Drought Could Impact Aquatic Life in Many Ways

by Barbara Maynard

The connection between aquatic life and drought might seem obvious. For organisms that live in water, less water means less habitat. However, drought affects not only water quantity, but also has the potential to degrade water quality.

“The critical issue is that there’s not as much flow to dilute substances from permanent point sources,” said Sarah Johnson, assessment unit manager for the Colorado Water Quality Control Division. Less dilution means that any contaminants in the water will be more concentrated.

High concentrations of nutrients such as nitrogen and phosphorus can boost productivity in streams, lake and reservoirs. On the surface increased productivity might sound like a good thing, but algal blooms are followed by algal die-offs, and the breakdown of dead organic matter drags down dissolved oxygen levels and increases pH and organic carbon levels, explained Michael Lewis, water quality specialist for the USGS Colorado District. These conditions can lead to fish kills, taste and odor problems, and increased costs of water treatment.

Lower flows can also mean warmer waters, which can stress organisms adapted to cooler temperatures. Warmer temperatures can also interact with pollutants to magnify their effects. “Ammonia is more toxic in warmer situations,” Johnson said.

These expectations fit with what Colorado Division of Wildlife fish biologist Randy Van Buren saw in the field. “I was seeing summer kills already in June, and I don’t see those,” he said. “I think the lack of dilution is the biggest factor.”

Warm conditions combined with increased nutrient concentrations and decreased dilution might also be conducive to pathogen growth, a concern in places where people are swimming or rafting, Lewis said.

Low flow can also alter the impact of discharges from wastewater treatment plants or other sources. Discharge permits are granted with low flow exceptions, Lewis explained. That is, the permits are not enforceable during low flow, which could exacerbate any problems.

With Colorado’s snowpack at 2 percent of average in June, and little rain over the summer, the resulting low flows could pose a threat to aquatic life in the state. In the long run, the drought could have one positive result by raising awareness of water quality issues and forcing a shift in priorities. “Water projects, say fixing the dam or dredging to make water quality better, were formerly thought of as too expensive. Now they are moving up the books,” Van Buren said. Over time, such projects will help to improve habitat.

Concerns about the potential impacts of drought on water quality, especially in the context of burgeoning population growth, prompted a special USGS study this past summer. The agency coordinated a statewide synoptic sampling at sensitive sites in all river basins across the state. Nine sampling sites were along the Big Thompson River. Parameters measured include major ions, metals, dissolved oxygen, pH, temperature, nutrients, bacteria, and emerging contaminants, which tend to be a concern for wastewater-impacted waters.

As the summer’s data are analyzed, USGS scientists will better understand which of the potential impacts of drought on water quality actually came to pass this year in the state.