Because it is hard to tell a HAB from non-harmful algal blooms, it is best to avoid swimming, boating, otherwise recreating in, or drinking water with a bloom. Keep reading to learn what to do if you spot a bloom. Click on the links below for more detailed information.

Most algae are harmless and are an important part of the food web. Certain types of algae can grow quickly and form blooms, which can cover all or portions of a lake. Even large blooms are not necessarily harmful. However some species of algae can produce toxins that can be harmful to people and animals. Blooms of algal species that can produce toxins are referred to as harmful algal blooms (HABs).

HABs are likely triggered by a combination of water and environmental conditions that may include: excess nutrients (phosphorus and nitrogen), lots of sunlight, low-water or low-flow conditions, calm water, and warm temperatures. Depending on the weather and the characteristics of the lake, HABs may be short-lived (appearing and disappearing in hours) or long-lived (persisting for several weeks or more).

• People, pets and livestock should avoid contact with any floating mats, scums, or discolored water. Colors can include shades of green, blue-green, yellow, brown or red.
• Never drink, prepare food, cook, or make ice with untreated surface water, whether or not algae blooms are present. In addition to toxins, untreated surface water may contain bacteria, parasites, or viruses that could cause illness if consumed.
• People not on public water supplies should not drink surface water during an algal bloom, even if it is treated, because in-home treatments such as boiling, disinfecting water with chlorine or ultraviolet (UV), and water filtration units do not protect people from HABs toxins.

If contact occurs:

Rinse thoroughly with clean water to remove algae.

• Stop using water and seek medical attention immediately if symptoms such as vomiting, nausea, diarrhea, skin, eye or throat irritation, allergic reactions, or breathing difficulties occur after drinking or having contact with blooms or untreated surface water.

Be prepared:

Before you go in the water, find out what waterbodies have blooms or have had them in the past. DEC maintains a HABs Notifications page of waterbodies that currently have blooms. Please note that if a waterbody is not listed, it does not mean that it does not have a bloom. It may have one that was not reported. Find out what waterbodies have had blooms in the past on the HABs Archive page. For additional information, please see the DEC Program Guide (PDF), updated in 2020.

DEC does not have the authority to close a lake in the event of a bloom, although they can close beaches that are operated by DEC (limited to a small number of beaches within the Adirondack and Catskill Parks). The NYS Office of Parks and Recreation, County officials or local Department of Health has the authority to close swimming beaches. Beach operators close beaches as needed to assure public health and safety. This can happen under a number of circumstances, including when excessive algal blooms are detected.

• If you suspect that you have seen a HAB, please report the HAB to DEC. Fill out and submit a Suspicious Algal Bloom Report Form (leaves DEC website). If possible, attach digital photos (close-up and landscape to show extent and location) of the suspected HAB in the web form. Email HABsInfo@dec.ny.gov if you are not able to complete the form.
• Please report any health symptoms to NYS Health Department at harmfulalgae@health.ny.gov and your local health department (leaves DEC website).