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Browse History: DO - hypoxia (2011)
Indicator Icon DO - hypoxia

Dissolved oxygen is critical to the survival of Chesapeake Bay's aquatic life. The amount of dissolved oxygen needed before aquatic organisms are stressed, or even die, varies from species to species.


Indicator Details

Indicator Icon DO - hypoxia

Forecast

2011 July Hypoxia Forecast dial

Given average Jan-May 2011 total nitrogen load of 341,417 kg per day from the Susquehanna River, July 2011 hypoxic (Dissolved Oxygen ≤2.0 mg·L‑1) volume forecast is 9.7 km3, with 95% confidence interval that the hypoxic volume will be between 8.6 and 11.1 km3. This is above average and the 6th highest in the post-1985 period.

July hypoxia forecast provided by Don Scavia and Mary Anne Evans, University of Michigan. For more information, please visit their website.


2011 Summer Hypoxia Forecast

The average volume of hypoxic water (Dissolved Oxygen ≤2.0 mg·L‑1) in the Chesapeake Bay is predicted to be 6.8 km³ for June to September 2011, with a 95% confidence interval that the hypoxic volumne will be between 5.7 and 8.0 km³. This would be the 4th highest hypoxic volume for the period of record (1985-2010).

Summer (June to September) Hypoxia forecast courtesy of Younjoo Lee, Chesapeake Biological Laboratory, UMCES.


Data


July 2011 Hypoxia Forecast

The average volume of hypoxic water (Dissolved Oxygen ≤2.0 mg·L‑1) in Chesapeake Bay for July 2011 is predicted to be 9.7 cubic kilometers, with 95% confidence that the hypoxic volume will be between 8.6 and 11.1 cubic kilometers. Compared to past years (1985 to 2010), this July is expected to have the 6th highest hypoxic volume. This forecast is based on a model that was developed to assess the impacts of changes in nitrogen loads on Chesapeake Bay hypoxia (Scavia et al 2006).

2011 July Hypoxia Forecast

July forecast provided by Don Scavia and Mary Anne Evans (University of Michigan). For more information, please visit their website.


Summer (June through September) 2011 Hypoxia Forecast

In 2010, we forecasted July + August hypoxia. However, this year we are presenting a forecast for the entire summer, June to September. Using June through September data in the model produced a better fit with the data (see Methods tab) and corresponds to the Chesapeake Bay Program's hypoxia timeframe. Additionally, it is when citizens are most likely using the Bay for recreational purposes, and therefore, the forecast is most relevant to them.

We used the Chesapeake Bay Monitoring Program data from 1985 to 2010, which consisted of 1 or 2 cruises during most months, to predict summertime hypoxia (Dissolved Oxygen ≤2.0 mg·L-1). Using the Data Interpolating Variational Analysis software package, dissolved oxygen (DO) fields were estimated for each cruise. Then, the hypoxic volume was calculated by summing the total volume of water with DO less than 2 mg·L-1. In order to predict hypoxic volume in the current year, the summer hypoxic volumes from June to September were used as a dependent variable, and Susquehanna River discharge (data from the U.S. Geological Survey), mid-bay chlorophyll a concentration (data from the Chesapeake Bay Program), and the cross-bay (east-west) wind speed (data from the Patuxent River Naval Air Station) were used as independent variables.

Summer Hypoxia time series with 2011 forecast

The average volume of hypoxic water (DO ≤ 2 mg·L-1) in Chesapeake Bay is predicted to be 6.8 km3 for June-September, 2011, with a 95% confidence interval that the hypoxic volume will be between 5.7 and 8.0 km3.


Summer (June-September) hypoxia forecast courtesy of Younjoo Lee, Chesapeake Biological Laboratory, UMCES

Methodology

July 2010 Hypoxia Forecast

The hypoxic forecast model predicts oxygen concentration downstream from point sources of organic matter loads using two mass balance equations for oxygen-consuming organic matter, in oxygen equivalents (i.e., BOD), and dissolved oxygen deficit. This approach to modeling coastal and estuarine hypoxia has also been used successfully for Gulf of Mexico hypoxia (Scavia et al. 2003, 2004). The original model was calibrated and tested against 1950-2003 nitrogen load and hypoxic volume estimates assembled by Hagy (2002). The Chesapeake Bay Program provided load and hypoxic volume updates for 1986-2008, and even though the new estimates varied little from the original ones; the model was recalibrated for this application to the new 1986-2008 estimates. The summer hypoxic volume forecast was generated using the following relationship. For more information, visit http://www.snre.umich.edu/scavia/hypoxia-forecasts/

Summer (June to September) Hypoxia Forecast

Since summer hypoxia is shown to be more related to late winter-spring processes than summer stratification, we have developed an effective tool to predict summer hypoxia based on river discharge, nutrient load, algal biomass, and wind conditions. The Susquehanna River contributes both nutrient loads from the land and buoyancy effects on estuarine dynamics.

