Reef-builders and sediment producers
Photo courtesy Hawaii Coral Reef Network
Jodi N. Harney
Modern coral reefs are specialized, colonial, hermatypic, symbiotic communities of coral and coralline algae that develop in warm (generally 18-30 degrees C), clear, well-lit, low-nutrient tropical and subtropical waters (0-28 degrees N/S latitude) supersaturated with CaCO3. Zooxanthellate corals, by nature of their photosymbiotic habit, are generally limited to shallow water environments; most grow in depths <20 m, although growth can occur to the base of the photic zone, often at 100 m. Coral reefs are generally removed from terrigenous input that brings fresh water, sediments, and nutrients that favor other competitors (e.g. fleshy algae and sabellariids). Zonation often occurs in tropical–subtropical coral reefs, such that certain communities and/or growth forms typify portions of the reef that are subjected to differential environmental conditions such as wave energy and water depth.
Tropical and subtropical reef communities are characterized primarily by scleractinian corals and coralline algae. Other contributors to warm, shallow-water systems include coralline and calcareous algae (such as Halimeda) bryozoans, sponges, serpulid and sabellariid polychaetes, vermetid gastropods, and oysters. Please visit my web pages on Hawaiian algae and sea life for photos and information on these organisms.
Reefs are important natural resources that protect coastlines, provide habitat for marine plants and animals, and offer an intriguing environment for humans to explore and appreciate. In addition, reefs are prodigious producers of carbonate sediment. In Hawaii, the white sandy beaches owe their existence entirely to constructive and destructive processes on shallow-water reefs. Every grain of carbonate sediment on Hawaii's beaches and in nearshore waters used to be a living animal or plant. Bioerosion and mechanical erosion of reef structures is the primary source of carbonate sand to most beaches here. Check out my web page on sand in Hawaii to learn more.
Although Hawaii is not known for its coral diversity (we have only 10 coral genera), one quarter of the species present are indigenous or endemic to the islands. The most common scleractinian (stony) corals found on Hawaiian reefs are shown here.