Mud, Marine Snow and Coral Reefs

Mud, Marine Snow and Coral Reefs The survival of coral reefs requires integrated watershed-based management activities and marine conservation Eric Wolanski, Robert Richmond, Laurence McCook and Hugh Sweatman oral reefs are the most diverse of mine tailings on them, fishing with ex- metabolites from which the corals de- Call marine ecosystems, and they plosives and cyanide, and land recla- rive much of their energy. (Corals con- are rivaled in biodiversity by few ter- mation. Reefs that experience such in- struct their hard habitat the way mol- restrial ecosystems. They support peo- sults often die; those that deteriorate lusks grow their shells, by accreting ple directly and indirectly by building but survive cannot recover to their calcium carbonate.) The main function- islands and atolls. They protect shore- original health as long as the distur- al components of a coral reef ecosys- lines from coastal erosion, support fish- bances continue. In other countries the tem include the hard corals, coralline eries of economic and cultural value, disturbances are more chronic than algae, filamentous and fleshy algae, provide diving-related tourism and acute. Reefs are assaulted by muddy blue-green algae (cyanobacteria), and serve as habitats for organisms that runoff, nutrients and pesticides from a host of invertebrates and fishes. produce natural products of biomed- adjacent river catchments, overfishing Coralline algae are essential to a ical interest. They are also museums of and global-warming effects. These dis- healthy reef because they cement reef the planet’s natural wealth and places turbances affect the key parameters structures and contain chemicals that of incredible natural beauty. permitting reef resilience: water and induce metamorphosis in coral planula Despite their recognized biological, substratum quality. As a result, corals larvae. Filamentous and fleshy algae economic and aesthetic value, coral fail to reproduce successfully, and the can be very abundant; indeed, the reefs are being destroyed at an alarm- coral larvae arriving from more pris- most telltale sign of a degraded coral ing rate throughout the world. Some tine reefs are unable to settle and thrive reef is the replacement of corals by al- countries have seen 50 percent of their on substrata covered by mud, cyano- gae (Figure 2). coral reefs destroyed by human activi- bacteria or fleshy algae. Coral popula- Three genera of corals dominate Pa- ties in the past 15 years. Some human tions thus fail to recover or reestablish cific reefs: Acropora, Porites and Pocillo- influences are acute—for example, themselves. pora. Acropora corals are the most spec- mining reefs for limestone, dumping Can science help save coral reefs? tacular; they are also framework Despite much talk about managing builders, providing habitat for a vari- Eric Wolanski received his Ph.D. in environmental coral reefs, the potential role of science ety of fishes and other reef organisms. engineering from the Johns Hopkins University in is limited. But it is important: Scientists They include the table, elkhorn, 1972. He is a leading scientist at the Australian can demonstrate the key processes con- staghorn and fast-growing branching Institute of Marine Science, where he studies trop- trolling the health of coral reefs and species. Porites corals include boulder ical coastal oceanography and its biological impli- how human activities damage them. or massive corals. Pocillopora corals in- cations for mangroves and coral reefs. Robert Rich- Then, we can hope, land-use managers clude both coarsely and finely branch- mond received his Ph.D. in biology from the State and marine-resources managers will be ing species, widely distributed across University of New York at Stony Brook in 1983. He is a professor of marine biology at the Universi- able to modify human behavior to re- the Pacific and into the Red Sea. ty of Guam Marine Laboratory. His research inter- duce or reverse damage to coral reefs. Natural disturbances—including ests include sublethal stresses on coral reefs. Lau- Toward this end we have developed a hurricanes (tropical cyclones or ty- rence McCook received his Ph.D. in biology from large-scale model for illuminating reef phoons), river floods (Figure 4), earth- Dalhousie University in 1992; he is a research sci- degradation and predicting the impact quakes and lava flows—have affected entist specializing in the ecology of algae and reef of future human activity. coral reefs for millions of years; they are degradation at the Australian Institute of Marine typically acute and have short-lived ef- Science, working with the Cooperative Research The Coral Reef Ecosystem fects. Reef areas away from human in- Centre for the Great Barrier Reef World Heritage The ecological functioning of a coral fluences often recover within a few Area. Hugh Sweatman received his Ph.D. from reef relies on the symbiotic association years if water and substratum quality Macquarie University in 1985; he is a research scientist at the