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Eyes on the Bay|
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Eyes on the Bay |
Chesapeake Bay Dissolved Oxygen Levels Return
to Normal in April (Updated 4/15/04)
Preliminary data from DNR's Chesapeake Bay Monitoring Program indicate that bottom dissolved oxygen levels are returning to normal levels in April 2004 (Figure 1), after record lows were reported for January through March 2004. For the 30 sites sampled and posted on the Eyes on the Bay web site thus far in the main stem Chesapeake Bay, and Potomac and Patuxent Rivers, 19 (or 63%) are above the historical (1985-2002) April average (Figure 2). Overall these sites are 0.6 mg/l above their historical averages for April and no stations were below the historical April minimums. Generally, April 2004 dissolved oxygen is lower than March 2004, which is consistent with historical trends of declining dissolved oxygen throughout the spring and into the summer (Figure 3). Only 2 sites had minimum dissolved oxygen less than 5 mg/l, a level considered to be potentially stressful to some living resources.
These more normal dissolved oxygen conditions can likely be attributed to some strong wind events in early April causing destratification (mixing) of the water column. Analysis by DNR scientists indicates that the low winter dissolved oxygen values were due to several circumstances, including strong stratification brought on by high freshwater flows in 2003, few large storms in January through March 2004 and significant ice cover that reduced winter water column mixing, higher than average nutrients levels, and a late winter algal bloom.
DNR Scientists will continue to review all available water quality data to evaluate water quality in the Bay and its tributaries throughout the year and results will be updated to the Department’s Eyes on the Bay website.
Figure 1. - Example of improved dissolved oxygen at site CB3.2 (Swan Pt.) above the Bay Bridge.
Figure 2. - Location of April 2004 sites falling above or below historical April dissolved oxygen levels. No sites set record lows in April 2004.
Figure 3. A Comparison of Dissolved Oxygen Levels for March and April 2004. Dissolved oxygen levels are closer to historical averages in April 2004 than they were in March 2004. Dissolved oxygen is generally lower in April 2004 compared with March 2004, which is consistent with historical trends of declining dissolved oxygen from February through June or July. For example, from 1985 to 2002, dissolved oxygen levels across all sites decrease an average of 2.2 mg/l from March to April.
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