Volume of Hypoxic Water in Chesapeake Bay

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Quick Summary

The amount of hypoxia in the Chesapeake Bay is estimated using a metric called the "hypoxic volume", which is the volume of water in the Bay with a dissolved oxygen concentration less than 2 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 Hypoxia Forecast Model 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 2014 through 2018 for comparison with 2019. 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.

 

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2019 Dead Zone Size

The forecast model provides an estimate of the amount of hypoxia in the Bay every night. The amount of hypoxia in the Bay is expected to increase from spring into summer and then decrease as summer progresses into fall. However, short-term weather will affect the amount of hypoxia in the Bay. Check back to see how the size of the dead zone increases seasonally and how daily weather changes the amount of hypoxia. Notable weather that may impact the amount of hypoxia are very windy days or periods of very calm wind. The below image will be continually updated throughout 2019.

2019_RT_HV_figure

 

historic_HV_table
Hypoxic Volume (HV) Metrics for Recent Years for Comparison to 2019 Forecast
Year   Maximum Daily HV [km3] Average Summer HV [km3] Hypoxic Duration [days] Total Annual HV [km3 days]
2014 7.7  (10%) 4.9  (6%) 115 625
2015 9.9  (13%) 4.6  (6%) 98 587
2016 10.7  (14%) 5.1  (7%) 101 664
2017 9.9  (13%) 5.3  (7%) 92 657
2018 10.4  (13%) 4.8  (6%) 123 645

Notes: 1 km3 equals about 400,000 Olympic-sized swimming pools of water.  Percents (%) represent the percent of the Bay that was hypoxic based on the volume of the Bay in the forecast model.

  • Maximum Daily Hypoxic Volume (km3): The maximum volume of Chesapeake Bay water experiencing hypoxic conditions on any given day
  • Average Summer Hypoxic Volume (km3): The average volume of hypoxic water from June through September
  • Hypoxic Duration (days): The number of days in a given year between the first and last day of hypoxic conditions exceeding 2 km3 in volume
  • Total Annual Hypoxic Volume (km3 days): The total amount of hypoxia in the Bay for a given year, calculated by summing the hypoxic volume on each day
2019_model_data_summary
Summary of 2019 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. One of the uses of these data is to calculate a data-based 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, it is expected they should be similar and follow a similar seasonal pattern. The below image and text comparing the model-based and data-based hypoxic volumes will be periodically updated throughout the summer as more data becomes available.

The model-based estimate of hypoxic volume in May is about average with recent years. However, the model-based hypoxic volume is lower than the data-based hypoxic volume for this first estimate of the severity of hypoxia. Water quality data was again collected between June 4 and 6 and used to estimate the amount of hypoxia in the Bay. The model-based estimate of hypoxic volume near the beginning of June matches very well with the data-based estimate. The model-based hypoxic volume suggests a peak in hypoxia around June 2 and then a decrease in the amount of hypoxia through mid-June.

 HV_mod_data_2019

 

 

2018_HV_summary
Synopsis of 2018 Dead Zone Size

The forecast model underwent a significant improvement in the estimated salinity in early 2019, which also affected the estimates of water temperature, dissolved oxygen and hypoxia. Some of the dead zone sizes will be different between these older results and those presented in the above 2019 realtime dead zone size summary.

During fall 2018, the Virginia Institute of Marine Science joined with Anchor QEA, LLC, to release their annual retrospective analysis of the severity of hypoxia in the Chesapeake Bay. The Annual Chesapeake Bay Hypoxia Report Card summarizes dissolved oxygen in the Bay as estimated by the team's 3-D, real-time hypoxia forecast model. The modeling team also generated dissolved oxygen statistics for 4 previous years for comparative purposes.

Springtime inflows from the Susquehanna River were high in 2018, resulting in the prediction that 2018 would have an above-average amount of hypoxia. However, wind speed and direction also play a large role in the severity of hypoxia during the summer. During 2018, the total annual hypoxic volume was similar to 2014 and 2017 through mid-July, but larger than in 2015 and 2016. Strong winds in the second half of July reduced the amount of hypoxia to near zero. Hypoxia increased rapidly again in early August and peaked at a higher value in early September than in previous years. Strong winds in September again mixed the Bay water and resulted in a large reduction in the volume of hypoxic water. Overall, the total amount of hypoxia in 2018 was estimated to be similar to 2017, but the seasonal patterns in hypoxia were very different; hypoxia was estimated to start earlier and last longer in 2018 than in recent years (Table below). The lack of hypoxia in late July was very unlike historical dissolved oxygen conditions, but consistent with the report released by the Maryland Department of Natural Resources for the Maryland portion of the Bay.

 
2018_model_data_summary
Summary of 2018 Model to Data Comparison (Model Accuracy)

The forecast model underwent a significant improvement in the estimated salinity in early 2019, which also affected the estimates of water temperature, dissolved oxygen and hypoxia. Some of the dead zone sizes will be different between these older results and those presented in the above 2019 realtime dead zone size summary.

Periodically throughout the year, Maryland Department of Natural Resources and Old Dominion University use boats to measure dissolved oxygen at many depths and numerous locations throughout the Bay. Many components of the Chesapeake Bay Hypoxia Forecast Model have been compared to these boat-based data to validate the accuracy of the forecast model. These comparisons have included both dissolved oxygen concentration and hypoxic volume. Some of the comparisons were published in scientific papers by Bever et al. (2013) and Irby et al. (2016).

Hypoxic volume is estimated from the boat-based data by interpolating (extrapolating) the measured dissolved oxygen concentration throughout the Bay and calculating the volume of water with dissolved oxygen concentration less than 2 mg/L. However, because the data is collected on different dates and times and is then extrapolated throughout the Bay, the data only provide periodic estimates of the amount of hypoxia. The 2018 hypoxic volumes estimated from the Chesapeake Bay Hypoxia Forecast Model were compared to the periodic data-estimated hypoxic volumes, to validate the seasonal pattern and amount of hypoxia estimated by the model. In this comparison, the model-estimated hypoxic volume is shown on the figure below for every day and the data-estimated hypoxic volume is shown as eight individual estimates.

Both the model and the boat-based data show hypoxia began in May and increased into June with relatively large hypoxic volume during the second data collection episode in June. Throughout July the model-estimated and data-estimated hypoxic volumes decreased to very low volumes and then increased rapidly in August. The model-estimated and data-estimated hypoxic volumes then decreased through September. This demonstrates that the model and data estimated the same seasonal pattern of hypoxic volume in which hypoxia was nearly absent near the end of July and in which:

  • Hypoxia began in May and increased through June
  • Hypoxia decreased during the last third of June to unusually low hypoxic volumes
  • Hypoxia increased rapidly in August and into September before decreasing into October

The model-estimated hypoxic volumes were very similar to the data-estimated hypoxic volumes, which were centered around June 5, July 10, July 26 and August 22. The model-estimated hypoxic volume was marginally lower than the data-estimated hypoxic volume for the remaining four data-collection episodes. This demonstrates that the model-estimated hypoxic volumes were very accurate based on half the data-collection dates and were slightly low for the other half of the dates, and that the model was accurate enough to use for understanding the variations in hypoxia at different times and different places in the mainstem portion of the Chesapeake Bay.

2018_HV_and_data