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Оглавление5.4. Mangrove Forests of Papua
DANIEL M. ALONGI
TANGROVE FORESTS are one of the major ecosystems within the coastal zone of Indonesia. They develop best where low wave energy and shelter foster the deposition of fine sediments, and are the only woody plants living at the confluence of land and sea. Evidence of their success is the fact that the standing crop of mangrove forests is, on average, greater than any other aquatic ecosystem.
Unlike other tropical forests, mangroves are architecturally simple, being composed of relatively few tree species and often lacking an understory of shrubs and ferns (Figure 5.4.1). Mangrove trees possess morphological and physiological characteristics that make them uniquely adapted to the tidal zone, including aerial roots, salt-excreting leaves, and viviparous water-dispersed young seedlings (i.e., seeds that germinate while still on the parent tree).
Mangroves forests are heavily used traditionally for food, shelter, timber, fuel, and medicine. These tidal forests occupy a crucial niche along the Indonesian coast, as they are a valuable ecological and economic resource. Mangroves provide important nursery grounds and breeding sites for fishes, reptiles, birds, crustaceans, shellfish, and mammals; accumulation sites for sediment, contaminants, carbon, and nutrients; protection against coastal erosion; and a renewable source of wood (Alongi 2002).
Figure 5.4.1. A mature, mixed Rhizophora-Bruguiera forest in the Fly Delta, Papua New Guinea.
Photo: P. Dixon.
This chapter describes the mangrove forests and associated ecosystems in Papua. As the mangroves of Papua are not structurally and functionally different from those in Papua New Guinea, the mangroves of the entire (800,000 km2) oceanic island of New Guinea will be reviewed here. Information about mangroves and their ecology on the other islands of Indonesia can be found in chapters of the other volumes of this series (e.g., Chapter 19 in Tomascik et al. 1997).
Distribution
The island of New Guinea has large tracts of mangrove forest with the greatest species diversity of mangroves in the world due to its location bordering the Australasian and Indo-Malesian centers of diversity (Duke 1992). There may be as many as 43 species in New Guinea (Table 5.4.1) with fewer species on the north coast than on the south coast. This disparity of species richness is indicative of a floral discontinuity between the northern and southern sides of the island. Duke (1992) maintains that this is convincing evidence of a fusion of boundaries between two previously isolated and different mangrove floras. Similar floristic discontinuities have also been described for upland plants on the island (Heads 2001).
These discontinuities are associated with tectonic events of the collision between the Pacific and Australian plates. The southern coast of New Guinea is a part of the stable Australian plate and has been subjected to alternating episodes of submergence and emergence as a result of glaciation that last took place around 18,000 years ago, when sea level was about 100–150 m lower than at present. In contrast, the north coast of New Guinea lies at the northern edge of the Australian Plate, which has remained submerged. Saenger (2002) notes that the mangroves along the northern shore of the island represent more ancient forests than those along the southern coast; the northern flora is derived from the Indo-Malesian mangroves but the southern flora is largely derived from northern Australia. The geographical isolation of the mangrove flora of the southern and northern coasts of the island is maintained by the high mountain ranges which form the backbone of New Guinea (Milliman 1995).
The island of New Guinea contains approximately 34,739 km2 of mangrove forest, of which 13,820 km2 are in Papua and 5,399 km2 are in Papua New Guinea (Darsidi 1984; Soemodihardjo 1986; Soemodihardjo et al. 1993; Spalding, Blasco, and Field 1997; Tomascik et al. 1997). The area of mangroves on the island is subject to considerable uncertainty; the area of Papua mangroves is a crude estimate only, as figures range from 13,000–26,000 km2 (Tomascik et al. 1997). What it is certain is that the mangroves of Papua constitute by far the largest area of mangroves (69–80%) in Indonesia.
Mangrove forests in New Guinea are situated in the deltas of large rivers and along the banks of 253 small and medium rivers (Figure 5.4.2). The large rivers on the island are the Mamberamo, Sepik, Ramu, Markham, Purari, Kikori, Bamu, Fly, Digul, and Palau-Palau rivers, which cumulatively discharge 1.7 billion metric tons of sediment to the adjacent coastal ocean (Milliman 1995). This high fluvial discharge is a result of high rainfall on the island and facilitates the development of large river deltas colonized by extensive inland freshwater and estuarine mangrove forests that can often penetrate quite deeply inland. For example, Sonneratia caseolaris occurs 75 m above sea level and several kilometers inland in southern New Guinea; in the Fly Delta, mangroves can be found 500 km upstream (Saenger 2002) and in Bintuni Bay on the west coast of Papua, mangroves can be found 30 km inland. Often there is little or no discontinuity from the sea to upland forests; the coastal vegetation progressively changes from mangroves to inland freshwater swamp and terrestrial forest (Taylor 1959). Generally, the mangrove forests along the southern and western coasts of New Guinea are more expansive than on the northern and eastern coastlines.
Mangroves thus develop best in areas associated with high rainfall. It is in the largest river deltas, where high rainfall and subsequent runoff transports and deposits mud, that the most luxuriant mangrove forests develop. In dry regions of the island, such as near Port Moresby, mangrove forests are reduced in height and are of lower species diversity (Frodin and Huxley 1975).
Figure 5.4.2. Map of New Guinea showing the major river systems and mangrove forests (blackened areas).
Forest Structure and Zonation
The physical settings of mangrove forests are based on the dominance of key physical characteristics: rivers, tides, waves, and sediment type and origin (Wood-ruffe 1992). The majority of mangrove forests in New Guinea inhabit river- and tide-dominated settings (Cragg 1987), but a great variety of composites of these settings are known (Johnstone and Frodin 1982).
Mangroves are typically distributed from mean sea level to highest spring tide with the most conspicuous feature being the sequential change in species either perpendicular or parallel to shore. Mangroves in New Guinea often consist of narrow crowned trees that can attain 30–40 meters in height, although emergents are common in Nypa palm stands. Rhizophora stylosa and Rhizophora apiculata are emergents in Bruguiera cylindrica and Bruguiera exaristata forests (Figure 5.4.3), with a dense ground layer or understory of Nypa proximate to the river bank. Similarly, Avicennia species are emergents in Ceriops tagal forests. The understory, when present, most often consists of Acanthus ilicifolius, Acrostichum speciosum, Excoecaria agallocha, Dalbergia candenatensis, and Maytenus emarginata, and various lianas and scrambling vines (Johnstone and Frodin 1982).
