Scientific research indicates that in recent years, the frequency and geographic distribution of harmful algal blooms (HABs) have been increasing nationally and globally. HABs can be detrimental to human health, animals, aquatic ecosystems, and local economies. In 2014, a major HAB in Lake Erie caused the city of Toledo, Ohio, to issue a "do not drink" order for tap water that left more than 500,000 people without drinking water for two days and had an estimated impact of $65 million in lost benefits. In the summer of 2016, a massive HAB in Lake Okeechobee—Florida's largest freshwater lake—resulted in beach closures, losses to the tourism industry, and negative impacts on marine life. According to the U.S. Environmental Protection Agency (EPA), between January 1 and August 12, 2016, states reported at least 266 notices for freshwater HABs, including cautions, warnings, public health advisories, and public health warnings. Congress, many federal agencies, states, localities, and other partners have taken and continue to take steps to address the rising trend in HABs and their impacts. However, there are many gaps in current scientific understanding of HABs among the research and management communities and considerable debate as to how best to address the issue from a regulatory standpoint. This report explores these issues as they pertain to HABs in freshwater systems. Specifically, it addresses the conditions and activities that contribute to the occurrence of freshwater HABs; steps that Congress, federal agencies—particularly EPA—and their partners are taking to address and mitigate their occurrence; and the current knowledge gaps on this issue. This report is focused on freshwater HABs, not marine or coastal HABs or issues associated with HABs in drinking water supplies. Algal communities are naturally occurring components of healthy aquatic ecosystems, such as lakes, rivers, and estuaries. However, under certain environmental conditions, such as increased temperatures and nutrient concentrations (e.g., nitrogen and phosphorus), colonies of algae can grow excessively, or "bloom," and produce toxins that pose a threat to human and aquatic ecosystem health and potentially cause economic damage. These HABs sometimes produce discolorations in the water that can appear as scums, paint-like slicks, clotted mats, or foam that may vary in color (i.e., light to dark green, yellow, red, or brown). Even when visible signs of a bloom are absent, however, algal toxins may still cause harmful effects. Figure 1 shows an aerial view of a HAB that produced visible green scums in Lake Okeechobee, Florida, in July 2016. While many types of algae can cause HABs in bodies of freshwater, cyanobacteria (sometimes referred to as blue-green algae) typically cause the most frequent and severe blooms. Cyanobacterial HABs pose a threat to human and aquatic ecosystem health and can kill pets, livestock, and wildlife. Some species of cyanobacteria produce toxins, called cyanotoxins, which can cause hepatic (liver-related), neurologic, respiratory, dermatologic, and other symptoms. These may be acute or chronic, mild or severe, and in some cases may be fatal. Humans may be exposed to cyanotoxins by consuming tainted drinking water, fish, or shellfish; swimming or recreating in waters with certain concentrations of cyanotoxins present; or inhaling aerosolized toxins. The cyanotoxins associated with these HABs can contaminate fish, interfere with a variety of recreational activities, and cause other economic and environmental damages. There are many types of cyanotoxins, which may have multiple variants with a wide range of toxicities. The most commonly occurring and most studied cyanotoxin is microcystin. Although cyanobacterial HABs are considered to be the most prevalent and toxic types of HABs, blooms of "golden algae" ( Prymnesium parvum ) are an emerging problem and likely the most problematic of non-cyanobacterial freshwater HAB taxa (i.e., group of related organisms classified as a unit). Golden algal HABs have caused large fish kills worldwide, including millions of fish in Texas. Most of the major fish kills have occurred since 2000. Golden algae thrive in brackish water, such as the rivers and reservoirs found in areas of Texas, Oklahoma, and Wyoming. The toxins produced by golden algae target gill-breathing organisms, such as fish, clams, and mussels. According to information from two states that experience golden algal blooms, there is currently no evidence that golden algal toxins pose a direct threat to humans, other mammals, or birds. Effects of cyanobacterial HABs and golden algae HABs are detailed in Table 1 . In addition to the effects of algal toxins on human and animal health, HABs can also contribute to deteriorating water quality and ecosystem health. An overabundance of cyanobacteria or other