2006 Severn Water Quality Monitoring Results--Dissolved Oxygen

   A series of depth profiles for dissolved oxygen was obtained for each monitoring station, and plots constructed of dissolved oxygen concentration vs depth, as is standard for oceanographic data.  As an example, data for our 40-foot-deep mid-Severn station at Joyce below Round Bay is shown.  The Y axis in these graphs is unusual in that it starts with zero at the top and goes down, but this should be thought of as depicting the water being measured.  The depth is given in meters, so that the ~40 foot bottom is show at ~13 meters.  The depth of deepest point varies varies according to the tide at the time of measurement. The dissolved oxygen concentration is shown in milligrams per liter (mg/l), plotted on the X axis.  Each colored line is a profile at this station obtained from a monitoring trip whose date is shown in the legend in the lower right. 
   All profiles show the expected high levels of dissolved oxygen near the surface, which is in contact with abundant atmospheric oxygen.  These near-surface DO levels can be elevated, especially on sunny days, due to oxygen generated by microscopic phytoplankton during photosynthesis.  Going down from the surface, DO levels diminished within 2-3 meters.  At this station a constant DO level was generally observed at about 6-8 meters, and throughout most of July-August these bottom DO levels were below 1 mg/l.  This station is representative of the deepest areas in the Severn.  Much of the the Severn mainstem bottom is 6-9 meters (20-30 feet) deep, including our lower Severn monitoring stations SR1 off the USNA, and SR2 near the Rte 50 bridge.  These shallower stations gave profiles that looked generally like the top portion of this graph, without the bottom plateau.  Complete profiles for all Severn stations monitored are presented in groups on the following pages:  Severn mainstem stations; lower Severn creek stations; Round Bay stations; Upper Severn creek stations.
     All our DO profiles showed the lowest DO levels near the bottom.  This is expected as this water is generally saltier and thus more dense than surface water, and is the least able to mix with the abundant atmospheric oxygen.  Because the bottom water bathes the bottom-dwelling (benthic) organisms such as oysters, clams, worms, and amphipods, low DO levels will limit the survival of these organisms, which form an important part of the food chain for fish and crabs. A DO level of 2 mg/l, termed severe hypoxia, is generally recognized as a minimal level for a healthy benthic habitat.  A map showing the average summer (July and August) bottom DO levels at each Severn monitoring station is shown below.

As can be seen from the map, all our Severn mainstem stations above the Rte 50 bridge, and two creek stations showed average summer DO levels near the bottom that were severely hypoxic.  These creeks, Asquith Creek and Martin's Pond, have compromised circulation with the Severn mainstem due to shallow bars blocking their entrance.  The four Round Bay stations that were more than 1/4 mile for shore all showed mean bottom DO levels less than 1 mg/l, while the station near shore at the Sherwood Forest pier was only slightly hypoxic.  Further up the Severn, our Narrows station also showed a mean bottom DO level of less than 1mg/l.  At the head of the Severn, the Indian Landing station showed severe hypoxia in spite of its shallow (6-8 foot) depth.
   Sites showing consistently low bottom DO levels were additionally tested with a water sampling device, providing an opportunity to test for the presence of hydrogen sulfide by its pronounced rotten egg odor (see photo). Hydrogen sulfide was readily detectable in near-bottom water samples at the Asquith Creek, RB West, Mid RB, and RB North stations, but was not present in samples taken from higher in the water column at these stations, nor in near-bottom samples taken from other stations.  
    No station showed an average bottom DO greater than 5 mg/l, a level considered "healthy" by the Chesapeake Bay Program and Maryland Department of the Environment.  In some cases such as the Joyce (SR3) station pictured above, that level was not achieved even half way between the bottom and the surface.  However, the dissolved oxygen profiles of different monitoring stations varied considerably over the course of the summer, and the detailed graphs of the different station should be examined to get a feel for this.  In general it appears that the lower Severn stations closer to the Chesapeake had higher DO levels than those above the Rte. 2 bridge.

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