There are subtle and complex patterns of species distribution across the inter-tidal seascape and upstream-downstream, relating to individual species tolerances to abiotic factors (e.g., soil salinity, nutrient status, degree of anoxia [lack of oxygen], degree of soil wetness) and to biotic factors (e.g., competition, predation). Some of these factors come into play over different temporal and spatial scales to control the distribution of tree species, prohibiting generalizations about the mechanisms governing zonation. Many such physical and ecological variations are often expressed within a single estuary (Duke, Ball, and Ellison 1998). For an individual tree, several factors operate to regulate tree growth, including temperature, nutrients, solar radiation, oxygen, and water.
Figure 5.4.3. A mature (> 30 m tall) mixed Bruguiera forest with a dense understory close to the river bank, Fly Delta, Papua New Guinea.
Photo: D. M. Alongi.
Mangrove forests in New Guinea are often naturally disturbed by storms, lightning, tidal surges, and floods, and may take decades to recover (Johns 1986). For instance, many river deltas in Papua and in Papua New Guinea experience tidal bores which are powerful tidal surges that can sweep up a river to destroy entire forests (Figure 5.4.4). Other natural events, such as disease and pests, may not immediately kill trees but can cause stunted growth, slow death, or the replacement of a species. Dieback of mangrove stands has been observed in Papua New Guinea (Arentz 1988) and attributed to either lightning strikes or periods of drought. Johns (1986) similarly reported the death of stands of mangrove forest in New Guinea, attributed to lightning strikes. Regeneration of mangrove seedlings was recorded, but analysis of aerial photographs suggested that mangrove forests affected by previous events had required over 200–300 years to recover fully.
In the river deltas of New Guinea, the zonation and distribution of some man-grove forests corresponds to the ‘‘classical’’ zonation parallel to shore, but most do not, as various zonation schemes for mangroves have been overemphasized. Brass (1938), Percival and Womersley (1975), Floyd (1977), Paijmans (1976), Paij-mans and Rollet (1977), Green and Sander (1979), Spenceley (1981), Johnstone (1983), and Gylstra (1996) have described zonation of mangroves in various sites around New Guinea and adjacent islands (e.g., the Aru Archipelago). For the mangroves in New Guinea, Johnstone and Frodin (1982) presented a more realistic depiction of patterns of zonation based on the following factors: tidal range and inundation frequency, degree of wave action, drainage, salinity, substrate type, and composition of biota. Zonation patterns are inconspicuous or absent in flat areas, but become more obvious with increasing ground slope, as water depth and frequency of tidal inundation control the seaward limit of mangroves. A good example of the importance of this factor is the mangrove flora of Galley Reach on the southern coast of New Guinea (Paijmans and Rollet 1977) where there are two large-scale zones of ‘‘true’’ mangroves and ‘‘transitional’’ mangroves. Many species occur in both zones, but some species are restricted to either zone: Bruguiera cylindrica, Bruguiera gymnorrhiza, Rhizophora mucronata, Sonneratia alba, Sonneratia caseolaris, and Xylocarpus granatum in the true mangrove zone and Avicennia rumphiana, Exocoecaria agallocha, Heritiera littoralis, Lumnitzera racemosa, Acrostichum aureum, and Acanthus ilicifolius in the transitional zone. The transition is very distinct, suggesting that floral discontinuity is related to the tides.
Figure 5.4.4. The destructive power of tidal bores in the Fly Delta, Papua New Guinea.
Photo: D. M. Alongi.
Drainage and substrate type appear to also be important factors controlling mangrove distribution. The well-drained banks of mangrove creeks are often inhabited by Rhizophora mucronata and Avicennia officinalis, whereas Sonneratia caseolaris is common on poorly drained banks. On coarse-grained and rocky substrates, Aegialtis annulata and Osbornia octodonta are common. Heritiera littoralis, Acrostichum speciosum, and Acanthus ilicifolius frequently occur on biogenic structures such as callianassid lobster (Thalassina anomala) mounds that can approach one to two meters in height. In sandier habitats, Avicennia marina, Heritiera littoralis, Ceriops tagal, Ceriops decandra, Lumnitzera racemosa, Lumnitzera littorea, Avicennia rumphiana, and Xylocarpus mekongensis are frequently found, although the latter species often occurs in mud.
Salinity is one of the major factors regulating community composition of Papuan mangroves. Along vast expanses of river banks of low salinity, the mangrove palm Nypa fructicans and, to a lesser extent, Sonneratia caseolaris, dominate the vegetation. In high salinity areas where rainfall is low (e.g., near Port Moresby), Ceriops tagal is usually the last mangrove species to be found before the transition to open ground (Frodin and Huxley 1975).
Despite that fact that no single factor or simple set of factors regulate the distribution and zonation of Papuan mangroves, large-scale patterns have been defined in specific locations around New Guinea. An ‘‘open coast’’ pattern (where wave action is significant) has been described for the northwest side of Hood Lagoon (Johnstone and Frodin 1982) and in the Raja Ampat Islands in far western Papua (Takeuchi 2003). From the sea to the land, the discernible assemblages in Hood Lagoon are: a beach fringe of Avicennia marina and Sonneratia alba followed by denser stands of Rhizophora stylosa, Rhizophora apiculata, Bruguiera cylindrica, and Bruguiera gymnorrhiza. Further inland, Ceriops tagal and stunted A. marina are common. In the Raja Ampat Islands, mangroves are sparse and species-poor compared with mangroves on the main island of New Guinea, and consist of Bruguiera gymnorrhiza-Rhizophora mucronata associations along the banks of the Gam and Kasin rivers, and a well-developed upstream sequence of Rhizophora mucronata-Ceriops tagal, Bruguiera gymnorrhiza, and Nypa fruticans with a brackish-freshwater zone composed of Xylocarpus granatum, Dolichandrone spathacea, and Heritiera littoralis.
