In the on-going discussions of the health of the coral reef and water quality in general, there is a lot of talk about algae. For many, the word conjures up images of the slimy green stuff that coats rocks and old shells and sometimes even the inside of your aquarium. But the story is not that simple. There are many types of algae. Some are natural and even crucial to the coral reef ecosystem. Others are opportunistic and may take advantage of an imbalance in the natural system and actually change that system.

In reality, one type of algae is the foundation of the coral reef. Tiny unicellular algae called zooxanthellae live within the coral animal polyps. The algae tends to be green or yellow in color and gives the coral polyps their pigment. While it is true that the coral animals deposit the calcium carbonate (limestone) skeletons that eventually build the reef structure, they could not do it without the zooxanthellae. Their photosynthesis provides between 75% and 95% of the nutrition for the stony corals. In addition, this algae partner provides a specialized boundary layer that allows corals to add calcium skeletons by establishing an alkaline environment on the coral cell surfaces that is necessary for the deposit of the mineral.

In the realm of minerals, there are other algae that are important contributors to the structure of tropical reefs. Coralline algae are abundant on the reef, both in encrusting and branching forms. Unlike zooxanthellae, coralline algae are brightly colored in hues of red, pink, white, and yellow. These colorful algae are helpful cementing agents along the coral reef because they deposit calcium carbonate within their cell walls.

A very familiar algae that adds calcium carbonate to the reef ecosystem (calcareous algae) is Halimeda. This leafy green macroalgae is abundant here in the Keys, both on the reef and on hardbottom areas. It provides a sturdy mat on shallow hardbottom areas that helps stabilize sediment. If you pick up a handful of sand you may notice an abundance pure white, flat, round pieces. These are the broken calcium skeletons of Halimeda.

Algae that live outside of the coral cells are as important to the coral reef ecosystem as are the zooxanthellae. But they thrive on nutrients in the water column, and if there is an unnatural level of nutrients in the water column, they can quickly grow into large communities. All of these algal communities trap sediments which shade, smother and can quickly kill coral tissue. Corals work best when their surfaces are sediment-free, when their symbiotic partners can get enough light to create nutrition through photosynthesis. If they are encrusted with macroalgae and sediment, they die quickly. Microbes then claim the freshly exposed skeleton, allowing the macroalgae to expand into the dead area. If there is nutrient pollution in the water, these algae feed vigorously on it and grow faster than the stony corals. If you've been snorkeling down here for a number of years you may have seen this process first-hand in nearshore patch reefs. First a small area of a coral head may appear damaged. After a while, you may see that the entire head has lost its coral and is now a dead rock encrusted with a variety of algae.

There are many other types of algae as well. The green, brown and sometime even red hues of water that at times flow out of Florida Bay and the west coast are caused by microalgae or phytoplankton that live in the water column. Certain levels of these, too are natural and provide the basis for the entire food chain. But if these "blooms" occur over the corals and sponges, or if they hang around for too long, they can block out crucial sunlight. And some types like the red algae that cause red tides can be directly toxic to organisms, or even cause disease like the one that causes black band on stony corals. Finally there are some algae that do not typically occur on coral reefs that show up and grow rapidly on very small levels of nutrients, like some blue-green algaes.

The keys to all of this is balance. These natural systems have an amazing array of checks and balances that keep them operating efficiently. When something is thrown out of whack, whether it be by natural phenomenon or human intervention, one organism may be given an advantage over another, and the entire complexion of an ecosystem will be changed.

Alyson Matley
Florida Keys National Marine Sanctuary