What Are Harmful Algal Blooms?

Frequently Asked Questions

Lake Dora HABs. Source: NOAA.

General Harmful Algal Blooms (HABS)

What is a harmful algal bloom?

A harmful algal bloom, often called a HAB, is any large increased density of algae that is capable of producing toxins. In freshwater, such as Lake Erie, those algae tend to be cyanobacteria — more commonly known as blue-green algae — that are always present in the water to some extent, but which grow excessively under certain conditions.

What makes cyanobacteria bloom?

Harmful algal blooms are caused by a combination of warm water temperatures (above 60 degrees Fahrenheit) and high concentrations of phosphorus in the water. Typically, a high concentration of phosphorus and nitrogen in cold weather will produce a bloom of diatoms, in cool weather we would expect a bloom of green algae, and in warm weather we often see blue-green algae.

Why do we focus so much on phosphorus?

Phosphorus is one of the three major components in most fertilizers, followed by nitrogen and potassium. In freshwater, the single essential nutrient that is in the shortest supply is phosphorus (in saltwater, it is usually nitrogen). Therefore, reducing phosphorus supply will limit algal blooms.

How does phosphorus get into the water?

In inland lakes such as Lake Erie, more than 65% of the phosphorus that causes HABs comes from agricultural fertilizer and manure runoff, when heavy rain washes soil and fertilizer into rivers and streams that eventually enter the lake. Some phosphorus also comes from sewage treatment plants, combined sewer overflows, water treatment plants, cleaning products, faulty septic tanks and residential lawn fertilizers.

What toxin is of concern with HABs? What does it do to people?

The toxin of greatest concern tends to be microcystin, which causes skin rashes, GI problems and varying degrees of nervous system, liver and kidney damage. While most healthy adults recover from contact with the toxin, it can be more problematic to children, the elderly and people with pre-existing conditions that weaken their systems. Exposure has also killed people in other parts of the world. The toxin can also be fatal to pets that drink or come in contact with contaminated water.

What do toxin levels mean?

The World Health Organization (WHO) sets the maximum allowable concentration of microcystin in drinking water at 1 part per billion (ppb) — about equivalent to 1 drop of toxin in an Olympic-sized swimming pool. U.S. Environmental Protection Agency guidelines, which advise microcystin limits of 0.3 ppb for children under 6 and 1.6 ppb for the general population.

How is toxin removed from the water?

Water treatment plants use activated charcoal (also called activated carbon), as well as UV rays and other techniques, to remove toxic substances from the water. The toxins, such as microcystin, bind to the charcoal particles, which are then filtered out of the water again.


Lake Erie HABs 2011. Source: NASA.


HABS in Lake Erie

If toxins have been detected in one part of the lake, is the water in the whole lake unsafe?

Blooms are generally limited by water currents, winds and where nutrients enter the water. Toxin can persist in the water for more than 30 days, but is rapidly diluted and quickly reaches safe levels when the bloom dissipates and as one moves away from the bloom. Water treatment plants in Lake Erie’s western basin routinely monitor the water they bring in for human use, so affected areas can know about a problem quickly.


When does the majority of phosphorus enter Lake Erie?

The largest phosphorus load, about 80-90%, happens during heavy rain storms when fertilizer and other phosphorus sources are quickly washed into rivers and streams that flow into Lake Erie.



Where and when do HABs start in Lake Erie?

HABs usually occur first in Maumee Bay at the mouth of the Maumee River and in Sandusky Bay at the mouth of the Sandusky River. Both bays are very warm and shallow and the watersheds of both rivers have very high percentages of farm land (the Maumee is the largest tributary to the Great Lakes and drains 4.2 million acres of agricultural land). As a result, both streams contain very high concentrations of phosphorus.

Why do we have more HABs in Lake Erie than the other Great Lakes?

Lake Erie is the southernmost, shallowest and warmest of the Great Lakes. Its watershed has the least forest, the most agricultural land and the second-most urban/suburban land. Therefore, Lake Erie gets more sediment and nutrients (fertilizer runoff, sewage, etc.) than the other lakes, while also having environmental conditions that favor algal blooms.