Bioerosion

Bioerosion

What is bioerosion?

• Bioerosion is the naturally occurring process which entails the removal of calcium carbonate substrate by bioeroders.

• The skeleton of corals is regularly being broken down into sand and rubble by bioeroders to make room for new corals to grow.

• However, if bioerosion is excessive then coral destruction will be faster than coral growth and the mechanical structure of the corals is weakened.

• Bioeroders include three main groups:

  *Microborers ==

           *organisms that bore microscopic holes into the surface of the coral substrate
           *This includes tiny endolithic algae, cyanobacteria, and fungi
           *In Australia, microborers erode about 0.35 kg of calcium carbonate per meter square in a year. If this rate is accurate for 
           the entire Great Barrier Reef, then microborers alone could produce enough sand to fill 1.1 million school buses in one year (1)

 *Macroborers
           *animals that erode the internal structure of coral making it weak and fragile
           *This includes small marine worms, sponges, bivalves, and barnacles.

 *Grazers
           *animals that scrape the surface of the coral skeletal framework often while trying to eat the algae that lives on the reef
           *These scrapings change the surface area of the framework which has implications for physical processes, such as water exchange, 
           on the reef.
           *This includes mainly urchins and fish

History:

• Limited evidence of Microborers reaches as far back as the Precambrian period (~800 MYA)

     *More conclusive research found Microborers in fossilized reefs from the Triassic period (~250 MYA)

• Macroborers (boring worms) are first seen in reefs in the Lower Cambrian period (~500 MYA)

     *Boring sponges and bivalves became important in reefs in the Triassic period

• Grazers were not relevant to reef bioerosion until later geologic periods

     *Patellids (Sea Snails) and Echinoids (Sea Urchins, Sand Dollars, etc) impacted bioerosion starting in the Jurassic period (~175 MYA)
     *Scarid Fishes are first seen in the Miocene period (~20 MYA)

• The intensity of bioerosion has changed over these extremely long time spans

     *The general trend has been an increase in bioerosion as more types of bioeroders evolved

Effect on reefs:

• Coral reefs are able to absorb about 90% of energy from wind-driven waves before they reach the shore. This helps protect the shoreline and coastal property from destruction, especially during storms and tsunamis. When the skeleton becomes weakened, these natural events are much more destructive and can wreak havoc on coastal communities.

• Coral reefs provide shelter for sea animals, so if the reefs are broken down, there is less shelter available for fish, crabs and shrimp. This is a problem for the species in that they do not have a habitat and for the humans that eat these ocean species.

• If bioerosion occurs at a faster rate than corals build, the reefs will not be able to keep up with sea-level rise. Humans have contributed to the increased speed of coral reef degeneration.

• About 114 reef fish species, like butterflyfish, parrotfish and damselfish, eat the surface of live coral without damaging the underlying skeleton. Damage to coral reefs takes this food source away from them (2).

Why is it happening:

• Bioerosion is a naturally occurring and necessary process in a healthy coral reef environment
• However, recent research has shown that human-induced factors could accelerate bioerosion rates and negatively impact the delicate balance between bioerosion and coral growth
• Ocean Acidification caused by global warming has been shown to significantly increase bioerosion by the sponge, Cliona orientalis
• Increasing Ocean Temperature is detrimental to coral reefs but research into its impact on bioerosion has been inconclusive

Sources

1. Tudhope AW, Risk MJ. 1985. Rate of dissolution of carbonate sediments by microboring organisms, Davies Reef, Australia. J Sediment Petrol 55:440-447.

2. "Threats to Coral Reefs." Endangered Species International. N.p., 2012. Web. 22 Feb. 2015. <http://www.endangeredspeciesinternational.org/coralreefs7.html>.

3. Glynn, Peter W. "Bioerosion and Coral Reef Growth: A Dynamic Balance." Bioerosion and Coral Reef Growth: A Dynamic Balance (2012): 69-98. University of South Florida. Web. <http://www.marine.usf.edu/reefslab/documents/evol_ecol2007/Glynn%28inpress%29.pdf>.

4. Vogel, Klaus. “Bioeroders in Fossil Reefs.” Facies (1993): Volume 28, Issue 1, pp 109-113. <http://download.springer.com/static/pdf/97/art%253A10.1007%252FBF02539732.pdf?auth66=1424793780_4823023e5d8645903321f3ec3461c4dc&ext=.pdf>

5. P.D. Taylor, M.A. Wilson, Palaeoecology and evolution of marine hard substrate communities, Earth-Science Reviews, Volume 62, Issues 1–2, July 2003, Pages 1-103, <http://www.sciencedirect.com/science/article/pii/S0012825202001319>

6. Wisshak et. al., “Effect of Ocean Acidification and Global Warming on Reef Bioerosion - Lessons from a Clionaid Sponge.” Aquatic Biology: Volume 19, pp 111-127. <http://www.int-res.com/articles/ab_oa/b019p111.pdf>

7. Bird, E. C. F. Coastal Geomorphology: An Introduction. 2nd ed. Chichester: Wiley, 2000. Print.