Types of Coral Reefs

Although there are many types of coral reefs, their formation begins the same way. The formation of a coral reef begins when a free-swimming coral larva called a “planula” settles and attaches itself to a hard substrate located in shallow, warm and clear waters (Lalli and Parsons 1997; Spalding et al. 2001). As more corals continue to settle and grow, the combination of their skeletons creates a calcareous platform that promotes the settlement and growth of other coral species (Lalli and Parsons 1997; Spalding et al. 2011). Over time, this process creates vertical growth in the calcareous platform(Lalli and Parsons 1997; Spalding et al. 2001).

Fringing reefs are formed from the vertical growth of a calcium carbonate platform along a shelving coastline (Jones and Endean 1977; Lalli and Parsons 1997; Spalding et al. 2011). Platform growth occurs more rapidly in shallow areas, and declines with depth. Therefore, the basic structure of a fringing reefs includes a shallow platform that extends out to a defined edge, called the reef crest, where it begins to slant down to the seafloor (Jones and Endean 1977; Lalli and Parsons 1997; Spalding et al. 2001).Barrier reefs are formed farther offshore and are separated from the coastline by a deep body of water called a lagoon (Jones and Endean 1977; Lalli and Parsons 1997; Spalding et al. 2001).

There are a few different ways that barrier reefs can be formed. Barrier reefs can form from fringing reefs over time or may simply form in offshore locations, separated from the coastline by a deep lagoon. There are two main ways that a barrier reef can be formed from a fringing reef. Barrier reefs can be formed from fringing reefs that continue to grow around a subsiding landmass. As the fringing reef continues to grow upward, a lagoon forms between the reef and the coastline, creating a barrier reef. In other cases, a lagoon can form between a fringing reef and the coastline from the effects of sea level rise, creating a barrier reef (Spalding et al. 2001).Atolls are circular reef structures that surround a deep lagoon and are common if oceanic areas, far from any landmass (Spalding et al. 2001). Charles Darwin was the first person to understand that atoll reefs develop from the upward growth of a barrier reef system that forms around a subsiding landmass. Bank or platform reefs are reefs that form away from the coastline. These structures lack a lagoon and therefore cannot be considered barrier or atoll reefs (Spalding et al. 2001).

There are many other reef types that lack the specific characteristics of fringing, barrier and atoll reef systems and cannot be labelled as such (Spalding et al. 2001). Bank barriers are common in the Caribbean (Spalding et al. 2001). These structures lie offshore and are separated from the coastline by a shallow lagoon and therefore cannot be true barrier reefs (Spalding et al. 2001). The long reef tracts observed off Florida and Cuba are examples of bank barrier reefs (Spalding et al. 2001).

Near atolls are coral reefs that surround the remnants of an island and therefore cannot be considered true atolls. Atoll-like platform reefs are coral reefs that look like true atolls but do not develop around a subsiding volcanic island, and patch reefs are small coral reef systems that form in various shallow water areas (Spalding et al. 2001). Fringing reefs, bank-barrier reefs and patch reefs are common in the US Virgin Islands (Rogers et al. 2008; Rothenberger et al. 2008).

Community composition varies dramatically across a coral reef due to changing environmental factors such as bathymetry, depth, wave action, current strength, light penetration, temperature range and turbidity (Lalli and Parsons 1997; Spalding et al. 2001; Schwartz 2006). As a result ,coral reefs are divided into different zones that vary among coral reef types(Lalli and Parsons 1997; Spalding et al. 2001; Schwartz 2006). Fringing reefs have three zones called the reef flat, the reef crest and reef front/slope or fore reef (Lalli and Parsons 1997; Spalding et al. 2001;Schwartz 2006; Heemsoth 2014).In addition to these zones, barrier reefs and atolls have a lagoon that separates the back reef from the coastline (Lalli and Parsons 1997; Spalding et al. 2001; Schwartz 2006; Heemsoth 2014)

Lagoon and back reef zones

If one were to make their way across a barrier reef, starting at the shoreline and moving out towards the open ocean, they would first encounter a lagoon that slopes upward into a back reef zone((Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).The composition of species found in a lagoon can vary substantially between coral reefs due to differing environmental conditions (Lalli and Parsons1997; Spalding et al. 2001; Heemsoth 2014).For example, shallow lagoons that are semi-enclosed can have extreme temperature fluctuations and higher levels of sedimentation and nutrient deposition (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014). Conversely, lagoons may have environmental conditions that facilitate coral growth and survival such as clear, calm water (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014). Seagrass communities are also commonly observed in lagoons (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).The back reef zone is located between the lagoon and the reef crest of a coral reef.Back reef environmental conditions are similar to those found in the lagoon and vary between coral reefs (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).

The Reef Flat

In a fringing reef, the reef flat zone extends directly from the coastline and is the first zone encountered if one were to make their way across a coral reef. In barrier reef systems, this zone is located on the ocean side of the back reef zone. In both reef systems, the reef flat is characterized by extreme environmental conditions such as shallow depths, substantial temperature fluctuations, low water circulation and in some cases, exposure to air during low tides (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).The base of the reef flat is composed of coral skeletal material, coral rubble and sand that may be colonized by algal and sea grass species (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014). This zone provides permanent habitats for many invertebrate species and at low tide, many other organisms migrate to the reef flat to search for food (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).

The Reef Crest

The reef crest zone is located on the ocean side of the reef flat and is characterized by constant wave action and occasional exposure to air (Lalliand Parsons 1997; Spalding et al. 2001; Heemsoth 2014). Since the reef crest is located at a deeper depth than the reef flat zone, water circulation is more constant and temperature fluctuation are less extreme than the latter (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).Certain species of branching coral such as Acropora palmata (elkhorn coral) dominate this part of the coral reef (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014). These corals form extensive fields within this zone providing habitats for many reef fish and invertebrate species (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014)

The Reef Front or Slop

Past the reef crest is the reef front or slope (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).This area has higher levels of species diversity and abundance than any other zone on a coral reef (Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014).Within the reef front or slope, environmental conditions change substantially with depth(Lalli and Parsons 1997; Spalding et al. 2001; Heemsoth 2014). In the most shallow parts of the reef front, where wave action can be extreme, conditions are similar to those of the reef crest and branching corals such as Acropora palmatadominate (Hubbard 1988; Jackson 1992; Aronson and Precht 2001; Pandolfi and Jackson 2006; Bellwood and Hughes 2006). However, in deeper waters where environmental conditions are more stable, diverse coral communities flourish (Lalli andParsons 1997; Spalding et al. 2001; Heemsoth 2014).