Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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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