Raviolus, and Green 2001). The only areas of relatively extensive shallow water and true continental shelf are a platform west of the Vogelkop Peninsula and to the south where Papua shares a common continental shelf with northern Australia. Surface currents are somewhat mixed in this region, however a northward current flows between Papua and Halmahera and an eastward current flows along the north shore of Papua during the northeast monsoon. This pattern reverses during the southeast monsoon.
The Raja Ampat Archipelago includes the four large islands of Waigeo, Batanta, Salawati, and Misool and hundreds of smaller islands. Ecological Rapid Assessments through the Raja Ampat Islands in 2001 and 2002 visited a total of 45 and 59 sites, respectively, surveying coral, mangrove, seagrass, other marine habitats, and turtle nesting beaches (McKenna, Allen, and Suryadi 2002; Donnelly, Neville, and Mous 2003). Substantial shallow seagrass meadows of T. hemprichii, C. rotun-data, and H. uninervis were reported on Sayang Island, in the bays on the southeastern side of Kawe, on reefs off the northern Waigeo coast and at Deer Island off the northern coast of Kofiau (Donnelly, Neville, and Mous 2003; Hitipeuw 2003). In the south of the archipelago, extensive seagrass meadows of E. acoroides and S. isoetifolium have been reported on the reef flat of Batanta Island (Tomascik et al. 1997; Scheltze-Westrum 2001). Seagrasses were also recorded at Kri Island, Pef Island, Waigeo Island (Mayalibit Passage), Wruwarez Island, the northwest side of Batanta Island, North Fam Island, Batang Pele Island, Wofah Island and Yeben Kecil Island (McKenna, Allen, and Suryadi 2002).
The seagrasses reported from northern coast of the Vogelkop, east of Sorong District, include S. isoetifolium, C. serrulata, C. rotundata, H. pinifolia, H. spinulosa, H. ovalis, T. hemprichii, and E. acoroides (Tomascik et al. 1997, Kuriandewa et al. 2003; see Table 5.3.3). There is however, little or no information describing the reef communities further east around Vogelkop Peninsula.
An ecological Rapid Assessment of Biak and the Supiori Islands in 1996 found nine seagrass species (T. hemprichii, C. rotundata, C. serrulata, H. uninervis, H. pinifolia, E. acoroides, H. ovalis, H. minor, and S. isetofolium; MREP 1996), of which C. rotundata and T. hemprichii were the most widely distributed, creating high density monospecific meadows (1,276 shoots per m2; Kuriandewa et al. 2003).
A prominent feature of the reefs in the Padaido Islands (south of Biak Island) is the presence of extensive reef-top seagrass meadows (530 ha) dominated by Cymodocea spp., E. acoroides, and T. hemprichii (Tomascik et al. 1997). The western extremity of the Padaido Islands has a dense coverage (95–100%) of seagrass over 529 ha of shallow reef flat, consisting of seven species (T. hemprichii, C. rotundata, C. serrulata, H. uninervis, H. pinifolia, E. acoroides, and H. ovalis). Similarly, extensive reef top seagrass meadows (T. hemprichii, C. rotundata, H. ovalis, and H. pinifolia) have been reported on Numfoor Island (Tomascik et al. 1997) and along the southern coast of Yapen Island.
Cenderawasih Bay National Park (established in 1994) is the largest marine park in Southeast Asia and the only marine park in the region. Extensive lagoonal seagrass meadows (T. hemprichii, C. rotundata, H. uninervis, E. acoroides, and H. ovalis) are present along the mainland coast of southwestern Cenderawasih Bay, particularly in Wandammen Bay (Nietschmann et al. 2000). The vast seagrass meadows in this bay are reported to harbor a large dugong population (Petocz, 1989). Maruanaya (2000) reported three species of seagrass in the same region covering an area of 24 ha on the seaward side of mangrove areas with average density of 56 shoots per m2. Fringing reefs surrounding the many small islands in the region are also covered with seagrass meadows, including Pepaya Island (near Nabire) (Chou et al. 2002).
Virtually nothing is known of marine ecosystems along the north coast of Papua. The north coast is almost reef free and continues as such from Cenderawasih Bay to Sarmi, with only very occasional areas of fringing reef about some of the small islands (Spalding, Raviolus, and Green 2001). Further east, fringing reefs are believed to follow a large proportion of the coastline to the border with Papua New Guinea. For the most part these are poorly described, but reef flats are estimated to reach 300–400 meters wide in places. Tomascik et al. (1997) reported six seagrass species (C. rotundata, E. acoroides, H. ovalis, C. serrulata, T. ciliatum, and T. hemprichii) on the near continuous fringing reefs from Jayapura to the border with PNG.
