Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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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