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​Why HABs Happen

HABs can occur in freshwater, marine, and brackish water bodies.  The factors that contribute to HABs vary from waterbody to waterbody, but HABs most commonly occur in waters with:  (1) calm, slow-moving, stagnant water; (2) high concentrations of key nutrients, like nitrogen and phosphorus; and (3) warm temperatures.

Calm, slow-moving waters

HABs most commonly occur in relatively slow-moving bodies of water such as lakes, ponds, reservoirs, bays, and estuaries.  HABs also can occur in the pools of dammed river systems, such as the Ohio River.  HABs less commonly occur in faster-moving waters, like high-gradient headwater streams, but cyanobacteria and cyanotoxins can be transported through faster-moving waterways from HABs occurring in upstream bodies of water.

High-nutrient waters

Waters with high concentrations of key nutrients, such as nitrogen and phosphorus, are more likely to experience HABs.  The most common sources of excess nitrogen in phosphorus in waterways and waterbodies include:  stormwater runoff from agricultural, urban, and suburban landscapes; discharges from point source discharges like wastewater treatment plants and concentrated animal feeding operations; malfunctioning septic systems; cracked sewer pipes; and atmospheric deposition.

Warmer waters

HABs typically occur during warmer months (i.e., mid-June to early September).

Prevention and Management of HABs

Although we still have a lot to learn about the complex and dynamic factors that contribute to HAB formation in specific waters at specific times, there are many existing resources and programs that address the general factors known to facilitate HAB formation, including: