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 to cover and smother the coral because the coral is attached and cannot move upward to get above the surface of the sediment. As a result of these processes, coral reefs are not found near the mouths of rivers that release huge volumes of suspended fine sediment into coastal waters. In Papua reefs are found around small islands, and along the western mainland coast. However there are no reefs in the eastern part of the province, where the landmass is large and larger rivers such as the Mamberamo, Pulau, and Digul empty into the ocean. Because the island of New Guinea is a geologically young, very high island in an area of very high rainfall (over 3,000 mm/yr in Papua; Tomascik et al. 1997), runoff of fresh water and sediment is very high. If the temptation to reap large quick profits by cutting the rainforests of Papua is not resisted, the amount of sediment runoff will grow many-fold, and coral reefs will be rapidly killed.
Reef building corals need light to live and grow. Suspended sediment in the water scatters and absorbs light, reducing its availability to coral. Although there are limits to how much sedimentation corals can tolerate, some are able to thrive in areas with moderate levels of sedimentation. A coral reef named Middle Reef about a kilometer offshore of Townsville, Queensland, Australia, survives with high coral cover in water that has a visibility of only about one to two meters. The coral species on this reef are quite different from those found on the nearby outer edge of the Great Barrier Reef, in clear oceanic waters. Further, sediment input to the nearshore waters of Queensland increased dramatically when sheep and cattle were introduced to the area about 100 years ago (McCulloch et al. 2003). This increased sedimentation stresses the surviving corals and renders them less resilient, perhaps to the point of being unable to recover from other types of disturbance.
TEMPERATURE
Coral reefs are restricted to warm waters where the minimum temperature is above about 18 C. The world’s most northern coral reefs are at Kure Atoll in the northwest Hawai’ian Islands, and in Japan, and the most southern are at Lord Howe Island, off southeastern Australia. Some coral communities can be found at even higher latitudes, but they do not accumulate calcium carbonate, and therefore do not form coral reefs. An example is in the Solitary Islands off New South Wales, Australia, where occasionally storms with waves up to ten meters tall sweep most corals off into deep water. Newly settled coral recruits then grow on the non-carbonate rocks, rebuilding the coral community, but their skeletons do not accumulate (Harriott, Smith, and Harrison 1994). Such coral communities can also be found in the tropics near the equator in marginal environments, such as sandy, high sediment, or low circulation areas. In addition, at the extremes of latitude, coral reefs tend to be small, and to have low diversity (Yamano et al. 2001, discussed below). Papua is situated in an area close to the equator, where temperatures are warm year-round (27.5–28.5 C in the Java Sea; Tomascik et al.1997) and nearly ideal for coral reefs.
Most coral reefs are found in warm, clear tropical water. The water is clear because it contains few of the tiny drifting plants and animals, which together are known as plankton. Temperate and polar waters are often opaque with a green or brown color, due to masses of plankton. The tiny drifting plants are called phytoplankton, and they require nutrients such as nitrogen and phosphorous, just as do other plants. In the tropics, the hot sun heats only the surface water but does not penetrate deeper water. Hot water rises above cold water because it expands slightly when heated. The boundary between the warm surface water and colder deep water begins at a depth of about 50 m in Indonesia, and extends down to about 300 m depth (Tomascik et al. 1997). The fact that warm surface waters float on top of the cold deeper water means that these two bodies of water do not mix. Phytoplankton absorb nutrients from the surface water as they perform photosynthesis and grow. The phytoplankton are fed on by zooplankton (tiny drifting animals), which are in turn fed on by larger animals in a food chain or food web. When any of these organisms die,
