Chesapeake Bay's water quality condition has been recovering: Insights from a multimetric indicator assessment of thirty years of tidal monitoring data

Highlights

• Chesapeake Bay's water quality history was assessed by using an indicator framework.

• The indicator has a positive long-term trend (p < 0.05) and reached its peak in 2014–2016.

• The indicator was responsive to extreme weather events but can recover afterwards.

• Improvement of indicator score in 2014–2016 over its long-term average was driven by open water and deep channel dissolved oxygen.

• The improvement in Baywide attainment was statistically linked to the decline of total nitrogen input.

Abstract

To protect the aquatic living resources of Chesapeake Bay, the Chesapeake Bay Program partnership has developed guidance for state water quality standards, which include ambient water quality criteria to protect designated uses (DUs), and associated assessment procedures for dissolved oxygen (DO), water clarity/underwater bay grasses, and chlorophyll-a. For measuring progress toward meeting the respective states' water quality standards, a multimetric attainment indicator approach was developed to estimate combined standards attainment. We applied this approach to three decades of monitoring data of DO, water clarity/underwater bay grasses, and chlorophyll-a data on annually updated moving 3-year periods to track the progress in all 92 management segments of tidal waters in Chesapeake Bay. In 2014–2016, 40% of tidal water segment-DU-criterion combinations in the Bay (n = 291) are estimated to meet thresholds for attainment of their water quality criteria. This index score marks the best 3-year status in the entire record. Since 1985–1987, the indicator has followed a nonlinear trajectory, consistent with impacts from extreme weather events and subsequent recoveries. Over the period of record (1985–2016), the indicator exhibited a positive and statistically significant trend (p < 0.05), indicating that the Bay has been recovering since 1985. Patterns of attainment of individual DUs are variable, but improvements in open water DO, deep channel DO, and water clarity/submerged aquatic vegetation have combined to drive the improvement in the Baywide indicator in 2014–2016 relative to its long-term median. Finally, the improvement in estimated Baywide attainment was statistically linked to the decline of total nitrogen, indicating responsiveness of attainment status to the reduction of nutrient load through various management actions since at least the 1980s.

Introduction

Like many other estuaries around the world, Chesapeake Bay and its tidal tributaries (the Bay) have suffered from a long history of cultural eutrophication that has resulted in ecological degradation. Key symptoms have included excessive algal growth, poor water clarity, decreased submerged aquatic vegetation (SAV) acreage, and low dissolved oxygen (DO), related to excessive nutrient and sediment inputs from its watershed (Hagy et al., 2004; Kemp et al., 2005; Murphy et al., 2011; Zhang et al., 2015; Zhang and Blomquist, 2018).

In 1983, the first Chesapeake Bay Agreement was developed, through which the U.S. Environmental Protection Agency (USEPA) and four Bay jurisdictions (Maryland, Virginia, Pennsylvania, and the District of Columbia) committed to the protection of water quality and habitat conditions necessary to support the living resources in the Bay ecosystem. In 2003, the Chesapeake Bay Program (CBP) partnership published a guidance framework entitled “Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll-a for the Chesapeake Bay and Its Tidal Tributaries” (USEPA, 2003a). These water quality criteria, applied over a 92-segment management grid (Fig. 1), were adopted into states' water quality standards to define which waters are impaired under the Clean Water Act (Table S1). In the 2003 framework (USEPA, 2003a), water quality criteria are established for aquatic habitats for open water (OW), deep water (DW), deep channel (DC), migratory spawning and nursery (MSN), and shallow water (SW) designated uses (DUs), which reflect the seasonal nature of water column structure and the life history needs of living resources (Fig. 2; Table S1) (USEPA, 2003b; USEPA, 2004b).

The 2003 framework also establishes the foundation of water quality criteria assessment procedures (USEPA, 2003a). The procedures are based on the most recent CBP segmentation scheme, which divides the Bay into 92 segments (USEPA, 2005). Since 2003, the assessment procedures have been periodically refined as new scientific understanding became available, leading to the publication of a series of technical addendums (USEPA, 2003a; USEPA, 2004a; USEPA, 2007a; USEPA, 2007b; USEPA, 2008; USEPA, 2010a; USEPA, 2017). For a summary of these addendums up to 2010, see Tango and Batiuk (2013).

To achieve consistent assessment over time and among jurisdictions, a multimetric indicator was proposed by the CBP partnership to provide a means for measuring progress toward attainment of water quality standards in the Bay (USEPA, 2017). This indicator uses available data - and applies a set of decision rules to account for missing data otherwise required - to perform a complete assessment of all criteria in order to compute an index score (Table S1). The index score represents a surface-area-weighted estimate of water quality standards attainment that quantifies the fraction of tidal waters estimated to meet all applicable season-specific criteria thresholds for each applicable standard in 3-year moving assessment windows. Due to data limitations, this indicator should not be treated as a full accounting of water quality standards for DO, water clarity/SAV, and chlorophyll-a as stated by state regulations. Also, this indicator does not consider other parameters that may impair water quality including pH, bacteria, or toxics.

The main objective of this work was to apply the multimetric indicator approach to three decades of monitoring data of DO, water clarity/SAV, and chlorophyll-a in the Bay to track the progress in water quality standards attainment for the 92 segments that are listed in the Chesapeake Bay Total Maximum Daily Loads (USEPA, 2010b). For the first time in the scientific literature, the status and trends of Chesapeake Bay water quality standards attainment are documented, which provide essential information to the Bay management and research community. One immediate use of such information is for assessing the effectiveness of management interventions after decades of public investment in the restoration of Chesapeake Bay. More broadly, this work highlights Chesapeake Bay as an example where a long-term, collaborative monitoring network has allowed for the development, refinement, and implementation of analyses to assess the ecological status of a complex ecosystem. This work can serve as a model for other coastal and inland systems, either for comparison with existing assessments, or for development of similar monitoring and assessment frameworks (Borja et al., 2008; Bricker et al., 2008; Patrício et al., 2016; Schiff et al., 2016; Sherwood et al., 2016; Trowbridge et al., 2016).