Mahogony tides, also known as red tides, have been the focus of much study in recent years. Many people who live near the Chesapeake Bay have heard of something called a red tide, but may not be exactly sure what it is. Perhaps they have been sailing along in their boat and noticed that the water in their wake was colored reddish brown and looked kind of muddy.
Picture: A red tide. Photo courtesy of Peter J.S. Franks, Scripps Institution of Oceanography. See the original photo.
Well, red tides aren't always as red as the water in the picture, and red tides aren't really tides.
The discoloration is caused by dinoflagellates, a specific kind of phytoplankton— the microscopic plants that float in the water. With enough food and the right temperatures, dinoflagellates can reproduce extremely quickly. Their rapid growth is called a phytoplankton bloom and the large number of dinoflagellates gives the water its reddish-brown color.
One of the main causes of red tides is the large amount of nutrients that run off from the surrounding land and reach the water. If, for example, heavy rains occur after crop fertilization, much of this fertilizer may run off into the watershed and eventually into the Bay. Combined with warm temperatures, these excess nutrients fuel the algal blooms.
Picture: Smithsonian scientist collecting a water sample of a mahogany tide. Picture courtesy Smithsonian Environmental Research Center/Sharyn Hedrick.
Although the dinoflagellates produce oxygen in the growth phase of a bloom, the algae use up a lot of oxygen when they die and are decomposed by bacteria. The decay of the algae can cause anoxia, a situation of low oxygen levels in the water, which has been the cause of fish kills when there is no oxygen left for the fish to breathe.
Another problem of these massive algal blooms is that they 'muddy' the water so much that sunlight can no longer reach submerged aquatic vegetation (SAV). Without sunlight, these plants cannot grow and they start to die. When SAVs disappear, other aquatic animals, like fish and the blue crab, loose an important habitat and food source.
References and further reading
The Smithsonian Environmental Research Center has a phytoplankton guide to the Rhode River and Chesapeake Bay. The site includes pictures of a particular blooming phytoplankton, Procentrum minimum and of its bloom.
Woods Hole Oceanographic Institute (WHOI) has a Harmful Algae Page.
The Public Health Fact Sheet by the Massachusetts Department of Public Health has more information about red tides.