Dissolved oxygen criteria attainment in Chesapeake Bay: Where has it improved since 1985?

Many estuaries, including Chesapeake Bay, have suffered from undesirable conditions of algae blooms, poor water clarity, and low dissolved oxygen (DO). To better understand the status and trends of DO criteria attainment deficit (AD), we conducted a comprehensive assessment using monitoring data in the period of 1985–2022 and focused on the comparison of trends among 13 tidal systems. Our results provide timely and updated information to help address a critical management question, i.e., where has the DO criteria attainment improved since 1985? The overall AD showed statistically significant improvements in both the long term (1985–2022) and the short term (2006–2022), which can be mainly attributed to open water and deep channel designated uses, respectively. Among the 13 tidal systems, ten showed improving trends in the long term, with Mainstem Bay, Pocomoke, Potomac, and Tangier having statistically significant improvements. Over the short term, nine systems showed improving trends, with Chester and Upper Mainstem Bay tributaries having statistically significant improvements. The only statistically significant degrading trend was observed in York. In addition, a report card summary was produced to provide a concise depiction of the trends for the 13 systems and illustrate how their trends are affected by the designated uses, which is the first of its kind in the context of DO criteria assessment. These spatially explicit results can help target locations for further assessment and restoration. Although many of the systems and their applicable designated uses have not improved over time, the detected significant improvements in some systems present new evidence on ecosystem recovery in the context of DO criteria attainment. Overall, this work highlights the combined value of collecting long-term water-quality monitoring data and developing scientific assessment approaches, which is fundamental to understanding the status and trends of complex ecosystems, including Chesapeake Bay and other estuaries. © 2024 Elsevier B.V.