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Macroalgal Overgrowth

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Uploaded on Mar 10, 2010

The declining health of coral reefs is associated with a phase-shift from predominantly coral to macro-algal dominated reefs (Done 1992). Top-down control by herbivores and bottom-up processes such as eutrophication are critical factors that affect the level of algae on coral reefs (McCook et al. 2001, Lirman 2001). As the algae encroach over the coral reef, the activity of coral associated microbial communities increase dramatically at the boundary between the invasive algae and coral tissues. Oxygen levels are found to be low around coral polyps adjacent to the invasive algae due to increased microbial activity, while coral polyps distanced from the algae have oxygen levels similar to healthy reefs (Smith et a. 2005). The deficiency in the levels of oxygen reaching tissues is analogous to suffocation of the coral animal. This results in a positive feedback loop with both the algae and coral microbes invading the corals tissues. The eventual outcome is an ecological shift from a healthy coral dominated reef to an algal dominated system.

Credits
Cinematography: Dr. Forest Rohwer
Edited by: Neilan Kuntz
Written by: Dr. Olga Pantos
Location: Christmas Island, Line Islands, Central Pacific (2005)

Done TJ (1992) Phase shift in coral reef communities and their ecological significance. Hydrobiologia 247: 121-132.

Lirman D. (2001) Competition between macroalgae and corals: effects of herbivore exclusion and increased algal biomass on coral survivorship and growth. Coral Reefs 19: 392-399.

McCook L.J., J. Jompa, G. Diaz-Pulido (2001) Competition between corals and algae on coral reefs: a review of evidence and mechanisms. Coral Reefs 19:400-417.

Smith JE, M. Shaw, R.A. Edwards, D. Obura, O. Pantos , E. Sala, S.A. Sandin, Rohwer F (2005) Effects of algae on Coral: algal-mediated, microbe-induced coral mortality. (in review)

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