Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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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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