Real-time Estimates of Hypoxic Water Volume

Quick Summary

Scientists estimate the amount of hypoxia in the Chesapeake Bay using a metric called the "hypoxic volume," which is the volume of water in the Bay with a dissolved oxygen concentration less than 2 milligrams per liter (mg/L). This volume represents an approximate size of the Dead Zone in the Bay. In the spring, the size of the summer dead zone is forecast based on the amount of nutrients supplied to the Bay. Once or twice a month, boats are used to collect observations and estimate the hypoxic volume. However, both these methods provide very infrequent estimates of hypoxic volume.

The Chesapeake Bay Environmental Forecast System estimates the hypoxic volume every day. The daily hypoxic volume forecasts are used to calculate the total annual hypoxic volume throughout the year. This metric provides a single number that represents the severity of hypoxia in a given year. The model was used to estimate hypoxic volume for each day from 1985 through 2022 for comparison with 2023. These daily estimates are based on complex computer models that continue to be improved; therefore, past estimates may be updated as improvements are made to the models.

Historical Hypoxic Volumes

The table below provides yearly metrics of model-estimated hypoxic volumes and the duration of hypoxia from 1985 through 2022. These metrics of hypoxia were derived using daily hypoxic volumes from long model simulations. The daily hypoxic volume and total annual hypoxic volume were calculated from the daily-averaged 3D model output following Bever et al. (2018). The duration of hypoxia was calculated from the daily estimates of hypoxic volume following Bever et al. (2013). The average summer hypoxic volume is the average volume of hypoxic water on each day from June through September. These yearly metrics of the severity of hypoxia in Chesapeake Bay are thus based on computer models that continue to be improved; therefore past estimates, such as those in the below table, may be updated as improvements to the model are made.

Summary of 2023 Model to Data Comparison (Model Accuracy)

The Maryland Department of Natural Resources and Old Dominion University periodically collect dissolved-oxygen data from the water surface to the seabed as part of the long-term Water Quality Monitoring Program. We use these data to calculate an estimate of the hypoxic volume, which we compare with the hypoxic volume estimated using the model. The data-based and model-based estimates of hypoxic volume will not be the same because different methods are used for each. However, we expect they should be similar and follow a similar seasonal pattern. The image and text below compare the model-based and data-based hypoxic volumes; we periodically update the data-based hypoxic volumes through the summer as more data become available. The black lines above and below the gray dots show the uncertainty in the data-based estimate of hypoxic volume.

Both the model-based and the data-based hypoxic volumes show hypoxia starting in mid-May. The model-based and data-based estimates for June both show relatively low amounts of hypoxia. Although, the data-based estimate is noticably lower than the model-based estimate. Both the model-based and data-based estimates for July suggest a low to moderate amount of hypoxia, continuing the trend of 2023 being a relatively mild year for hypoxia in the Bay. Continue to check back to see how hypoxia progresses throughout the remainder of the summer.

Synopsis of 2022 Dead Zone Size

Hypoxia in 2022 started relatively late in the season, similar to in 2020. Around the beginning of June, hypoxia started to spread through the mainstem of the Bay. Although the amount of hypoxia increased from June through July, the volume of hypoxic water stayed relatively low compared to the historical volumes and the mid-summer peak in hypoxia was slightly less than the historical average. Hypoxia quickly decreased following the mid-summer peak and was effectively ended by the passing of the remnants of Hurricane Ian around the beginning of October. Overall, 2022 was a relatively good (low amount) year for hypoxia in Chesapeake Bay. 

Summary of 2022 Model to Data Comparison (Model Accuracy)

The Maryland Department of Natural Resources and Old Dominion University periodically collect dissolved-oxygen data from the water surface to the seabed as part of the long-term Water Quality Monitoring Program. We use these data to calculate an estimate of the hypoxic volume, which we compare with the hypoxic volume estimated using the model. The data-based and model-based estimates of hypoxic volume will not be the same because different methods are used for each. However, we expect they should be similar and follow a similar seasonal pattern. The image and text below compare the model-based and data-based hypoxic volumes; we periodically update the data-based hypoxic volumes through the summer as more data become available. The black lines above and below the gray dots show the uncertainty in the data-based estimate of hypoxic volume.

Both the data-based and model-based estimates of hypoxia show the onset of hypoxia starting relatively late in the season, with very little hypoxia until June. The model-based estimate of hypoxic volume has hypoxia starting a little later in the season than the data-based estimate. Both the model-based and data-based estimates of hypoxic volume increase through the beginning of August. Hypoxia then decreased until late-September to early-October, when the remnants of Hurricane Ian effectively ended hypoxia for the year in both the model-based and data-based estimates of hypoxia. The data-based estimate of hypoxia suggests a mid-summer low in early July, which the model-based hypoxic volume did not capture. Ongoing research is focused on evaluating the cause of the decrease in the data-based hypoxic volume and the overprediction in model-based relative to data-based estimates of hypoxic volume for this data collection event.