Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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plates. There are many areas of uplift in eastern Indonesia, and many places that have terraces on slopes above the waterline, including along the northwestern coast of Papua (Tomascik et al.1997). On the Huon Peninsula of northeastern Papua New Guinea, continuous uplift of the landmass along with oscillating changes in sea level have produced a stair step series of fossil coral reefs on land. During ice ages, huge amounts of water are withdrawn from the oceans and locked up in giant ice sheets on land in North America, Europe, and Asia, much like in Antartica today. In addition, lower temperatures in the oceans cause the water to contract. These two processes together cause sea levels to drop substantially during ice ages. The last ice age peaked at about 22,500 years ago, causing a drop in sea level of about 120 m. This is well below the lower limits (usually around 30 m) of all presently living coral reefs. Thus all presently living coral reefs were exposed to air at that time and died. The change in sea level happened sufficiently slowly that larvae of sessile organisms such as corals were able to attach farther down and remain alive in the water, and begin building reefs farther down the slopes of islands and continents.
In an area like the Huon Peninsula where land is rising steadily, coral reefs build up along the shore when the water levels rise at about the same rate as the land rises. Then when the sea level drops, the reef is left out of the water and new corals attach farther down. There have been a whole series of ice ages, and so the hillsides on the coast of the Huon Peninsula have a series of benches made of coral reefs up and down their slopes (Figure 5.2.2). A living fringing coral reef is in the water along the shore, and the series of raised reefs on the hillside begins with the youngest at the bottom and progresses to the oldest at the top. There are nine fossil reefs, spanning a period of 95,000 years. A comparison of the species on the living reef and in the fossil reefs shows that the reefs have the same species composition even though they differ in age by up to nearly a hundred thousand years. Complex reef assemblages have been able to reconstitute themselves in the same form time after time over a very long period of time. Differences were actually greater between points along the coast than at the same reef over 95,000 years (Pandolfi 1996). Similar areas of rising land in Papua are likely to have experienced the same series of events, and have similar fossil reefs. In areas where the land is not rising or is even sinking, reefs produced during lower sea level stands are likely to be found underwater. Such reefs produce terraces or ledges, such as the one reported from Cenderawasih Bay.
ZONATION
Coral reefs have several zones. The term zonation refers to situations where the type of organisms present (i.e., species composition) changes along some environmental gradient. Rocky intertidal zones in temperate climates have zonation, with some organisms living only high in the intertidal, others living in the middle intertidal, and still others in the lower intertidal. On coral reefs there are a series of zones encountered as one moves out from shore (Figure 5.2.3). On a barrier reef, the zone closest to shore is the lagoon, which has a sandy bottom and may have seagrasses and algae on the bottom, along with patches of coral. Farther out, a shallow, hard calcareous (composed of calcium carbonate) bottom is called the reef flat and may have scattered corals. The crest is where waves break on the reef, and is usually dominated by crustose coralline algae. This type of algae forms a smooth hard layer over underlying coral rubble or rock, cementing it together and withstanding the force of breaking waves. From the crest, the reef slopes downward in what is called the forereef slope. On the reef slope there may be a series of ridges and gullies running down the slope called spur and groove, tongue and groove, or ridges and sand channels. On some reefs the slope ends in a vertical drop-off that can be called a wall. Walls commonly have overhangs that alternate with steeply sloping sections. Some reefs have caves on reef slopes or walls, and most reefs have small holes in the reef that may extend in a maze through the reef. At the base of a wall, there is usually a slope of sand and debris, but few or no reef-building corals.
Figure 5.2.2. Coral reef benches on the north shore of Huon Peninsula, Papua New Guinea.
Figure 5.2.3. Zones on a barrier reef.
Each of these zones is a distinct habitat. The zones differ in exposure to waves and currents, with lagoons being the most protected from waves and sometimes having restricted circulation. Organisms living in lagoons need to be able to burrow in, or attach to, or live on sand, which is the substrate. The reef flat usually has wave action and strong currents from the waves breaking over the crest, pumping water into the lagoon. Organisms here are also subjected to intense solar radiation on clear days. On the crest, organisms are battered with powerful waves and exposed to intense solar radiation. On the reef slope, wave surge and solar radiation decrease with depth. Coral diversity is usually highest on the reef slope, moderate on the reef crest, and lowest on the reef flat (Karlson, Cornell, and Hughes 2004). On walls, wave surge is usually nonexistent, and solar radiation decreases rapidly with depth. There is enough sunlight on steep slopes on a wall for organisms that need light, like coral and algae, to grow. But overhangs do not have enough light for such organisms, and have a strikingly different community of organisms. Overhangs are usually dominated by sponges, soft corals, and coralline algae (which need light but can grow in lower light levels than most corals). Caves have increasingly lower light levels with distance from their opening. Water circulation decreases with distance into caves and holes, and yet is still sufficient for some types of organisms. Oxygen levels may also decrease with distance inside holes in reefs as organisms use up the oxygen coming in on a limited flow of water. The zonation with decreasing light in caves may parallel over short distances the zonation on the wall or slope below the reef over a larger depth range. Thus an organism (such as a black coral or sclerosponge) that is found in deep water below the reef may also be found in shallower water under overhangs or within caves.
Corals grow most rapidly between depths of about 10 and 30 m. Walls often begin at depths of around 20 m, though in some places they can start in just a few meters (such as several sites in the Philippines) or well below 30 m depth (Discovery Bay, Jamaica). Coral cover is usually highest at depths around 5–20 m, but this does not hold at all locations. Coral cover decreases with depth on most reefs below a depth of around 30 m, though the depth at which this begins is variable. Corals become quite rare most places below about 50 m depth. The deepest corals that require light have been said to be at about 100 m depth, but in Hawai’i living coral has been found as deep as 187 m (Chave and Malahoff 1998). The lower depth of some reefs is determined by habitat, with the reef ending in a sandy slope. Such a sandy slope can be reached at virtually any depth, with some beginning at less than 10 m depth and a few beginning as shallow as 5 m.
Many corals have relatively broad depth ranges, yet some are quite restricted in their depth ranges. I found Acropora aspera to be present only on reef flats less than one meter deep in American Samoa, and Acropora cf. pinguis in Malaysia to be totally restricted to depths of less than two meters. Acropora digitifera is common only in shallow water (about 0–3 m). A. robusta and A. pulchra are rare except in shallow water (about 0–7 m deep). Acropora nana, a species with very thin delicate branches, is restricted to shallow water (about 0–2 m) and is somewhat surprisingly most common in heavy surf zones. The genus Leptoseris is largely restricted to low light level areas such as deep water and overhangs, as are the black corals (Antipatharia). Giant clams, Tridacna sp. and Hippopus sp., are most common in shallow water and densities drop off quickly with depth. Coral communities in lagoons may be dominated by corals that are rare on reef slopes and vice versa.
SEDIMENTATION
Coral reefs are found in warm, shallow, clear tropical saltwater. Corals can only live in saltwater; none are found in freshwater or even brackish water. They are rarely found near the mouths of rivers, and almost never found near large rivers. Most corals thrive best in clear water, and cannot survive in water containing large amounts of sediment or in mud. Most corals require a hard surface to attach to, though there are some corals that do not attach or only attach for a short period in their life cycle. Sediment in the water that settles on a coral can be cleared off by the action of tiny hair-like structures called cilia. The cilia can remove small amounts of sediment but not large amounts. Further, if sediment buildup occurs on the bottom, sediment will begin