Most often harmful algal blooms in freshwaters are caused by cyanobacteria (aka blue-green algae). Algae are simple, often single-celled, plants that are naturally occurring and form the base of the food webs. A small percentage of algae can produce toxic compounds.
Nutrient pollution, warm water, high pH, stagnant water and lots of sunlight can lead to excessive blooms. Introduction of invasive species, such as non-native fish species, may also lead to excessive blooms. Nutrient pollution can come from wastewater treatment plants, residential on-site wastewater treatment systems, agricultural, urban and forestry runoff and natural sources. Introduced fish species also can recycle nutrients within a lake, allowing for more intense blooms. Warm water, high pH, stagnant water and sunlight are conditions that are harder to control in lakes and large rivers than nutrient pollution.
More information about specific lakes can be found in the Atlas of Oregon Lakes.