A simple model for forecasting the effects of nitrogen loads on Chesapeake Bay hypoxia

The causes and consequences of oxygen depletion in Chesapeake Bay have been the focus of research, assessment, and policy action over the past several decades. An ongoing scientific re-evaluation of what nutrient load reductions are necessary to meet the water quality goals is needed. While models can provide insights and advice for public policy on load reduction goals, they are caricatures of nature, and it is wise to use independent modeling approaches. In this paper, we describe our simple, biophysically based model that offers a middle ground between statistical models and complex dynamic models. Our model suggests that the target total nitrogen load reduction of 35% will reduce hypoxic volumes by 36-68%, which, on average (53% or 3.4 km(3)) is lower than values reported for 1950-1970 (4.2 km(3)), and roughly half of the values reported for 1980-1990 (7.2 km(3)). By pursuing a simple model construct, we were able to quantify uncertainty to a greater extent than is possible with the more complex numerical models. Yet, by retaining some mechanistic detail we could validate the model against state variables and process rates, an advantage over simple regressions.