Algae Bloom On The Big Hole

August 2020

After a heads up from local outfitters who first noticed the issue, we conducted a survey of an abrupt algae bloom that showed up in the middle river.

Hot temperatures, excess nutrients, and suspended sediment have likely combined to trigger this bloom here and also places like the Gallatin & Blackfoot Rivers.

Here’s the rundown, after a couple of weeks of monitoring, sampling, and working with Montana DEQ….

Conditions were ripe for a cladophora bloom (water velocity, bright sun, elevated water temps, pH, and excess nitrogen & phosphorus). Nutrient levels were above state thresholds for July at Wisdom & Mudd Creek. These include unnatural levels of nitrogen and phosphorus. It is often extremely difficult to pinpoint the sources of such nutrients because it’s rarely from a single point (i.e. a pipe emptying into the river). That is the nature of non-point source pollution and also the ability of phosphorus molecules to attach to sediment and remain in the river for long stretches, until taken up by growing algae.

A significant rain event (i.e. flash flood) in the Maiden Rock & Divide areas in late July is likely responsible for flushing excess nutrients into the middle river at a time of ideal growth conditions, thus triggering the bloom.

In addition to our monitoring, DEQ also mobilized some resources to get a surface water technician down here to collect algae and water samples.

For this year, the bloom is an eyesore and a recreational nuisance. Once year of such a bloom will not necessarily impact a fishery. However, if blooms continue it may shift aquatic insect communities and affect the fishery.

Through our monitoring program & lab support partnership with DEQ, we’ll continue to record conditions of the river, document changes, and submit that data to state water quality reports.

Though we’re seeing the importance of this program after only one year, a long-term data set is going to be key!