The Task Force received multiple reports in 2018 that algae growth in the Gallatin River was noticeably more prolific than previous years.

We have collected data that confirmed excess algae growth in the West Fork, Middle Fork of the West Fork, and South Fork of the West Fork over six different years (2005, 2008, 2013, 2014, 2016, and 2018). We have not been able to identify whether or not algae is becoming more widespread/prolific over time from these results; however, the inability to determine a long-term trend may be due in part to limited data, changes in Montana Department of Environmental Quality (MT DEQ) algae monitoring protocols, and the complex factors impacting algae growth.

Algae biomass data from 2005 to 2016 from the West Fork and South Fork Gallatin River. The MT DEQ uses 125 mg/m2 as a critical value for chlorophyll-a and 35 g/m2 for ash-free dry weight. The MT DEQ changed their sampling protocol in 2006 and data collected before 2006 cannot directly be compared to data collected after 2006.

In order to understand long-term trends, researchers must compare decades of data, rather than just a few years of measurements. To date, we do not have sufficient data to determine whether or not there are any long-term trends. In addition, the MT DEQ changed their algae monitoring protocol in 2006 making it difficult to compare data collected before and after that time. Finally, algae growth is affected by several factors that vary greatly year-to-year, including water temperature, sunlight, water clarity, nutrients, and streamflow.

Moving forward, the Task Force will continue to monitor the situation by conducting interviews with long-time river users about algae, continuing to take annual photos of algae at all of our field sites, and performing regular algae biomass assessments.