The Impact of Human Sewage on Tropical Oceans: What You Can Do

A hallmark of tropical ecosystems is they are low in the nutrients needed by living creatures, like nitrogen and phosphorous, because those nutrients cycle very quickly and don’t linger in a usable form. Organic material breaks down rapidly in the warm climate, and plants quickly take up any nutrients that are released to fertilize their lush growth. The result of this enthusiastic growth is that the soil and water are usually low in nutrients. In cooler climates, nutrients like nitrogen or phosphorous might linger in the soil or water, but not in the tropics. Creatures of the tropics, like the reef-building corals, evolved in a world where nutrients were not abundant, and they are well adapted to this nutrient-poor environment.

Humans can change the balance of nutrients by disturbing soils for construction, by adding chemical fertilizers directly to soils, as we do when farming or maintaining golf courses, and by the release of our wastes in the form of sewage, which is very rich in nutrients. Recent human activities on the Riviera Maya have increased nutrient levels substantially, and these nutrients are a real threat to the coral reefs that protect the coast of the Yucatan Peninsula.

The reef environment is a very diverse one, with many species of fish, invertebrates and other creatures making their homes among the coral formations. In fact, coral reefs are among the most diverse and productive ecosystems in the world. Without the coral animals constantly maintaining their reefs, the structures erode and this very rich ecosystem is lost, along with the many species that call it home.

When nutrient levels go up, the tiny coral animals that build coral reefs feel the strain. Increasing levels of nitrogen, especially, cause algae to grow. Algae can grow right across the surface of a coral reef, blocking sunlight from reaching the coral animals underneath. Corals are really a partnership between the coral animal itself and small photosynthetic algae called zooxanthellae. The zooxanthellae live inside the corals, using the sunlight that penetrates deeply into the clear waters of the tropics to make food for themselves and for the corals in which they live. When algae grow over the surface of the coral reef, the zooxanthellae can’t photosynthesize, and the corals starve.

When conditions are stressful for the corals, they lose their zooxanthellae, a condition known as coral bleaching. Corals around the world have been bleaching at alarming rates over the last few decades. Research shows that coral bleaching is associated with rising water temperatures, and that a heavy load of nutrients in the water can make the bleaching worse.

Adult coral animals, called polyps, stay in one place, secreting around themselves the calcium carbonate shells that make up the beautiful lacy patterns of the reef. The fragile little coral animals build these hard, sharp reef formations that protect them from predators and from waves and currents. In their youth, though, coral larvae float free in the ocean. At adolescence, each tiny animal must find a spot to settle on an existing coral reef, joining others of its kind and settling into a position in the complex community where it will spend the rest of its life. If algae are growing over the surface of the coral reef, the juvenile corals can’t settle onto the reef and join the community. When juveniles can’t be recruited, the future of the coral reef is threatened.

The little coral animals are susceptible to diseases, too, and a number of coral diseases caused by bacteria or fungi can rapidly kill various types of corals. Important research on coral diseases is being done on the reefs near Akumal, and one research group, headed by John Bruno from the University of North Carolina, has shown that when nutrient levels in the water are high, several coral diseases spread more rapidly and have more serious consequences. His work has shown that diseases can be more deadly for corals in polluted waters than for those in clean, low-nutrient environments. According to Dr. Bruno, half of the world’s reef-building corals have already been lost.

The Riviera Maya is protected by an enormous coral reef system that stretches the length of the Yucatan Peninsula. The Mesoamerican Barrier coral reef is the second largest in the world, surpassed only by Australia’s Great Barrier Reef. The Mesoamerican coral reef, known for its beautiful inhabitants that include many species of colorful fish and invertebrates like octopus and shrimp, attracts visitors from all over the world to dive and snorkel in its warm waters. Ironically, though, the Mesoamerican coral reef is threatened by its very popularity, because those visitors put stress on the coastal communities and lead to more nutrients in the pristine waters of the Riviera Maya.

Increasing tourism means that the mangrove trees, which naturally filter the water running off the mainland, are cleared to make way for resort hotels and villas. Increasing tourism means more nutrient-rich waste from food preparation and human sewage. It means a rising permanent population along the fragile coastline, as workers move in to serve the needs of the tourists. It means that lawns and golf courses are fertilized with synthetic fertilizers, and that when it rains, as it does a lot in the rainforests along the Riviera Maya, those fertilizers wash into the ocean and add to the nutrient load.

