Arizona State University

The plight of the world’s oceans is dire, according to recent studies, through insults from human-derived activities depopulating and damaging reefs, altering coastlines, and creating pollutants, such as nitrogen runoff from terrestrial watersheds.

A study by 31 aquatic biologists involving 72 stream sites in the United States and Puerto Rico has found that one critical buffer to excess nitrogen run off from agricultural and urban areas turns out to be small streams and rivers. The findings are published March 12 in the journal Nature.

“We found that nitrate was filtered from stream water by tiny organisms such as algae, fungi and bacteria,” says Patrick Mulholland, lead author of the study and a member of Oak Ridge National Laboratory’s Environmental Sciences Division, with a joint appointment at the University of Tennessee. “Further, our model showed that the entire stream network is important in removing pollution from stream water.”

The study used a rare nitrogen isotope to examine the effects of nitrogen loading in streams. The researchers analyzed its removal relative to the amount of nitrogen present in the stream overall. The results showed that much of the nitrogen was removed by bacteria, in a process called denitrification that releases harmless nitrogen gas to the atmosphere. However, the study also demonstrated that as nitrate loads increase, the efficiency of removal was reduced.

“Our study shows that nitrogen loading compromises the ability of streams to retain or transform nitrate, a major pollutant that has been associated with lake and stream eutrophication, groundwater pollution, and coastal dead zones,” says Nancy Grimm, an ecologist at Arizona State University who has been involved with the project since the 1980s.

Presently it’s believed that small streams and rivers remove three-quarters of the excess nitrogen contamination before it reaches the oceans by acting as “sinks.” However, the researchers’ findings published in Nature suggest that as land use changes, and shifts to increasing nitrogen loads occur, that this buffering capacity could be overwhelmed. Nitrogen pollution could generate algal blooms, oxygen depletion (dead zones) and death to coral, fish and shellfish in coastal zones.

Grimm believes that the long-term, collaborative nature of the project supporting this study, which has incorporated two separate experiments each conducted in a range of ecosystems, was key to “advancing understanding of stream nitrogen dynamics far beyond what could be accomplished with a single-investigator grant focused on one region.”

As a professor in ASU’s School of Life Sciences, Grimm is no stranger to long-term collaborative efforts. For the last 10 years she has led the Central Arizona-Phoenix Long-term Ecological Research (CAP-LTER) project centered on the analysis of urban-semi-arid ecosystem relationships. The co-director of CAP-LTER is anthropologist Charles Redman, director of ASU’s School of Sustainability.

With her collaborators, Grimm has established a conceptual basis for including human choice and action in theory of urban ecosystem dynamics. Grimm and her counterparts’ empirical work on biogeochemistry, species distribution and abundance, and designed aquatic ecosystems in cities have revealed that many ecological features are best explained by combinations of social and biophysical drivers. Grimm was also the first to describe nitrogen cycling in desert streams, work that led directly to the long-term collaboration and the experiments described in the Nature article.

The findings published in Nature underscore the critical interplay that exists between human action and ecosystems dynamics and capacity, and emphasizes “the management imperative of controlling nitrogen loading to streams and protecting or restoring stream ecosystems to maintain or enhance their nitrogen removal functions.”

Along with Mulholland and Grimm, other collaborators on this study include scientists from the Woods Hole Marine Biological Laboratories, University of Georgia, Athens; Eco-Metrics; University of Wyoming, Laramie; Michigan State University; University of Notre Dame; Oregon State University; University of New Mexico; Kansas State University; Institute of Ecosystem Studies; U.S. Forest Service; University of New Hampshire; Virginia Tech; and Ball State University.