Jan-May Susquehanna flow

2011 Jan-May Susquehanna TN loads

 

The concentration of spring chlorophyll a, a proxy for spring algal biomass, is also associated with the initiation and duration of hypoxia. Moreover, cross-bay wind is significantly correlated with summer hypoxia, which is influenced by regional climate.

Feb-April Chlorophyll a Concentrations

 

Feb-April Wind Speed

 

We found that there was a strong statistical relationship between January to April cross-bay (east-west) wind speed and summer hypoxia. Although the mechanistic link between wind speed in spring and summer hypoxia is not clearly understood, the cross-bay wind significantly improves the output of the regression model. We believe that it may influence the transport and/or deposition of biomass via lateral circulation. For example, more biomass may be accumulated over shallow areas (rather than deep areas) due to stronger cross-bay wind, resulting in less hypoxia, and vice versa. Table 1 lists the variables used in a multiple linear regression analysis for predicting summer hypoxia (June-September).

 

Variables

Period

Unit

Dependent

(predictand)

Mean Summer Hypoxic Volume (Hypoxia)

Jun-Sep

km3

Independent

(predictor)

Mean Susquehanna River Discharge (River)

Jan-May

m3/sec

Mean Total Nitrogen Load from the Susquehanna River (TN)

Jan-May

kg/day

Mean Concentration of Chlorophyll-a in the Mid-Bay (Chla)

Feb-Apr

μg/l

Mean Cross-bay Wind at Patuxent River Naval Air Station (Uwind)

Feb-Apr

m/s

 


A multiple linear regression is used to model the relationship between a dependent variable (predicted) and independent variables (predictors). It is based on a least squares method to minimize the sum-of-squares of differences between observed and predicted values. The model expresses the values of a summer hypoxic volume as a linear function of four independent variables as listed in Table 1 and an error term:

Hypoxia = a0 + a1·River + a2·TN + a3·Chla + a4·Uwind + e

where, a0 is regression constant, a1, a2, a3, and a4 are coefficients on the independent variables, and e is an error term. The current year hypoxic volume is predicted using all years of data from 1985 to 2010.

2011 Summer Hypoxia Model




Reference:
Lee, Y.J. and K.M.M. Lwiza. (2008). Characteristics of bottom dissolved oxygen in Long Island Sound, New York, Estuarine, Coastal and Shelf Sciences, 76, 187-200.


Background

All animal life in Chesapeake Bay, from the worms that inhabit its muddy bottom, to the fish and crabs found in its rivers, to the people that live on its land, need oxygen to survive. We breathe oxygen, which lets us extract energy from the food we eat. Our bodies use this energy to function. This process is essentially the same in all species with one major difference: worms, fish, and crabs use some form of gills instead of lungs to extract oxygen from the water. As water moves across the gills, dissolved oxygen is removed from the water and passed into the blood. As dissolved oxygen concentrations in water decrease, the animals that inhabit the Bay struggle to extract the oxygen they need to survive.

Chesapeake Bay Organisms

These organisms need dissolved oxygen to survive in Chesapeake Bay.

Chesapeake Bay scientists generally agree that dissolved oxygen concentrations of 5.0 mg·L‑1 (milligrams of oxygen per liter of water) or greater will allow the Bay's aquatic creatures to thrive. However, the amount of dissolved oxygen needed before organisms become stressed varies from species to species. Although some are more tolerant of low dissolved oxygen than others, in some parts of the Bay dissolved oxygen can fall to the point where no animals can survive. When the levels drop below 2.0 mg·L‑1, the water is hypoxic, and when it drops below 0.2 mg·L‑1 the water is considered anoxic.


organism DO requirements figure

In an estuary such as Chesapeake Bay, there are several sources of dissolved oxygen. The most important is the atmosphere. At sea level, air contains about 21% oxygen, while the Bay's waters contain only a small fraction of a percent. This large difference between the amount of oxygen results in oxygen naturally dissolving into the water. This process is further enhanced by the wind, which mixes the surface of the water. Two other important sources of oxygen in the water are phytoplankton and aquatic grasses. Phytoplankton are single-celled algae and aquatic grasses are vascular plants; both produce oxygen during photosynthesis. Another source of dissolved oxygen in the Bay comes from water flowing into the estuary from streams, rivers, and the Atlantic Ocean.

See Methodology tab for factors that influence dissolved oxygen.

See Dissolved Oxygen newsletter for more information.


Additional Information

Relevant Web Sites

Don Scavia - Hypoxia Forecasts
Gulf of Mexico hypoxia