The wonderful geology of the Yucatan Peninsula contributes to this situation. There is very little surface water in this area, because the rocks that underlie the peninsula are mostly limestone formed around the fossilized skeletons of ancient corals. These rocks are very porous, and give the area its characteristic underground rivers and, in places where the domes of caves have collapsed to reveal these hidden waterways, magical cenotes. This geology also means that nutrients from sewage, fertilizers and kitchen waste can rapidly enter the groundwater, and once there will flow swiftly into the shining turquoise waters of the Caribbean.

There is much that can be done to protect this exquisite region from being loved to death by its visitors. For more than a decade, Centro Ecológico Akumal (CEA, pronounced “SAY-ah”) has been a leader in this effort, with programs that focus on water quality and conservation of the area’s fragile resources. The first constructed sewage treatment wetlands were built at CEA in 1996 in cooperation with Mark Nelson. Dr. Nelson later was a designer of the wetlands in the Biosphere 2 project, and he was one of the “biospherians” who lived in that experimental closed system in Arizona designed to increase understanding of the complex functions that take place on planet Earth. That original sewage treatment wetland is still working well, removing bacteria and nutrients from wastewater, and a second CEA wetland provides excellent treatment for sewage from the public restrooms that serve the Turtle Bay and Cueva del Pescador restaurants and local shops, as well as housing for CEA volunteers. Many homeowners and hotels in the area have followed suit, constructing wetlands to remove nutrients before sewage is released; it is estimated that over 200 wetlands are in operation along the Riviera Maya, with more than 50 of these in the Akumal area.

These constructed sewage treatment wetlands are generally shallow concrete pools that are filled well above the water line with gravel to prevent contact with the wastewater. They are usually planted with ornamental foliage, and are often quite beautiful. They take the place of the lateral lines that would be used with a traditional septic system, which are not practical in on the Riviera Maya due to very thin soils and very porous geology. A septic tank provides settling, and then the wastewater flows through the wetland, where beneficial bacteria break down the wastes further. The nutrients are a benefit to the plants, allowing for lush and decorative growth, and they are thus removed from the water before it is released into the environment, preventing them from polluting the sensitive coral reefs.

Research by our group and others has shown that these constructed treatment wetlands can substitute for the water cleaning functions that would previously have occurred in the mangrove swamps along the coast. Removal of the bacteria typically found in sewage (coliforms and enterococci) often exceeds 99%, and removal of nutrients that would be harmful to the reef, especially nitrogen, can be substantial if the wetland is not overloaded. The cost of construction for these wetlands is low when compared with the infrastructure needed to support centralized sewage treatment plants, and they have the advantages that sewage does not have to be pumped long distances through pipes, and the owner or manager retains control of the system. In addition, a properly managed wetland can be an attractive addition to the landscape.

CEA has also built composting toilets to serve the CEA Center itself as well as nearby La Ecocina restaurant and the Akumal Dive Shop. The composting toilets are self contained units that serve as a good example of another way that human waste can be prevented from entering the fragile Riviera Maya ecosystem. The composting toilets for use by visitors to nearby Yal Ku lagoon, a popular snorkeling destination, decorated with ocean-washed stones of fossil corals, are another excellent example of a beautiful solution for containing nutrients and protecting the lagoon.

Sewage treatment varies widely along the Riviera Maya. Many villas and hotels are very conscientious, with constructed wetlands or other effective means of sewage treatment. Some, however, are less responsible, releasing sewage into the groundwater with little or no treatment. This practice threatens to pollute the entire area, with its potential for spreading disease and the flow of nutrients onto the sensitive coral reefs. When booking your Riviera Maya vacation, it’s a good idea to ask the property owner or manager how your sewage will be treated, and take this into consideration when selecting your accommodations. The corals will appreciate your concern, you will be ensuring that this beautiful reef will be there for future generations to enjoy, and no doubt you will feel better to know that you are not swimming in your own sewage!

Rebecca V. Ferrell, PhD
Professor of Biology
Metropolitan State College of Denver
303-556-4764
ferrellr@mscd.edu




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