A ‘‘deltaic’’ pattern (where muddy soils and quiescent conditions predominate) has been described for a variety of river deltas and sheltered embayments, such as the Purari and Fly deltas discharging into the Gulf of Papua (Cragg 1983; Robertson, Daniel, and Dixon 1991), Bintuni Bay on the sheltered west coast of Papua (Erftemeijer et al. 1989) and on the banks of the Ajkwa and Tipoeka estuaries in southwestern Papua (Ellison 2005).
The mangroves of Bintuni Bay are the most developed and extensive mangrove forests of Papua, covering an area of 618,500 hectares. The most seaward stands are dominated by seedlings and saplings of Avicennia marina and Sonneratia alba. Further upstream, the vegetation is dominated by stands of Rhizophora apiculata, Bruguiera parviflora, and Bruguiera gymnorrhiza. Overwash islands, colonized mostly by Rhizophora apiculata and, to a lesser extent, by Bruguiera parviflora and Bruguiera gymnorrhiza, abound within the embayment. In the Ajkwa and Tipoeka estuaries, Ellison (2005) identified five major mangrove forest types, recording extensive Bruguiera- dominated forests, consisting of Bruguiera cylindrica, Bruguiera parviflora, and Xylocarpus mekongensis, mostly north of the main Ajkwa River mouth. Nypa fruticans and mixed mangrove-floodplain forest dominated areas landward in both lower salinities and at higher elevation. At the seaward margins were found Rhizophora -dominated forests, mostly composed of R. stylosa, R. apiculata, and R. mucronata, whereas accreting mudbanks were colonized by pioneering stands of Avicennia marina and Sonneratia caseolaris. Within river channels, high structural diversity was found, apparently in relation to microscale topography. Ellison (2005) noted that tree heights for Bruguiera and Rhizophora often exceeded 25 m.
In the Fly Delta of Papua New Guinea, mangroves cover 87,400 hectares mostly on the delta islands (Robertson, Daniel, and Dixon 1991). Twenty-three species of mangroves were recorded, classified into three major forest types: Rhizophora apiculata-Bruguiera parviflora (salinities > 10); Nypa fruticans (salinities 1–10); and Sonneratia lanceolata-Avicenna marina (accreting banks). On accreting banks in very low salinity areas, S. lanceolata was found in large monospecific stands.
In the Purari delta further east of the Fly delta, Cragg (1983) recognized three major types of mangrove forest: fringing, main, and transitional. He also classified mangrove associations for the southern coast of New Guinea and identified groups related primarily to salinity regime (Table 5.4.2). Sonneratia lanceolata is the dominant mangrove found in fringing stands, ranging from the seaward edge to many kilometers inland. In lower salinity, Sonneratia alba, Avicennia eucalyptifolia, and Aegiceras corniculatum are major members of fringing forests, while palms (Pandanus sp. and Nypa fruticans) dominate fringes below a salinity of 2. The main mangrove species is Rhizophora apiculata, followed closely by Bruguiera parviflora and Bruguiera sexangula. Greatest diversity is encountered in the transitional areas between zones, where true mangrove species, mangrove associates, terrestrial intruders, epiphytes, and climbing plants coexist. The most common mangrove and mangrove associates in this zone are Bruguiera sexangula, Camptostemon schultzii, Dolichandrone spathacea, Diospyros spp., Excoecaria agallocha, Heritiera littoralis,
R. apiculata, and Xylocarpus granatum. Several freshwater swamp species invade this zone and frequently develop root structures similar to mangroves. These species include Calophyllum sp., Intsia bijuga, Myristica hollrungii, and Amoora cucullata. In this zone, an understory of Barringtonia, Brownlowia, Inocarpus, Hibiscus, and Cerbera with scattered small palms Areca, Arenga, Metroxylon, and Nypa is often formed.
There are marine macroalgae associated with mangroves, particularly with stilt roots of Rhizophora and pneumatophores of Avicennia and Sonneratia (Coppejans and Meinesz 1988; King 1990). In Bintuni Bay, the red alga Gracillaria crassa is very common on the pneumatophores of Sonnertia alba. In the Madang region of Papua New Guinea, 25 species of macroalgae have been recorded, including a "Bostrychia-Caloglossa" association and the genera Caulerpa, Halimeda, Neomeris, Chnoospora, Cutleria, Dictyota, Padina, Catenella, Laurencia, Murrayella, Peyssonnelia, Polysiphonia, and Stictosiphonia (King 1990). Further information is fragmentary, but it appears that macroalgae associated with mangroves in New Guinea are derived from inshore reefs (Tanaka and Chihara 1988).
Forest Biomass and Production
The mangrove forests of New Guinea are among the largest on earth, rivaling the height and mass of even the largest tropical rainforests. Figure 5.4.5 provides best estimates of the above-ground biomass of the world’s mangrove forests, including the few data from New Guinea (nearly all of the values between 2˚ and 8˚ S Latitude). Mangrove forest biomass ranges from 48 to 580 metric tons dry weight per hectare, with most mature forests being between 100–400 metric tons/ha in weight. The mean weight of all New Guinea mangroves is 285 metric tons/ha. Arguably the New Guinea mangroves are the largest stands yet recorded.
Critical to our ability to estimate the role of mangroves in fisheries and wood yield is an accurate estimation of net primary production. This is because primary producers and the carbon they fix via photosynthesis are the crux of mangrove food chains. About 2% of the radiant energy reaching the earth’s surface is used by plants to assimilate atmospheric CO2 into organic compounds used to construct new leaf, stem, branches, and root tissue, as well as to maintain existing tissue, create storage reserves, and provide chemical defense against insects, pathogens and herbivores.
Net production is the balance between gross photosynthesis and leaf dark respiration, and represents the amount of carbon available for growth and tissue maintenance. Photosynthesis varies with many factors, especially light intensity, temperature, nutrient and water availability, salinity, tidal range, stand age, species composition, wave energy, and weather. Five methods have been used to measure mangrove forest primary production: litter fall and incremental growth of the stem, harvesting, gas exchange of leaves, light attenuation/gas exchange under the canopy; and demographic/allometric measurements of trees.