Relatively little is known about the seagrasses along the shores of southern Papua. This area of the coastline has extensive mangrove forests. Over half the area of mangroves in Indonesia are located in Papua (Spalding, Blasco, and Field 1997). Bintuni Bay, which contains more than 1.1 million acres of mangroves, is the world’s third largest mangrove area and the second largest in Asia.
However, the only seagrass known in the bay is an anecdotal report from Berau Bay (the west side of Bintuni Bay) (Jamartin H. S. Sihite, pers. comm.). No sea-grasses were located anywhere else within Bintuni Bay or within the open, deeper waters (up to 60 m) towards McCluer Gulf (Erftemeijer, Allen, and Zuwendra 1996). This is possibly a consequence of the high turbidity throughout Bintuni Bay: secchi depths (a parameter used to measure the clarity of surface waters) of 11 to 85 cm in the mangrove area (creeks and rivers) and maximum 157 cm in the open waters of the bay (Erftemeijer, Allen, and Zuwendra 1996). Unfortunately Bintuni Bay marine ecosystems are increasingly threatened by overharvesting, logging, and clearing to make way for coastal shrimp farm facilities. Although the Bintuni Bay Nature Reserve affords some protection, there are no seagrasses in the Reserve and economic development is increasing due to a new liquified natural gas field in the bay, and the human population is expanding rapidly.
Although there are other significant areas of mangroves and wetland areas with sago palms in the gulf near Timika, Mimika district, around the Asmat region, and surrounding Yos Sudarso Island, Merauke (Spalding, Blasco, and Field 1997), the presence of seagrass communities is unknown. Large amounts of sediment are found along the southeastern coast (apparent in remote images; see http://eosweb.larc.nasa.gov/; http://eol.jsc.nasa.gov/), which possibly prohibit reef development in this region. Significant land clearing, logging, and mine tailings may exacerbate sedimentation, further prohibiting seagrass growth in localized areas. For example, large tracts of mangrove were also cleared at the mouth of the Timika River to construct the Amamapare seaport and mine tailings are now polluting nearby coral reefs. As part of the Freeport-McMoRan Copper & Gold Inc. mine Long-Term Environmental Monitoring Program, monitoring of benthos occurs at 14 sites in the estuaries (e.g., Minajerwi River) and 40 sites in the Arafura Sea (http://www.fcx.com/envir/wtsd/2004/env-perform.htm). The monitoring indicates no impact of tailings on the marine benthos in the Arafura Sea outside of the tailings management area.
It is likely however, that seagrasses are present throughout this region as significant seagrass meadows surround the nearby Aru Islands, a group of about 95 low-lying islands (8,563 km2) in the Arafura Sea (Moluccas), southwest of Papua. These meadows are predominately E. acoroides, T. hemprichii, C. rotundata, and S. isoetifolium. H. decipiens has also been reported from the deeper waters in the north of the islands (Nietschmann et al. 2000) and possibly extends northward to the Papuan mainland coast. Expansive seagrass meadows, which support significant Green Sea Turtle populations, also surround the adjacent Kai Islands, Kai Kecil and Kai Besar (Suárez 2001).
South of this region are the expansive seagrass meadows of the Torres Strait. The Torres Strait is a shallow (mostly 10–20 m depth) body of water formed by a drowned land ridge extending from Cape York to southwestern Papua New Guinea. Seagrass communities occur across the open sea floor, on reef flats and subtidally adjacent to continental islands. The large expanses of open water bottom are covered with either sparsely distributed Halophila or mixed species (Halodule, Thalassia, and Syringodium) communities (Coles, McKenzie, and Campbell 2003). It is likely that these meadows may extend northward to the Merauke coast, but surveys (e.g., Long et al. 1995) have not included this region.
