Doctor of Philosophy (PhD)


Oceanography and Coastal Sciences

Document Type



Human endeavors, particularly the agricultural and industrial activities of the last half century, now produce more biologically available nitrogen (N) than all other natural sources combined. The increased N availability can have consequences for the health of aquatic biota on the local, regional, and global scales. One manifestation of this problem is the formation of coastal hypoxic zones where terrestrial N loading creates eutrophic conditions in coastal waters. This dissertation examines a century of changes in land use and water quality to quantify the relationships between agricultural land use practices and riverine N yields in the Mississippi River Basin and 56 other watersheds across the continental U.S. A novel and spatially-explicit geographic information system database encompassing the continental U.S. from 1840 to present was developed to test the hypothesis that land use affects water quality. The database was compiled from semi-monthly water quality monitoring records from the U.S. Geological Survey, and from county-level Census of Agriculture data from the U.S. Bureau of the Census and Department of Agriculture. The results of this research indicate that intensive agricultural land use is statistically associated with riverine N yields at the beginning and end of the 20th Century. These findings imply that agriculture was already affecting N export at the turn of the century, but that intensive contemporary management practices have significantly increased the N export per hectare of cropland in the latter half of the century. The baseline conditions for N export from minimally impacted watersheds, however, did not change over the course of the last century indicating that rehabilitation of surface water quality is possible. Additionally, decreasing in-stream N yields are quantitatively associated with increasing landscape diversity, increasing perennial farmland cover, and decreasing commercial fertilizer applications in agricultural watersheds across the continental U.S. The results also suggest that government farm payments can be re-distributed to implement land use practices that effectively reduce in-stream N concentrations at the sub-basin scale, while potentially improving groundwater quality, soil quality, and biodiversity. These improvements will require changing land use practices on large spatial scales, however, in ecological units ranging from upland watersheds to coastal bays.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

R. Eugene Turner