Australian Institute of Marine Sci- between corals and dinoflagellate algae remain high. Acute, natural distur- ence where he leads the long-term reef-monitoring (zooxanthellae). In this system, the di- bances thus help maintain diversity on program. Address for Wolanski: AIMS, PMB No. noflagellates reside as symbionts with- coral reefs by knocking back dominant 3, Townsville MC, Qld. 4810, Australia. E-mail: in the cells of the coral host; the sym- species and allowing less competitive [email protected] bionts take in nutrients and produce species to reestablish themselves. © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 44 American Scientist, Volume 91 Figure 1. Healthy coral reefs such as this section of Australia’s Great Barrier Reef not only are aesthetic treasures but also possess the greatest biodiversity of any aquatic ecosystem. In particular, reefs in which various species of the fast-growing, branching Acropora corals dominate provide habitat for a spectacular array of life both large and small. Despite the recognized value of reefs, worldwide they have been degraded dra- matically over the past several decades owing to anthropogenic effects, and little has been done to halt their decline. A large part of the prob- lem has been a lack of tools that would allow land- and marine-resource managers to estimate the relative benefits to reef health of various regulatory actions. The authors have developed a model that permits managers to estimate the effects on reefs of regulating various human activities. (Except where noted, photographs by the authors.) The synergistic and cumulative effects hesive bridges of exopolymer (mucus). suspension readily attaches to the of human disturbances superimposed The aggregates resemble snowflakes sticky marine snow, forming muddy over natural disturbances make recov- and hence are called “marine snow” marine snow. The clay particles act as ery less likely and, in some cases, result (Figure 5). ballast that makes the flocs settle onto in stable states dominated by algae. Human activities on land often re- coral reefs. Muddy marine snow flocs sult in increased nutrient concentra- settle fast, typically at a speed of about Marine Snow tions in coastal coral reef waters, which 5 centimeters per minute—about 1,000 The water around coral reefs some- enhances the growth of algae. In turn, times faster than individual mud parti- times looks clear, but it can contain a increased nutrients also result in an in- cles settle. The settled muddy marine variety of suspended matter, starting creased prevalence of marine snow. In snow has detrimental or even lethal ef- with inorganic particles such as resus- the nutrient-enriched coastal waters of fects on small coral reef organisms. pended calcareous material, fecal ma- Guam and the Great Barrier Reef, ma- Corals and reef organisms such as terial, organic detrital particles, and rine snow flocs can exceed several cen- barnacles are able to clean themselves mucus secreted by plankton, algae and timeters in diameter. Wave-exposed of small settling flocs as long as the silt bacteria. Corals themselves secrete mu- waters have smaller flocs than do more content remains low—less than 0.5 mil- cus to cleanse their colony surfaces and sheltered areas. ligrams per square centimeter. At high as a metabolic by-product. This mucus Marine snow is almost neutrally siltation levels (4 to 5 milligrams per prevents corals from becoming clogged buoyant and can remain in suspension square centimeter) or when flocs are with sediment particles and also for hours in turbulent reef waters. large (particles 200 to 2,000 micrometers shields against desiccation if they are Near-shore waters, however, may con- in diameter), the coral polyps initially exposed to air at low tide. Particles in tain additional suspended fine clay exude thick layers of mucus and die af- suspension in water are rapidly aggre- particles, rich in nutrients and detritus ter less than one hour of exposure—a gated by flocculation as well as by ad- derived from land runoff. This mud in short time compared with the rate of © 2003 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 2003 January–February 45 natural marine snow deposition on coral reefs. Katharina Fabricius, at the Australian Institute of Marine Science, found, from laboratory experiments, that settling muddy marine snow flocs are far more hazardous to young than to older corals; the mortality rate is 10 times greater for young coral recruits (newly established animals) than for adult corals. Coral recruits typically die after 43 hours of exposure to muddy marine snow, a threshold that is rou- tinely exceeded in nutrient-enriched coastal waters of the Great Barrier Reef but not in areas farther offshore. The presence of muddy marine snow on coral is often short-lived, resulting from Figure 2. Most coral-reef decline, including this example at Palau, is tied in subtle but pre- a river flood or from resuspension by dictable ways to sediment-laden runoff from human activities within adjacent watersheds.

Load more