Anthropogenic and climatic controls on carbon and nitrogen exports from Mississippi river basin to Gulf of Mexico during 1800 – 2100: Implications for hypoxia and ocean acidification

The enlarged size of dead zone in the Gulf of Mexico in 2015, resulting from high summer precipitation and nutrient runoff from agriculture and other human activities in Mississippi river basin, has aroused plenty of scientific attentions and public concerns. Although recent-decade patterns of water/carbon/nitrogen exports from the US land to Gulf of Mexico have been intensively investigated through gauge station monitoring and empirical-based modeling, our understanding of its historical and future long-term trends and the underlying mechanisms is still limited. Climate variability and change, land cover/land use change (e.g., cropland shift from eastern US to Midwest US) and evolving land management practices (e.g., nitrogen fertilizer use in corn belt) are all important drivers regulating interannual, decadal and century-long variability in riverine carbon and nitrogen exports. In this study, we explore river discharge and carbon/nitrogen exports from US drainage basins in a 300-year period covering both historical and future eras (1800 – 2100) and further quantify the contributions of climate, land use, nitrogen fertilizer use, and atmospheric chemistry by using a process-based land ecosystem model (DLEM) with networked river system incorporated. The results indicate that spatial distribution and shift of agricultural land is of critical importance in shaping land-to-aquatic mass flow and coastal water quality. Historical pattern and future scenarios of climate variability and change play an important role in the trend of water yield and enhanced inter-annual variations of river discharge and carbon/nitrogen exports. Atmospheric nitrogen deposition and agricultural nitrogen fertilizer uses in land ecosystem largely contributed to land-to-aquatic nitrogen exports. Our sensitivity analyses with DLEM suggest that precipitation in the basin as well as nitrogen fertilizer use in US corn belt are important determinants of nutrient export and hence the size of dead zone in the Gulf of Mexico. These findings imply that we need to consider both climate and anthropogenic changes taking place in land ecosystems for better developing land management strategies in mitigating hypoxia and ocean acidification.