Tracing nutrient sources in the Mississippi River Basin, United States of America

To provide information for the development of management strategies to reduce N loads and enhance N attenuation mechanisms, isotopic techniques have been used to investigate the sources and cycling of nutrients at a number of sites in the Mississippi Basin (which includes the Ohio and Missouri River Basins). About half of the POM in the Mississippi (and other big rivers in the USA) is composed of plankton and/or heterotrophic bacteria. This suggests that in-situ productivity may be a significant source of bioavailable organic matter contributing to the hypoxia in the Gulf of Mexico. Monthly samples from 19 river sites in the Basin sampled over 5 years showed that δ 15N and δ 13C were quite useful in discriminating among four major categories of POM: terrestrial soil, fresh terrestrial vegetation, aquatic macrophytes, and plankton/bacteria The isotopic data, along with ancillary chemical and hydrologic measurements, were also useful for documenting seasonal changes in in-situ processes. A pilot study in 2000-2001, designed to investigate the usefulness of isotopic techniques for determining nutrient sources in 24 medium and large watersheds in the Basin, found that nitrate and POM from basins with different land uses (e.g., row crops, animal farming, urban development, and undeveloped) had moderately distinctive isotopic compositions. The nitrate δ 18O and δ 15N values of the large rivers sites resembled the compositions seen in sites dominated by row crops. Sites with livestock tended to have high δ 15N values characteristic of manure, and urban and undeveloped sites tended to have higher δ 18O values characteristic of a significant fraction of atmospheric nitrate. The δ 18O data were critical in showing abrupt changes in nitrate sources with discharge. A more thorough study of nutrient sources in the Ohio River Basin was initiated in 2002. For this study, nitrate, POM, and water were collected 15-20 times each year at 6 small NAWQA-program watersheds in the White River- Miami River basins, and at the 7 large river NASQAN-program sites in the Ohio River Basin. Nitrate samples were analyzed for δ 15N and δ 18O, POM for δ 15N and δ 13C, and water for δ 18O and δ 2H. The δ 15N and δ 13C of fish were used as indicators of nutrient sources. Other studies have indicated that POM consists primarily of phytoplankton and is transported in the water column, particularly size fractions < 1-mm diameter, were the primary food source for food webs in the Ohio and Upper Mississippi Rivers