Tropical Marine Ecology. Daniel M. Alongi

likely is 160 387 km² with a moderate to high confidence (Figure 4.5), but possibly 266 562 km² with lower confidence (McKenzie et al. 2020). Seagrass meadows in the tropical Atlantic (44 222 km² with moderate–high confidence) and in the tropical Indo‐Pacific (87 791 km² with moderate–high confidence) make up 82% of the global total and tropical seagrass meadows make up 85% of the global total if the low confidence estimates are included (McKenzie et al. 2020).

Like seagrasses and mangroves, the global distribution of coral reefs (Figure 4.6) reflects the influence of long‐term environmental conditions that are most suitable for establishment and growth. Species richness of corals is greatest in the Coral Triangle in Southeast Asia (Figure 4.6, dark red area). Corals are found in three principal areas: the Caribbean, the Red Sea, including the Indian Ocean islands such as the Seychelles, and in the Indo‐West Pacific. Corals are found in a broad band throughout the tropics, although there are areas out of this band where warm currents permit the existence of corals, such as on the west and east coasts of Australia and as far north as the southernmost islands of Japan. Coral reefs cover about 600 000 km² of the global ocean, comprising only about 0.17% of total ocean area and roughly 15% of the world’s sea floor <30 m (Spalding et al. 2001). This small area belies their importance to the structure and function of the tropical coastal ocean as they are important areas of fisheries, marine biodiversity, and carbonate production.

FIGURE 4.4 Global distribution of mangrove forests. The green bars outside the box indicate relative distribution of mangrove with latitude (right) and longitude (bottom).

      Source: Bunting et al. (2018), figure 4, p. 10. Licensed under CC BY 4.0. © MDPI.

FIGURE 4.5 Global seagrass area relative to the maximum potential seagrass area within each of the global seagrass bioregions which are represented by scaled circles. Seagrass area in each bioregion: 1. Temperate North Atlantic = 3229 km²; 2. Tropical Atlantic = 44 222 km²; 3. Mediterranean = 14 167 km²; 4. Temperate North = 1866 km²; 5. Tropical Indo‐Pacific = 87 791 km²; 6. Temperate Southern = 9112 km².

      Source: McKenzie et al. (2020), figure 4, p. 7. Licensed under CC BY 4.0. © IOP Publishing.

FIGURE 4.6 Global distribution of coral reefs. The coloured areas indicate species richness of hermatypic coral reefs in each region.

      Source: www.coralsoftheworld.org (accessed 3 January 2021). © Japanese Coral Reef Society.

Six major physical factors limit the development of coral reefs: temperature, light, depth, salinity, sedimentation, and emersion (Montaggioni and Braitwaite 2009). Hermatypic corals are found in waters bounded by the 20 °C isotherm with the lower temperature set at 18 °C for reef formation. Optimum reef development occurs at mean temperatures of 23–25 °C, although some corals can tolerate higher temperatures. The absence of reefs from the west coasts of Central and South America and the west coast of Africa is due to the upwelling of cold water. Reefs are also depth and thus light‐limited and do not form in waters > 50–100 m. Most grow in 25 m or less and are restricted to continental margins or islands due to light limitation. Light compensation depth is the water depth where light intensity is 1–2% of incident light at the ocean surface. Corals do not grow below 50–60 m in the Pacific but grow as deep as 100 m in the Caribbean due to greater light penetration.

      Another factor limiting the development of coral reefs is salinity as corals have a narrow tolerance for salt. Freshwater runoff can occur in proximity to reefs which they can tolerate for short periods of time, but generally corals thrive in areas where there is little if any decreases in salinity and increases in sedimentation that clogs feeding structures and reduces available light by turbidity or mixing (Montaggioni and Braitwaite 2009). Corals can also tolerate short periods of exposure to air, but generally their growth is limited to the tide mark of mean low water.

      There are four types of coral reefs: fringing reefs, barrier reefs, patch reefs, and atolls (Sheppard et al. 2018). While they all differ in their geomorphology, they are all part of a series of forms that develop in the same basic manner. Corals will grow where conditions are suitable, especially in clear shallow waters and they can grow along tropical rocky coasts to about 45 m depth. Corals grow upward until limited by emersion into air and begin to spread outward. Fringing and barrier reefs are found along continental coasts and off islands while atolls are mostly found in the Indo‐Pacific area. Atolls are oceanic and circular in shape with a series of sandy cays enclosing a deep lagoon. They form when a submarine volcano develops a fringing reef and as it sinks over time the coral will grow upward. The top of the volcano then subsides to eventually form a deep lagoon in the centre of a group of coral reefs.

      Barrier reefs can be located further offshore with a broad, wide lagoon compared to fringing reefs. Patch reefs are generally oval along the axis of the prevailing winds and may have a sandy cay on the leeward side. In some areas where there is enough shelter, patch reefs can develop into islands where they become low wooded islands and may even have mangroves and seagrasses in a patchy lagoon.

Reefs display a variety of zonation patterns depending on the water depth, wave action, and exposure (Sheppard et al. 2018) but the ‘classical’ zonation pattern is of a reef front or slope culminating in a wave break zone, followed by a reef crest then a reef flat which leads to a back reef or lagoon (Figure 4.7). The reef front or slope extends from the low tide mark to deep water and it is here that coral growth is most rapid; the slope is dominated by large corals such as Acropora and Monastrea within the upper 15–25 m. Wave action and light intensity are reduced below this depth; light is reduced to only about 25% of the surface so only small branching corals predominate. At about 25–40 m depth, corals become patchy as light becomes scarce and there is some accumulation of sediment. Gorgonian corals can dominate at this depth range. The wave‐break zone and reef crest bear the full brunt of the waves and there is often a pattern of groove and spurs which forms because of the constant wave action. The reef crest zone is exposed at low tide and varies in width from a few m to tens of m and is dominated by very hardy coral species that can withstand strong wave action. The reef flat can be tens of m in length and is one of the largest areas of the reef by area. It receives less wave action than the more forward zones but is still exposed at low tide and consists of a wide mixture of corals and turf algae and can often have quiescent pockets of sandy patches where a variety of invertebrates and fish exist. The reef flat deepens into the back lagoon where unconsolidated sediment prevails and where there can be ‘bommies’ or hummocks of massive coral skeletons on which grow a variety of organisms, including young corals. The back reef can be exposed at low tide and often has a dominant biota of calcified green algae, such as Halimeda, along with various species of seagrasses and hummocks of corals, such as Porites. It extends outward from the shore to the lagoon and reef flat and may be any area in size from a few tens to hundreds of m in length. The