Figure 5.4.5. Mangrove forest biomass as a function of latitude. Nearly all data points between 2–8 S Latitude are from New Guinea.
Source: Modified from Alongi (2006).
Litter fall is by far the most common method used because it is inexpensive and easy to measure, but it only measures leaf production and not growth of the remainder of the tree. Two studies have measured mangrove litter fall in New Guinea, but unfortunately, both took place near Port Moresby where rainfall is less than on the rest of the island (Leach and Burgin 1985; Bunt 1995). Both sets of values indicate very high rates of litter fall (> 1,000 g dry weight per m2 per yr). Seasonally, as in other places, maximum litter fall is cued to the onset of the summer wet season (January–March), although different species flower at different times of the year.
Harvesting is labor intensive and slow, and accounts for only above-ground production. It often does not account for leaf production. Gas exchange is precise and rapid, although subject to error due to the problem of extrapolating from an individual tree to an entire stand. Moreover, relying solely on gas exchange measurements overestimates net production as it does not account for most tree respiration.
Combining measurements offers the best hope of accounting for production of all, or most, tree parts. Measuring litter fall and incremental growth of the trunk accounts for all above-ground production, but not below-ground production. Arguably one of the best methods is to measure light attenuation. The method relies on relating the amount of light absorbed by the mangrove canopy to the total canopy chlorophyll content. The early efforts (e.g., Bunt, Boto, and Boto 1979) provided rapid and relatively easy estimates of potential net primary production. The method, however, suffers from lack of actual photosynthesis measurements and a number of untested assumptions based on light attenuation models from temperate forests. Four workers subsequently modified the light attenuation method, combining measurement of light attenuation with a more robust method of calculation of photon flux density at the bottom of the canopy and empirical measurements of leaf photosynthesis (Gong, Ong, and Wong 1991; Gong, Ong, and Clough 1992; Clough 1997; Clough, Ong, and Gong 1997).
Litter fall underestimates, and gas exchange overestimates, net primary production, but the modified light attenuation method gives the most reasonable estimate of total production, while litter fall plus incremental growth can give reasonable estimates of above-ground production (and excluding below-ground root production). The modified light attenuation method is most reasonable because it measures total net fixed carbon production and incorporates the most robust assumptions based on tree physiology and carbon balance. A number of recent studies have measured above-ground production using allometry (relationships of tree weight to stem diameter) coupled with litter fall or leaf turnover (Duarte et al. 1999; Coulter et al. 2001; Ross et al. 2001; Sherman, Fahey, and Martinez 2003).
The estimates of net primary production made using the modified light attenuation method include below-ground production but there is currently no clear understanding of how carbon is allocated to different parts of the tree. As pointed out by Clough (1998), it is not yet possible to construct a robust model of carbon balance for mangrove trees because of the lack of empirical data and the difficulty of measuring root processes and respiration of woody parts. However, some preliminary carbon data for mangroves suggest that roughly half of carbon incorporated into the tree is respired, an estimate that is in agreement with similar estimates for terrestrial trees (Barnes et al. 1998).
If we accept the data obtained using the modified light attenuation method as the most comprehensive estimate of net primary productivity of mangroves, the average rate of net primary production in New Guinea and surroundings (Table 5.4.3) is 51 tons dry weight per ha per yr. There is considerable range between values, but the figures do suggest that mangroves are significant primary producers. This is supported by empirical measurements of rates of leaf photosynthesis in mangroves (Clough and Sim 1989). Measuring gas exchange characteristics and water use efficiency for various mangrove species located on the Era, Wapo, and Ivi rivers along the Gulf of Papua, Galley Reach, and Motupore Island, Clough and Sim (1989) measured the most rapid rates of carbon dioxide uptake, stomatal conductance, and water-use efficiency yet measured. These high rates of CO 2 as-similation and other physiological attributes are a reflection of favorable climatic conditions.
Plotting all available data on mangrove productivity against latitude (Figure 5.4.6) gives a significant negative relationship, indicating that mangrove production declines away from the equator, mirroring the latitudinal decline in mangrove biomass (Figure 5.4.5) and litter fall (Saenger and Snedaker 1993). These graphs show that the mangroves of Papua are well placed geographically and climatically to grow to immense size and are as productive as any other tropical forests in the world.
Fauna
Mangrove forests in Papua support a wide diversity of biota, ranging in size from bacteria to crocodiles, and like most mangroves, house species originating from both land and sea. The fauna of the Papuan mangroves is poorly known. Most information comes from faunal surveys along the southern coast of Papua New Guinea and the west coast of Papua. Generally, there is a high level of similarity between the northern Australian and Papuan faunas (Macnae 1968); differences in species recorded are likely due to the lack of surveys in New Guinea rather than any real biogeographical anomalies.
Figure 5.4.6. Latitudinal changes in net primary production measured using a modified light interception method.
Source: Data from Gong, Ong, and Wong (1991); Gong, Ong, and Clough (1992); Atmadja and Soerojo (1991); Robertson, Daniel, and Dixon (1991); Sukardjo (1995); Clough, Ong, and Gong (1997); Clough (1998); Alongi and Dixon (2000); Alongi, Tirendi, and Clough (2000); Alongi et al. (2004).
The most comprehensive studies of the mangrove fauna of the island have been conducted in the Purari and Fly river deltas bordering the Gulf of Papua (White and White 1976; Liem and Haines 1977; Bayley 1980; Cragg 1983; Liem 1983; Pernetta 1983). The mangrove-associated fish fauna of Bintuni Bay in Papua has also been surveyed (Ecology Team 1984; Erftemeijer et al. 1989), but most of the species lists reflect attention to species of commercial or subsistence use and must be considered incomplete.
Of the mangrove vertebrates, 30 species of reptiles, 12 species of amphibians, 250 species of birds, 50 species of mammals, and 195 species of fish have been recorded on the island thus far (Appendix 8.3). Mangrove invertebrates have not been as well investigated, with the exception of commercially valuable groups, such as crabs and shrimps. The best described groups include the mollusks and insects, the most conspicuous of the latter being the Anopheles mosquito which is the vector for malaria and filariasis.