algae can block out sunlight and clog fish gills. In addition, as the algae die and decompose, they consume oxygen, leaving waterways in a hypoxic (or low oxygen) state, sometimes forming "dead zones"—areas where life cannot survive due to lack of oxygen. Low oxygen areas can suffocate and kill fish and bottom-dwelling organisms such as crabs and clams. According to EPA, over 166 dead zones have been documented nationwide, including in waterbodies such as the Chesapeake Bay and the Gulf of Mexico. Significant economic losses have occurred as a result of hypoxia. Many factors may influence the occurrence and prevalence of HABs in freshwater, including nutrient concentrations, water temperature, availability of light, pH, and water circulation. Nutrient enrichment is widely recognized as one of the key causes of HAB formation. Nutrients, such as nitrogen and phosphorus, are essential to plant growth and natural parts of aquatic ecosystems. However, when high levels of nutrients enter a body of water, they stimulate plant and algal growth, which can lead to depletion of dissolved oxygen (as explained above), reduced transparency (i.e., turbidity), changes to the biological community (e.g., loss of sportfish, such as bass), and degradation of the aesthetic appeal of the water (i.e., from odor and scums). This process is called eutrophication. While some sources of nutrients in water bodies are natural, many anthropogenic activities contribute nutrients to waterbodies from a number of point and nonpoint sources. Point sources include municipal and industrial wastewater discharges and concentrated animal feeding operations (CAFOs). Nonpoint sources include urban stormwater runoff, failing septic systems, atmospheric deposition of nitrogen from fossil fuel emissions, runoff from fertilized cropland, and manure runoff from cropland, pastures, and animal feeding operations. See Table 2 for more information on these sources. Studies also indicate that increased temperatures and changes in frequency and intensity of rainfall associated with climate change may also favor HAB formation. HABs generally proliferate in warmer waters. In addition, some studies have found that swings between flooding and drought may enhance HAB formation. For example, if intense rainfall is followed by a drought, the nutrients washed into receiving water bodies may remain in them longer, increasing the potential for HABs. Scientists largely agree that the frequency and distribution of HABs, the economic losses from them, the types of resources affected, and the number of toxins and toxic species have all increased in recent years. Some scientists note that factors such as better detection methods and increased reporting have contributed to the upward trend. HABs, including cyanobacterial HABs, have been recorded in the waters of all 50 states, with some HABs crossing state lines. Figure 2 shows the generalized distribution of selected freshwater HAB events (cyanobacterial HABs and golden algal HABs) that took place between 2006 and 2015 across the United States. The findings of EPA's most recent national assessment of lakes are consistent with other reports of the rising trend in HABs. In EPA's 2012 National Lake Assessment, EPA concluded that there was little change from its 2007 survey of lakes, with two exceptions—trends in algal toxin and nutrient measures. In 2012, EPA and its partners detected microcystin in 39% of lakes, a 9.5% increase from 2007. EPA noted, however, that for both studies, the concentrations of microcystin remained low and rarely exceeded the levels of concern established by the World Health Organization for recreational uses (see " Regulatory Efforts and Guidelines "). EPA also found an 8.3% increase in the percentage of lakes in the "most disturbed condition" category when analyzing the density of cyanobacterial cells (i.e., an indicator of risk for exposure to algal toxins because the cells may produce toxins). Finally, EPA found an overall increase in the median concentration of phosphorus across all lakes and a "dramatic" decline (18.2%) in the percentage of lakes with low nutrients and high oxygen levels. These findings are important because in many lakes, phosphorus is considered the limiting nutrient, meaning that the available quantity of this nutrient controls the pace of algal production. It also means that even small increases in phosphorus can lead to very rapid increases in algal growth. More broadly, the study found that nutrient pollution is a widespread problem across the country. Approximately 35% of lakes have excessive levels of total nitrogen, and 40% of lakes have excessive levels of total phosphorus. Congress has recognized the increasing frequency of HABs and has passed legislation in an effort to address public health, economic, and environmental consequences of HABs.