Johnstone (1982) considered that seagrass zonation, where it occurs, was fairly similar across New Guinea and seems to be determined by comparable biotic and abiotic parameters. It can be safely assumed that such zonation would also be relevant in Papua, where species and habitats are similar. From intertidal to subtidal, the zonation pattern of seagrasses generally begins with a zone of one or two species (mostly H. uninervis, H. pinifolia, or H. minor). Subsequently, in the lower eulittoral zone, other seagrass species join in a mixed seagrass meadow generally dominated by C. rotundata, H. uninervis, and T. hemprichii, with isolated patches of H. ovalis. In the upper sublittoral zone, the mixed seagrass meadow is dominated by T. hemprichii and E. acoroides, with isolated patches of S. isoetifolium, C. serrulata, and H. uninervis. The lower edge of the meadow consists of a combination or two to four species when a reef plateau is present, or monospecific H. decipiens or H. spinulosa at the deepest depths on the sublittoral sandy slopes. The remaining species are less common and not widely distributed. Monospecific patches of T. ciliatum have been reported to occur on coral rubble banks in 6–8 meters depth on the deeper edges of the reef slopes (e.g., Jayapura; Johnstone 1982). Monospecific patches of T. ciliatum are also common on reef edges in the nearby Torres Strait between Papua and northern Australia (Coles, McKenzie, and Campbell 2003).
Local conditions may determine which seagrass species are present. Extensive mixed seagrass meadows are the dominant community type in the bays, harbors, and sheltered capes along the coasts of the Papuan mainland and large continental islands. These extensive seagrass meadows are dominated by T. hemprichii and/or
E. acoroides, with up to nine other species present to varying degrees. H. decipiens meadows sometimes occur in the deeper areas and meadows of E. acoroides occur in shallow lagoons and border the gentle sloping mangrove fringes in the more protected bays. This species is common in sheltered bays and on reef platforms throughout the tropics in water depth less than two meters at low tide. This is a species that must be able to reach the surface to pollinate and so is restricted to shallow and sheltered waters. Throughout the rest of Papua most seagrass occurs in shallow lagoons or on the reef platforms and leeward shores of small vegetated islands. These communities are dominated by colonizing and intermediate species, such as T. hemprichii, C. rotundata, and H. uninervis, which can survive a moderate level of disturbance. E. acoroides occurs in small protected bays or behind the reef crest on the sublittoral reef flat, as it has low resistance to perturbation (Walker, Dennison, and Edgar 1999). Smaller islands are generally characterized by relatively small fringing reef platforms, where seagrass communities dominated by C. rotundata and T. hemprichii, with small quantities of H. ovalis, are restricted to locations with shallow lagoons (0–2 m depth).
Heijs and Brouns (1986) studied the Wewak coastline of northern Papua New Guinea, which consists of several bays separated by headlands (capes) with extensive mixed species seagrass communities generally located on the fringing reef platforms from Wewak to Vanimo near the Papuan border. These mixed meadows are dominated by T. hemprichii with E. acoroides, S. isoetifolium, and C. rotundata. Other species such as H. uninervis, H. ovalis, and C. serrulata occur occasionally. On the landward side, seagrass meadows are dominated by a narrow band of H. uninervis mixed with C. rotundata. The seaward side of the meadows are generally of combination of two to four species, which become monospecific H. decipiens in the deeper areas. The distribution of E. acoroides is either interspersed or forming small isolated patches behind the reef crest.
FLORA AND FAUNA ASSOCIATED WITH SEAGRASS
Although few studies have examined the macro- and mega-fauna in seagrass meadows in Papua, some general remarks can be made. The most conspicuous macrofauna is often the abundance of holothurians (sea cucumbers), the most common is the black sea cucumber Holothuria atra. Echinoids (sea urchins and sand dollars) are also common in the mid-and lower eulittoral areas, and the genus Tripneustes is abundant. Asteroidea (true starfish) are abundant, particularly in seagrass meadows with sandy substrate. Reef platform seagrass meadows support a wide range of mollusks, fish, holothurians, and decapods (shrimp, lobster, and crabs). Common gastropods (snails) found associated with seagrasses include Strombidae, Cypraidae, and Conidae. Most of these occur in the eulittoral and sublittoral areas. Other mollusks such as the trochus shell Trochus niloticus found in seagrass meadows are collected as a source of cash income. Similarly the Holothurians have been a valuable source of cash income although now heavily over-fished (Uthicke and Conand 2005).
The existence of productive commercial shrimp fisheries in the coastal waters of the Aru Islands, Moluccas, is largely due to the presence of extensive seagrass meadows in the area. An average shrimp catch of about 490 tons per year has been reported from commercial trawling grounds south of the Aru Islands (Tomascik et al. 1997).
Megafauna such as Green Sea Turtles and dugong depend on the seagrass meadows present throughout Papua which are recognized as significant foraging grounds. Such areas include Cenderawasih Bay, northwest of Biak Island, and Sahul Shelf (Arafura Sea) near the Aru Islands. Many of these locations are adjacent to important nesting beaches for Hawksbill and Green Sea Turtles, such as Ingressau Beach on the northeastern coast of Yapen Island. Many of the dugong and turtle populations supported by the seagrass meadows are also traditionally hunted.
