Red tide

A red tide is a marine phenomenon that occurs when simple plant organisms called algae grow out of control and produce harmful or toxic conditions in an aquatic ecosystem. In scientific terms, the condition is referred to as a harmful algal bloom (HAB). The name "red tide" comes from the predominately rust-red color that the algal bloom can cause in the water, though this color is not always present, and HABs can have other colors or be colorless. Many red tide events release toxins that can kill marine life and make fish and shellfish dangerous to eat. Others can consume the oxygen in the water or overwhelm an ecosystem with sheer numbers. The conditions are a cause for concern in coastal regions, where they can devastate wildlife and lead to illnesses in humans.

Background

Algae are a group of aquatic organisms that produce oxygen through photosynthesis—the process of using sunlight to convert carbon dioxide and water into food. Algae are different from land plants in that they do not have root systems or leaves and do not circulate water through their bodies. Algae are found in both freshwater and seawater and range in size from microscopic single-celled organisms to large seaweed that can grow to lengths of about 200 feet (60 meters).

Most algae species are beneficial to the environment. A classification called cyanobacteria, or blue-green algae, are ancient single-celled organisms believed to be responsible for generating the oxygen in the atmosphere of the young Earth. A freshwater algal bloom, such as the green pond scum that forms on the surface of a lake, is caused by cyanobacteria.

Eukaryotic algae are more complex, multicelled algae that evolved from different common ancestors. Among the categories of eukaryotic algae are dinoflagellates, a diverse species of whip-like organisms. Most dinoflagellates exhibit photosynthesis, but some can act as parasites, relying on fish or other algae species to survive.

Overview

Under the right conditions, algae or colonies of algae can rapidly multiply and form algal blooms. Many blooms are harmless and may be invisible or simply discolor the water. The HABs responsible for red tides are primarily caused by three species of coastal marine dinoflagellates. Alexandrium fundyense are algae commonly found in the waters of the North Atlantic from New England to Canada. Alexandrium catenella is a species of algae found in coastal Pacific regions from Mexico to Alaska and Japan to Australia. In the Gulf of Mexico, the species Karenia brevis is responsible for creating HABs that can affect the Florida and Texas coasts in late summer or early fall.

Unchecked growth of these species typically colors the water a rusty shade of red; however, the algae may also make the water take on a brown, green, or purple appearance. The first possible account of a red tide event came from sixteenth-century Spanish explorers who wrote that the native people of Florida warned against a "red water" that was deadly to birds and fish. Scientists in the United States first began to study red tides along the coast of Florida in the mid-nineteenth century. The first recorded human illness from eating infected shellfish occurred in 1884.

HABs can produce neurotoxins, substances that can interfere with normal nerve and muscle activity. The algae normally produce the substances as a defense mechanism to prevent being eaten by small animals. When large quantities of the algae form in the ocean, the toxins produced can kill the surrounding marine life. Outbreaks of Karenia brevis in the Gulf of Mexico have been blamed for the mass deaths of millions of fish, birds, and other wildlife. Marine life can be affected by the toxin by absorbing it directly into their bodies from the water or by eating other organisms that have consumed the substance.

Humans can be affected by breathing in the algae if it washes ashore and becomes airborne or by consuming contaminated fish or shellfish. Exposure to airborne toxins can cause coughing, shortness of breath, chest constriction, and asthma attacks. Because the toxins accumulate in the organs of fish, eating the fish can result in ciguatera, a foodborne illness that causes nausea, pain, headaches, and numbness. Eating contaminated shellfish can cause paralytic shellfish poisoning (PSP), an illness that can lead to neurological symptoms such as tingling in the hands and feet, loss of muscle control, and difficulty breathing. In rare instances, PSP can cause respiratory paralysis and death.

Several factors contribute to HABs, including low salt content, warm surface water temperatures, and an increase in nutrients in the water. Red tide events have been reported after natural occurrences such as floods, storms, or shifting water currents. In 1972, a powerful storm altered the wind patterns and water currents in the North Atlantic. Algae from cold Canadian waters were pushed south into the warmer seas off Maine, New Hampshire, and Massachusetts. The resulting bloom had a significant effect on the local fishing industry. The algae also became a permanent part of the ecosystem and continued to plague the region into the twenty-first century.

The red tide phenomenon occurs every year in the United States and, according to a 2016 estimate from the Woods Hole Oceanographic Institution, results in average annual economic impacts of $50 million. These impacts comprise public health effects; losses in industries like fisheries, recreation, and tourism; and the costs of environmental monitoring and management.

In the United States, red tide is often associated with the waters of the Gulf of Mexico, and in particular with Florida. While HABs are a regular occurrence, some years have seen notably heavy blooms, with impacts extending beyond the marine ecosystem to affect people and the economy. In 1947 a large red tide outbreak along the Gulf Coast of Florida lasted for nearly a year and effectively crippled the region's fishing industry. As the spray from the ocean sent it airborne, the algae became a health concern that prompted the closure of beaches, schools, and hotels. In 2004, about one hundred dolphins were killed by a red tide in the Gulf of Mexico; in 2013, a red tide killed nearly three hundred of the state's five thousand manatees, about 5 percent of the total population. A year later, a large red tide in the Gulf about 60 miles (96.5 kilometers) wide and 90 miles (145 kilometers) long killed more than a thousand fish, crabs, and other marine life.

Another major Florida red tide event occurred in 2018 and early 2019, with leftover blooms from November 2017 contributing to especially heavy red tide conditions. The event had a detrimental impact on the fishing and tourist industries and sickened multiple people. At the same time, a freshwater algal bloom occurred in Florida's Lake Okeechobee. These events led Florida governor Rick Scott to declare states of emergency to bring in extra resources for cleanup efforts. The 2017–19 red tide was also blamed for the first known death of a whale shark due to HAB, along with thousands of other animal deaths, including more than 400 sea turtles, about 150 dolphins, and over 100 manatees. That red tide appeared to abate along the Florida Gulf Coast by February 2019, after about sixteen months; however, red tide re-erupted in southwestern Florida in October of that year. Though this red tide did not appear as severe or widespread as the previous year, it had impacted beaches in several counties and led to the death of some fish and other marine life through November.

While most red tide events are natural, they can also be caused or worsened by the introduction of nitrogen- or phosphorus-based fertilizers into oceans. The nutrient-rich chemicals can run off from lawns and farms and eventually flow into the sea, where the algae overfeed and rapidly multiply. As a result, some areas affected by HABs have limited or banned the use of fertilizers. Waterworks and treatment facilities are other approaches being considered for prevention.

In addition, some scientists have suggested that climate change effects could lead to an increase in HABs. Disruptions that could promote HABs include hotter water, increased saltwater intrusion into freshwater, higher amounts of carbon dioxide, storm-related nutrient runoff, more areas of shallow coastal water, and coastal upwelling.

Research into HABs remains ongoing. Volunteer citizen scientists observe conditions and collect data that aid federal environmental monitoring programs. Scientists continue to examine the contributing factors and mechanisms by which HABs arise and to improve red-tide detection methods. Researchers are also studying ways to eliminate the excess algae and their toxins while protecting the marine ecosystem.

Bibliography

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"What Is a Red Tide?" National Ocean Service, 16 June 2024, oceanservice.noaa.gov/facts/redtide.html. Accessed 19 Nov. 2024.