Ecology of Indonesian Papua Part Two. Andrew J. Marshall
Reach
Headwater streams (Figure 5.5.1) drain first and second order catchments lying at elevations above 800 m or possessing gradients in excess of 30%; streams of this type in montane regions possess both such attributes. The substratum is usually bedrock or coarse alluvium such as large rocks and boulders; current speeds are generally high; water temperature is less than 18 C (most typically 12–15); conductivity is less than 50 µmhos (with dissolved solids less than 40 mg/l); and pH is often slightly acidic. The fauna of such small, steeply dropping streams is dominated by insects, particularly Ephemeroptera in the family Baetidae, Trichoptera in the families Glossosomatidae, Hydrobiosidae (Apsilochorema), Hydropsychidae (Hydropsyche), Hydroptilidae (Hydroptila), and Leptoceridae (Mystacides, Triae-nodes), Diptera in the families Dolichopodidae and Ephydridae, Odonata in the families Platystictidae (Drepanosticta), Megapodagrionidae (Argiolestes), and Platycnemididae (Idiocnemis), Coleoptera in the families Dytiscidae (Platynectes), Hydrophilidae (Enochrus), and Gyrinidae (Merodineutes, Macrogyrus), and Heteroptera in the families Naucoridae (Nesocricos, Tanycricos), Gelastocoridae (Nerthra), and Veliidae (Rhagovelia, Papuavelia, Tarsovelia), with crustaceans scarce, and fishes generally absent above 2,000 m (Allen 1991). Alpine streams, lying at elevations above 3,000 m, lack most of these biotic elements except for a very limited and specialized assemblage of Trichoptera in the families Hydrobiosidae (Apsilochorema), Hydropsychidae, and Hydroptilidae and Diptera in the family Chironomidae.
Fig. 5.5.1. This cascading tributary of the Kikori River, on Mt Bosavi in south-central New Guinea, is a typical high gradient headwater reach of a perennial stream, with extensive bedrock exposures in the stream channel.
Photo: D. A. Polhemus.
Midreach
The midreach zone (Figures 5.5.2 and 5.5.3) is intermediate in environmental conditions between the headwater and terminal reaches (for discussion of terminal reaches, see below). Depending on the length and gradient of an individual stream catchment, this can either be a brief and highly foreshortened zone, or conversely may comprise the majority of a stream’s length. The substratum is usually mixed alluvium consisting of boulders, rocks, and gravel, with occasional sand or cobble bars developing on the inner margins of bends, and water temperatures typically range between 18 and 24 C. The fauna of such streams in New Guinea is diverse, including plotosid catfishes, rainbowfishes (Melanotaenidae), grunters (Terapontidae), and gobioid fishes (Gobiidae and Eleotridae), Crustacea in the family Parastacidae (Charax), Ephemeroptera in the families Baetidae, Leptophlebiidae (Thraulus), and Prosopistomatidae (Prosopistoma), Trichoptera in the family Hydropsychidae (Cheumatopsyche, Hydropsyche, Macrostemum), Zygoptera in the families Calopterygidae (Neurobasis), Chlorocyphidae (Rhinocypha), Coenagrionidae (Palaiagria, Pseudagrion, Teinobasis), Platycnemididae (Idiocnemis), and Protoneuridae (Nososticta, Selysioneura), Anisoptera in the family Libellulidae (Huonia), Lepidoptera in the family Pyralidae, Diptera in the family Chironomidae, Coleoptera in the family Gyrinidae (Rhombodineutes), and Heteroptera in the families Gerridae (Ptilomera, Tenagogonus, Limnometra, Stygiobates, Metrobatoides), Veliidae (Rhagovelia, Strongylovelia, and numerous genera of Microveliinae), Mesoveliidae (certain Mesovelia), Ochteridae (Ochterus), Leptopodidae (Valleriola), Hebridae (Hebrus), Nepidae (Cercotmetus), and Naucoridae (Sagocoris, Aptinoocoris, Cavocoris, Idiocarus).
Figure 5.5.2. The upper midreach zones of perennial stream networks on New Guinea are composed of clear, rocky creeks, such as this tributary to the Ziwa River in the central mountains of the island. Such streams are numerous in forested upland terrain, and support extremely diverse biotas of native aquatic insects but only a limited array of native fishes.
