A novel threshold-based indicator for assessing dissolved oxygen criteria attainment deficits, buffers, and trends in estuarine waters

Low dissolved oxygen (DO) conditions pose a persistent threat to aquatic life in coastal ecosystems. In Chesapeake Bay, DO criteria are used to evaluate restoration progress, but traditional assessment methods—such as binary pass/fail outcomes and attainment deficit metrics—can obscure important spatial and temporal variability across tidal segments. This study introduces a novel criteria sensitivity indicator, which quantifies the proportion of a segment’s water volume that meets or exceeds a range of DO thresholds. The indicator provides a continuous, threshold-based view of criteria attainment, capturing both the severity of attainment deficits and the strength of attainment buffers. To enable comparisons across space and time, each criteria sensitivity curve can be summarized using the 'area under the curve' metric. We applied this framework to more than three decades (1985–2022) of summer-season (June—September) DO data across 92 tidal segments in Chesapeake Bay. The results reveal clear spatial patterns and long-term trends: open-water and deep-water habitats exhibited more improving areas over this period, while deep-channel habitats showed more degrading areas. This indicator supports more targeted and adaptive restoration by identifying not only where DO criteria are met or missed, but also how close segments are to transitioning between attainment states. It is particularly well-suited for identifying segments showing ecological improvement, persistent impairment, or emerging vulnerability—patterns often masked by traditional approaches. The framework is potentially transferable to other coastal systems with threshold-based water-quality criteria and long-term monitoring, offering a practical tool for comprehensive assessment, restoration planning, and adaptive management.