The Trouble With Red Tides

By Mrunmayi Joshi

A red tide is a phenomenon in which water bodies such as oceans, bays, and estuaries display an unusual color, most commonly a deep, rusty red, but which can also be a shade of pink, brown, yellow, or purple. These events are visually unique and beautiful, but can cause extensive harm to the surrounding wildlife and even humans. Red tides have been infamously mythologized as the plague of water turning to blood; however, these stunning-yet-dangerous occurrences have a cause unrelated to divine retribution.

Image Credit: Wikimedia Commons @ melvil

In reality, red tides are the result of a sudden burst in the population of dinoflagellates, a type of algae that are marine plankton. During this population explosion, known as an algal bloom, millions of these unicellular organisms aggregate and reproduce, causing the water to change in color. This is due to the various pigments that algae produce; therefore, while blooms are often red, some can be green or blue. As such, some scientists argue that the proper term for a red tide is “harmful algae bloom”, or a HAB, so as to be nonspecific towards the coloring of these growths. Yet pigmentation aside, excessive quantities of nutrients such as phosphorus and nitrogen are a key factor in the growth of all dinoflagellates. Additionally, low water salinity and warm surface water temperatures contribute to algal blooms. When these three conditions are met, as they are more and more frequently, vast swathes of the ocean can become discolored.

Most of these large-scale, periodic algal blooms can be attributed to three species of dinoflagellates: Karenia brevis, which lives in the Gulf of Mexico and affects Florida and Texas, Alexandrium fundyense, which lives along the Atlantic coastline around New England, and Alexandrium catenella is found along the Pacific coast from Mexico to Canada. Although these three have localized areas for spread, algal blooms can be pushed into new areas by wind or hurricanes. Additionally, all these species produce potent neurotoxins called brevetoxins, which are released through the deaths of individual cells. These neurotoxins bind to voltage-sensitive sodium channels in the nervous system, which are used to let sodium into the neuron so that an action potential can proceed. As a result, the voltage-sensitive sodium channels always remain open and the nerves are constantly firing, producing disastrous effects on the nervous systems of most animals.

Besides causing problems for tourists and residents of beach towns alike, red tides generate immense stress on the environment. This is because the brevetoxins cause the deaths of fish in massive numbers, as well as the associated deaths of birds and other animals that feed on these fish. Smaller creatures such as shellfish also accumulate large quantities of the toxins. Even larger animals, such as manatees, dolphins, and squid, have seen a rapid reduction in numbers. Additionally, through contact with contaminated shellfish, humans can contract parasitic shellfish poisoning, a condition that can cause respiratory paralysis and subsequent death. Just living alongside a shore with red tide can cause respiratory problems, as waves enable the toxins to reach the air. Swimmers who have come in contact with red tide have reported rashes and breathing issues, showing how damaging its effects can be.

The prospect of widespread climate change caused by human activity only threatens to make this problem worse. As waters warm, the algal bloom season will extend and the blooms themselves will become more intense. As sea levels rise, there will be an increased number of hurricanes, which spread the algae outside its usual habitat and stir up nutrients from deeper water; a large hurricane on the Florida coast has almost always resulted in a more widespread and potent bloom. Additionally, increased use of fertilizer will provide more nutrients for the algae to consume; algal blooms have been on the rise since the 1980s due to increased use of fertilizers containing phosphorus and nitrogen. It is clear that permanently solving the issue of red tides will require a major change in human activity in these areas.

Educational Content

Q: What are the conditions that promote the growth of algal blooms? How has human activity contributed to the increased frequency of algal blooms?

A: The conditions that promote the growth of algal blooms are the presence of nutrients such as phosphorus and nitrogen, as well as decreased salinity and warm surface water temperatures. Climate change caused by human activity has resulted in warmer waters, where the algae can thrive, and more hurricanes; this means that the algae is introduced into new areas and can feed off nutrients swept up from the colder parts of the ocean. Increased use of fertilizer has provided the algae with a food source.

Q: How do algal blooms impact the surrounding fauna?

A: The dinoflagellates release neurotoxins called brevetoxins, which force the nerves to constantly fire and lead to rapid systemic failure in fish, causing massive fish kills and harming animals like birds and dolphins, which feed on them. They also accumulate in shellfish, causing respiratory distress in humans who consume them. Direct contact with an algal bloom can lead to rashes and breathing difficulties.