Nutrient pollution from nitrogen and phosphorus in streams has long been known to increase carbon production by algae, often causing nuisance and harmful algal blooms.
But according to results of a new study, nutrient pollution can also result in the loss of forest-derived carbon--leaves and twigs--from stream ecosystems, reducing the ability of streams to support aquatic life.
"Most people think of nitrogen and phosphorus pollution in streams as contributing to algae blooms," said Diane Pataki, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research.
"But streams contain a lot of leaf litter, and this study shows that nutrient pollution can also stimulate carbon losses from streams by accelerating the breakdown of that litter. That helps us better understand how fertilizer runoff affects carbon transport and emissions from streams and rivers."
What matters: How long a leaf or twig floats in a stream
The findings, published today in the journal Science, demonstrate that the in-stream residence time of leaves and twigs, which provide energy to fuel stream food webs, may be cut in half when moderate amounts of nitrogen and phosphorus are added to a stream.
"This study shows that excess nutrients reduce stream health in a way that was previously unknown," said Amy Rosemond, an ecologist at the University of Georgia (UGA) and the paper's lead author.
"By increasing nutrients, we stimulate decomposition, and that can cause the loss of carbon that stream life depends on."
Stream food webs based on photosynthesis, leaves and wood
Stream food webs are based on carbon from two main sources.
One is algae, which uses photosynthesis to transform carbon dioxide in water into food.
The other is leaves and bits of wood from streamside forests. This forest-derived carbon usually persists year-round, making it a staple food resource for stream organisms.
Nitrogen and phosphorus play essential roles in decomposition of carbon by microbes and stream-dwelling insects and other invertebrates, but cause problems when they are present in excess amounts--as they increasingly are.
Widespread nutrient pollution
Nutrient pollution is widespread in the United States and worldwide, primarily due to land use changes such as deforestation, agriculture and urbanization.
Its effects on algae are well-known and very visible in the form of algal blooms.
Little was understood about how nutrient pollution affects forest-derived carbon in stream food webs, so Rosemond and her colleagues devised a set of experiments to find out.
Working at the Coweeta Hydrological Laboratory, an NSF Long-Term Ecological Research site in North Carolina, they set up a system to continuously add nutrients to several small headwater streams.
The first experiment ran for six years in two streams, and the second for three years in five streams, with different combinations of nitrogen and phosphorus to mimic the effects of different land uses.
The researchers found that the additional nutrients reduced forest-derived carbon in streams by half.
"We were frankly shocked at how quickly leaves disappeared when we added nutrients," said Rosemond. "By summer, the streams looked unnaturally bare.
"This is comparable to the doubling of carbon from algae that can occur with nutrient pollution, but it's not a zero-sum game.
"Increasing one form of carbon and decreasing another does not equate. These resources have unique roles in stream food webs, and nutrients are affecting their relative availability."
Many streams lack enough light for algae to grow, making forest-derived carbon their main source of energy. But forest-derived carbon is more than a source of food.
Leaves and twigs in streams take up pollutants
"Leaves and twigs, and the microbes that live on them, are also important in taking up pollutants like nitrogen and phosphorus," Rosemond said.
"Ironically, by stimulating the loss of these resources with nutrients, we lose a lot of their capacity to reduce the nutrients' effects. That means that more nutrients flow downstream where they can cause problems in lakes and estuaries."
Rosemond said she hopes the study's findings will be incorporated into policies aimed at reducing nutrient pollution.
"Our results provide a more complete picture of nutrient effects in streams."
Co-authors are Phillip Bumpers, David Manning and Bruce Wallace, all of UGA; Jonathan Benstead and Keller Suberkropp of the University of Alabama; Vladislav Gulis of Coastal Carolina University; and John Kominoski of Florida International University.