Photo: D. A. Polhemus.
Terminal Reach
The terminal reach (Figures 5.5.4 and 5.5.5) is the section of a watercourse below the first sharp gradient that bars upstream migration of itinerant marine fishes, such as flagtails (Kuhlia). The elevation is generally less than 50 m (although though this may not be reached until far inland on large systems such as the Digul, Wapoga, and Mamberamo), and gradient is less than 5%. The substratum in larger streams and rivers is primarily fine sediment, intermixed to varying degrees with sand, gravel, and rocks. The water temperature is greater than 24 C (generally 25–27); conductivity exceeds 80 µmhos (mainly 100–150 µmhos); dissolved solids are 60–100 mg/l; and pH is neutral to slightly basic, ranging from 6.5–7.8. Large lowland rivers such as the Digul and Mamberamo fall within this division, as do the lower courses of many rivers draining from the southern face of the central mountain ranges to the Arafura Sea. Characteristic fauna includes Barramundi (Lates calcarifer), ariid catfishes, numerous gobioid fishes (Gobiidae and Eleotridae), Ephemeroptera in the family Palingeniidae (Plethogenesia), Odonata in the families Coenagrionidae (Agriocnemis, Argiocnemis, Pseudagrion, Xiphiagrion) and Libellulidae (Agrionoptera, Huonia, Neurothemis, Orthetrum, Rhyothemis, Tetrathemis), Coleoptera in the family Gyrinidae (Spinosodineutes), Heteroptera in the families Gerridae (Rhagdotarsus, Limnometra, Limnogonus, Ciliometra), Veliidae (Microvelia), Mesoveliidae (certain Mesovelia), Notonectidae (Nychia) and Corixidae (Micronecta). The terminal reaches of the larger lowland rivers in Papua experience periodic mass hatches of large-sized palingeniid mayflies, with body lengths exceeding 3 cm, which create an impressive sight on the water and represent an important food source for native fishes.
Figure 5.5.3. The midreach of the Minajerwi River as it exits the southern foothills of the Papuan Central Ranges north of Timika is indicative of the high velocity and base flow exhibited by many rivers in the mountains of New Guinea. The coarse alluvial character of the bed is in marked contrast to the fine sediments of the terminal reach channel occupied by this same river just a few kilometers downstream.
Photo: D. A. Polhemus.
The above three stream divisions may each be further segregated into two zones, erosional and depositional. Erosional zones include waterfalls, rapids, riffles, and other areas where there is a net loss of substrate or organic material due to the action of flowing water. Depositional zones include pools, the inner margins of stream bends, and other areas where such material is deposited. Since these two types of zones alternate and intergrade along the length of any given reach, they are not considered as discrete ecosystems per se, but they are often important habitat determinants for individual taxa. For example, the goby genus Stenogobius is generally restricted to depositional zones.
Figure 5.5.4. Terminal reach streams in lowland areas, such as this rainforest creek in the Wapoga River basin, typically occupy low gradient beds of fine alluvium, and are among the faunally richest aquatic ecosystems in New Guinea in terms of both fishes and insects.
Photo: D. A. Polhemus.
INTERMITTENT STREAMS
Intermittent streams (Figure 5.5.6) comprise seasonally flowing waters in discrete channels, with flow decreasing in volume to slow-exchanging pools prior to desiccation. Examples of such ecosystems include small creek beds in lowland alluvial forests, such as those seen near Kuala Kencana in the Timika area, and periodically flooded overflow channels adjoining larger rivers (Figure 5.5.6), which carry water briefly during spates, or for more prolonged periods during the rainy season. Pools in such systems generally persist for at least a few weeks to a few months, usually as discontinuous surface manifestations of diminishing hyporheic (subsurface) flow. Ecosystems of this type most frequently develop in porous, sandy channels where flow can readily retreat subsurface, and although water quality is variable along such reaches, in some cases becoming stagnant, it is often high due to some slight degree of flow coupled with natural sand filtration.
Because intermittent streams differ environmentally from perennial streams in terms of flow regime and water continuity, they are biologically distinct in lacking many diadromous species (those that migrate between fresh and salt water), but by contrast often contain abundant insects and