Monitoring data shows areas of the Severn were "Dead Zones"

   The most striking finding of our 2006 monitoring project was that, near the bottom, much of Round Bay was either severely hypoxic or anoxic throughout most of the summer, and this condition also existed in Asquith Creek.  These findings are surprising because "dead zones" have been described in the deep channel of the Chesapeake's mainstem, but not in Chesapeake tributaries.   Whether or not our monitoring results lead to "dead zone" labels could be debated, as we did not have any stations with average metered DO levels of less than 0.2 mg/l, generally considered anoxic.  However, at oxygen levels below 2 mg/l our two meters showed inconsistent low readings and did not agree with each other very well, so we cannot make a strong claim for strictly-defined anoxia based on our DO numbers.  However, additional observations with a water sampling device lend strong support to the "dead zone" label.  While the great majority of our measurements at each monitoring station were done with the YSI 85 meters, in August we began to sample stations showing consistently low bottom DO levels with a bottle-type water sampler.  Samples brought up from close to the bottom at the Asquith Creek station and three Round Bay stations (SR5, RBW, and RBN) gave a pronounced rotten-egg smell of hydrogen sulfide.  This toxic gas is produced by anaerobic bacteria only in the absence of oxygen, and hydrogen sulfide itself is unstable in the presence of oxygen.  Based on these observations we characterize most of Round Bay and at least part of Asquith Creek as "dead zones" for at least part of the summer.   This seems surprising because both Round Bay and Asquith Creek are aesthetically very pleasant water bodies, with relatively low levels of shoreline development visible (relative to other parts of the Severn), and both have seen recent remarkable increases in submerged aquatic vegetation in their shallows. However, the hypoxia we observed was only near the bottom (about 25 feet down), and could only have occurred if vertical mixing was minimal.

   It may also seem surprising that hypoxia/anoxia was not previously described in an area as large and well known as the Severn's Round Bay .  Part of the answer is that monitoring programs are always faced with limited resources, and it is never possible to monitor more than a small fraction of desirable sites.  As discussed previously,  the Maryland DNR has excellent long-term data from the Rte 50 Bridge station, and in the summers of 2002 and 2003 carried out  technically demanding continuous and mapping measurements of DO in the Severn.  These monitoring programs very likely failed to detect the low DO levels we found in Round Bay because they only made measurements at a depth of 1 meter, where severe hypoxia is unexpected (and where we did not find it).  Other monitoring programs examining DO levels in the Severn were undertaken by the DNR Fisheries division to assess potential yellow perch habitat, and while considerable hypoxia was found in creeks in the upper Severn, Round Bay was apparently not monitored.  However, one existing dataset from an extensive Bay-wide monitoring program shows results entirely compatible with ours.  This monitoring was carried out by EPA's MAIA program, and their Severn data are from single measurements made at multiple stations throughout the Severn in August, 1997.  When their Round Bay stations are selected, one can see a strong dependence of DO readings on the depth of the station, such that, with one exception, all stations below 5 m in depth showed sever hypoxia (the exception was a station near our RBS station, where we also found milder hypoxia).  This MAIA dataset also includes data for benthic biomass, a measure of total living organisms in the top layer of bottom.  These numbers are strongly correlated with the DO levels, and indicate that the deeper parts of Round Bay were indeed "dead", i.e., devoid of benthic organisms.  The correlation supports the obvious hypothesis that hypoxia caused the depletion of this important component of the estuarine food chain, making it a "dead zone".  Further evidence for degraded benthic habitat in the upper Severn including Round Bay comes from measurements of the "benthic index of biological integrity" by Versar, Inc for the Chesapeake Bay Program (left).  These data show that the benthic habitat in the top half of the Severn including Round Bay is not healthy, and in some cases, devoid of life.  While in principal this could be caused by toxic chemicals, there is no strong evidence to support this, and oxygen depletion is a much more compelling explanation.
   Looking over our dissolved oxygen results for the Severn as a whole, all of the mainstem and the Creek stations showed some hypoxia (DO < 5mg/l) near the bottom, and most showed healthy oxygen levels near the surface.  It appears that episodes of severe bottom hypoxia at most stations did not persist as they largely did in Round Bay and Asquith Creek.  Although the criterion of 5 mg/l for "healthy" DO levels applies to fish, benthic organisms such as worms, clams, and oysters are adapted to low oxygen conditions and can tolerate DO levels down to 2 mg/l for substantial periods.  However, even looking at this less stringent criterion, one can see that the benthic habitat is stressed in much of the Severn and its creeks over the summer months.

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