A lack of oxygen caused by an excess of rotting vegetation is killing life in four more P.E.I. rivers, and provincial officials are blaming the weather.

'Putting fertilizer on your garden or on an agricultural crop … all those are contributing factors.'— Cindy Crane, Environment Department

Cindy Crane, surface water biologist with the Department of Environment, said Monday when cloudy, rainy weather, such as P.E.I. experienced in July, is following by intense heat like the 30 C-plus temperatures that blanketed the Island earlier this month, it can lead to a lot of dead plants such as sea lettuce in the water.

As those plants rot, they consume oxygen, creating what is known as anoxic conditions. Without oxygen in the water, fish and shellfish die.

Crane told CBC News human activities that add nutrients to water systems add to the problem.

"Whether it's putting fertilizer on your garden or on an agricultural crop or having a septic system or having some other kinds of development near a water course, all those are contributing factors," said Crane.

"We do tend to see these a little more on the north shore of P.E.I. That's because the rivers on the north shore of P.E.I. just have a lot less potential to have the excess nutrients flushed out of the system."

The areas most recently affected are:

• The Suffolk branch of the Winter River at Tracadie Bay.
• Upper Covehead Bay.
• Brackley Bay near Shaw's Beach.
• The Huntley branch of the Kildare River.

In those rivers there is rotting sea lettuce and the water has turned a milky white or green colour and has a foul odour, Crane said.

Department officials are also investigating a large mortality on an oyster lease in the Fox Island area of Mill River. While the river was not anoxic, Crane said, the mortality may be the result of large mats of decaying vegetation on top of the oysters and smothering them.

Earlier this month, the Environment Department reported anoxic conditions in the Southwest River upstream of Long River, the upper Hunter River between Rusticoville and New Glasgow, the Wheatley River in the Oyster Bed area and the Montague River near Dewar's Point.

Several hundred small, dead fish have been discovered in the upper Southwest River and in a small tributary of the Hunter River. Crane said conditions in the Southwest River were slightly improved when readings were taken in the area last week.

Anoxic events can last from several days to a week or more, depending on weather conditions and prevailing tides.