Official USGS algae report to be released by end of year

RBC | Official results of a United States Geological Survey (USGS) study looking into the cause of algae blooms on the White River are still on hold, for now. In the meantime, residents from both ends of the county got a glimpse into the data last week at a presentation by USGS Biologist Natalie Day.

“All of our data and all of our analyses should be available in two reports, they should probably be available by the end of the year,” Day said.

She shared an overview of “key points” from the study Thursday, noting that all information shared was technically preliminary, and thus “subject to review.”

The study, which involved three years of data collection (2018-2020) tested two hypotheses. First, that “stream bed movement during peak runoff limits algae growth.” Second, that “physical and chemical characteristics like light availability, water temperature, and nutrient availability would promote algae growth.” The results seem to suggest that both of these hypotheses were correct.


“There was a relatively large, late and long lasting peak streamflow in 2019, and we saw a pretty noticeable decrease in the algae” said Day, describing how above average snowpack in 2019 created conditions that were less favorable to new algae growth. In particular, large quantities of water during peak runoff season helped to shift rocks and sediment on the streambed, which can scrape off algae. Day said the positive effects of 2019 streamflow “scouring” may have lasted more than a year, based on subsequent algae


By comparing algae collections to water temperature data from Colorado Parks and Wildlife/Trout Unlimited during the same period, Day and others identified a threshold at which the filamentous algae seemed to thrive, which is around 55 degrees Fahrenheit (approx. 13 degrees celsius). Based on mean temperature data in August, water temperatures “exceed the threshold favorable to algae at many sites starting at above Fawn Creek on the North Fork,” according to Day.


Similar to temperature, scientists set thresholds for nutrient (phosphorus and nitrogen) loads in rivers, lakes and streams. If those nutrient thresholds are exceeded, new algae blooms can occur, and existing ones can worsen. “You can see that many sites on the mainstem exceed that criteria,” Day said, referencing datasets which indicate that filamentous green algae can thrive in “nitrogen limited” environments, where availability of another nutrient (phosphorus) is higher.


“What we found by measuring these variables across 20 different sites for multiple years is that sites with greater stability, or bigger rocks, higher water temperatures and greater phosphorus availability had greater algal biomass,” said Day, reiterating that none of these factors were solely responsible for algae growth, rather that all factors “act on each other and are interrelated.” As an example, she noted how reductions in streamflows during summer months can influence things like water temperature, dissolved oxygen, dilution potential and light availability.

“We’ve been in drought conditions for the past 20 years, the lows are much more noticeable than the previous 20 years,” said Day, adding that long term streamflow data also shows decreases in max flows during May-June, decreases in minimum flows during August and September, and increases to April’s max flows.

These metrics may indicate a shift toward earlier melt happening, and overall smaller peak, which can allow algae to start growing sooner in the year. It could also mean that “there’s not as many of those big flows to blast algae away.” According to the same data, over the last 40 years, annual flows have decreased by 66%.

Long term nutrient data reflects some regional trends, which are a decrease in nitrogen availability and an increase in phosphorus availability in the river. When asked why this might be the case, Day noted that increased emissions standards had reduced nitrogen pollution. Meanwhile a variety of factors could be increasing phosphorus loads including wind-blown, nutrient rich dirt landing on snow, wildfire aerosols and the release of “legacy phosphorus” from old agricultural fields. Air temperature increases might also play a role in increased phosphorus. “This region in particular has a much higher rate of air temperature change than anywhere in the country,” said Day, noting that the temperature changes “can do weird things to nutrients stored in soil” such as releasing naturally occurring phosphorus from the forest.


“Factors contributing to algal blooms are complex and interrelated, and increased knowledge about them can be used by local management groups to implement BMPs to limit nuisance levels of algal biomass,” said Day. Some examples of BMPs (best management practices) would be riparian habitat protection and improvement, water management and water use efficiency, and nutrient awareness and management. Day also talked about how continued monitoring of algae could help inform new BMPs, and would be needed to track the success of old ones.

During question and answers after the presentation, a variety of other subjects came up including river dredging, insecticide spraying and more. Ultimately, Day emphasized that while the existing data did not make suggestions towards any particular kind of action, it could be used as a guide for determining what types of activities may promote algae growth, and what may limit it.

“You’re gonna have to decide as a basin, ‘are we just gonna throw up our hands and say, oh it’s out of our control?’ Or are you gonna adapt and make what changes you can in the basin to reduce nutrients and to influence the factors that we identified as affecting algae?


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