What Controls Nuisance Algae in Colorado’s Upper White River?

Benthic algae grow attached to streambeds and make their own food from sunlight. Nuisance amounts of them are showing up more and more in the rivers and streams of the western United States, where they can spoil a river's looks, limit recreation, clog water infrastructure and harm aquatic life.

In 2016, Colorado Parks and Wildlife found Cladophora glomerata — a widespread nuisance green alga — in the White River of western Colorado. From 2018 through 2021, the U.S. Geological Survey, working with the White River and Douglas Creek Conservation Districts, the Colorado River Basin Salinity Control Forum and the Colorado River Water Conservation District, studied what might control algae in the upper White River Basin.

Benthic algae on the North Fork White River at Buford (site 8), summer 2018. Photograph by Natalie Day, U.S. Geological Survey.

What the study tested

Two hypotheses:

1. Snowmelt floods that move the streambed may change the conditions that limit how much algae can grow.
2. Streambed particle size, water temperature, light and nutrients affect how much algae grows.

The team used more than 20 years of streamflow and nutrient records from long-monitored sites, collected new data at 20 water-quality sites during 2018–21, and used a linear mixed-effects model to weigh each factor's effect on algal biomass.

The upper White River Basin, with selected USGS water-quality sites. Irrigated-lands layer modified from Colorado Decision Support Systems (2020).

The algae

• Samples were taken each summer, timed for when algae were most abundant.
• Algal biomass (measured as chlorophyll-a) ranged from 0.7 to 309 mg/m². The highest medians were at North Fork White River at Buford (site 8; 51 mg/m²) and White River above Dry Creek (site 15; 50 mg/m²).
• Overall, biomass was highest in 2018.
• Surveys in 2020 and 2021 found four genera of filamentous green algae — Cladophora, Stigeoclonium, Ulothrix and Spirogyra — and many genera of cyanobacteria, some able to produce toxins and taste-and-odor compounds.

At 20 sites in 5 subbasins: (A) mean August water temperature, 2019 and 2020; (B) mean riparian canopy cover; (C) streambed surface particle size; (D) chlorophyll-a (algal biomass). Graph from Day and Henneberg, 2023.

Streamflow

When high and low flows come, how big they are and how long they last all affect algae. In the upper White River, flow peaks with snowmelt in late spring or early summer.

• High spring flows can reduce algae by scouring or rearranging the streambed. Snowmelt water is also cold and murky, cutting the light algae need.
• Low summer flows mean warmer water, which favors algae. In years when snowmelt comes early, water warms sooner and the growing season lasts longer.

Factors that may control algae growth in the upper White River Basin, 2018–21.

Records from 1980 through 2020 showed significant drops in August and September flow on the White River above Coal Creek (site 18). Lower May and June flows, with higher April flows, may point to earlier snowmelt.

Field measurements tested whether the streambed moved during peak runoff:

The model found that larger streambed particles went with more algae: big stones give algae a stable place to settle and slow the water flowing past, helping them take hold.

Nutrients

Nitrogen and phosphorus feed algal growth. They come from weathered rock and soil, decaying organic matter, the atmosphere and human activity.

• At four long-term sites (2000–2020), phosphorus rose over time in both concentration and load, while nitrogen fell.
• Phosphorus rose fairly evenly on both the North and South Forks, suggesting regional causes; similar increases were seen in the nearby Yampa River.
• The balance between the two shifted: from about 2012, both forks went from limited by both nutrients (colimited) to nitrogen-limited — meaning more phosphorus was available.
• The model linked more nitrogen-limited conditions (more available phosphorus) with more algae.

A one-time synoptic sampling in October 2020 — a snapshot, not an explanation of long-term trends — found:

• a large phosphorus yield from the lower North Fork subbasin;
• high total phosphorus and orthophosphate loads at North Fork White River below Marvine Creek (site 5);
• a jump in total nitrogen between the two farthest-downstream White River sites (19 and 20), probably from organic nitrogen.

Water temperature

Cladophora glomerata grows best above about 13 °C.

• That threshold was passed by early to mid-June in 2018, 2020 and 2021.
• In 2019, with high flows and a long snowmelt, it wasn't reached until July.

Warmer water went with more algae. Every site whose mean August temperature stayed below the threshold had consistently low algae. Water was warmer at downstream sites, which also had more algae.

Streamside shade

Tree cover along a stream controls how much sun reaches the water; less shade means warmer water and more algae. Canopy cover at the study sites was low, from 0 to 26 percent.

Conclusions

• Algae were found so widely that blooms probably have basin-wide causes rather than a single one.
• Long-term changes in nutrients and streamflow — especially the timing and size of high and low flows — may affect how often and how badly blooms occur.
• Big peak flows that move the streambed may be the main control on blooms from year to year. A large, late, long-lasting peak, like 2019's, may suppress algae that year and the next: biomass stayed low in 2019 and 2020, even though 2020 had warm water, low summer flow and no streambed movement.
• When the streambed isn't disturbed, nutrients and temperature may matter more.

Sources

Based on U.S. Geological Survey Fact Sheet 2023–3005, "Potential factors controlling benthic algae in the upper White River Basin, Colorado, 2018–21," which summarizes Day (2023) and Day and Henneberg (2023); a work of the United States government in the public domain.