REEF CONSTRUCTION AND BIOEROSION

Corals need warm tropical water. The best reefs are found within the tropics (equator± 23.5 degrees north and south) where the temperature in the winter months is less than 5 0C lower than the summer months. The minimum winter temperatures usually cited for coral reef development is 20 oC, but reefs can form in waters that may get a degree or two colder, although they are often less diverse. Most coral reefs lie between the latitudes of 30 degrees north and south where ocean temperatures meet the minimum requirements.

 

Corals need access to sunlight (the intracellular dinoflagellates- the zooxanthellae need sufficient  light for photosynthesis), and so reefs are best developed in shallow  water. If the water is too shallow, the mechanical energy of wave and current action will inhibit reef development, as will the intense solar radiation (heat and UV light) damage hosts and their symbionts. If the water is too deep or has significant amounts of particulate material, light penetration will be reduced as explained previously. In very clear oceanic water, reef development may persist to 70 m or more.  In most locations however, reef growth is diminished below 50 m. In Florida, reef growth rarely persists below 30 m. For reasons explained previously, coral reefs do not grow close to the mouths of rivers even within the tropics.

 

Reef Building

We have already seen the effect that reef organisms have on reef growth. 

REEF GROWTH=

 

REEF CONSTRUCTION 

 

MINUS

 

REEF EROSION
 
Reef growth is the absolute value of reef accretion less bioerosion. In other words:                                       

 

 

 

 

 

 

 

 

 

Overall, we can divide reef formation into three components:

  1. FRAMEWORK BUILDERS

 

  1. SEDIMENT PRODUCERS

 

C.  CEMENTING AGENTS
 
 

 


      

 

 

PRINCIPLE FRAMEWORK BUILDERS

 

            • SCLERACTINIAN CORALS

            • NON-SCLERACTINIAN CORALS (e.g., Millepora)

            • CORALLINE ALGAE
 
 

 

 

 

 

 


 

 

 

 

 

 

REEF SEDIMENT PRODUCERS

• fragmented corals and coralline algae

• forams

• Halimeda segments

• sponge spicules

• worm tubes

• mollusk shell fragments

• echinoderm ossicles and spines
 
 

 

 

 

 

 

 

 

 


 

 

 

Echinoderm ossicles, agglutinated sediment  worm tube and echinoid spines
 

Cementing agents:

            • sheet-like growth of stony corals, Millepora

              and coralline algae

            • sessile  forams (Homotrema, Gypsina
 
 

 

 

 

 

 


 

 

 

Bioerosion

 

The opposite of reef growth or accretion is bioerosion. Two types are distinguished. Direct bioerosion is caused by organisms boring into, biting or grinding carbonate substrate. The second type is indirect bioerosion, which is caused by organisms that facilitate the invasion of coral substrate by  killing or injuring coral tissue. We have discussed many of these agents, some of which are illustrated below.

 

                    BIOEROSION AGENTS

• endolithic algae

• boring sponges

• boring bivalves

• sipunculans

• echinoids
• scarids (parrot fish)
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bore holes of many different bioerosion agents can be identified as shown below. The genera  C and S are Cliona and Spheciospongia,  boring sponges.

L is Lithophaga, a boring bivalve. Sipunculan boreholes (referred to here as sipunculids) are also shown.