The mangrove forests of Papua support a rich molluscan and crustacean fauna consisting of approximately 95 and 80 species, respectively (Kartawinata et al. 1979; Sabar, Djajasamita, and Budiman 1979; Kastoro et al. 1991). Numerically, gastropods are the dominant group of mollusks, with Littorina scabra frequently found at the seaward margin in large numbers, with Monodonta labio a co-occurring species (Soemodihardjo 1987). In comparison, bivalves are represented by only a few species, with the genus Enigmonia being dominant in many intertidal regions of the island.
At Tatawori estuary in Bintuni Bay, the mangrove fauna is dominated by gastro-pods and crabs with densities of >120 individuals/m2 of each group, with biomass averaging 10 g DW/m2 (Erftemeijer et al. 1989). The gastropods on the seaward forest edge are dominated by Melampus, but Nerita spp. and Littorina spp. dominate the forest, foraging on algae and detritus on mangrove roots and tree trunks. As in mangroves throughout the Indo-Pacific, the cosmopolitan species Telescopium telescopium dominates the high intertidal areas, scraping detritus and algae off substrata. Ocypodid and grapsid crabs dominate the crustacean fauna in the bay with Uca species being most ubiquitous, followed by species of Sesarma.
About one-half of the world’s 60 Uca species are found in Indonesia, with these species exhibiting complex niche patterns to maintain high species diversity compared to other invertebrates (Susetiono 1989). These zonation patterns are the result of niche partitioning of sediment by grain size and organic content. The crabs feed on detritus and microbes attached to the sediment particles using specialized feeding maxillipeds that are unique to each species. In Papua, most of the mangrove species found throughout the rest of Indonesia probably occur, but only Uca dussumieri dussumieri, U. vocans vocans, U. coarctata coarctata, U. lactea annulipes, and U. seismeela have been recorded in Papuan mangrove forests thus far (Moosa and Aswandy 1983).
The most commercially important crab, Scylla serrata, is a large carnivorous scavenger that lives in deep burrows within the forest floor and along river banks throughout Indonesia. It exhibits some color plasticity, being dark green in the western archipelago and dark brown in Papua. This species inhabits mangroves throughout its adult life, but females migrate to spawn in waters offshore. Megalopa (crab larvae) move into mangrove waters and by post-larval stage they are sedentary and grow to adulthood in mangrove environments. No quantitative ecological studies are available for the benthic fauna of Papua. Extensive species lists of gastropods, scaphopods, bivalves, and crabs exist for other Indonesian forests and can be found in Tomascik et al. (1997, Chapter 19). Table 5.4.5 summarizes the molluscan species recorded thus far in Papua.
The faunal distribution of the mangroves may be considered within four categories (Cragg 1983): permanent residents; animals that also occur in adjacent forest; animals that are strictly estuarine and marine; and animals that spend their early stages in mangroves. Among the permanent residents are the mudskipper (Periophthalmus spp.), the Mud Lobster (Thalassina anomala), the Mud Crab (Scylla serrata), as well as numerous species of isopods (Ceratolana papuae, Bruce 1995) and brachyuran and sesarmid crabs (Paracleistostoma laciniatum, Baruna trigranulum, Rahayu and Ng 2003; Perisesarma foresti and Perisesarma cricotus, Rahayu and Davie 2002) endemic to New Guinea and the other islands of Indonesia. There are also many other as yet described species that occupy restricted niches in the mangroves of Papua. A variety of wood-boring bivalves (family Teredinidae) are also specialized for life in mangrove wood. Specialist mangrove fauna generally exhibit clear zonation patterns usually in relation to frequency of tidal inundation and salinity (Cook, Currey, and Sarsam 1985; Cragg and Aruga 1987).
Animals that occur in both mangrove and neighboring terrestrial forests and swamps are within the second category, and this fauna includes mostly insects, birds, and mammals. These include water rats, bandicoots, bush pigs, wallabies, sugar gliders, possums, bats, and birds such as the Magpie Goose, cassowary, and Brush Turkey (Appendix 8.3). One of the most complex ecological relationships in this category is that between Philidris ants and epiphytic myrmecophytes, Hydnophytum moseleyanum, in mangrove forests in northern New Guinea (Maeyama and Matsumoto 2000). Both Philidris ants and the epiphyte are common throughout the forests of New Guinea, also occupying mangrove trees. The relationship is mutualistic. The epiphyte gets sustenance by absorbing nutrients from the detritus stored inside tree cavities by the ants, and the ants obtain honeydew secreted by scale insects attracted to the shoot tips of the host mangrove tree.
The third category, strictly estuarine and marine organisms, enter mangrove creeks and waterways on the rising tide and depart as tides recede. Fish are the predominant animals in this category. As Haines (1983) describes for the Purari delta, few fish species are confined to any one zone but many species are confined to a range of zones with species of the same or closely related genera replacing each other along salinity gradients. Some species are wide ranging, such as the Archer Fish (Toxotes chatareus). The Saltwater Crocodile (Crocodilus porosus) lives only in the seaward limits within waterways, while the Freshwater Crocodile (Crocodilus novaeguineae) occurs in river waters upstream.
Some marine invertebrates, such as the penaeid prawns (Table 5.4.6) and the Giant Freshwater Prawn (Macrobrachium rosenbergii), use the mangroves as nursery grounds during their early life stages. The most abundant and widespread species of commercial importance is the Banana Prawn (Penaeus merguiensis). This species is more abundant in regions of high salinity but it exhibits the life cycle typical of all penaeids. During the first stage, planktonic post-larvae settle in estuaries where they feed and grow until adolescence. An oceanic stage begins when the prawns emigrate from the estuaries to coastal waters where, after a period of growth to adulthood, they move into deeper waters to spawn. Penaeus merguiensis, Metapenaeus demani, and Metapenaeus eborancensis spawn in waters 10–20 meters deep, whereas other species such as Metapenaeus ensis and Penaeus semisulcatus spawn in waters up to 60 m deep. The larvae then move shoreward via currents, initially settling in the headwaters of mangrove-lined creeks and waterways. Juveniles and adults congregate in these river mouths before migrating out to sea. The cycle is continuous throughout the year.
