Excess nitrates cause a bloom of aquatic plants, such as here in Hunter River. (CBC)

Decaying aquatic plants have caused a severe shortage of oxygen in at least 13 P.E.I. waterways and killed large numbers of fish, environment officials say, and they think the problem is likely larger still.

Cindy Crane, a biologist with the P.E.I. Environment Department, told CBC News Tuesday the largest fish kills seem to have occurred in the Wheatley River, but rivers all over the province are suffering.

The government on Tuesday reported the anoxic conditions — a lack of dissolved oxygen in the water caused by large amounts of rotting vegetation. Excess nitrates in the water act as fertilizer that combines with the hot weather to cause disproportionate growth in aquatic vegetation.

As the plants die and decompose they use up oxygen in the water, which turns milky white or green, and gives off a rotten egg smell — and fish die.

"We don't really have a value or an actual number," said Crane.

"It did appear to be fairly significant: very numerous numbers of fish, mostly mummichogs and killifish and smaller fin fish and some eels and flatfish — those types of things."

There are likely anoxic rivers that have not yet been reported, said government biologist Cindy Crane. (CBC)

Officials know about significant mortality of fish and shellfish in the past week in the Cardigan River, in eastern P.E.I., and the Wheatley River, in the north-central part of the province. Other rivers with problems found by Tuesday included the Hunter, French, Mill and Montague. On Wednesday the province added the Trout/Stanley, Granville and Southwest rivers to the list.

Some of this week's events were reported by the public, while others were discovered by environment department staff. Crane said there are probably more affected estuaries that haven't been reported yet.

Long-term problem

Environment officials will be working Wednesday to determine the extent of the current problem, but anoxic events are an annual occurrence with deep underlying causes.

A joint federal-provincial commission is currently studying the problem of nitrates in P.E.I.'s water. An interim report released earlier this month said any solution to the problem of nitrates in the Island's water would take decades.

Unlike previously reported fish kills on P.E.I., which were mass poisonings caused by the washing of pesticides into rivers by heavy rains, the aquatic blooms caused by excess nitrates have many causes, the interim report says. Fertilizers from farmer's fields and golf courses, septic systems and other human development along rivers are all sources of nitrates.

'This is absolutely not run-off from fields.'— Environment Minister George Webster

Environment Minister George Webster was quick Wednesday morning to point out the difference between the two types of fish kills.

"Oh no, this is absolutely not run-off from fields. It's probably caused to some extent by nitrate levels higher in the water table, and phosphate levels," said Webster.

"They're coming from fields, yes, they're coming from sewer systems, from human activity."

The carcasses of dead fish will likely not be removed from either of the fish kill sites, Webster said.

Environment officials were out Wednesday morning investigating the rivers and taking water samples. While current events give these research trips a certain urgency, officials have been investigating rivers for similar problems in the weeks around the end of July every year since 2001.