Algal blooms are rapid, explosive growths of algae or cyanobacteria (often called blue-green algae) in lakes, rivers, estuaries, and coastal waters. They happen when excess nutrients, warm temperatures, and calm water conditions combine to let microscopic organisms multiply far beyond their normal levels. Some blooms are relatively harmless, turning water green for a few days. Others produce dangerous toxins that can sicken people and animals, kill fish, and shut down drinking water supplies.
What Causes Algal Blooms
The core ingredient is nutrients, specifically nitrogen and phosphorus. These elements enter waterways through agricultural runoff (fertilizers and animal waste), urban stormwater, wastewater discharge, and septic system leaks. Nitrogen is a building block of algal cells, while phosphorus drives photosynthesis, energy conversion, and enzyme activity. When both accumulate in a lake or bay beyond normal levels, the water becomes “eutrophic,” essentially overfed.
Nutrients alone aren’t enough. Blooms typically form when water temperatures are high, sunlight is abundant, and water flow is slow. A warm, still lake in late summer with heavy nutrient loading is the classic setup. Wind matters too: calm conditions let algae concentrate near the surface where they can absorb more light. Once a bloom takes hold, it can reinforce itself. Certain cyanobacteria absorb sunlight and release heat, raising surface water temperatures even further and giving themselves an additional growth advantage over competing species.
Why Some Blooms Are Dangerous
Not every bloom is toxic, but the ones caused by cyanobacteria frequently are. These organisms produce several categories of toxins that affect different organ systems. Liver toxins (hepatotoxins) are the most common. Neurotoxins target the nervous system. Others damage skin or cells more broadly.
The most widely studied toxin, microcystin, attacks the liver and kidneys. Ingesting water contaminated with microcystin can cause nausea, vomiting, diarrhea, headache, fever, loss of appetite, and fatigue. In more serious exposures, symptoms can include blood in the urine, acute hepatitis, and jaundice. Other cyanobacterial toxins, like anatoxin-a, cause neurological symptoms: tingling, numbness, drowsiness, muscle twitching, and speech disturbances. At high doses, certain neurotoxins can cause progressive muscle paralysis.
You don’t have to swallow the water to be affected. Skin contact during swimming can cause rashes, and inhaling water droplets near a bloom (from waves, wind spray, or boating) can irritate airways. Even the smell of a bloom, often described as grassy or septic, can cause nausea in some people.
How To Identify a Toxic Bloom
Cyanobacteria blooms have a few distinctive visual signatures. They often look like blue or green paint spilled into the water, or a thick, puffy foam (sometimes called scum) floating on the surface. You might see a blue or green crust along the shoreline, or swirling colors just beneath the surface. Some blooms appear white, brown, or red instead of the classic blue-green. They tend to accumulate near shorelines and shift position with wind and wave action.
Long strands of green algae, duckweed, and filamentous algae that look like wet hair are commonly mistaken for toxic blooms but are generally harmless. The key difference: cyanobacteria blooms look more like a thick liquid or paint than like individual plant strands. If you’re unsure, treat it as toxic and stay out of the water. The EPA’s recommended safety threshold for recreational water is 8 micrograms per liter for microcystin, a level that can only be confirmed through lab testing, not by looking at the water.
What Blooms Do to Ecosystems
Even when a bloom isn’t producing toxins, it damages the ecosystem in predictable ways. A dense bloom blocks sunlight from reaching underwater plants, which depend on light to survive just like land plants. Without those submerged plants, fish and invertebrates lose habitat and food sources.
The worst damage often comes after the bloom dies. As bacteria decompose the massive amount of dead algae, they consume dissolved oxygen in the water. When oxygen levels drop low enough, fish, crabs, and other aquatic animals suffocate. These low-oxygen areas are called hypoxic zones, or dead zones. The Gulf of Mexico’s annual dead zone, fed by nutrient runoff from the Mississippi River watershed, is one of the most well-known examples, but smaller dead zones form in lakes and bays across the country every summer.
Climate Change Is Making Blooms Worse
Warmer water temperatures directly favor cyanobacteria, which grow faster in warm conditions than most competing algae species. Cyanobacteria also have a structural advantage: they can migrate up and down the water column, pulling nutrients from cool, dark bottom layers and then rising to warm, sunlit surface layers to photosynthesize. Other algae species can’t do this and end up shaded out.
Changing rainfall patterns compound the problem. Heavier rainstorms wash more nutrients off farmland and into waterways. If intense rain is followed by extended drought, those nutrients stay concentrated in shrinking water bodies, creating ideal bloom conditions. The pattern also threatens coastal areas: extreme rainfall can flush freshwater blooms and their toxins from rivers into estuaries and marine waters, seeding new blooms in places that historically had fewer problems. Major blooms in Lake Erie in 2011 and 2015 were linked to exactly this cycle of intense rainfall and nutrient loading.
Economic Costs
Harmful algal blooms cost the U.S. an estimated $10 to $100 million per year on average, according to NOAA, and a single major event can account for tens of millions in losses by itself. Those costs hit multiple sectors: commercial fisheries lose catch when fish die or fishing grounds are closed, tourism drops when beaches are shut down, and water treatment plants face significantly higher costs to make tap water safe. In 2014, a bloom in Lake Erie forced Toledo, Ohio, to issue a “do not drink” order for roughly 500,000 residents, a scenario that’s becoming less unusual as blooms intensify.
Reducing the Nutrients That Feed Blooms
Because nutrient pollution is the primary driver, prevention focuses on keeping nitrogen and phosphorus out of waterways in the first place. On farms, this means applying fertilizer and manure at rates matched to what crops actually need, based on soil and manure testing. Timing matters: applying phosphorus right before a heavy rainstorm washes much of it into nearby streams. Incorporating manure into soil through plowing instead of leaving it on the surface has been shown to reduce phosphorus in runoff by as much as 20-fold.
Livestock operations can reduce phosphorus at the source by adjusting animal feed. Reducing the phosphorus content of pig diets by roughly one-quarter (from 0.48% to 0.38%) cuts phosphorus in manure by 30 to 35 percent. Combining specialized feed enzymes with low-phosphorus corn varieties has reduced phosphorus in swine waste by 60 percent. Treating poultry litter with aluminum sulfate before land application has been shown to cut phosphorus in runoff by 85 percent.
Landscape-level strategies also help. Cover crops protect bare soil from erosion during the off-season. Buffer strips of grass or trees along streams filter sediment and nutrients from runoff before it reaches the water. Terracing slows water movement across sloped fields. Fencing livestock away from streams prevents direct nutrient inputs and bank erosion. In urban areas, upgrading wastewater treatment, managing stormwater, and reducing fertilizer use on lawns and golf courses all contribute to lowering the nutrient load.