Food Web Dynamics
Although little if any information exists on the trophic ecology of New Guinea mangroves, it is assumed that the dynamics of food webs in Papuan mangroves are similar to those in other tropical mangroves. Mangrove links with coastal fisheries have received a lot of attention, but the food webs of mangroves are mostly detritus-based, with most trophic activity focused on interactions among fauna either directly or indirectly through consumption of tree material such as leaves, flowers, propagules, wood, bark, and roots (Robertson, Alongi, and Boto 1992).
Direct grazing on mangrove tissue, mainly by insects and arboreal crabs, generally constitutes a small proportion of energy flow. More recently, evidence of the trophic importance of algal food resources in mangrove ecosystems has emerged, demonstrating that a number of key faunal groups depend on phytoplankton, benthic microalgae, or macroalgae growing on above-ground roots and other tree parts, for food. From a nutritional perspective, algae are a better food than detritus derived from mangrove trees because of easier digestion and relatively higher nitrogen content.
Various species of mammals, insects, and birds permanently or temporarily reside in some mangrove canopies. The feeding ecology of mangrove-associated birds is fairly well understood. Bird communities can be spatially and trophically complex and include up to eight feeding guilds: granivores, frugivores, piscivores, aerial hawkers, and hovering, gleaning, fly catching, and bark-foraging insectivores (Kathiresan and Bingham 2001).
On and beneath the forest floor, crabs are generally the keystone group driving food webs. Sesarmid crabs (Grapsidae) are the most conspicuous organisms, but fiddler crabs (Ocypodidae) are also abundant, being highly efficient consumers of benthic microalgae. Recent work throughout the world has shown that large proportions of leaf and other litter deposited on the forest floor is consumed or hidden underground by crabs (Kathiresan and Bingham 2001). This pathway has profound effects on energy and carbon flow within mangrove forests, as the quantities of material available for export from forests are reduced, and the cycling of nutrients to support forest primary production is enhanced. Material that is consumed or hidden by crabs underground must eventually be decomposed by microbial communities in the sediments.
The major pathway of trophic dynamics in mangrove sediments is via detritus/ algae to microbe to crab. This is, of course, an overly simplistic depiction of fairly complex interrelations among bacteria, fungi, protozoa, nematodes and other worms, algae, detritus, crabs, and other invertebrates (Figure 5.4.7). In mangrove waters, large swimming organisms, such as fish and prawns, are at the apex of a fairly complex food web in which ‘‘the microbial loop’’ forms a crucial part (Figure 5.4.7). The ‘‘microbial loop’’ is fuelled by the dissolved exudates of phytoplankton, especially from those algal cells broken up by ‘‘sloppy feeding’’ zooplankton. The rates of microbial activity in mangrove waters are thus tightly linked to rates of phytoplankton production.
In New Guinea waters, rates of primary production vary greatly depending on the extent to which suspended particulate loads and tides affect turbidity and the availability of light. In the Fly delta, phytoplankton production is highly variable, with rates depending greatly on water clarity (Robertson et al. 1993; Robertson, Dixon, and Alongi 1998). Inside the delta where waters are most turbid, rates are low, ranging from 22 to 95 mg C per m2 per day. At the delta mouth where waters are deeper and less turbid, rates were considerably higher, ranging from 188 to 693 mg C per m2 per day. Rates of bacterial production mirror those of the phytoplankton, suggesting a close trophic link. Zooplankton biomass can be highly variable, weakly correlating with phytoplankton biomass but most often associated with large pieces of mangrove debris floating down river. A similar trophic connection exists in the Purari delta, where phytoplankton production is low in turbid waters but microbial activity is high (Pearl and Kellar 1980). In Indonesian man-grove waters, rates of phytoplankton production are most often light-limited (Soemodihardjo 1987).
Figure 5.4.7. A conceptual model of food webs within mangrove forests and in adjacent waterways, dominated by trees, crabs, and ‘‘the microbial loop.’’
A complex consortium of microbes is responsible for colonizing and decomposing organic particles, including algal cells, and being the food for many larger planktonic organisms, such as larval invertebrates. Unfortunately, actual rates of trophic transfer from microbes to zooplankton are unknown for Papuan and Papua New Guinea waters. Larger animals such as birds and crocodiles, although highly conspicuous, generally do not play a major role of mangrove energy flow.
In the Indo-West Pacific region, most mangrove forests occur in estuaries or as dense forests with intersecting tidal waterways in relatively protected embayments, and have a high proportion of forest to open water. Within such habitats, man-grove vegetation is likely to be the dominant contributor to food webs. Work using stable isotopes confirms that many consumers in mangrove habitats have an isotope signal close to that of mangrove tissue (e.g., Rodelli et al. 1984). In more open mangrove habitats, such as fringing mangroves with open canopies, algae appear to be more important as a food source (Bouillon et al. 2002).
Mangrove waterways are often dominated by zooplankton and fish, with densities usually greater than in adjacent habitats. It is generally believed that the higher numbers of organisms in mangroves compared with adjacent habitats is a reflection of greater availability of food, as well as the increased availability of refugia from large predators. In one of the more detailed surveys of fish and their feeding relationships, Haines (1983) found that the fish fauna of the Purari delta is deficient in herbivores and plankton-feeders compared to the fauna in offshore waters. This implies that most fish in mangrove deltas feed primarily on detritus, insects and other invertebrates, and other fish, rather than on algal foods (Table 5.4.7). Prawns are a particularly important prey item for mangrove fish in the Purari, and are the largest contributors to fish biomass. A similar demersal fish fauna is found off the Mamberamo River in northern Papua (Muchtar 2004), suggesting a similar trophic function for the fish species off the northwestern coast of Papua. The same is true for the Markham delta off the coast of East Kalimantan (Dutrieux 1991), implying that there is a consistent fish community structure and function in coastal Indonesia.
