Jodi N. Harney

Reef-builders and sediment producers

Pocillopora eydouxi
Photo courtesy Hawaii Coral Reef Network

Jodi N. Harney
Ph.D. Student, Coastal Geology Group
Dept. of Geology & Geophysics, University of Hawaii

Coral reefs are unique ecosystems, sites of underwater beauty, protectors of coastline, and factories of sediment. Here in Hawaii, where a siliciclastic source of sediment is absent, marine organisms living on and within corals reefs are the sole source of sand for beaches and nearshore environments. Corals and calcareous algae build a submerged reef framework that is constantly undergoing breakdown by physical and biological erosion. The product of such reef erosion is rubble, sand, and fine sediment. Some of the sediment is lost, some remains locked up within the reef framework, and some is released to the surrounding environment. Released sediments may be stored in submarine channels and depressions or deposited onshore as beaches and dunes. Are sediment supply and storage "balanced"? Check out my current research, a sediment budget of Kailua Bay, to investigate that question.

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.

Phylum Cnidaria
Class Anthozoa
Order Scleractinia


Montipora verrucosa Montipora patula Montipora flabellata
rice coral ringed rice coral blue rice coral


platy morphology Porites lobata

Porites compressa
finger coral

Porites evermanni Porites rus Porites furcata


Pocillopora meandrina Pocillopora eydouxi Pocillopora damicornis
cauliflower coral antler coral lace coral


Pavona varians Pavona duerdeni
false brain coral


Leptastrea purpurea Leptastrea bottae Cyphastrea ocellina




Fungia scutaria Psammocora stellata Tubastrea coccinea

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