Coral Reef Info

Why Do Reefs Look the Way They Do in Florida?

Originally published in the Florida Keys National Marine Sanctuary Newsletter, then updated in 1999 — this is a work in progress.

By Dr. Steven Miller, Director, NOAA’s National Undersea Research Program at the University of North Carolina at Wilmington.


I think it’s fair to say that most people take it for granted that actively growing and beautifully developed coral reefs are part of our heritage in the Florida Keys. And if the reefs are not in great shape it’s due to some kind of human interference. It’s easy to see why this perception is so strong. Major ship groundings that smash reefs are measured in the thousands from colonial times. Harvesting fish, corals, live rock, and sponges are often inheritances passed from generation to generation; and people are capable and inclined by nature to overfish. Maybe not individuals, but as more and more people come to depend upon a resource, the chances are less and less that the resource will sustain itself. Over–harvesting is not just a commercial problem, but includes a substantial recreational component too. The human assault also includes polluted nearshore waters (mostly from sewage) that threaten off–shore reefs. However, if you try to make the case that people are totally responsible for whatever changes occur to the reefs in Florida, you would be wrong.

Changes to reefs in Florida and the Caribbean

It’s important to understand that human–caused damage to reefs is just part of the story that describes why reefs look the way they do in Florida. For example, reefs in Florida represent the northern extension of a well developed Caribbean flora and fauna. Today, active reef growth stops south of Miami, but less than two thousand years ago actively growing coral reefs were found just off Fort Lauderdale. Something happened to shift the northern boundary, before people were capable of damaging the system. In recent times, cold weather fronts have moved from the north to sufficiently reduce water temperatures to kill large numbers of corals. This emphasizes that things can happen to reefs that are not the result of human activity, especially in Florida where our corals live in a more subtropical, rather than tropical, region. Sea level rise in the last several thousand years is also probably responsible for the demise of certain reefs too, when back reef lagoons develop that cause sediment buildups and subsequent declines in water quality.

Things get even more complicated when we look at more recent factors affecting coral reefs. Major events have changed the way reefs look over relatively short time scales, and while scientists were around to observe the changes they still don’t understand the causes. For example, in the early 1980s a disease epidemic killed off almost all the long–spined black sea urchins that lived in the Caribbean and south Atlantic. If you remember the urchins you might be inclined to say good riddance, but the urchins were the sheep of the reef and they helped to keep seaweed growth in check. Now that they’re mostly gone it’s uncertain if the additional seaweed found on reefs is due to nutrient pollution, or the absence of urchins.

In the 1980s and 1990s a disease epidemic killed a significant amount of the fast growing branching corals (the Elkhorn and Staghorn species) throughout the Keys and Caribbean, significantly reducing the three dimensional structure of the reefs and eliminating two of the major reef building corals from many locations. The loss of Elkhorn and Staghorn coral is terrestrially equivalent to the loss of trees from Dutch Elm Disease or Chestnut Blight — both the seascape and landscape have been significantly changed by disease (for more information about coral diseases please see the statement prepared by the International Society for Reef Studies). Also, in the last two decades we have seen coral bleaching events more frequently, and more intensely than ever before. This past year (1998) was the worse year on record, worldwide. In the Keys, serious coral bleaching was also seen the year before (1997). These consecutive bleaching events are causing alarm among local scientists, managers, and the public (for more information about coral bleaching please see the statement prepared by the International Society for Reef Studies).

Water quality concerns
The number one issue highlighted in our daily newspaper headlines is related to declining water quality in south Florida, with problems extending from south of Lake Okeechobee, through the Everglades, into Florida Bay, and ultimately to include the offshore coral reefs. In the Florida Keys, sewage disposal is blamed for the decline of our nearshore ecosystems (hardbottoms and seagrasses close to shore) and represents a major threat (most people believe) to our offshore reefs too. Few data exist to suggest that the changes we’ve seen in our offshore reefs are caused by local sewage problems. However, that doesn’t mean the threat is not real, only that data do not support the idea that polluted water from sewage in the Keys is making its way out to the reefs.

There is a tremendous amount of natural system variation in the delivery of nutrients to our offshore reefs, and this natural system variation needs to be considered relative to whatever sewage is being added to the system. Natural variation in nutrients is substantial in the Keys because the Gulf Stream delivers huge amounts of water to the system every day. High current speeds in the Gulf Stream have two effects: 1) a substantial total amount of nitrogen and phosphorus is delivered to the reef (low concentrations times high flow rates equals substantial delivery RATES — rate is the key, not concentration); and 2) the potential for pollution is reduced because reefs in the Keys are part of an “open system,” where Gulf Stream waters and the tides have the potential to flush large amounts of pollution offshore — away from the reefs. There is, however, a building body of evidence to suggest that the entire Caribbean and Gulf of Mexico are polluted from the Great Rivers of South America and the Mississippi. If so, this might explain some of the regional declines seen in coral reefs that occur far from large population centers and coastal development.

