More on Coral Reefs


Factors influencing reef development include:
  • Light
  • Temperature
  • Sedimentation
  • Salinity

Coral reefs occur along coastlines in tropical and subtropical regions wherever environmental conditions are
suitable for their development.

Light is critical in maintaining the symbiotic association between corals and symbiotic algae (zooxanthellae). The
intensity of light greatly affects photosynthetic rates of the zooxanthellae, indirectly impacting coral growth and
survival. Abundance of corals decreases rapidly with depth due to reduced light levels. In clear tropical waters,
corals may live as deep as 150 feet (48 m), with very limited species found beyond that depth.  

Seawater temperatures can be tolerated between 61-95°F (16-35°C), with optimal coral growth occurring at
temperatures of 73-77°F (23-25°C). These temperatures exist throughout most of the tropics with the exception of
cool water currents off the west coasts of Africa and Australia. However, where currents move warm water from the
tropics towards the north, reefs can survive in subtropical regions as near Bermuda.  

Reefs can only develop in areas lacking nearby rivers that bring silt and freshwater into marine environments.
Excessive sedimentation reduces available light, inhibiting photosynthesis by the symbiotic algae. Silt also settles
on the coral surface, blocking feeding and respiration.

The amount of dissolved salts in the water is referred to as salinity and is measured in parts per thousand (ppt).
Corals can tolerate a narrow range of salinities, between 30 and 40 ppt.


Distribution of coral reefs:
  • Worldwide
  • Florida

Reef corals are found throughout tropical and subtropical oceans in the Indo-Pacific and Atlantic, normally
between the Tropic of Capricorn and Tropic of Cancer (30ºS, 30ºN latitude). However, they also occur outside this
range where warm water currents travel outside this band, as is the case where the Gulf Stream Current brings
warm tropical water from the Caribbean north to the islands of Bermuda.  

Along the Atlantic coast of Florida, Oculina reefs dominate in the north, transitioning southward to tropical reefs.
South and west of Miami, offshore of the Florida Keys archipelago, is the only tropical coral reef found along the
North American coastline. This region, also known as the Florida Reef Tract, is the third largest barrier reef
system in the world. Toward the coastline, these reefs are surrounded by the Florida Keys, Biscayne Bay, and
Florida Bay. Moving out to sea from the reef is the Florida Current. This current brings tropical waters into the area
that moderate winter water temperatures as well as large volumes of plankton and larvae-rich water from the


Coral reef communities within Florida waters are categorized as:
Patch Reef
Bank Reef
Hardbottom Community
Close to shore
Low species diversity
Dominated by gorgonians, algae, and sponges

Hardbottom reef communities are found close to shore over limestone rock covered by a thin sandy layer.
Hardbottom communities have low species diversity, dominated by gorgonians, algae, sponges, and a few stony
coral species. Hardbottom habitats provide important cover and feeding areas for many fish and invertebrates.  

Hardbottom communities are often divided into two types:·        
  • Nearshore restricted hardbottom communities are subject to limited water movement. This bottom
    community is dominated by algae including epilithic algae that attaches itself directly to the limestone
    bottom as well as drift algae.

  • Nearshore high-velocity hardbottom communities are exposed to strong currents. Gorgonians, easily
    recognized with their rod-like appearance and flexibility, and sponges dominate these communities.  

Stony corals have adaptations, including a mucous layer used to removed sediments from their polyps, enabling
them to survive in these communities. These coral species include:
  • smooth starlet coral (Siderastrea radians)
  • mustard hill coral (Porites asteroides)
  • golfball coral (Favia fragum)
  • elliptical star coral (Dichocoenia stokesii)
  • common brain coral (Diploria strigosa)

A variety of other animal species, including anemones, mollusks, crabs, seastars, sea cucumbers, spiny
lobsters, and fish, are associated with hardbottom communities. Fish that forage in these areas include grunts
(Haemulon spp.), snappers (Lutjanus spp.), groupers (Epinephelus spp.), and great barracudas (Spyraena
barracuda). Tangs (Acanthurus coeruleus) and surgeonfish (Acanthurus bahianus) form large feeding schools
over hard botttoms.  

Gorgonians provide an increase in surface area for attachment of invertebrates including tunicates, anemones,
mollusks, and worms.
Florida Keys National Marine Sanctuary