Last but by no means least, an abundant array of fishes uses seagrass meadows’ different tide regimes during parts of their life history. Some fish are herbivorous, feeding either on the seagrass leaves or the epiphytes, such as Siganids. Maruanaya (2000) studied seagrass associated fish in Cenderawasih Bay and recorded 55 sea-grass fish species dominated by sardines (Stolephorus bucanieri), rabbitfish (Siganus canaliculatus), and Gerres kapas. Some indication of the likely use of tropical Pacific seagrass meadows are reports that 154 species of tropical invertebrates and fish feed directly on seagrasses (Klump, Howard, and Pollard 1989), and that Coles et al. (1993) listed and classified 134 taxa of fish and 20 shrimp species found in tropical Australian seagrass meadows. Other fish such as the Lutjanidae (snappers) use the seagrass as shelter when they are juveniles, and some Syngnathids (seahorses and pipefishes) permanently reside or shelter in seagrass meadows. Pyle (1999) lists at least 3,392 fish described as reef and shore fish from the Pacific Islands but it is not possible to distinguish which are from seagrass meadows. However, Allen (2003) reported from the ecological Rapid Assessment conducted of the Raja Ampat Islands, that although the region has one of the world’s richest coral reef fish faunas, other habitats such as silty bays, mangroves, seagrass meadows, and pure sand-rubble areas were consistently the poorest areas for fish diversity. Sea-grass meadows throughout Papua are of significant importance to subsistence fisheries for Siganids (rabbitfish), Hemirhamphidae (garfish), holothurian species, and shellfish.
LOSSES AND THREATS
Tropical seagrass meadows are known to fluctuate in size seasonally and across years (Erftemeijer and Herman 1993; Mellors, Marsh, and Coles 1993; McKenzie 1994; McKenzie et al. 1996), and losses have been reported from most parts of the world, sometimes from natural causes such as cyclones and floods (Poiner, Walker, and Coles 1989; Campbell and McKenzie 2004). More commonly, loss has resulted from human activities such as dredging, land reclamation, industrial runoff, oil spills, or changes in land use and agricultural runoff (Short and Wyllie-Echeverria 1996).
The major changes in Papuan seagrass meadows have occurred since World War II and are related to coastal development, agricultural land use, and population growth. However there is insufficient information and no long-term studies from which to draw direct conclusions about historic trends. Munro (1999) reported that 2,000 year old mollusk shell middens in Papua New Guinea have essentially the same species composition as present day harvests, suggesting indirectly that the habitats, including seagrass habits and their faunal communities, are stable and that any changes occurring are either short-term or the result of localized impacts. Dependence on coastal marine ecosystems for protein remains high and subsistence fishing is widespread.
Localized impacts are likely to occur from sedimentation, that increases turbidity of marine waters, and is related to coastal agriculture (palm oil plantations), land clearing (upland logging and mining), bush fires, and from the discharge of mine tailings (e.g., from Freeport-McMoRan Copper & Gold Inc., in Timika; Coles and McKenzie 2005). Inappropriate coastal development or construction often result in beach erosion. Major impacts result from collecting beach sand for construction materials, construction of airports, hotels, and other structures too close to beaches or in offshore waters, and sand mining. In other locations in Indonesia (e.g., Seribu Islands and the coast of Bali), heavy coral mining and collection from reef flats have resulted in the deterioration of seagrass meadows (Humoto and Moosa 2005).
Other that negatively impact seagrass ecosystems include sewage discharge, industrial pollution, and overfishing. For example, there have been reports that dugong are disappearing from Cenderawasih Bay National Park because the shallow water seagrass meadows are being destroyed by trawl fishing as well as sedimentation resulting from deforestation (Putrawidjaja 2000). Most of these impacts remain localized and relatively small and can be managed with appropriate environmental guidelines. However, in the future climate change and associated increase in storm activity, water temperature, or sea level rise has the potential to damage seagrasses in the region or to influence their distribution.
All identified seagrass habitats have high ecological or economic value, whether supporting fisheries or biodiversity. Estuary/lagoonal and coastal habitats are considered to be the most threatened, due to extensive coastal development. However, the limited knowledge of deeper water seagrass habitats suggests that impacts on these habitats are extremely difficult to assess.
