Evaluating Confidence in the Impact of Regulatory Nutrient Reduction on Chesapeake Bay Water Quality

Isaac D Irby,Marjorie A M Friedrichs

doi:10.1007/s12237-018-0440-5

Abstract

Excess nutrients derived from anthropogenic activity have resulted in the degradation of coastal water quality and an increase in low-oxygen and hypoxic events worldwide. In an effort to curb these impacts and restore water quality in the Chesapeake Bay, a maximum load of nutrients has been established based on a framework of regulatory standards and models. This research aims to evaluate the projected changes in water quality resulting from the implementation of these nutrient reductions by applying the regulatory methodology to two different models that have been previously shown to have similar model skill. Results demonstrate that although the two models differ structurally and produce a different degree of absolute change, they project a similar relative improvement in water quality along the main stem of the Chesapeake Bay and the lower reaches of the tributaries. Furthermore, the models largely agree on the attainment of regulatory water quality standards as a result of nutrient reduction, while also establishing that meeting water quality standards is relatively independent of hydrologic (wet/dry) conditions. By developing a Similarity Index that compares model results across habitat, time, and methodology, this research identifies the locations and causes of greatest uncertainty in modeled projections of water quality. Although there are specific locations and times where the models disagree, overall this research lends support and increased confidence to the appropriateness of the nutrient reduction levels and in the general impact of nutrient reduction on Chesapeake Bay water quality under current environmental conditions.

Highlights

As the largest estuary in the continental USA with a watershed supporting a growing population of over 18 million people, the Chesapeake Bay (~ 11,000 km2) is prone to water quality degradation as a result of human activity
When the Total Maximum Daily Load (TMDL)-WIP nutrient reduction scenario is applied to both models for 1993–1995, the models produce similar changes in summer dissolved oxygen (DO) concentrations; especially at the bottom, the relative changes are more similar than the absolute changes (Fig. 4)
The TMDL-WIP nutrient reduction scenarios result in the absolute increase in summer DO in ChesROMS-ECB being roughly 1 mg L−1 higher than CH3D-ICM along the central main stem (Fig. 4c, d); again the relative increase is remarkably similar in magnitude between the two models with increases in DO

Summary

Introduction

As the largest estuary in the continental USA with a watershed supporting a growing population of over 18 million people, the Chesapeake Bay (~ 11,000 km2) is prone to water quality degradation as a result of human activity. Federal, and private partners to improve the Bay’s water quality eventually resulted in the 2010 Chesapeake Bay Total Maximum Daily Load (TMDL), which established location-specific mandated pollutant reductions throughout the six states and Washington, D.C. that make up the Chesapeake Bay watershed (Fig. 1 and Table 1). The nutrient reductions required to improve the water quality of the Chesapeake Bay are estimated to cost in the tens of billions of dollars (Nelson 2014). With such astounding potential costs, it is Estuaries and Coasts (2019) 42:16–32

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