The Guadalupe Estuary is one of seven major estuarine systems centrally located along the Texas Gulf Coast and is among the top three Texas estuaries with the most productive resource base for commercial bay fisheries. Two major river basins drain to the Guadalupe Estuary, the Guadalupe and San Antonio River Basins. The two basins differ in size, population, urban land use, precipitation and number of permitted discharges. This study aims to investigate effects of terrestrial-origin nutrient loadings on estuarine ecological processes, and is composed of two main components: 1) landscape analyses using a geographic information system (GIS) coupled with historical water monitoring data; and 2) ecosystem box modeling. Land-cover data was obtained from the Coastal Change Analysis Program (C-CAP) of the National Oceanic and Atmospheric Administration (NOAA) and included in the landscape analyses. Multivariate statistical methods were employed to correlate land-cover/land-use changes with nutrient loadings from 17 watersheds in the Guadalupe and San Antonio River Basin. Increased human population densities, changes in land-use patterns and various agricultural practices have been attributed to increases in concentrations of inorganic and organic nutrients. To better understand human activities and subsequent estuarine ecosystem responses, a multi-compartment ecosystem model was developed. The model domain includes two segments, neighbored with two upstream boundaries (i.e., watersheds that belong to the two basins), and one segment at seaward boundary (Gulf Inlet). Each segment has 7 ecological components, including phytoplankton, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), zooplankton, detritus, benthic suspension and deposit feeders. The model was calibrated and validated with long-term time series data sets (1987-2007). In a preliminary analysis, we found that mean concentration of DIN for the period of 1976-1997 was an order of magnitude higher in the Lower San Antonio River Watershed than in the Lower Guadalupe River Watershed. Results of sensitivity tests revealed different ecosystem responses to different nutrient loadings. We anticipate this interdisciplinary approach will help distinguish differing basin characteristics and their contribution to nutrient loadings, and subsequently, on estuarine ecological processes.

Key words: estuary, land-cover/land-use, GIS, nutrients loading, ecological process, modeling