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Browse History: Dissolved Oxygen (2010)
Indicator Icon Dissolved Oxygen

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.

Chesapeake Bay - Indicator Details:

  Indicator Selection Diagram

Threshold Map

This map shows how often (% of sampling times) dissolved oxygen levels were above or below the threshold between June and September 2010. Good water quality is characterized by having dissolved oxygen levels that are frequently above the threshold (indicated by shades of green), while poor water quality tends to have dissolved oxygen that is frequently below the threshold (indicated by shades of yellow and orange). In 2010, many regions of the Bay had dissolved oxygen levels that were frequently above the threshold (i.e., in good condition). Regions that frequently had poor dissolved oxygen levels in relation to the threshold include the Lower Western Shore (MD), the Pataspco and Back Rivers, and the Elizabeth River.

Creating this map
Dissolved oxygen was measured at approximately 144 stations, 8 times between June and September 2010, using a water quality probe positioned just above the bottom. The proportion of time that dissolved oxygen was below the threshold at each station was calculated and then interpolated to provide estimates between the stations.

Data Map

This map shows average bottom dissolved oxygen levels for summer (June to September) 2010. The most extensive areas of low average dissolved oxygen occurred in the deeper channels of the mainstem Bay and lower Potomac River. Unlike other years, however, there was not a sharp decline from good oxygen to no oxygen (which would be indicated by red). Rather, there is a wider than average band of low dissolved oxygen (1-2 mg l-1) that continues from the mainstem into the Potomac River. Small areas of low dissolved oxygen were also observed in the lower reaches of many smaller tributaries including the Chester, Patapsco, and Choptank Rivers.

Low dissolved oxygen in the Bay's mainstem is controlled primarily by the quantity of organic matter and nutrients flowing out of the Susquehanna River during the preceding spring. While this map provides a broad-level assessment of dissolved oxygen in the Bay, it does not accurately capture small and short-term low dissolved oxygen events that are often attributed to causing fish kills.

Understanding this map
Dissolved oxygen was measured at approximately 144 stations, up to 6 times between June and September, using a water quality probe positioned just above the bottom. Average dissolved oxygen levels at each station were calculated and levels between stations estimated using spatial interpolation software.


Comparison of Bay Health Index scores for 2010 () compared to

Score Legend
 Score (%)
 0               20               40               60               80              100
Upper Western Shore  
James River  
Choptank River  
Lower Bay  
Rappahannock River  
Lower Eastern Shore (Tangier)  
Overall Bay  
Upper Bay  
Upper Eastern Shore  
Potomac River  
Mid Bay  
York River  
Patuxent River  
Lower Western Shore (MD)  
Patapsco and Back Rivers  
Elizabeth River  

This figure ranks each region from best to worst dissolved oxygen scores for 2010. As the number of sampling times and locations that dissolved oxygen is below the threshold is relatively few, the scores for most regions are relatively high. However, as shown on the threshold map, many regions of the Bay have dissolved oxygen levels that are frequently below the threshold. The highest ranked regions in 2010 were the Upper Western Shore and James River regions. The lowest ranked regions in 2010 were the Patapsco and Back Rivers and Elizabeth River regions.

Excel Spreadsheet

Threshold Levels

Designated Use SeasonCriteria Thresholds (mg L-1)‡
Open WaterJun-Sep≥5.0
Deep WaterJun-Sep≥3.0
Deep ChannelJun-Sep≥1.0

‡ U.S. Environmental Protection Agency (2003)