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Browse History: DO - late anoxia (2015)
Indicator Icon DO - late anoxia

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 - late anoxia

Forecast

A diagram illustrates the forecasted anoxic volume for the late summer of 2014.

The late summer anoxic volume is forecast to be 1.16 km3, with 95% confidence interval that the anoxic volume will be between 0.0 and 2.4 km3. This volume is slightly smaller than average compared to the past 30 years. 

Anoxia forecast was recreated by Jeremy Testa (UMCES) from the original analysis conducted by Rebecca Murphy, with collaboration with Bill Ball (Johns Hopkins University), Malcolm Scully, Michael Kemp (UMCES-HPL).


Data


A graph compares the observed anoxic volume of past years to the forecasted anoxic volume for 2014

Anoxic (Dissolved oxygen ≤0.2 mg L-1) volume in late summer of 2015 could be the 22nd lowest compared to the previous 30 years. The average mainstem late summer anoxic volume is predicted to be 1.34 km3.  

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.

Dissolved Oxygen Conceptual Diagram

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-1) 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 used Chesapeake Bay Program data (http://www.chesapeakebay.net/data_waterquality.aspx) from 1985 to 2012 which consists of 1 or 2 data collection cruises every month. For each cruise, we used a statistical interpolation method (Murphy et al. 2011) 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.

Late Summer Model

A different model was used for the late summer anoxia forecast. Research has shown that the persistence of anoxia during the summer is correlated most strongly with late spring and early summer nutrient loads through the Susquehanna River. Stratification factors that play a role in early summer are not nearly as predictive of late summer anoxia. In general, it is fairly difficult to predict late summer anoxia due to the effects of events that sometimes occur in August and September (such as hurricanes or droughts).

Nutrient Loads are represented by total nitrogen from the Susquehanna River in January through May. 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.

A graph compares the total nitrogen loads found in the Susquehanna River from 1985-2014

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.

A graph illustrates the relationship between the predicted late summer anoxic volume and the observed late summer anoxic volume.

 

Reference:

Murphy RR, Kemp WM, Ball WP (2011) Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading. Estuaries and Coasts (in press) DOI: 10.1007/s12237-011-9413-7

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.

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.