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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 back reef is shallow and sheltered from wave action. Here, water circulation is less rapid, and sediment tends to accumulate, contributing to poor coral growth; benthic invertebrates are common.
FIGURE 4.7 Schematic of an idealised coral reef showing various reef zones from the reef front to the back reef. Zones are not to scale.
The geological development of coral reefs is controlled by temperature, nutrient availability, hydrology, and changes in sea‐level and ocean chemistry. Most research has focused on sea‐level changes in relation to ancient reef development and evolution (Montaggioni and Braitwaite 2009). Changes in sea‐level are related to the availability of habitats suitable for coral reef development and such changes, when large enough, have triggered mass extinctions (Chapter 5).
Biotic controls play a role in reef development (Montaggioni and Braitwaite 2009). The evolutionary history of coral reefs shows an increase in biological disturbance such that there was an increase during the Cretaceous and Cenozoic in predators specialised for corals, including bioeroders and herbivores. These specialised organisms influenced the community structure of coral reef ecosystems. Such organisms limit the distribution and abundance of sessile organisms, such as corals, which require a stable substrate and quiescent sedimentological conditions.
Coastal lagoons can be most simply defined as natural enclosed or semi‐enclosed water bodies parallel to the shoreline. Lagoons are sometimes confused with other coastal ecosystems, such as estuaries and coral reef lagoons. Thus, coastal lagoons can be most precisely defined as ‘shallow aquatic ecosystems that develop at the interface between coastal terrestrial and marine ecosystems and can be permanently open or intermittently closed off from the adjacent sea by depositional barriers’ (Esteves et al. 2008). The waters of coastal lagoons can span the range of salinities from fresh to hypersaline depending on the balance of hydrological drivers, including local precipitation, river inflow, evaporation, groundwater discharge, and seawater intrusion through or directly via the depositional barrier.
The geophysical characteristics that contribute to the formation and maintenance of a coastal lagoon are important and help in identifying different types of lagoons (Eisma 1997). The first characteristic is whether the coastal lagoon has a connection to the sea. Some lagoons are lentic non‐tidal, that is, without permanent connection to the sea or lentic micro‐tidal, permanently connected to the sea. The second characteristic of a coastal lagoon is its origins. Most lagoons have originated from the flooding of lowland coastal areas due to the global rise in sea‐level during the Late Quaternary marine transgression (Esteves et al. 2008). Lagoons originating in this way generally have large surface areas and are located parallel to the coastline which increases the probability for marine intrusions through or over the depositional barrier. Other lagoons have originated by the build‐up of sediments at the mouths of rivers due to the working of waves and tides to form a barrier. Such lagoons have a branched configuration and a high perimeter to area ratio and are formed by the flooding of river valleys. Due to their geomorphology, high levels of dissolved and particulate materials from land enter such coastal lagoons.
Perhaps no other coastal environments are as complex as coastal lagoons. The
