Analysis of the Chesapeake Bay Hypoxia Regime Shift: Insights from Two Simple Mechanistic Models

Abstract

Recent studies of Chesapeake Bay hypoxia suggest higher susceptibility to hypoxia in years after the 1980s. We used two simple mechanistic models and Bayesian estimation of their parameters and prediction uncertainty to explore the nature of this regime shift. Model estimates show increasing nutrient conversion efficiency since the 1980s, with lower DO concentrations and large hypoxic volumes as a result. In earlier work, we suggested a 35% reduction from the average 1980-1990 total nitrogen load would restore the Bay to hypoxic volumes of the 1950s-1970s. With Bayesian inference, our model indicates that, if the physical and biogeochemical processes prior to the 1980s resume, the 35% reduction would result in hypoxic volume averaging 2.7 km 3 in a typical year, below the average hypoxic volume of 1950s-1970s. However, if the post-1980 processes persist the 35% reduction would result in much higher hypoxic volume averaging 6.0 km 3 . Load reductions recommended in the 2003 agreement will likely meet dissolved oxygen attainment goals if the Bay functions as it did prior to the 1980s; however, it may not reach those goals if current processes prevail.

Exploring estuarine eutrophication sensitivity to nutrient loading

“…The parameter for Chl conversion efficiency (a), and its variation among estuaries, was similar to that found by Liu and Scavia (2010), despite the model changes made to incorporate DO ( Fig. 5; slope 5 1.2, intercept 5 0.09, R 2 5 0.92), suggesting a robust modeling framework.…”

“…The mineralization rate k m was set to 0.8 d 21 , the carbon to Chl ratio (ca) was set to 50 g C : 1 g Chl (Riemann et al 1989), zooplankton grazing rate (L) was set to 0.2 d 21 , the fraction of zooplankton grazing converted to detritus (a) was set to 0.2, sinking rate (v s ) was set to 1 m d 21 (Lucas et al 1998), the exchange coefficient between the two layers (k d ) was set to 0.2 d 21 , and the concentration of organic matter in the ocean (B o ) was set to 0.3 mg L 21 . The initial test, using a range of parameter values, showed that the model was relatively insensitive within these ranges, with the exception of k d .…”

“…Parameters were perturbed, one by one, by up to 620%, and in all cases, model fit (deviance) changed by a much smaller percent than the parameter change. The value for k d can vary by more than an order of magnitude in observations and model studies (Borsuk et al 2001;Liu et al 2010), so we tested this parameter over a larger range than the others. Our analysis indicated that model fit deteriorates at the lower end of the observed range (values less than 0.1), so we selected a k d value within the range where model fit was less sensitive to this parameter.…”

“…However, the sensitivity of bottom water DO to TN loading has also been shown to vary through time (Liu and Scavia 2010;Liu et al 2010;Scully 2010) and among estuaries (Zhang et al 2010). Here, we expand on these analyses to explore the relative simultaneous sensitivity of Chl concentration and DO depletion to TN loading in a cross-estuary context.…”

“…Clear links exist between nutrient loads, especially total nitrogen (TN), and bottom water DO, for example, in the Gulf of Mexico (Turner et al 2008;Greene et al 2009) and in Chesapeake Bay (Kemp et al 2005;Liu and Scavia 2010). However, the sensitivity of bottom water DO to TN loading has also been shown to vary through time (Liu and Scavia 2010;Liu et al 2010;Scully 2010) and among estuaries (Zhang et al 2010).…”