Algae are natural components of marine and freshwater flora, performing many roles that are vital for the health of ecosystems. Most algae are not harmful, and the proliferation of algae generally provides the energy source to fuel food webs. However, when certain conditions are favorable, algae can rapidly multiply, causing “blooms” and increasing the risk of toxin contamination of water. When algae bloom in significant numbers and produce toxic or harmful effects, such events are termed HABs. The risk is especially great for blooms caused by some species of cyanobacteria, historically known as blue-green algae, which typically occur in freshwater but may also appear in marine environments. Cyanobacterial HABs are of special concern because of their potential impacts on drinking and recreational waters. Toxins produced by cyanobacteria (cyanotoxins) can cause allergic and respiratory issues, attack the liver and kidneys, or affect the nervous system in mammals, including humans. Cyanobacterial HABs can also cause detrimental effects on aquatic ecosystems. High biomass blooms, whether of toxic or nontoxic species, can accumulate as thick scums and mats, which decompose, causing excessive oxygen consumption—which, in turn, leads to an increased mortality rate in local fish, shellfish, invertebrate, and plant populations because of hypoxia. Cyanobacterial HABs may also adversely affect some types of flora and fauna because they cause decreased light penetration. Figure 1 shows a satellite image of the then- record-setting cyanobacterial HAB in Lake Erie in September 2011 (an even larger Lake Erie HAB set a new algal biomass record in 2015), overlaid on a map of the lake’s tributaries. This image shows the algal bloom (in green) covering the entire western basin and beginning to expand into the central basin of Lake Erie, where it continued to grow until October 2011. Coastal waters of the United States are subject to most of the major HAB impacts and poisoning syndromes. Marine HAB impacts on animals and plant life include fish kills, shellfish mortalities, widespread marine mammal mortalities, and loss of submerged vegetation. Specifically, some types of harmful algae produce potent toxins that cause illness or death in humans and marine organisms—fish, seabirds, manatees, sea lions, turtles, and dolphins are some commonly affected animals. Other types of harmful algae are nontoxic to humans but cause harm to fish and invertebrates by damaging or clogging their gills or by forming such large blooms that the death, and subsequent decay, of the algae lead to hypoxia in the bottom waters of marine environments, forcing animals to either leave the area or die. Birds can also get sick by eating algae, drinking contaminated water, or eating contaminated fish or shellfish. Exposure to marine HAB toxins can occur through direct contact by swimming (dermal exposure); breathing in aerosolized toxins (toxins in water turned into tiny airborne droplets or mist); or eating toxin- contaminated seafood, including shellfish and finfish. Impacts on human health through contaminated seafood include amnesic shellfish poisoning, ciguatera fish poisoning, diarrhetic shellfish poisoning, neurotoxic shellfish poisoning, and paralytic shellfish poisoning—illnesses that have been reported in the United States. Two major groups of marine algae— diatoms and dinoflagellates—produce HAB toxins that cause these syndromes. According to NOAA’s website, the toxins that cause these syndromes have been found in U.S. marine waters, and these syndromes have adverse human health impacts, as follows: Amnesic shellfish poisoning, which is caused by several species of Pseudo-nitzschia that produce the toxin domoic acid, produces gastrointestinal and neurological effects. Mild cases arise within 24 hours of consumption of contaminated shellfish. Symptoms include nausea, vomiting, diarrhea, and abdominal cramps. In more severe cases, neurological symptoms occur, which include headaches, hallucinations, confusion, short-term memory loss, respiratory difficulty, seizures, coma, and, in extreme cases, death. These toxic species have been found on the Pacific Northwest coast from Canada to mid-California and the Atlantic northeast coast of Canada, as well as the Gulf of Mexico. Ciguatera fish poisoning is the most common seafood-toxin illness reported in the world, and certain algal species have caused this syndrome in many tropical and subtropical regions with coral reefs in the United States, including Caribbean and Pacific Islands, Florida, and the Gulf of Mexico. Ciguatoxins are transferred through the food chain from small toxic algae to large fish, such as grouper, and produce gastrointestinal, neurological, and cardiovascular symptoms that usually begin developing within 12 to 24 hours of eating contaminated fish. Gastrointestinal symptoms include