Climatic Variability and Extremes, Interacting with Nitrogen Storage, Amplify Risks of Coastal Eutrophication

Despite three decades of basin-wide nutrient-reduction efforts, severe hypoxia continues to be observed in the largest estuary in the U. S., the Chesapeake Bay. Here we show the critical influence of interaction between climatic variability and nitrogen (N) storage on Susquehanna River dissolved nitrogen (DN) loads, which are known to be strongly related with the hypoxia in the Bay. We analyzed reported river flows and DN loads, and used the process model LM3-TAN to produce 9 DN-load distributions following 9 different histories of climatic preconditioning. We found that high DN-load anomalies arise after prolonged dry spells even if precipitation returns to a normal state, and that this is explained by flushing of accumulated N and by altered microbial processes. Specifically, we illustrate that after 1- to 4-year dry spells, the likelihood to exceed a threshold DN load of 68 kt yr^-1 increases by 31 to 86%. High precipitation generally results in high DN loads. When this fact is combined with the sensitivity to dry preconditioning, we can make the generalization that climatic variability and extremes act to increase high DN-load anomalies. These mechanisms can help explain increasingly extensive hypoxia in the Bay, and possibly also in other coastal ecosystems. LM3-TAN is capable of capturing decadal-to-century changes in vegetation-soil-river N storage in response to climate and land-use changes, which allows us to explore such complex mechanisms and provides an implication that effective mitigation strategies might account not just for reducing anthropogenic N inputs and for climatic mean trends, but also for changes in interannual climatic variability. We will further discuss preliminary results for its global implementation, which would provide dynamic riverine N loading for ocean models, currently relying on prescribed data.