Deciphering the role of dissolved organic nitrogen in stream nutrient cycling

University Of New Hampshire, Durham NH

Investigators

Abstract

Many surface waters are yellow or brown in color due to the presence of organic molecules which come from many natural sources such as leaves, soils, and wetlands. These molecules affect a wide range of characteristics within streams such as the ability of light to penetrate the water and the acidity of the water. The organic molecules also can provide streams with energy and essential nutrients such as nitrogen and phosphorus. One particularly important form of these organic molecules is dissolved organic nitrogen, which is found globally in forests, arctic tundra, estuaries and streams. However, very little is known about when and where dissolved organic nitrogen serves as a source of energy, a source of nitrogen, or both to biological communities. This project will examine the role of dissolved organic nitrogen in streams across five distinct forested regions spanning a diverse set of environments in an effort to shed light on the role of dissolved organic nitrogen. This research is relevant to water resources management because dissolved organic nitrogen is a source of excessive nutrient loading causing toxic algal blooms in estuaries, and when it is found in drinking water supplies it can produce, during water treatment, disinfection by-products that can harm human health. This research will also integrate under-represented groups into the laboratory work and field work needed for the project and provide training opportunities for a graduate student and a post-doctoral scientist. Differentiating among the competing roles for dissolved organic nitrogen (DON) is difficult, as direct manipulation of the DON pool in aquatic ecosystems is most easily accomplished by addition of specific compounds such as amino acids, which are commercially available but are not particularly representative of the DON found in streams and rivers under ambient conditions. Based on initial experimental results, the researchers will conduct a series of novel field manipulations in streams across New England, Puerto Rico, Spain, Czech Republic, and Siberia to better understand the extent to which DON serves as an energy source or nitrogen source in streams globally. Initial experimental observations in New England show that the DON can serve as both a nitrogen source and an energy source, but its role appears to vary over space and time. Here the researchers will perform additional experiments where nitrate pulses are added to streams across a global network to assess how frequently, and under what conditions, DON serves predominantly as a nitrogen source, versus an energy (carbon) source. This set of experiments will provide some of the first experimental evidence to assess the importance of these two competing drivers in regulating the levels of ambient DON in natural waters.