CONSERVATION
Currently there is no legislation in Indonesia that specifically stipulates that the function of seagrass ecosystems should be maintained (Indonesian Seagrass Committee 2002). However, seagrasses do not exist in nature as a separate ecological component and are often closely linked to other community types. Associations are likely to be complex interactions with mangrove communities, algae beds, salt marshes, and coral reef systems. Worldwide, many management activities to protect seagrasses have their origins in the protection of wider ecological systems or are designed to protect the overall biodiversity of the marine environment. The protection of seagrass habitat for species listed as threatened or vulnerable to extinction (e.g., Dugong and Green Sea Turtle), and their importance as habitat for juvenile fish and crustaceans which form the basis of economically valuable subsistence and commercial fisheries, have become motivating factors for the protection of seagrasses.
In Indonesia existing legislation relevant, directly or indirectly, to the management of seagrass ecosystems is considered sufficient for the adequate protection of seagrass ecosystems in the near future. However, there is an urgent need to reach common understanding regarding the vision and mission required to implement these laws in the field (Indonesian Seagrass Committee 2002). Law enforcement is still weak and ineffective; hence pollution and degradation of seagrass ecosystem continue to occur (Indonesian Seagrass Committee 2002).
The Indonesian Seagrass Committee in 2002 assessed many of the problems of legal aspects relevant to seagrass management in Indonesia and made a number of recommendations. These recommendations suggest that the following five steps be taken. First, an institution must be assigned specific authority to coordinate the campaign against pollution and degradation of the sea. Second, many legal acts (including Fisheries, Management of Living Environment, and acts concerning natural resources and their ecosystems) must be revised. Third, terrestrial spatial planning should be integrated with that of coastal areas and the sea on the basis of integrated ecosystems; Provincial and District/Municipality governments should designate new conservation areas in accordance with the land-use plan. Fourth, the division of authority between Provincial and District/Municipality governments in administrative aspects be publicized so that the management of ecosystems are assessed holistically and in integrated manner, and free of bureaucratic complications. Fifth, forest cutting, which directly affects coastal and marine ecosystems (including river banks, greenbelts of dams, lakes, rivers, and coast lines) be prohibited.
Implementation of such recommendations may require several approaches. Coles and Fortes (2001) separated these into three components: a prescriptive legal approach; a non-prescriptive broad-based approach ranging from planning processes to education; and a reactive approach designed to respond to specific issues, such as a development proposal. These approaches may overlap and be used simultaneously in many cases.
Prescriptive management of seagrass issues might range from local laws to a Presidential Decree. In Southeast Asian countries such as Indonesia and in the Pacific Island countries, protection is often strongest at the village or district level by government-supported agreements or through local level management (Coles and Fortes 2001). At the village level, successful enforcement is heavily dependent on community support.
While no international legislation specifically protects seagrass, there are international conventions that recognize the importance of wetlands and coastal areas, such as the Ramsar Convention on Wetlands. In some cases, seagrass meadows have been inadvertently protected because they are located within protected areas, such as Cenderawasih Bay National Park and Kamiali Wildlife Management Area. It is hoped that recognition of the global significance of areas such as Raja Ampat, which also include seagrasses, will also provide some degree of future protection.
Prescriptive management can include establishment of Marine Protected Areas (MPAs). A Marine Protected Area is an area of sea that is dedicated to the protection and maintenance of biological diversity and of natural and associated cultural resources, and is managed through legal and other effective means (IUCN 1994). A Marine Protected Area may be a ‘‘no-take’’ area like a terrestrial national park or it may comprise a multiple-use area, zoned in such a way to minimize conflicts and allow extractive activities to occur in specific areas. Establishing even a small Marine Protected Area is a complex process and includes a needs assessment and requires the involvement of all stakeholders and government agencies in defining a border and specifying permitted uses.
An alternate and complementary non-prescriptive approach is a Locally-managed Marine Area (LMMA). A Locally-managed Marine Area is an area of nearshore waters (including include coral reefs, seagrass meadows, mudflats, mangrove, and other areas) that is actively being managed by local communities or land-owning groups, or is being collaboratively managed by local communities together with local government and other partners based in the immediate vicinity. For example, Yayasan Rumsram, a nongovernmental conservation organization in Biak and the Padaido Islands, is pioneering the use of Locally-managed Marine Areas (LMMAs) in Indonesia through traditional marine resource management and customary prohibition (sasizen) practices.
