Walter Jaap: Associate research scientist with the Florida Marine Research Institute
where he has worked for 26 years.
Article Source:  "The Florida Keys Environmental Story" produced by the
Monroe County Environmental Education Council

Seaward of the Florida Keys lies a unique state and national treasure. This treasure is not gold, silver or diamonds; the treasure is the subtropical marine ecosystem of mangrove forests, seagrass meadows, and coral reef communities. The Florida Keys is the only place in North America featuring these natural systems. This ecosystem is important for many reasons. It supports important recreational and commercial fisheries, attracts millions of tourists, protects the coast from storms, and provides a vast array of recreational opportunities. Its economic value can only be estimated; however, the value of an individual coral reef may exceed $400 million. Although this section describes only coral reefs, mangroves and seagrasses also play an important role. The interdependent relationships among these components should be recognized when considering this ecosystem.

To illustrate, let’s consider the life cycle of the spiny lobster. Lobster larvae are released from the female lobsters in seaward coral reefs in the Atlantic Ocean and remain floating in plankton for a considerable length of time (six to nine months). Young lobsters settle on the sea floor, near the coast, in algal patches. Juvenile lobsters move to the hardgrounds and patch reefs. At sexual maturity (about three years old), they move into dens in offshore reefs. The behavior pattern of adult lobsters also reflects the use of various ecosystem components. During the day, lobsters seek refuge in their dens; after dark, however, they emerge to forage for food in seagrass meadows and sandy areas away from the reef. They feed on small mollusks and worms that inhabit these communities.

There are numerous animals with similar life cycles throughout the coastal ecosystem. To maintain and sustain the ecosystem, it is important to ensure environmental quality for all the components.

Coral reefs are wave-resistant limestone masses found on the sea floor, constructed by plants and animals. These limestone masses provide shelter, food and breeding sites for the multitude of other plants and animals found on a reef. Coral reefs concentrate immense biological activity into a small area. In this they are similar to tropical rain forests because they have many thousands of species, provide numerous habitat types (canopy: upper level; understory: middle lever; substory: lower level), recycle limiting chemical nutrients, and they exhibit strong interspecies dependencies (symbiotic interrelationships). The corals are the equivalent of the trees and the fish are the equivalent of the birds. Coral reefs are complex structural and functional biological communities. There is a richness in the number of species and methods by which these plants and animals obtain food and nutrition.

Florida Keys coral reefs are similar in structure and resident species to reefs located in the Bahamas and the Caribbean. In fact, many of the plants and animals found on Florida reefs probably have ancestral heritage with reefs in the Caribbean. Many reef organisms give birth to planktonic young that are carried by wind and currents to settle in areas distant from their parents. The Florida Current, that portion of the Gulf Stream found in the Straits of Florida seaward of the Florida Keys, is an excellent transport system to supply the Florida reefs with species from other Caribbean locations. This supplements the young that are born in the Florida coastal ecosystem.

Geography and geology are important to the distribution of coral reefs off the Florida Keys. Geologists report that Florida’s recent reefs came into existence about 6,000 to 7,000 years ago. Sea level was much lower and seawater temperature was slightly reduced. About 4,000 years ago, Florida Bay flooded as sea level rose and began to influence the distribution of coral reefs.

Since Florida Bay is a shallow basin, climate has a significant influence on its water quality. Heavy rainfall rapidly dilutes salinity. Winter cold fronts chill the water quickly; summer doldrums (periods of very hot weather) increase the salinity through evaporation (water evaporates, the salts do not). Temperature and salinity also influence the oxygen dissolved in seawater; hot saline water contains reduced concentrations of dissolved oxygen.

The larger keys form a natural dam that restricts water flow from Florida Bay into the Atlantic Ocean, which in turn affects the distribution of coral reefs. For example, Key Largo is a long key with a single opening from the bay into the Atlantic; reef development seaward of Key Largo is extensive. Navigation charts reveal that coral reef development is concentrated in the region from Elliott Key to south Key Largo and to the southwest, from Big Pine Key beyond Key West. In the region between south Key Largo to Big Pine Key, reefs are not as numerous nor as well developed. Large openings in the middle portion of the Florida Keys permit unrestricted flow of water from Florida Bay into the Atlantic, e.g. Long Key and Bahia Honda channels. The climate in the Florida Keys is characterized as being sub-tropical; however, extremely cold and hot temperatures are often experienced. Prolonged exposure to water temperatures of less than 64 degrees F or greater than 85 degrees F are stressful to reef corals and, in some cases, deadly. Hurricanes are a natural disturbance that can cause severe damage to the reefs. Recovery following a moderate hurricane usually requires 5 to 10 years.

Corals are the principal architects of the reef. They are primitive life forms; on an evolutionary scale, they are one step beyond the sponges and less advanced than the flat worms. Their closest relatives are the anemones, jellyfish and Portuguese man-o-war. Several types of coral are found in Florida reefs. Octocorals are more commonly referred to as soft corals, sea fans, sea whips, and gorgonians; the fire corals are so named for their potent sting. Stony corals or hard corals form the mass of the reef with their skeletons. Additionally, there are a number of anemone-like coverings over the reef surface.

True stony corals are master reef builders. Some 65 species reside in Florida Keys reefs. However, the elkhorn coral and massive star coral are responsible for the majority of reef construction. Reef building corals are composed of a limestone skeleton (calcium carbonate mineral), thousands of tentacled polyps (animal), and millions of microscopic algae (plants) living within the coral tissue.

The life cycle of the coral is a contrast of a free-living larva and a sedentary (attached to the sea floor) adult. Following fertilization of an egg and subsequent larval development, the larva floats or crawls to a suitable location. Once attached to a rock or reef platform, the larva develops into a coral polyp. Limestone cement anchors the coral firmly to the sea floor, and a wall of limestone is secreted around the polyp. Tentacles covered with microscopic stinging cells protect the coral from would-be predators and stun planktonic prey. Growth proceeds from division (cloning) of daughter polyps and the skeleton. The more rapid-growing staghorn and elkhorn corals add between four and five inches a year. On the other hand, the massive star and brain corals grow only one half inch per year.

Corals and other reef builders (algae, mollusks, etc.) are able to use the carbon dioxide, a product of respiration, in their biological processes and combine it with calcium from seawater to form a limestone skeleton. In the case of the corals, most of the energy required to support these processes comes from the microscopic algae within the coral tissue. The photosynthesis carried on by these algae (called zooxanthellae) provides the sugars, carbohydrates and fats necessary to support coral metabolism. This strong interrelationship is termed endosymbiosis, the coral and algae are both dependent upon each other for success and survival. The number of zooxanthellae in a square inch of coral tissue can range from 1 million to 2 million. Since reef corals have evolved in tropical regions that are deficient in chemical nutrients, the endosymbiosis allows for the production of energy needed to support maintenance, growth, and reproduction of reef corals and other animals containing algae within their tissues.

Strong sunlight is a prerequisite for reef growth; hence, clear water (lacking sediments, particulate material, and plankton) is a very important environmental requirement of coral development. Areas with chronic silt and runoff from the land usually lack coral reefs. A trip from the coast to five or six miles seaward of the Florida Keys would progress through a mangrove forest, limestone rock, sand, seagrass meadows, patch reefs, more seagrasses and sand, bank reefs, deep reefs, and end in sand and rubble flat areas beyond 150-foot depths. Extensive limestone hardgrounds are found in the complex of reefs, seagrasses and sand. Hardgrounds provide homes for many sponges, algae, corals and fish. They differ from reefs in that they do not project upward growth into the water column.