Other Information:

According to the EPA, the primary sources of nutrient pollution in water bodies are: 

1. Agricultural runoff

2. Stormwater runoff

3. Wastewater (from failing septic tanks and ageing wastewater treatment plants)

4. Household waste (fertilizers, yard/pet waste, soaps/detergents)

5. Fossil fuels (nitrogen going into the air and then coming down via precipitation)

https://19january2017snapshot.epa.gov/sites/production/files/2015-03/documents/facts_about_nutrient_pollution_what_is_hypoxia.pdf 

Definitions of Harmful Algal Blooms (HABs)

The EPA definition: 

“Harmful algal blooms are over growths of algae in water. Some HABs produce dangerous toxins in fresh or marine water, but even nontoxic blooms hurt the environment and local economies.” The EPA considers non-toxic algae to be part of HABs. Some agencies have started calling toxic algal blooms “CyanoHABs”.

The USGS definition:  

“An algal bloom may be called harmful because of resulting reductions in dissolved oxygen concentrations, alterations in aquatic food webs, unsightly scums along shorelines, production of taste-and-odor compounds that cause unpalatable drinking water and fish flesh, or the production of toxins potent enough to poison aquatic and terrestrial organisms. Many different types of algae can cause harmful algal blooms in freshwater ecosystems. However, the most frequent and severe blooms typically are caused by cyanobacteria, the only freshwater “algae” with the potential for production of toxins potent enough to adversely affect human health.”

How nutrient pollution leads to HABs

Nutrient inputs to lakes are essential for the growth of algae and aquatic plants, which form the base of all lake food webs. As nutrient inputs to a lake increase, the algae will become much more abundant (i.e. they “bloom”). The increased algae reduces the amount of light available for submerged aquatic vegetation. Zooplankton may initially increase in abundance with the increase in their food source, however they will eventually decline as the species composition of the algae shifts towards less edible/nutritious taxa. Some of these algal species, like cyanobacteria, may produce toxins (see below). Bacterial decomposition of dying algal cells causes reductions in the amount of dissolved oxygen in the water, which can lead to the death of fishes and invertebrates.   

https://www.dec.ny.gov/chemical/77118.html 

Toxic HABs

Some Harmful Algal Blooms (HABs) cause an increase of toxins in the water and are a current major threat to freshwater lakes. Toxic HABs occur when too many nutrients make their way into lakes, often over decades, from a variety of sources (as noted above).  The algae that cause toxic HABs (blue-green algae, also known as cyanobacteria) are common in lakes around the world, but they are typically not abundant. When the perfect conditions occur (late summer with unusually warm weather, calm winds), the species of cyanobacteria that cause toxic HABs begin to move up from the lake bottom and experience daily migrations in the lake. Eventually, the algae move to the lake’s surface, where they appear as a distinct bright green film on the water.  Should the calm weather become windy, the floating algae can be blown into one end of the lake. Each algal cell releases a tiny amount of toxin; when millions of algal cells are concentrated in one area of a lake, the amount of toxin can become concentrated enough to pose a threat to people and their pets.  Impacts of the toxin (i.e. microcystin) on people can be found here at the Centers for Disease Control and Prevention.

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