Coral Reef Videos‎ > ‎Short Movies‎ > ‎

Threats to Reefs

Sedimentation

Removal of natural buffers between land and sea, such as mangroves, allows the products of terrestrial erosion to wash straight into marine habitats. The coral animal is able to tolerate acute episodes of sedimentation by secreting mucus as a defense mechanism, helping to keep their surfaces clean. Nonetheless, destructive activity on land allows the continual input of sedimentation. This form of pollution, in particular the fine silt fraction of the sediment, directly smothers coral reefs blocking out the sunlight required for photosynthesis (Loya 1976). The short-term effect can interfere with corals by lowering growth rates, metabolism and fecundity (Rogers 1990). Prolonged smothering and burial eventually causes the coral to die. In addition, the silt covers the hard substrate that was available for settlement of juvenile corals making recruitment of the propagules impossible and reducing their reproductive success (Gilmour 1999).

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

Gilmour J (1999) Experimental investigation into the effects of suspended sediment on fertilization, larval survival and settlement in a scleractinian coral. Marine Biology 135: 451-462.

Loya Y (1976) Effects of water turbidity and sedmentation on the community structure of Puerto Rican corals. Bulletin Marine Science 26: 450-466.

Rogers CS (1990) Responses to coral reefs and reef organisms to sedimentation. Marine Ecological Progress Series 62: 185-202.



Marine Snow

As the name would suggest, marine snow resembles snowflakes suspended in the oceans' water column. Marine snow is the aggregations of a variety of suspended material consisting of calcareous algae, organic detritus, and mucus secreted by plankton, algae, bacteria and corals. Increased nutrient concentrations in coastal waters enhance algal growth, which indirectly increases the levels of marine snow (review by Wolanski et al. 2003). Increased carbon levels resulting from sewage runoff and mucus secretion* also directly influence marine snow. Until recently, the significance of marine snow and coral reef health has been neglected. The adhesive property of marine snow means that it readily attaches itself to suspended sediment (fine clay) from coastal runoff resulting in it becoming negatively buoyant. This muddy marine snow is detrimental and even lethal to coral reefs as it settles on the reef smothering it (Fabricius and Wolanski 2000). Rich in carbohydrates, marine snow is a source of energy available to microbes and therefore affecting microbial activity (Kline et al. 2005) including those associated with the coral tissues. The coral-associated bacterial populations dramatically increase with increased carbon availability, which has been seen to be lethal to the coral (Kline et al. 2005). The following footage shows marine snow in high levels in a severely impacted coral reef in Borneo, Malaysia.
* See video on Mucus Production in Coral Reef Dynamics

Credits
Cinematography: Neilan Kuntz & Dr. Forest Rohwer
Edited by: Neilan Kuntz
Written by: Dr. Olga Pantos
Location: Borneo, Malaysia (Sipadan) (2003)

Fabricius K. E., E., Wolanski (2000) Rapid smothering of coral reefs organisms by muddy marine snow. Estuarine, Coastal and Shelf Science 20: 115-120.

Kline, D.I., N.M., Kuntz , M., Breitbart, N., Knowlton, F., Rohwer (2005) The critical and unexpected role of carbon in coral mortality.” In press. Marine Ecological Progress Series
Wolanski, E., R., Richmond, L., McCook, H., Sweatman (2003) Mud, marine snow and coral reefs. American Scientist 91: 44-51.


Macroalgal Overgrowth

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)



Fishing in No-Taking Reserves

Globally, marine fisheries are under increasing threat due to unprecedented levels of exploitation. Coral reef areas are heavily targeted due to increasing human populations. "No-take" marine reserves are a form of resource management that regulates human activity in order to conserve and protect local marine habitats. Target species are expected to increase in abundance and biomass inside the no-take reserves and eventually influence surrounding areas that are fished (Russ 2002). For example, the density and biomass of the highly target coral trout, Plectropomus spp, was found to be significantly higher in the protected "no-take" zones than in fished zones (Williamson et al. 2004). To be an effective management tool, no-take reserves must have the compliance of local fishing communities and regulation enforcement. Unfortunately, most coral reefs reside in some of the poorest regions in the world where proper infrastructure and enforcement do not exist. The following footage shows an artisan fisherman searching for lobster in an area protected as a "no-take" reserve demonstrating how ineffective this form of resource management is without proper enforcement and compliance of local communities.

Credits
Cinematography: Neilan Kuntz
Edited by: Neilan Kuntz
Written by: Neilan Kuntz
Location: Bocas del Toro, Panama (2003)

Russ, G.R. (2002) Yet another review of marine reserves as reef fisheries management tools. In: Coral Reef Fishes: Dynamics and Diversity in a Complex Ecosystem, ed. P.F. Sale, pp. 421-443. San Diego, USA: Academic Press.

Williamson, D. H., G.R., Russ, A.M., Ayling (2004) No-take marine reserves increase abundance and biomass of reef fish on inshore fringing reefs of the Great Barrier Reef. Environmental Conservation 31(2): 149-159.



Crown of Thorns

One of the most dominant predators of Pacific corals is the crown-of-thorns seastar, Acanthaster planci. A. planci is considered a generalist coral predator with a preference for branching species such as the acroporids (Carpenter, 2004). The seastar feeds on the coral by everting its stomach over the live coral and secreting a protease enzyme that breaks down the coral tissue (Birkeland and Lucas, 1990). Coral consumption is slow, requiring a leisurely 4-6 hours to digest a coral branch or a small colony. It is estimated that a single seastar can consume 5-6 m2 of live coral per year (Birkeland 1989). The immediate ecological impact of A. planci outbreaks on most reefs is the modification of community structure and the colonization of algae in newly available space following coral mortality (Birkeland and Lucas, 1990). The following footage shows slow-moving crown-of-thorns.

Credits
Cinematography: Dr. Forest Rohwer
Edited by: Neilan Kuntz
Written by: Neilan Kuntz
Location: Borneo, Malaysia (Sipadan) (2003)

Birkeland, C., and J.S. Lucas. (1990) Acanthaster planci; major management problem of coral reefs. CRC Press, Boca Raton, Florida, 257 p.

Birkeland, C. (1989) The influence of echinoderms on coral-reef communities. In M. Jangoux and J.M. Lawrence (eds.), Echinoderm studies, pages 1-79. A.A. Balkema, Roterdam.

Carpenter, R.C. (1997) Invertebrate Predators and Grazers. In: C. Birkland (eds) Life and Death of Coral Reefs. Chapman & Hall New York, New York