NOAA's Reporter's Coral Reef Tip Sheet
September 29, 1997 - Week 39

Today, throughout the Florida Keys and Bahamas, almost anything that lives on the rocky bottom is being overgrown by algae, transforming exquisite coral gardens into algal-covered rocks. According to coral researcher Phillip Dustan of the University of Charleston, SC. The culprit may be a deadly combination of sediment and algae working together.

Dustan points out that throughout the Caribbean and Western Atlantic coral cover is decreasing, algal cover is increasing, and coral regeneration is slowing. Signs of stress are most dramatic on coastal reefs near population centers. Algae is abundant on the outer reefs of Key Largo, Molasses Reef has thick rug-like algal mats and Carysfort Reef has mats with finer filaments. All of these types of algal communities trap sediments which shades, smothers, and quickly kills coral tissue. Since corals work best when their surfaces are sediment-free, their metabolic efficiency diminishes and tissue death occurs. Microbes quickly claim the freshly exposed skeleton which is followed by establishment of an algal turf or macroalgal community, especially in the presence of nutrient pollution. Algae vigorously strip nutrients from the water column enabling them to grow faster than the stony corals. To complete the vicious cycle, the death rate of coral from sediment influence increases when algae are present. The algae also shade coral tissue causing bleaching and eventually tissue damage. Large algal colonies can also scrape the soft coral tissue as they wave in the surge. Small algal filaments at the edge of corals also form sediment "dams" which prevent corals from cleaning their surface, slowly suffocating the live tissue.

So where is all this sediment and nutrient load coming from? Dr. Dustin points out that ecosystems on the land are naturally "conservative." They don’t easily give up elements to the sea. Whatever sediment and nutrients are dislodged tend to get trapped and contained. Coral reefs have adapted to the resulting clear water and low levels of nutrients. But the modern processes of development have severely disrupted the land’s capacity to hold onto elements. Agriculture, urban development, forestry and other practices have made it easy for rains to wash sediments and nutrients out to sea.

Locating the exact source of the increased levels of nutrients and sediments found in any specific location is difficult. In the Florida Keys, the pollution of cities, towns, farms and a watershed too vast to control slowly bleeds into the sea through canals, rivers, and coastal bays. The origin can either be a steady and well-defined point source stream or an effluent that seeps from the land with each rainfall. Both push sediments and nutrients into the sea. More of it upwells from injected sewage, some leaches from shallow septic tanks, urban lawns, agricultural lands, or vacant lots. Some washes into the sea along the west and east coast of Florida, the Everglades, the Mississippi, and lands that are farther downstream. Bits and pieces from this wide and diffuse array of sources contribute to a pervasive level of stress on the reef.

Dustan is frequently asked which single factor is responsible, sediments or nutrients? His answer is that we are seeing an accumulation of a whole series of nested stresses which are as local as the fisherman, as regional as the landowner, sugar cane field, or village, and as global as deforestation in Amazonia, the ozone hole, and greenhouse effect. Each factor compounds the rest, a synergy towards death for the reef.

Contact:
Phil Dustan
Department of Biology
University of Charleston
Charleston S.C. 29424
telephone 803-953-8086
fax 803 953-5453
pdustan@zeus.cofc.edu

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