Voice of the River

Understanding Algae Blooms on the Gallatin

If you have spent time along the Gallatin, you have undoubtedly seen for yourself or heard the chatter about long, stringy green algae growing on rocks throughout  the river. This is an algae called Cladophora glomerata (Cladophora). Cladophora is naturally occurring in the Gallatin; however, in 2018 for the first time it grew in excess, followed by another similar bloom in 2020. 

Excess  algae growth can be harmful to aquatic life, such as trout and aquatic insects, and negatively affects the experience of river recreation, like  boating and fishing. 

After the first algae bloom in 2018, the Gallatin River Task Force (Task Force) partnered with the Montana Department of Environmental Quality (DEQ) to conduct annual assessments on the Upper Gallatin to determine drivers of the blooms. Excess algae growth can be caused by a number of factors, including elevated nutrient concentrations, warmer temperatures, high amounts of sunlight, and low streamflow. The Task Force is looking to find out why blooms only occur some years and not others. 

Water data from 2018 to 2020 suggested that warmer water temperatures, lower streamflows, adequate nutrients, and an abundance of bright sunny days may have been the cause of the 2018 and 2020 blooms.  We were therefore surprised when there  was not a large-scale bloom on the Gallatin in 2021. 

The water data collected in 2021 indicated that nitrogen and phosphorus levels were below state standards. Similar to years past, the nitrogen state standard was exceeded in the lower West Fork. Sources of excess nitrogen include fertilizer, stormwater runoff, land application of treated wastewater, leaks from private septic systems, and pet waste. Sources of phosphorus are the same as nitrogen but include soil erosion as well.

Nutrients are just one piece to the puzzle of why algae blooms occur. Air temperatures were higher in June, July, and September for the 2021 season than past years. Water temperatures were generally warmer in 2021 than in previous years. By July, all sites measured on the Gallatin and its tributaries were within the growing temperature range for Cladophora. Streamflow throughout the summer was consistently below the historical average, with the greatest difference reaching 42% of average in July. 

If conditions were ideal for Cladophora to grow, then why didn’t a widespread bloom occur in 2021? To answer this, the Task Force looked more closely at sunlight. The summer of 2021 was notable in that wildfire smoke moved into the area early and stuck around for most of the summer. Analysis of sunlight levels from the past few years showed that incoming light for July of 2018 and 2020 when algae blooms occurred was higher than the levels of 2019 and 2021 when algae blooms did not occur. These lower levels could explain why there was not a bloom in 2021 when there were warmer water temperatures, sufficient nutrient levels, and lower streamflow. 

The Task Force will continue to collect data to better understand the algae blooms that occur in the Gallatin, and the reasons behind why they occur in some years, and not in others. This information will be critical to identify the most efficient strategies to mitigate future algae blooms. Stay tuned for the release of the 2021 Monitoring report for more in-depth findings!

The Task Force will be out on the rivers again this summer collecting data with the help of a dedicated team of volunteers. Feel free to join us to try your hand at collecting important water data or stop by and say hi. See you on the river! 

If you have any questions or need more resources, you can visit our website, or call 406-993-2519. And remember, every drop counts 💧

Subscribe to the GRTF Newsletter

Join our mailing list to receive news and updates from the Task Force.

You have Successfully Subscribed!