Modeling how land use affects nutrient budgets in the Guayas Basin-Ecuador: Ecological and economic implications (Ecuador)

This study embraces the complexity of understanding the interactions between biophysical and human systems using the fluxes of nutrients as the main indicator. I present a comprehensive spatial land use nutrient budget model that estimates the inputs of nitrogen (N) and phosphorus (P) into the Guayas Basin. These spatial fluxes (N, P) are then coupled to the seasonal budgets of N and P of the Guayas estuary and the N/P Redfield ratio distribution in the Gulf of Guayaquil. Subsequently, I applied a tradeoff analysis of three agricultural technology scenarios to explore how different agricultural land use and management practices can alter the nutrient balance and fluxes within the basin and the coastal zone.; Overall, synthetic fertilizers are the largest input to the Guayas Basin (56% for N and 53% for P), and the largest outputs are N and P in crops. Losses of N and P in river export account for 14--38% of total N and P inputs. Although the nutrient balance of the entire Guayas Basin is positive, there are negative balances in sub-watersheds with extensive banana, coffee and permanent crops suggesting nutrient depletion leading to decreases in soil fertility, agricultural yield, and ultimately economic profitability.; This study confirms that, as in other tropical ecosystems, the Guayas Basin is an N-limiting system and that agricultural activities increase this N limitation downstream and into the Guayas Estuary. If the current agricultural land use management practices persist this research suggests that there will be a steady increase of N and P into the system, with a progressive decline in the N:P ratio leading to eutrophic conditions in the Guayas Estuary.; The tradeoff analysis reveals that under current agricultural practices there is a positive relation between profit and nutrient depletion. While technology is available to improve the agriculture productivity and reduce nutrient depletion there are economic and socio-cultural barriers that prevent the implementation of such technology. These results suggest that there is a need to create incentives to implement a nutrient management plan for agriculture activities in the Guayas watershed. Finally, the results and methodological tools used in this study can be applied to the delineation of an integrated management plan for coastal watersheds.