Indicator Details

Forecast

Anoxic conditions (no dissolved oxygen) in the Bay's mainstem are predicted to be moderately poor for early summer (June to mid-July), with the average anoxic volume forecast to be 0.8 ± 0.3 cubic kilometers.



The average anoxic volume forecast for late summer (mid-July to September) is predicted to be 1.1 ± 0.36 cubic kilometers.

Compared to the previous 25 summers, 2010 could have the 7^th smallest anoxic volume if these predictions hold true.

Anoxia forecast courtesy of Rebecca Murphy (Johns Hopkins University), with collaboration with Bill Ball (Johns Hopkins University), Malcolm Scully (ODU), Michael Kemp (UMCES-HPL), Jeremy Testa (UMCES-HPL), and Jeni Keisman (UMCES-CBP).

Data

Methodology

There are many factors that determine the dissolved oxygen content of the tidal waters of Chesapeake Bay. Nutrient loading, water column stratification, wind and tidal mixing, and water temperatures are but a few of these factors. The two most important determining factors are water column stratification and nutrient loading.



Water column stratification is caused by density differences between the surface and deeper waters of the Bay. Cooler, saltier (more dense) water from the ocean flows underneath the warmer, fresher (less dense) water from the rivers that flow into the Bay. Between the lighter surface water and heavier deeper water is a boundary called the pycnocline. Oxygen consumed beneath the pycnocline cannot be replenished from above, and this leads to lower dissolved oxygen concentrations below the pycnocline. The pycnocline is typically strongest in spring and early summer when fresh water flows are usually at their highest.

Nutrient inputs to the Bay from the land are directly related to precipitation and therefore river flow. Nutrient loads from land-based sources (agriculture, urban runoff, etc.) are higher in the spring when river flows are typically at their highest. Nutrients that flow directly into the Bay from a pipe (sewage treatment plants, industry, etc.) are generally less sensitive to flow and are more consistent through the year. There is a direct relationship between the magnitude of these nutrient loads and the severity of low DO the Bay experiences. Nutrients-nitrogen and phosphorus-fuel the growth of the phytoplankton that make up the base of the Bay's food web. Unconsumed phytoplankton settle below the pycnocline and are decomposed by oxygen–consuming bacteria living in the mud on the bottom of the Bay. Since this is occurring below the pycnocline, this oxygen is not replenished from surface waters. This process occurs every year in Chesapeake Bay, fueled by spring flows that wash large amounts of nutrients into the Bay. 

Recent research has shown that in many years, there is a significant difference between anoxic volume (water with DO <0.2 mg/L) in the early and later parts of the summer. This happens due to changing conditions during the summer such as large summer nutrient loads, storm events, or prevailing wind patterns that affect stratification. To improve the forecast of anoxia, this year we are providing two forecasts: early summer (June through mid-July) and late summer (mid-July through September). The early summer forecast is released in early June and the late summer forecast will be released in July.

The first step to generating the anoxic volume forecast is to calculate what the anoxic volume was in previous years. We use Chesapeake Bay Program data (http://www.chesapeakebay.net/data_waterquality.aspx) from 1985 to 2009 which consists of 1 or 2 data collection cruises every month. For each cruise, we used a statistical interpolation method (Murphy et al. 2010) to estimate DO concentrations everywhere in the Bay from the samples collected along the main channel. The anoxic volume is calculated by summing the total volume of water with DO less than 0.2 mg/L. The monthly anoxic volumes are averaged to get early summer and late summer volumes. These early and late summer anoxic volume averages are then used with nutrient load and stratification-related data to build a model that can be used to predict anoxic volume in the current year.

The model used for the early summer anoxia forecast takes into account both nutrient loads and stratification.

Nutrient Loads are represented by total nitrogen loads from both the Susquehanna and Potomac Rivers from January to April plus the below fall-line point source releases of nitrogen in the Potomac River and north of the Potomac into the Bay. Because point source loads are only available until 2007, we fit a simple linear regression model to predict point source loads for 2008-2010. Total nitrogen loads through the Susquehanna and Potomac were available for each year, including this year, from the USGS.

Stratification is strongly influenced by freshwater flow and wind, and freshwater flow is highly correlated with nitrogen loads.  Since nitrogen loads are already included in the model, including freshwater flow does not improve the model fit.  However, for the first time this year, we are including wind effects in the statistical model. The direction, speed, and duration of wind events over the Chesapeake Bay can impact the strength and depth of the pycnocline. To account for some of these effects, we used the fraction of hours that wind from the southeast has blown over the Bay in recent months. The time period March through May was selected because that is the most recent wind data available, and during that period the long-term wind frequency pattern is similar to the pattern in June and early July. Wind from the southeast is correlated with a smaller anoxic volume because it causes increased mixing of the water column (Scully 2010 ). Wind data was available for every year from NOAA.

Model Details: We used a generalized linear multiple regression model to predict anoxia volume. This model has two variables: total nitrogen load and SE wind frequency. In the model equation shown here, b₀, b₁, and b₂ are fitted coefficients and log(anoxic volume) represents a method used to prevent predicting a negative anoxic volume.

log(anoxic volume) = b₀ + b₁(total nitrogen) - b₂ (SE wind)

The model was fit using all years of data from 1985 to 2009 except for 1993 because it was an extreme outlier year for early summer anoxia.

Nutrient Loads are represented by total nitrogen from the Susquehanna River in January through May plus the point source loads directly into the upper Bay. Loads through the Susquehanna appear to have the longest term impact on the Bay throughout the summer, as opposed to the Potomac River loads that have a larger impact in early summer. Loads were available from USGS.



Model Details: We used a linear regression model to predict anoxia volume. This model is very similar to the early summer model, but with only total nitrogen as a variable.


Reference:

Murphy RR, Curriero FC, Ball WP (2010) Comparison of Spatial Interpolation Methods for Water Quality Evaluation in the Chesapeake Bay. ASCE Journal of Environmental Engineering 136(2):160-171

Scully ME (2010) The importance of climate variability to wind-driven modulation of hypoxia in Chesapeake Bay. Journal of Physical Oceanography 40:1435-1440

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.



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.

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

Chesapeake Bay Environmental Observatory (CBEO)
COSEE Coastal Trends Dead Zone module
Chesapeake Bay Program