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Water Quality

Didymosphenia geminata in British Columbia Streams

What is Didymosphenia geminata?

In recent years, the Ministry of Environment has received several reports of dense mats of algae, called Didymosphenia geminata, growing in several streams throughout the province. Although the white algal mats can cover large areas of stream bed, they are not generally considered an indication of degraded water quality.

Didymosphenia geminata is a species of algae belonging to a group known as the diatoms. Diatoms are unique among algae because they consist of a silica shell, called a frustule, which fits together in two halves like a pill box. Diatoms are generally free-floating plankton; however, there are species that grow attached to the stream bed. The frustules of Didymosphenia are attached to rocks by a gelatinous stalk that form mats when growing in high densities.

stalked  Didymosphenia geminata frustule

Close-up photograph of a stalked Didymosphenia geminata frustule

Where does Didymosphenia grow?

Didymosphenia geminata is normally present at low levels throughout the province. It thrives in clear, warm, shallow and nutrient-poor water, and is influenced year to year by weather and rainfall patterns.

Didymosphenia has been reported to grow to nuisance levels in a number of river systems around the province — especially throughout central Vancouver Island, where it was first reported in 1989. It has also been found in significant quantities in the Bulkley, South Thompson, Kettle, Columbia and Kootenay Rivers.

It is thought that Didymosphenia may proliferate because of increased exposure to ultraviolet (UV) radiation. UV radiation may reduce the aquatic insect population that normally limits the accumulation of Didymosphenia, or perhaps Didymosphenia out-competes other species of algae under relatively high UV conditions. Increased UV exposure due to loss of streamside vegetation, warmer winters, reduced ice cover and reduced flows may all favour the growth of Didymosphenia — if the UV theory is correct.

As sunlight decreases at the end of summer, Didymosphenia stops growing and dies off. The growth of Didymosphenia in a given stream is unpredictable and may not occur from one year to the next.

What are the mats?

Didymosphenia has a somewhat unusual growth habit in that each diatom frustule (the silica shell) is found on the end of relatively long stalk. The stalks are multi-branched, giving an overall appearance similar to that of a microscopic palm tree.

When conditions are favourable, Didymosphenia can form mats that will cover large areas of stream bottom. The mat consists mainly of the stalks, which contain no chlorophyll, giving the mat a colour that ranges from pale yellow-brown to white. As stream levels drop, the drying mats remain on the rocks and can be mistaken for toilet paper, causing concerns about possible upstream sewage discharges.

Are there any health concerns?

Based on the available information, Didymosphenia does not appear to affect the safety of drinking water, although taste and odour problems may be a concern. Swimmers have occasionally complained about itchy eyes after swimming in areas downstream of large Didymosphenia mats. This is likely due to high quantities of the diatom frustules finding their way into swimmers’ eyes and causing a reaction similar to that triggered by pollen.

Didymosphenia mats

Didymosphenia mats forming in the Stamp River, Vancouver Island

Are there any fisheries concerns?

Fisheries concerns include possible reduced rearing habitat for salmonids — due to changes in invertebrate communities and populations, physical impacts such as gill irritations and clogging, and displacement of some fish species at high concentrations of Didymosphenia growth. The mats may also restrict the flow of water, and therefore oxygen, to the eggs and fry in gravel. Finally, the decomposition of the algal mats may deplete dissolved oxygen in the water.

Are there other concerns?

Water intakes (both domestic and those used for fish hatcheries) are sometimes clogged with growing mats of Didymosphenia or with algal material from sloughing mats. Many reports have been received from recreational users complaining about the appearance of their favourite swimming holes, slippery rocks and algae fouling fishing lines and gear.

Should I report Didymosphenia?

If you find an area that is significantly affected by Didymosphenia geminata, contact the nearest regional office of the Ministry of Environment.


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