Analysis of Terrestrial Discharge from Agricultural Watersheds and Its Impact on Nearshore and Offshore Reefs in Fiji

The coral reef ecosystems are rapidly deteriorating due to intense anthropogenic perturbations in the river basins in northwest Viti Levu Island, Fiji. This study aimed to assess the change in benthic cover as influenced by terrestrial loadings from the 14 adjoining agriculturally dominated (especially sugarcane) coastal watersheds using remote sensing, a geographic information system (GIS), and a physically based, distributed hydrological model. An integrated modeling framework with land-use change was constructed to simulate the runoff, sediment yield, and nutrient discharge using the Soil and Water Assessment Tool (SWAT) model. The impact of terrestrial discharge on the coastal ecosystem was investigated by spatio-temporal analysis (1992-2007) of the benthic cover using Landsat TM/ETM+ data. Due to the cropland expansion, a significant amount of surface runoff and sediment and nutrient discharge was generated during 1992-2007 from the steeply sloping watersheds. The hydrological response results from areas with different land-use conditions (1992-2007) revealed that the conversion of forestland (7.88%) and shrubland (7.59%) into agricultural (10.04%) and barren land (3.06%) had the greatest impact on the potential risk of surface runoff. To understand the discharge behavior of the watersheds and its impact on coastal ecosystems, we examined three hydrological response assessment indices, namely, the annual average, wet season average, and monthly maximum index. A change in the benthic community composition was observed as an increase in algae (139.3%), sea grass (70.6%), and degraded reef area (59.39%) around these coastal watersheds due to the increased sediments and nutrients, mainly from the sugarcane fields.