Global issues
Were people responsible for the demise of urchins and corals, and coral bleaching? Nobody knows, but the Caribbean–wide extent of the disease problems, and worldwide coral bleaching, suggests that answers will be found beyond local shores. It is possible that coral and urchin diseases might be caused by stress due to pollution, and coral bleaching might be caused by global warming (note: carbon dioxide that causes the greenhouse affect that warms our planet also affects the pH balance of the oceans, and as atmospheric carbon dioxide concentrations go up more becomes dissolved in our oceans — lowering pH with the potential to seriously affect organisms that calcify — like corals) due to human-caused greenhouse effects. Whatever the causes, however, an important (but frustrating) take–home message is that things are happening to coral reefs that are difficult to explain, and appear to be happening in ways that are beyond our control. However, we know for certain that human–caused damage can make things worse, faster. Thus, there are things we can do to help our coral reefs — if we’re willing to change longstanding human behavior. Not an easy task.

The “Health” of Coral Reefs

Note that I talk about change rather than “health” when I discuss the condition of reefs. You will frequently see the word health used in reference to ecosystems, but it is a term better applied to individual organisms. It’s also a term that doesn’t distinguish between human and naturally–caused changes to the reef. Health, when applied to ecosystems, is really a metaphor that’s used to describe some perception of what we think is good and right about how the ecosystem should look and function. It somehow legitimizes our view that we know what we’re talking about when we observe change. When you go to the doctor with a stomach ache you might also have a fever and chills. The fever and chills are signs that something is wrong. When you tell the doctor about your aching stomach, that’s a symptom that can mean many different things. Taken together, signs and symptoms can be used to make a diagnosis about your health. Can we do the same thing for a reef?

It’s difficult, if it can be done at all. We can observe change, which might be taken as a sign something is wrong. But the reef can't describe symptoms. And our perception about what a reef is supposed to look like is strongly biased toward what we see in documentary films, or what we grew up with. Neither case necessarily describes a long–term condition. A term that works better than health is something called “biological integrity.” In this approach, you separate human–caused change from natural system variation. You do this by asking, “What would the reef look like if people weren’t around?” This forces you to recognize that there are things happening out there that don’t have anything to do with people. It also allows you to distinguish between things that people do to reefs that can cause problems, and Mother Nature.

The Future

In many ways we are lucky to have reefs at all in Florida. Urchin dieoffs, disease epidemics, coral bleaching, extreme weather events including hurricanes and cold fronts, rising sea level, maybe even the average annual position of the Gulf Stream, all effect the way reefs look in Florida. That makes it all the more important that we try to minimize human–caused damage. Except for Hurricane Georges, the Keys have escaped storm damage for an unusually long period of time. History tells us that the average amount of time between storms is about seven years, much more frequent that the 60 years that have passed since the last major storm in the lower Keys. One of the major challenges to scientists and managers working in the Keys is to understand the effects of natural system variability and human caused damage. Why? Because the latter is manageable, the former is not.

Management issues

All is not doom and gloom. A tool that managers now have to help learn more about how reefs work, and why they look the way they do, is found in the Florida Keys National Marine Sanctuary Management Plan. Marine protected areas, called Sanctuary Preservation Areas (SPAs), are located from Key West to Key Largo (one larger Marine Reserve is located in the lower Keys that stretches from shore to the reefs, while the SPAs encompass only limited reef areas). The SPAs are designated as “no–take” zones where harvesting of marine resources is prohibited. Access to the SPAs by boaters, snorkelers, and divers is allowed. A small subset of the marine protected areas are further protected as Research Only Areas, where access is by permit and only for research. Taken as a whole, the marine protected areas allow previously harvested resources a chance to recover. Experience worldwide leads us to believe that these areas will eventually contain more and larger fish, and possibly more and larger invertebrates such as lobster and conch.

It is believed that changes to the fish and invertebrate populations are also likely to cause changes to what lives on the bottom, possibly affecting corals, algae, and other organisms. Further, economic considerations suggest that the worth of larger and greater numbers of fish, and possibly improved conditions on the reef (as a more natural assemblage of organisms develops due to changing proportions of carnivores and herbivores), far exceeds present values. For example, the worth of a ten pound grouper caught and served in a restaurant or taken home might exceed fifty dollars. However, the same grouper, growing to 40 or 50 pounds in a protected area, will attract tourists year–round who will pay for trips out to the reef to see fish that can’t be seen anywhere else. Thus, generating income and jobs worth many thousands of dollars.


This is an exciting as well as a critical time in the history of coral reefs. Managers and scientists who work in the Florida Keys National Marine Sanctuary have unprecedented opportunities to protect and study our reefs. World attention is focused on the Management Plan to learn about results that can lead to a better understanding about how reefs function in Florida, and why they look they way they do. Threats to coral reefs are real, and come from many different directions. The stakes are high and time is short if we hope to reverse human–caused damage to coral reefs in Florida. The management response in Florida is cause for celebration. The combination of NOAA’s vision to protect parts of the marine sanctuary from harvesting, and the Environmental Protection Agencies Water Quality Protection Program, give us a good chance to handle the human assault coral reefs face in Florida.