Nutrients a larger factor than temperature in Colorado mountain stream ecosystems, study finds

Amanda Budd, amanda.budd@uga.edu

Contact: Amanda Rugenski, atrugenski@uga.edu

A mountain stream in Colorado that was part of the study by Whitney Beck, Amanda Rugenski, and LeRoy Poff. Photo: Amanda Rugenski.

A new study demonstrates how nutrients and temperature interact to influence stream ecosystem processes. The findings, published in Freshwater Science, provide insight into how streams might respond to changes in climate and land use, and could inform algae mitigation strategies.

Scientists have long known that concentrations of nutrients, particularly nitrogen and phosphorus, affect ecosystem processes in streams. These processes include the growth of algae and the recycling of nutrients by aquatic insects like mayflies.

Environmental factors such as light, temperature and the speed of the stream current can enhance or inhibit the response of stream organisms to nutrients.

Researchers Whitney Beck and LeRoy Poff of Colorado State University and Amanda Rugenski of the University of Georgia Odum School of Ecology sought to untangle the relative influence of nitrogen concentrations and these environmental factors, which vary from place to place and seasonally.

They conducted an experimental study in eight Colorado mountain streams at varying elevations and at different times of the year. As elevation increased in these streams, temperature decreased and nitrogen levels rose. The researchers added nutrients to each stream in July and September 2016 and measured algal growth during both time periods and insect response to changes in temperature and nitrogen concentration in July.

The researchers initially expected that the lower temperatures at higher elevations would limit algal growth under higher nitrogen levels, but instead found that algae increased at these elevations, as did nitrogen ratios in waste excreted by insects.

“When temperature decreases, you often see the algae decrease as well because algae just don’t grow as well in the cooler temperatures, but we actually found the most algae at our cool, high-elevation streams,” Beck explained.

The same proved true for seasonal differences, with more algal growth in autumn than in summer.

The researchers believe this may indicate that temperature is not as important as nutrient levels to predict insect nutrient excretion and algal growth for the streams in this area.

Because algal growth and invertebrate excretion can influence stream ecosystem health and productivity, the findings of this study have implications for stream management strategies and climate change response.

“When we’re looking at how we’re going to mitigate climate change responses, it’s really important to know what areas are going to be the most vulnerable, and what factors are going to contribute to that vulnerability,” Rugenski said.

The research also provides insight on how streams should be managed to preserve water quality for recreation or other uses.

“This type of research that looks at what will work to control the growth of algae is very important to help manage nutrients and streams,” Beck said. “Here we found that the more nitrogen, the more algae, so if you can control your nitrogen sources, you can potentially keep the algal levels pretty low so that they’re not causing water quality problems.”

Part of this study was a component of a larger project called Evolutionary and Ecological Variability in Organismal Trait Response with Altitude and Climate (EvoTRAC), which is focused on investigating ecosystem sensitivity to warming in both tropical and temperate areas.

Rugenski said she is now working on a larger synthesis project with EvoTRAC to put their findings in this study into context and examine how climate change could impact aquatic communities.

Meanwhile, Beck said, further research is needed to gain a better understanding of the drivers of algal growth and their effects on ecosystems.

“Studies like this that can tease apart which one is more important, temperature or nutrients, and under what conditions, could be really helpful for thinking about how biological communities are going to respond to climate change,” Beck said.

This research was funded by the National Science Foundation.