Coral Reefs: Types and processes

Introduction

The term "coral reef" generally refers to a marine ecosystem in which the main organisms are corals that house algal symbionts within their tissues. These ecosystems require: 1) fully marine waters; 2) warm temperatures; and 3) ample sunlight. They are therefore restricted to shallow waters of tropical and subtropical regions.

Fig1. Distribution of the world's coral reefs, showing the relationship to average temperature of the ocean surface.

Corals that do not have algal symbionts can also form significant reef communities in deeper, darker, and colder waters, but these communities are distinguished as cold-water coral bioherms.

The more technical definition of "coral reef" includes an additional geological requirement that the reef organisms produce enough calcium carbonate to build the physical reef structure. The coral reef community lives only on the surface veneer of the reef, on top of already existing skeletal material left behind by previous reef-builders. Many processes act to break down the skeletal material and reef as soon it is laid down by organisms. These include mechanical processes such as waves and currents, and a wide array of biological processes (e.g., bioerosion). Some of the best known bioeroders are large organisms such as parrotfish and sponges, but much of the bioerosion occurs at the microscopic scale by organisms such as algae and fungi. A coral reef is produced only if the coral reef community produces more calcium carbonate than is removed. Indeed, some coral reef communities grow too slowly to build a reef.

Types of Coral Reefs

Coral reefs everywhere grow by the same processes, but their geomorphology is shaped by the foundation on which they grow and sea level history. Most coral reefs of today were established less than 10,000 years ago, after sea level rise associated with the melting of glaciers caused widespread flooding of the continental shelves. Once the coral reef communities were established, they began building reefs that grew upward in concert with continued sea level rise. Reefs that grew too slowly became covered by deeper and deeper water until they received too little light to support reef growth altogether. These reefs are sometimes referred to as drowned reefs.

Fig2. Darwin’s three stages of atoll formation. (Image courtesy of NOAA CORIS)

The geomorphology of reefs largely reflects two main variables: relative sea level rise and the nature of the underlying substrate. Reefs are commonly classified according to three main reef types: fringing reefs, barrier reefs, and atolls.

• Fringing reefs are simply reefs that grow attached to the shoreline (of either continents or islands). Fringing reefs begin growth in shallow water close to shore and tend to accumulate outward. Many reefs of the Florida Keys are fringing reefs.
  
  
• Barrier reefs typically grow along the outer edges of continental shelves, separated from the mainland by open water, and are actually discontinuous composites of many smaller reefs separated by channels. These often grow on top of one or more ancient reef structures that had grown during the previous “interglacial” but then dried out during the following glacial periods (the Great Barrier Reef is an example of this type of reef).
  
  
• Atolls are circular reefs enclosing lagoons, such as Bikini Atoll. Atolls begin by colonizing a seamount or volcano, and then grow upward as the seamount sinks and/or as sea level rises. Eventually, the seamount sinks below the sea surface, while the coral reef continues to grow upward resulting in the characteristic donut-shaped reef enclosing a central lagoon.

Fig3. Profile of coral reef with typical reef “zones”. (Image courtesy of NOAA CORIS)

Reefs are often categorized into more specific categories, such as patch reefs (small patches of reef that tend to grow within lagoonal back-reef areas), platform or bank reefs (isolated, flat-topped reefs that are larger than patch reefs and usually grow on mid-shelf regions), and ribbon reefs (long sinuous reefs that parallel the shelf break). At times, reefs are identified by additional terms such as deltaic, cuspate, and reticulate. Most of these more specific classifications simply reflect the general shapes of the reefs – which may or may not reflect the geomorphology of the underlying substrate – and are not widely used.

Most reefs exhibit a similar zonation pattern that reflects both vertical and horizontal gradients in water energy (mainly waves) and proximity to land.

Coral reef communities

Coral reefs by definition are composed predominantly of scleractinian corals (Phylum Cnidaria; Class Anthozoa; Order Scleractinia). Most scleractinians, and particularly those that build reefs, are colonial, anemone-like animals that house microscopic algae and secrete skeletal structures composed of calcium carbonate. The coral animal apparently derives energy and nutrients through photosynthesis by the algal symbionts, which enhances its growth and allows it to thrive in nutrient-poor conditions. Corals are usually the main contributors to reef accumulation and provide the main structural network, or “framework”, of the reef. However, algae that secrete calcium carbonate, such as coralline red algae and an abundant calcifying green alga known as Halimeda, are almost always significant contributors as well. While many calcifying algae contribute particles that rapidly become reef sediments, coralline red algae often have an encrusting growth form that tends to act as a “glue” that holds the reef fragments and sediments together. Another significant component to reef sediments are benthic foraminifera, protozoans that secrete calcium carbonate tests that range from microscopic to millimeters in length. These organisms produce the bulk of the reef structure and the basis for the entire coral reef community, which includes representatives from nearly every marine phylum, from unicellular organisms to fish and mammals. Coral reef communities similar to those of today first appeared after the Cretaceous-Tertiary extinction event 65 million years ago, and have increased in prevalence in the last few million years. 

Coral Reef Biodiversity

Coral reefs are often considered the marine counterpart to rainforests because of their high biodiversity (driven in large part because the reef itself provides a tremendous amount of structural complexity), the elaborate specializations of the resident species, and the many co-evolved relationships between species. The number of reef species is unknown, although estimates range from 600,000 to 9 million. The most biodiverse reefs occur in the Indo-Pacific region and the least biodiverse (with a tenth of that in the Indo-Pacific) occur in the eastern Pacific. The reasons for this stark difference in biodiversity range from temperature to the area of suitable reef habitat within 600 km of a reef.

Current Status of Coral Reefs

Coral reefs provide many ecosystem services that support human populations economically, culturally and aesthetically. They also provide ecological support for other ecosystems such as seagrass beds and mangroves. Despite the benefits of maintaining healthy coral reefs, many have been seriously degraded due to overuse and abuse by human activities such as over-harvesting of fish and other reef resources, destructive fishing practices, coastal development and poor land-use practices. Coral reefs are also vulnerable to elevated temperature, which causes corals to expel their symbiotic algae in a process called coral bleaching. Mass coral bleaching episodes have increased dramatically over the last 2-3 decades. Corals can recover from bleaching, but high mortalities are often reported over huge reef areas; some 16% of the world’s coral reefs experienced bleaching in 1997-1998, and about half of those have not recovered. A second global threat to corals and coral reefs is ocean acidification, or a lowering of ocean pH due to its uptake of atmospheric carbon dioxide and the subsequent formation of carbonic acid. Ocean acidification has been shown to decrease the rate at which corals and coralline algae secrete their calcium carbonate skeletons.