Nutrients could reduce streams’ ability to support aquatic life, researchers find

America’s fresh waters are being challenged as increases in nutrient pollution are causing a significant loss of terrestrial-derived carbon from these ecosystems. Nutrient pollution is widespread in the U.S. and globally, due primarily to agricultural fertilization and urban stormwater runoff.

This trickle down effect could be reducing the stream’s ability to support aquatic life.

A team of researchers, including FIU biologist John Kominoski, has found organic carbon from forests, including leaves and twigs, is cut in half when low-to-moderate nutrient pollution, such as an excess nitrogen and phosphorus, is added to stream ecosystems.

“Nutrient pollution’s effects often focus on highly visible, global incidents of harmful algal blooms. But little was known about how nutrients affect terrestrial-derived carbon, a major form of carbon in many inland aquatic ecosystems,” Kominoski said. “We found nutrient pollution is reducing terrestrial organic carbon by 50 percent, and this higher magnitude loss of carbon is less visible than smaller increases in algal-based carbon in forested streams.”

Organisms rely on energy from carbon, and nitrogen and phosphorous are natural parts of aquatic ecosystems. The balance of carbon and nutrients in ecosystems determines the food resources for organisms, like invertebrates and fishes. However, even low-to-moderate increases in these nutrients can cause state changes in the source and quantity of carbon available to dependent organisms.

“Many aquatic ecosystems have low light, so they do not accumulate algae-based carbon.  This requires these ecosystems to rely on terrestrial carbon for food.  However, organic carbon from terrestrial ecosystems is more than food, it helps remove excess nutrients from the water,” Kominoski said. “Leaves and wood are actively colonized by microbes that remove nutrients from the water, as they consume carbon.  By accelerating the loss of these carbon resources with nutrients, we reduce the capacity of aquatic ecosystems to remove excess nutrients.  This means that more nutrients are not retained where they enter streams and rivers, but rather they are transported downstream where they can cause problems in lakes and estuaries.”

David Manning (L), University of Georgia doctoral student, and John Kominoski (R) perform maintenance on the pump used to add nutrients to one of the streams in their experiment. Credit: Jon Benstead/University of Alabama

According to Kominoski, aquatic ecosystems are often managed for nutrient concentrations and subsequent algal blooms, not for the availability and persistence of terrestrial organic carbon. The more carbon stored in ecosystems, the more resilient they are to human- and nonhuman-derived environmental stressors. The researchers hope the study’s findings will be incorporated into policies aimed at reducing nutrient pollution.

“Ecosystem management needs to account for the concentrations and exposure levels of nutrients that reduce an ecosystem’s capacity to store carbon. We aren’t able to match the loss of terrestrial organic carbon with the gain of carbon from algae. It’s not a zero-sum game, it’s a net loss of carbon for ecosystems,” Kominoski said. “We should be concerned about how the functions of aquatic ecosystems are altered by carbon loss, because we rely on them for clean drinking water, recreational activities and food production.”

The study was funded by the National Science Foundation and published in Science this month. In addition to Kominoski, the research team included scientists from the University of Georgia, University of Alabama, and Coastal Carolina University.