Although crabs and other invertebrates process large amounts of mangrove detritus (Wada and Wowor 1989), most of the decomposition of this material is mediated by fungi (Ulken 1981) and bacterial assemblages, especially those that are anaerobic (i.e., do not require oxygen). In mangrove and adjacent intertidal muds of the Fly delta, Alongi (1991) and Alongi, Christoffersen, and Tirendi (1993) found that anaerobic bacterial assemblages were highly active, to the extent that these sediment deposits take up rather than release nutrients that may support food webs in the adjacent Gulf of Papua. Rapid growth of bacteria may be partially maintained by the decomposition and release of nutrients of mangrove roots and rhizomes. A close bacteria-nutrient-plant connection conserves scarce nutrients necessary for growth of the large mangrove forests in the delta (Alongi et al. 1993; Alongi and Robertson 1995).
Links to the Coastal Zone
FISHERIES
Mangrove forests are functionally linked to the biota and abiotic processes (sediment and nutrient flow, water circulation) of the adjacent coastal zone. Nearly all of the evidence of these linkages in New Guinea comes from the mangroves bordering the Gulf of Papua (Alongi and Robertson 1995; Robertson, Dixon, and Alongi 1998). Three types of mangrove-associated fishing practices occur in the Gulf of Papua: gill netting for Barramundi, trawling for prawns; and spearfishing for lobsters. These practices may, to some extent, be exemplary of other coastal zones of Papua and Papua New Guinea.
Barramundi (Lates calcarifer) spawn along the coastal strip of the western gulf, in salinities of 30 (Moore 1982; Moore and Reynolds 1982; Reynolds and Moore 1982). Most of the spawning aggregation occurs with the onset of summer, probably triggered by the coincidence of peak spring tides and strong onshore winds. Rich detrital material derived from mangroves and marshes carried offshore by ebb tides may be the trigger for spawning offshore. Eggs hatch and incoming tides carry larvae into shallow wetlands. Most predators are excluded by shallow tidal waters, and food is abundant to promote rapid growth of the young. From these nursery areas, most Barramundi migrate eastward to access mangroves inhabiting the river deltas to feed on the abundant prawn populations (Haines 1979). Localized fisheries take advantage of these predictable movements, especially during the summer spawning (Mobiha 1995). From 1971 to 1984, commercial landings declined dramatically from 394 tons/yr to 139 tons/yr. Annual catch from 1993– 1994 was seven tons in the western gulf while the total catch was 58 tons in the eastern gulf (Opnai and Tenakanai 1986; Dalzell, Adams, and Polunin 1996; Kare 1996). This decline can be partly attributed to overfishing and poor management practices.
Two prawn fisheries operate along the southern coast of Papua New Guinea. The Torres Strait fishery is dominated by Metapeneaus endeavouri (50%), Penaeus esculentus (40%), and Penaeus longistylus (10%). The fishery in the Gulf of Papua is dominated by Penaeus merguiensis and, to a lesser extent, Penaeus monodon. The prime nursery grounds for the species are seagrass meadows; as the prawns mature they move eastward to deeper waters. For Penaeus merguiensis, the larvae migrate inshore and settle to the bottom as they reach post-larval stage. The principal nursery area for this fishery is the mangrove-fringed islands and channels between the Purari and Kikori rivers (MacFarlane 1980; Evans and Kare 1996). Prawn post-larvae settle in the mangroves in November, grow and recruit to the fishery in February (Evans, Opnai, and Kare 1995). The trawler fleet is one of the largest in the South Pacific, with the annual catch generating up to 1,300 tons/yr (Dalzell et al. 1996). The Gulf of Papua annual average prawn landing was estimated to be 523 tons/yr for 1974–1993 for Penaeus merguinsis, and 844 tons/yr for all the other prawn species (Evans, Opnai, and Kare 1995). The total fish catch off the west coast of Papua in 1997 was 151,133 tons, with most of the catch being tuna, skipjack, and prawns.
Rock Lobsters (Panulirus ornatus) are a small, but important, fishery species in the region. Lobster larvae develop in the open ocean, taking about six months to grow to juvenile size, and then settle as post-larvae into the seabed of the Torres Strait (Pitcher 1991). When they are about two and one-half years old they begin a mass migration in August from the strait (MacFarlane and Moore 1986) to coastal reefs in the eastern gulf near Yule Island. Most of the lobsters arrive in poor condition and most apparently die after the breeding season (Dennis et al. 1992). However, it is suspected that some lobsters move to other, unknown, breeding grounds such as deep reef habitats on the edge of the continental shelf (Evans 1996).
The lobster exhibit great plasticity of habitat use due to their complex reproductive, migratory, and settlement processes. They are found in a wide range of environments from sheltered, turbid waters to very silty areas near rivers and mangroves. Their diet consists mostly of mollusks and crustaceans (Joll and Phillips 1986). The lobsters are fished at both ends of the migration route and, until recently, along the route as well. Because this species is susceptible to being caught by trawlers, lobster by-catch was marketed at up to 200 tons/yr, but concerns about the reduction of spawning stocks and subsequent effects on recruitment led to a ban on keeping the by-catch (Pitcher 1991). In the Daru area, the lobster catch peaked at 92 tons in 1994 with a minimum catch of 57 tons in 1987 (Evans and Polon 1995). The annual catch at the Yule Island end of the migration route is usually two to three metric tons (Dennis et al. 1992). There is a Bêche-de-Mer (Holothuria scabra) fishery in the Daru area which in 1995 had a total yield of 55 tons, but there is little known about holothurian biology or the sustainability of this fishery.
According to estimates of fish resources in the Arafura Sea along the southern coast of Papua (Dalzell and Pauly 1989), potential yields for small pelagic and demersal fisheries are equivalent to those in the Gulf of Papua, with small pelagic yields on the order of 2.8 and 2.5 tons per km2 per yr and demersal yields averaging 1.1 and 1.5 tons per km2 per yr for the Gulf of Papua and Arafura Sea, respectively.
