A new study on nutrient pollution co-authored by Vladislav Gulis, associate professor of biology at Coastal Carolina University, appears in the current issue of the journal Science. The findings from the study show that an important food resource has been disappearing from streams without anyone noticing - until now.

Gulis, an aquatic microbial ecologist who studies fungi and bacteria associated with plant litter in aquatic environments, was one of a team of researchers led by University of Georgia (UGA) ecologists who worked on the project for more than a decade.

According to the study, excess nutrients - nitrogen and phosphorous from farm runoff and other sources - cause a significant loss of forest-derived carbon from stream ecosystems, reducing the ability of streams to support aquatic life. The findings show that the in-stream residence time of carbon from leaves, twigs and other forest matter, which provide much of the energy that fuels stream food webs, is cut in half when moderate amounts of nitrogen and phosphorus are added to a stream.

"This study shows how excess nutrients reduce stream health in a way that was previously unknown," said the study's lead author Amy D. Rosemond, associate professor in the UGA Odum School of Ecology.

Stream food webs are based on carbon from two main sources. One is algae, which produce carbon through photosynthesis. Nutrient pollution has long been known to increase carbon production by algae, often causing harmful algal blooms. The second source is leaves and wood from riparian forests. This forest-derived carbon typically persists year-round, making it a staple food resource for stream organisms.

Nitrogen and phosphorus play essential roles in the breakdown of organic carbon by microbes and stream-dwelling insects and other invertebrates, but cause problems when they are present in excessive amounts - as they increasingly are. Nutrient pollution is widespread in the U.S. and globally, due primarily to land use changes like deforestation, agriculture and urbanization.

Nutrient pollution's effects on algal carbon are well known and highly visible in the form of algal blooms. Little was known 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, a U.S. Department of Agriculture Forest Service and National Science Foundation Long Term Ecological Research site in western 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 whole stream reaches by half. "We were frankly shocked at how quickly leaves disappeared when we added nutrients," said Rosemond. "By summer, the streams looked unnaturally bare."

"Nutrients stimulated activity of microorganisms such as fungi that grow on dead leaves in streams, accelerating carbon losses as CO2," said Gulis. "At the same time, build-up of microbial biomass made leaf litter more palatable to aquatic invertebrates. The combined effects of fungi and invertebrates led to the increased breakdown rates of leaves and faster disappearance of this important food resource. The reduction in the available carbon may negatively affect the abundance and diversity of aquatic life, as well as stream ecosystem functioning."

Many streams lack sufficient 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 and the microbes that live on them are also particularly 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. This means that more nutrients head downstream where they can cause problems in lakes and estuaries."

The scientists hope the study's findings will be incorporated into policies aimed at reducing nutrient pollution, which currently focus on algae.

"There has been limited national progress in efforts to reduce the impact of nutrient pollution in the U.S., according to a U.S. Geological Survey study," said Rosemond. "Our results provide a more complete picture of nutrient effects in streams, suggesting that increased efforts at addressing their impacts can now improve stream health in more than one way."

Support for the study was provided by the National Science Foundation. Additional coauthors are Phillip M. Bumpers, David W.P. Manning and J. Bruce Wallace of the Odum School of Ecology; Keller Suberkropp of the University of Alabama; and John S. Kominoski of Florida International University.

For the full article, visit http://www.sciencemag.org/content/347/6226/1142.full.pdf

For more information, contact Gulis at 843-349-2576 or vgulis@coastal.edu.