Coral Reef systems are highly complex with countless interactions between an immeasurable amount of species. Corals themselves are the most dominant species found growing on the substrata when the system, as a whole, is healthy. However, live coral cover has began to decline globally over recent years. One of the main factors contributing to this coral reef health decline comes as a result of negative human actions and inactions. This includes: pollution, run-off from agriculture & irrigation, land clearing and over fishing just to name a few human based factors that can greatly affect the health of coral reef systems. Corals are known as Bio-indicators, which basically means that when the system is “out of whack” corals are the first organism that shows signs of health deterioration on a large scale. This is due to corals being highly sensitive and requiring a very specific set of environmental and biological conditions to survive, including temperature, salinity, abundance of herbivorous fish, water clarity and nutrient levels. When one, or a few, of these factors are outside the preferred requirements for healthy coral growth then the door opens for other organisms to replace them as a dominant species.
Competition between differing organisms that grow on the substrata is intense and complex, and one of coral’s main competitors is algae, and in particular, benthic algae. This competition between corals and benthic algae is a key ecological process on reef systems, which determines not only the composition, but structure of benthic communities (Jompa & McCook, 2003). This interaction between coral and algae is highly complex with numerous mechanisms and variables. When in direct contact, algae employs differing mechanisms in order to compete with corals for the substrata, which does have various effects on coral tissue ranging from minor stress symptoms to severe consequences, such as, death. Smith et al. (2003) observed another one of these mechanisms, where they stated that autotrophs, such as algae, naturally secrete dissolved organic 
carbon (DOC) as excess photosynthate. This increases the microbial activity that causes stress in the coral due to the result of hypoxia. This mechanism when employed by algae can result in localized coral mortality (Smith et al. 2006). Infecting coral with disease is another mechanism used by algae to out compete coral. It is believed that an increase in the number of diseases from 2 to 18 has occurred in the last 30 years. It is known that algae can act as either a reservoir of pathogens or as a vector carrying other infected organisms, which has the potential to cause widespread mortality. However, not all direct contact from certain coral species and algae species cause coral to experience forms of localized mortality and/or stress; it all depends on the physical, biological and chemical properties of the algae (Jompa & McCook, 2003). Furthermore, the overall health of the reef system would be another deciding factor to the coral’s resilience to algae’s competitive nature. Remembering that when the coral reef system as a whole is healthy then corals have an easier chance to defend, in other words, kick algae’s arse and remain the dominant benthic species.
So why is this important?? Well, when the degradation of coral reef systems occurs, it tends to imply that a phase-shift is in progress, which is where algae become the dominant species on reefs instead of corals (Jompa & McCook, 2003). This results in the biodiversity of a healthy reef system decreasing significantly, losing a great deal of its signature colour and abundance of various organisms. As a frequent diver and marine biologist I enjoy hitting the tropical water and admiring the unique diversity and colour that is found on a healthy coral reef system. The importance of maintaining such a system is crucial on numerous levels, including tourism, bio-medical research and fisheries amongst others. The increase of negative human based activities that alters the health of this complex system opens the door a little more for alternative organisms, such as, algae to out compete corals and cause a phase shift. However, the basic concept that a more environmentally aware and conscience society can aid in changing lifestyles and reducing negative human actions that result in coral reef degradation. This in turn may provide a release trigger and reduce the coral’s susceptibility to algae’s competitive nature that causes disease and potential localized/widespread mortality. Hopefully, this will help to maintain healthy coral reef systems around for future enthusiasts, researchers, divers and photographers for future generations to come.
Tom Mitchell, BS Marine Biology
Tom is a marine biologist from the eastern coast of Australia specializing in corals in the Great Barrier Reef. Tom is visiting Canada for the second time and has fallen in love with the west coast and all the biodiversity that this area has to offer. He is a regular contributor to Pinnacle’s Dive Blog as well as a divermaster on the boat.
References
Smith JE, Shaw M, Edwards RA, Obura D, Pantos O, Sala E, Sandin SA, Smriga S, Hatay M & Rohwer FL. 2006. Indirect effects of algae on coral: algae-mediated microbe-induced coral mortality. Ecology Letters 9:835-845.
Jompa J & McCook LJ. 2003. Coral-Algal competition: macroalgae with different properties have different effects on corals. Mar Ecol Prog Ser. 258:87-95.
Nugues MM, Smith GW, van Hooidonk RJ, Seabra MI & Bak RPM. 2004. Algal contact as a trigger for coral disease. Ecology Letters. 7:919-923.