NUTRIENT AND SEDIMENT FLUXES
Mangroves are often considered to be accumulation sites for particulate nutrients and sediments, and this appears to be the case for the mangroves of New Guinea. Geological studies of mangroves bordering the Gulf of Papua and the Ajkwa and Tipoeka Rivers in west Papua (Thom and Wright 1983; Barham 1999; Brunskill et al. 2004; Walsh and Nittrouer 2004; Ellison 2005) have indicated that the man-groves are accreting. In west Papuan mangroves, Ellison (2005) estimated sedimentation rates in the range of 0.6–1.5 mm/yr, a rate in the same range as that measured by Thom and Wright (1983) in the Purari mangroves. In a series of contiguous cores, Brunskill et al. (2004) measured sedimentation rates in the Ajkwa mangroves on the order of 4.5–13 kg sediment per m2 per yr, which are well within the range of rates measured in other mangroves. Walsh and Nittrouer (2004) examined sedimentary history of the mangroves bordering the Gulf of Papua and measured accumulation rates ranging from 1.3–7.5 cm/yr (11–65 kg sediment per m2 per yr). Greatest rates of accumulation were observed on accreting banks in the mid-tidal zone, with lower rates above and below this tidal horizon. These figures are higher than those measured in the Ajkwa estuary, but remote sensing indicates that areas of accretion co-exist with areas of erosion in many mangrove/estuarine regions of the Gulf of Papua. Nevertheless, the net accumulation of coastal mangroves of the western Gulf of Papua is estimated to account for 2–14% of the total sediment load of the gulf. All of these sedimentation rates are testimony to the large volume of river sediment discharged from the land to the coastal zone of New Guinea. The discharge of water and sediment from the New Guinea highlands to the coastal plain translates not only into the deposition of particulate material in mangroves, but also into the export of nutrients to the adjacent coastal zone. It is believed that this export of dissolved and particulate material stimulates pelagic and benthic food webs in coastal waters, and supports the fisheries described above.
In the Gulf of Papua region, there is sufficient evidence to show that mangroves utilize significant quantities of dissolved riverine materials to sustain their high rates of primary production (Liebezeit and Rau 1987; Alongi, Christoffersen, and Tirendi 1993). A budget of carbon gains and losses in the Fly delta (Table 5.4.8) indicates that of the approximately 22.1 10 11 g C that comes into the delta from both river discharge and production by mangrove forests, roughly half is consumed in the delta and the other half is exported to the Gulf of Papua (Robertson and Alongi 1995). Roughly 25% of the organic carbon that is exported is man-grove-derived. Most of this material is low-quality detritus, such as leaves, roots, and bark. A study of the carbon-isotope composition of the sediments in the Gulf of Papua (Bird, Brunskill, and Chivas 1995) confirms that this material is exported from the Fly delta, but is limited to within a few kilometers of the coastline. The rapid decline of dissolved nutrient concentrations from the rivers to the adjacent coastal waters off the Fly, Purari, and Mamberamo deltas (Viner 1979; Robertson et al. 1993; Muchtar 2004) suggest similarly important, but geographically limited, export of mangrove material to the coastal ocean bordering the entire island.
Extrapolating the sediment and carbon discharge rates for rivers draining into the Gulf of Papua to the rest of New Guinea suggests total sediment and carbon discharge rates for the island similar to those of the Amazon River (Milliman 1995). There is circumstantial evidence that mangrove litter reaches the deep Coral Sea. Considering the much narrower distance from the rivers to the deep sea along the north coast of New Guinea, it is likely that proportionally more mangrove-derived matter reaches the deep ocean in the north (Kuehl et al. 2004).
Human Impacts
Most of the mangrove forests of Papua are still relatively pristine as human population density is low, and most human use is on a small scale. The only mangroves that have been subjected to a substantial degree of human impact are those near development projects and industries, such as copper mining, capture fisheries, wood chip extraction, and several oilfield projects. Mangrove losses in Papua have been small (< 10%) but for the islands fringing the Timor Sea, the losses of man-grove forest range from less than 5 to 50% (unpublished references cited in Morrison and Delaney 1996). Table 5.4.9 lists examples of the various human uses of mangrove forest in Papua.
The best-documented areas of human impact are the Ajkwa River estuary and Bintuni Bay. The main source of impact in the Ajkwa River estuary is the tailings from a copper-gold mine located some 3,700 m above sea level in the Moake Range. This mine is operated by PT Freeport Indonesia (PRFI) and has been open since 1972. Mine tailings are discharged into the estuary at a rate of about 125,000 tons/day. These tailings consist of sand and small pieces of ground rock, and deposit in a 130 km2 area contained by levees above the salt wedge of the estuary. Despite the levees, recent geochemical measurements (Brunskill et al. 2004) have found that copper accumulation rates have been enhanced 40-fold in mangrove sediments since the introduction of mining. The biological impact of these copper concentrations is unknown.
In Bintuni Bay, the mangroves have been increasing affected by wood extraction, fisheries, and oil and gas development (Brotoisworo 1991; Ruitenbeek 1992, 1994). Most of the primary forest bordering the bay has been allocated for timber concessions (Petocz 1987) with at least seven companies holding concessions of about 300,000 ha of the total mangrove area of 618,500 ha (Erftemeijer et al. 1989; Ruitenbeek 1994). An economic analysis of mangrove management options for Bintuni Bay (Ruitenbeek 1992, 1994) indicates that traditional non-commercial uses of mangroves have an estimated value of US$10 million/yr; commercial prawn fisheries are valued at US$35 million/yr and mangrove wood extraction are worth US$20 million/yr. Ruitenbeek (1994) suggested that the optimal management strategy was selective cutting of 25% of the harvestable mangrove for a total return of US$35 million/yr.
Oil concessions to four companies have resulted in extensive oil and gas production, with an estimated recoverable reserve of about 12.2 million barrels (Brotoisworo 1991). With the extensive logging concessions and the fact that the penaeid prawn production of Bintuni Bay was 1,375 tons/yr or about 20% of the total prawn production for Papua, there is significant overlap in resource use in the bay. At present, there does not appear to be effective reconciliation between the need for development and the need for conservation.
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