5.3. 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