diarrhea, abdominal pain, nausea, and vomiting occur, and neurological symptoms include numbness and tingling of hands and feet, dizziness, altered hot/cold perception, muscle aches, and low heart rates and blood pressure. In extreme cases, death occurs through respiratory failure. Diarrhetic shellfish poisoning is a gastrointestinal illness that has been reported worldwide. Symptoms usually occur within 30 minutes to a few hours after consumption of contaminated shellfish. Symptoms include diarrhea, nausea, vomiting, and abdominal pain. A full recovery is expected within 3 days, regardless of medical treatment, but long-term exposure may promote tumor growth in the digestive system. Various species of Dinophysis and their related toxins, which cause this syndrome, have been found along the Texas Gulf coast and the Chesapeake Bay. Neurotoxic shellfish poisoning produces gastrointestinal and neurological symptoms within 3 to 6 hours of ingestion of contaminated shellfish. Milder cases may include symptoms of headaches, diarrhea, and muscle/joint pain and commonly occur when beachgoers are exposed to toxic aerosols produced by wave action. Symptoms include irritation of the throat and upper respiratory tract, causing asthma-like effects. More severe effects are altered perceptions of hot and cold, difficulty breathing, or double vision. Neurotoxic shellfish poisoning is caused by Karenia brevis, a toxic dinoflagellate found in the Gulf of Mexico and the east coast of Florida. Karenia brevis “red tides” are typically characterized by patches of discolored water, dead or dying fish, and toxic aerosols. Paralytic shellfish poisoning symptoms include tingling sensations or numbness, headaches, fever, rash, dizziness, and gastrointestinal illness. In severe cases, symptoms include muscular paralysis, respiratory difficulty, and choking sensation. Despite the severity of this toxin, victims generally begin to recover within 12 to 24 hours of intoxication. In some severe cases, victims may die from paralysis and respiratory failure. A number of species of the dinoflagellate Alexandrium, which produce a group of toxins (called saxitoxins) responsible for paralytic shellfish poisoning, have been observed along the northeast and west coasts of North America, including Alaska. Another species associated with this syndrome, Pyrodinium bahamense, has also been found in Florida. The United States has reported persistent problems with cases of this syndrome since the mid-1960s. Each of these syndromes is caused by different species of toxic algae that occur in various coastal waters of the United States and the world. According to the U.S. National Office for Harmful Algal Blooms’ website, there are virtually no human populations that are free of risk, in part because of an increase in interstate and international transport of seafood. CDC officials stated that records of these syndromes are incomplete because their diagnosis is difficult, physicians might not always report cases to their health departments, and reporting to CDC is voluntary; however, CDC estimates that 15,910 cases of ciguatera fish poisoning occur in the United States annually. According to NOAA officials, the recent emergence of diarrhetic shellfish poisoning in U.S. waters, the massive domoic acid-producing bloom on the West Coast in 2015, and the rapid expansion of cyanobacterial blooms in freshwater indicate that HABs are an increasing threat in the United States. However, FDA officials stated that controls are in place to prevent contaminated seafood from entering interstate commerce. Beyond human health threats, NOAA officials noted, marine HABs can have other deleterious impacts. For example, massive fish kills drive away tourists, require costly beach cleanup, and impact fisheries management. Some HABs kill or weaken young stages of shellfish, damaging shellfish aquaculture and impairing shellfish restoration. Furthermore, NOAA officials stated, water discoloration and accumulation of algae on beaches affects tourism and property values. Many of these effects can have serious economic impacts on communities in coastal areas that depend on marine resources for their livelihoods. HABs have occurred in all 50 states in the past decade in marine waters, freshwaters, or both. According to the U.S. National Office for Harmful Algal Blooms’ website, all 50 states have been affected by cyanobacterial HABs, typically in many different lakes, rivers, streams, reservoirs, and other freshwater sources. In addition, 23 states have been affected by “golden algae” blooms caused by Prymnesium parvum. Furthermore, the dinoflagellate Karlodinium veneficum blooms along the mid-Atlantic coast; and brown tides caused by the rapid population growth of a minute alga, Aureococcus anophagefferens, have decimated multiple fisheries and seagrass beds in mid-Atlantic estuaries for three decades.