Anoxic Events

Anoxic Event - Upper Southwest River, August 17, 2021 (photo credit: David Thomas)

Anoxic events are a common occurrence in Island estuaries and ponds during the summer months. Symptoms of an anoxic event include:

  • floating mats of dead or dying algae,
  • milky white or green discolouration of the water, 
  • Sulfur (H2S) or 'rotten turnip' odour,
  • low (near zero) dissolved oxygen readings, and in some cases,
  • dead fish and/or shellfish. 

What causes anoxic events?

Elevated levels of nutrients (nitrogen and phosphorus) cause an overgrowth or bloom of algae. When large amounts of algae die the decay process can trigger anoxic events. Decay strips the water of oxygen and special bacteria that can survive without oxygen begins to grow. This bacteria is the cause of the bad odours and cloudy discolorations that are symptoms of an anoxic event. While the bacteria is not toxic, some people can be more sensitive than others to the other conditions that are part of the event. Low to nil dissolved oxygen levels in the water can stress or kill fish and shellfish. 

Where can I see data for anoxic events?

Data regarding current and past anoxic events is now located within the PEI Water Registry. You can also navigate directly to the Tracking Anoxic Events in PEI section of the Registry. 

Can I report an anoxic event?

The department encourages members of the public to report anoxic events. The Province relies on results of monitoring data and reports of observed anoxic conditions from the public to identify where anoxic events are occurring. If you suspect that an anoxic event is occurring, or if you are concerned about water quality because of something you have seen, you can contact the Department by:

  • filling out an online form to report the anoxic event; or
  • calling (902) 368-5044.

If you suspect that a problem may need immediate attention (like a chemical or petroleum spill, or dead or dying fish/shellfish), contact Environmental Emergency toll-free at 1-800-565-1633.


Published date: 
August 2, 2023
Environment, Energy and Climate Action

General Inquiries

Department of Environment, Energy and Climate Action
4th Floor, Jones Building
11 Kent Street
PO Box 2000
Charlottetown, PE C1A 7N8

Phone: 902-368-5044
Toll-free: 1-866-368-5044
Fax: 902-368-5830
Report an Environmental Concern