Didymo research grows interesting Kootenai River facts

A little less than a year ago, I wrote an article describing the start of a research project on the Kootenai River aimed at investigating a species of algae known as “didymo” (scientific name Didymosphenia geminata). Although didymo has probably always been present in the Kootenai River, it began forming noticeable dense growths or “mats” downstream of Libby Dam in the early 2000s.  

In addition to creating an unsightly eyesore on the Kootenai River, Montana Fish, Wildlife & Parks suspects that the dense didymo mats may be reaching levels high enough to reduce the density of many important aquatic insects eaten by fish. Although trout anglers on the Kootenai River in 2013 saw good action after the high water subsided, biologists were concerned enough to launch a detailed study to learn more about this nuisance algae and its impacts on the Kootenai River trout fishery.  

Fish, Wildlife & Parks collaborated with the U.S. Army Corps of Engineers at Libby Dam on a research project that will take a closer look at the factors responsible for the prolific mats of didymo and investigate if anything can be done to control this nuisance algae species in the Kootenai River. The first year of the three-year research effort is complete, and I thought I would share the results.  

This study uses an experimental flume system housed in two utility trailers on the north bank of the Kootenai River below Libby Dam, and grows didymo in shallow troughs using water pumped out of the Kootenai River. This small-scale flume system allows researchers to test the feasibility of adding various substances to limit the growth and proliferation of didymo. The hope is to develop a practical approach that could be considered for application to the entire river.  

While didymo is present in the entire Kootenai River throughout the year, it is most prevalent and forms the thickest mats immediately below Libby Dam, and the thickest mats typically occur during the winter months from February to April. Previous research conducted elsewhere has shown that didymo isn’t like most species of algae. Under normal nutrient conditions, didymo exists in the cellular form and does not produce the dense nuisance mats, but under low-nutrient conditions, it shifts from cell production to mat production, thus for didymo, the amount of material grown increases as nutrients decrease. This is contrary to most other species of algae. The two most common nutrients that most plants (including algae) require are nitrogen and phosphorus. Libby Dam traps 63 percent of the phosphorus and 25 percent of the nitrogen behind it, creating a mostly phosphorus-limited system downstream of the dam. These facts lead researchers to suspect that phosphorus was the logical starting place for the first year of research.  

Researchers decided to add three very low levels of phosphorous in addition to river water only, which was used to compare the phosphorous treatments. The three levels tested were 3, 5 and 8 parts per billion phosphorous. The research in 2013 lasted almost seven months, including a monthlong period of river water only to establish didymo in the experimental flumes, a three-month phosphorus addition period and a three-month period after phosphorous additions stopped.   

The most dramatic difference between the river water-only flumes and all three phosphorous levels was a complete algal community shift. All three levels of phosphorous saw increases in other species of algae including filamentous greens and other diatom species, while the river water only flumes were mostly dominated by didymo only.  

All three levels of phosphorous did increase the number of didymo cells and decrease the mat forming stalk form of the algae. While these results certainly were encouraging from a standpoint of developing a potential control for didymo, there were aspects that deserve further study prior to jumping straight to a river-wide application.

The dense proliferation of the other algae species was substantial and potentially high enough to suggest that we may simply be trading one problem for another should we proceed with one of the three phosphorous treatments at the river scale. All three levels of phosphorous reduced the length of the stalk material that forms the didymo mats, leading researchers to ponder if a lower phosphorous level may be equally effective at reducing didymo mats while limiting the proliferation of the other species of algae observed in 2013.

Due to some construction issues at the site, the research work completed in 2013 started in April, which is after the period which didymo mat formation usually peaks in the Kootenai River.  These issues prompted researchers to refine their plans for 2014, which will look at three even lower levels of phosphorous and conduct the research during the period when didymo mats are most severe.  

(Jim Dunnigan is a fisheries biologist with the Montana Department of Fish, Wildlife & Parks in Libby.)