VIMS

Real-time Estimates of Hypoxic Water Volume

Chesapeake Bay

HV_summary
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.

realtime_HV_table
2023 Dead Zone Size

The amount of hypoxia in the Bay is expected to increase from spring into summer and then decrease as summer progresses into fall, with hypoxia starting sometime in May. 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 image below will be continually updated throughout 2023 based on the daily forecast model.

Hypoxia in summer 2023 really began in mid-May, increased to a moderate level, and then leveled off. Hypoxia remained at low to moderate levels throughout June, July, August, and September. The spring-time nutrient supply to the Bay was relatively low which may have contributed to the low amount of hypoxia. In 2023, hypoxia was uncharacteristically small from June until ending in late-September. The quick end in hypoxia resulted from temperatures cooling and stronger winds from the passing of Tropical Storm Ophelia, both of which act to limit the amount of hypoxia in the Bay. Overall, the duration of hypoxia in summer 2023 was normal, and the total annual amount of hypoxia and the maximum daily amount of hypoxia were quite low, representing a very good year for hypoxic conditions in the Bay.

 2023_NV_realtime

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Hypoxic Volume (HV) Metrics for Recent Years for Comparison to 2023 Forecast
Year Maximum Daily HV [km3] Average Summer HV [km3] Hypoxic Duration [days] Total Annual HV [km3 days]
Historical 6.2 to 13.4 3.1 to 8.0 96 to 146 418 to 1,075
2018 17.3 (21%) 8.3 (10%) 133 1,125 ± 146
2019 20.2 (25 %) 11.6 (14%) 154 1,688 ± 219
2020 10.7 (13%) 5.0 (6%) 101 623 ± 81
2021 8.4 (10%) 5.0 (6%) 143 720 ± 94
2022 10.4 (13%) 5.4 (7%) 114 673 ± 87
2023 4.3 (5%) 2.4 (3%) 113 336 ± 44

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 and tidal tributaries in the forecast model. Historical values are based on a 37-year simulation and represent the normal range in conditions between 1985 to 2021 (median ± one standard deviation).

  • 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. Uncertainty in total annual hypoxic volume estimates from cruise-based observed data has been estimated at 13% (Bever et al. 2018, Table 4). The 13% was adopted here as a general estimate of the uncertainty of the total annual hypoxic volume for each year, represented by the plus and minus (±) value.
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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.

historic_HV_2
Year Maximum Daily HV [km3] Average Summer HV [km3] Hypoxic Duration [days] Total Annual HV [km3 days]
1985 7.4 (9%) 4.9 (6%) 193 771 ± 100
1986 9.4 (12%) 5.8 (7%) 116 780 ± 101
1987 10.0 (12%) 5.5 (7%) 123 714 ± 93
1988 8.0 (10%) 4.4 (5%) 97 565 ± 73
1989 12.2 (15%) 7.5 (9%) 118 945 ± 123
1990 9.4 (12%) 5.5 (7%) 136 747 ± 97
1991 9.3 (11%) 5.2 (6%) 129 763 ± 99
1992 5.8 (7%) 3.1 (4%) 99 379 ± 49
1993 17.3 (21%) 10.6 (13%) 130 1371 ± 178
1994 16.1 (20%) 9.9 (12%) 125 1257 ± 163
1995 7.1 (9%) 3.3 (4%) 90 421 ± 55
1996 13.9 (17%) 9.7 (12%) 133 1240 ± 161
1997 11.8 (14%) 6.8 (8%) 133 908 ± 118
1998 15.6 (19%) 9.4 (12%) 123 1196 ± 156
1999 4.6 (6%) 1.9 (2%) 57 246 ± 32
2000 11.2 (14%) 5.5 (7%) 115 736 ± 96
2001 6.0 (7%) 3.5 (4%) 102 450 ± 59
2002 4.8 (6%) 1.9 (2%) 76 236 ± 31
2003 18.5 (23%) 10.7 (13%) 146 1430 ± 186
2004 13.8 (17%) 9.5 (12%) 154 1444 ± 188
2005 13.6 (17%) 8.8 (11%) 121 1118 ± 145
2006 11.3 (14%) 5.6 (7%) 131 738 ± 96
2007 8.5 (10%) 5.0 (6%) 116 669 ± 87
2008 12.6 (15%) 6.6 (8%) 106 816 ± 106
2009 7.2 (9%) 3.9 (5%) 96 487 ± 63
2010 9.3 (11%) 5.4 (7%) 122 767 ± 100
2011 14.5 (18%) 8.9 (11%) 147 1238 ± 161
2012 7.5 (9%) 4.8 (6%) 130 716 ± 93
2013 7.5 (9%) 3.5 (4%) 92 443 ± 58
2014 9.8 (%) 5.9 (%) 117 733 ± 95
2015 7.9 (10%) 4.2 (5%) 92 514 ± 67
2016 6.8 (8%) 3.8 (5%) 90 466 ± 61
2017 9.8 (12%) 5.3 (7%) 96 655 ± 85
2018 12.8 (16%) 7.1 (9%) 137 905 ± 118
2019 17.1 (21%) 9.8 (12%) 131 1241 ± 161
2020 10.8 (13%) 5.0 (6%) 95 614 ± 80
2021 10.7 (13%) 6.6 (8%) 141 869 ± 113
2022 9.7 (12%) 4.5 (6%) 85 548 ± 71

  

2023_model_data_comparison
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.

2023_HV_somparison

 

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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. 

2022_realtime_HV

table1
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.

2022_HV_comparison