Non-prescriptive methods of protecting seagrasses generally have a strong extension or educational focus. Providing information is important because it encourages and enables individuals to act voluntarily act in ways that reduce impacts on seagrasses. Actions in response to such information could range from being more aware of the downstream effect of poor agricultural practices to lobbying politicians for stronger sanctions against decisions that lead to seagrass loss. Non-prescriptive methods range from simple explanatory guides to complex industry codes of practice developed in negotiation with the industry in question (Coles and Fortes 2001).
Reactive processes generally occur in response to a perceived operational threat, such as a coastal development proposal. Reactive processes can also include risk management plans that identify areas of seagrass to be protected in the event of an impact (e.g., oil spill or ship grounding). Reactive processes are generally identified in environmental impact statements, which also propose strategies (e.g., redesign, response, or by reducing future risk) to minimize the effects of a development or structure on the coastal environment, including seagrasses. The combination of project redesign in response to environmental impact statements and reactive environment management systems can provide enormous improvements to coastal seagrass protection.
Discussion
A key step in protecting seagrasses in this region will be to obtain better distributional and abundance data and to develop a better understanding of seasonal changes and local ecosystems. At the present time information is patchy at best and it is quite likely that large areas of seagrass could be lost without any formal record. Seagrass dieback has been recorded in nearby waters of the Torres Strait (Long et al. 1995) and is considered of sufficient concern to be a major focus of the Australian Cooperative Research Centre for the Torres Strait.
A survey of 3,000 kilometers of the northern Australia coastline has just been completed (Roelofs, Coles, and Smit 2005) using a helicopter as a cost-effective way of estimating seagrass area, abundance, and species over a large and remote area. Similar methods could provide an up-to-date map of Papuan seagrass with a precision suitable for quantifying future gains and losses.
What is recorded for Papua suggests distribution patterns of seagrasses are comparable to that found in other parts of the Indonesian archipelago, Papua New Guinea, the western Pacific islands, the Philippines, and northern Australia (Coles and Lee Long 1999; Coles et al. 2003; Green and Short 2003). Subsets of the same suite of tropical species occur and the zonation patterns described can be found in similar locations in all the adjoining countries and islands. The threats to sea-grasses are also relatively generic to the region, with local land clearing and resulting sediment run off, mine tailings, inappropriate fishing methods, and nutrients from sewage likely to be the major problems at a local scale. Population and development levels in Papua are generally low at the present time, but as they increase, transport infrastructure development issues will affect coastal seagrasses as they have elsewhere.
Climate change is likely to be the major variable in the medium to long term. Climate change is predicted to raise sea level and seawater temperatures, and to increase carbon dioxide concentrations in seawater. Rising sea levels could increase the distribution of seagrass because more inland areas will be covered by seawater. However, the sediment erosion that is likely to be associated with sea level rise could destabilize the marine environment and cause seagrass losses. Increasing concentrations of carbon dioxide in seawater could increase the area of seagrass because seagrasses will have more carbon available for growth and could increase photosynthetic rates. Increased seawater temperatures might raise the photosynthetic rate of seagrasses. However, in some places, seagrasses are close to their thermal limit and rising temperatures could cause ‘‘burning’’ and tissue death.
To provide an early warning of change, long-term monitoring and community engagement programs have been established as part of the Global Seagrass Monitoring Network (www.SeagrassNet.org, Short et al. 2002; www.seagrasswatch.org, McKenzie, Campbell, and Roder 2001). Establishing a network of monitoring sites in Papua would provide valuable information on temporal trends in the health status of seagrass meadows in this region and provide a tool for decision makers in adopting protective measures. Monitoring encourages local communities to become involved in seagrass management and protection. For example, one of the recommendations for conservation action after the 2002 ecological Rapid Assessment in Raja Ampat was the establishment of monitoring programs, including seagrass monitoring (Donnelly, Neville, and Mous 2003). Working with both scientists and local communities, this approach is designed to draw attention to the many local anthropogenic impacts on seagrass meadows that degrade coastal ecosystems and decrease their yield of natural resources.
Acknowledgments
We thank the participants of the University of New Hampshire and the David and Lucile Packer Foundation–funded Seagrass 3M Workshop: Mapping, Monitoring and Management of Seagrass Resources in the Indo-Pacific, held at The Nature Conservancy, Southeast Asia Center for Marine Protected Areas, Sanur, Bali, 9th to 12th May 2005, for their assistance. We also thank Yayu La Nafie and the members of the Indonesian Seagrass Association (id_seagrass@yahoogroups.com) for their encouragement and support.
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