The Role of Land-Use/Land-Cover and River/River-Margin Denitrification in the Regulation of Nitrogen Delivery to the Mobile Bay Ecosystem
Jonathan Pennock1 and Ronald Kiene2
1 University of Alabama,  Dauphin Island Sea Lab, Dauphin Island AL 36528
2 Department of Marine Sciences, University of South Alabama, Mobile AL 36688
World-wide, nutrient over-enrichment (eutrophication) has become a critical management issue in coastal environments as a result of the negative effect that it has on the production of aquatic food webs, commercial fishing harvest, recreational use and the aesthetic value of these ecosystems. Changes in the magnitude and timing of nutrient loading¨primarily that of nitrogen (N)¨caused by changes in land-use and atmospheric deposition of N are likely to have a significant impact on coastal ecosystem health as a result of their effects on biogeochemical processes and food web dynamics (e.g., phytoplankton production) in these ecosystems (Vitousek et al. 1997). 

Like most other large watersheds, the Mobile Bay watershed has been highly altered. Upland forests, riparian areas, and wetlands in the watershed have been removed for agriculture, timber harvesting, and urban growth. Quantifying how land-use/land-cover change has impacted non-point source N pollution is not possible due to the scarcity of historical nutrient values.  Problems associated with land-use and land-cover change¨e.g. increased use of fertilizer, deforestation, and destruction of wetlands and riparian vegetation¨are the primary sources of increased non-point source N pollution to estuaries. To understand the effects of these changes on coastal ecosystems, it is necessary to examine the relationship between land-use/land-cover and N dynamics in watersheds that are undergoing rapid change. It is also imperative that we understand how the major biogeochemical removal processes such as denitrification and storage in plant biomass, serve to modify anthropogenic nutrient loads to these systems.  

There are two primary goals in this study: 

To produce N mass balance models, based on the linkage between land-use/land-cover and hydrology, for three characteristic sub-watersheds of Mobile Bay; a predominately agricultural watershed, a predominately urban watershed, and a predominately pristine watershed located in the Mobile/Tensaw River delta. 

To use the relationships between basin land-use/land-cover and N dynamics gleaned from the study of these linkages at the finest spatial resolution possible to quantify N flux over an annual cycle from the lower Mobile Bay watershed, the portion of the watershed below the confluence of the Alabama and Tombigbee Rivers, to the Mobile Bay estuary.  

Understanding the relationship between land-use/land-cover and N biogeochemistry will increase the strength of predictions relating to how land-use/land-cover change will alter N delivery to Mobile Bay and the coastal ocean. Further, at a more basic level, this study will elucidate the role of land-cover in regulating ecosystem N biogeochemistry by examining the regulation of N input, loss and storage, and N output.