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Indicator Details

DO - anoxia

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Anoxia conditions changed from early to late summer

For the second year in a row, the anoxia (DO ≤ 0.2 mg l^-1) forecast was split into two portions—an early summer (June to mid-July) forecast and a late summer (mid-July to September) forecast. These two time periods are often characterized by different influencing factors, such as stratification and wind direction, and therefore the volume of anoxia in the mainstem Bay can be quite different between early summer and late summer.

Overall, summer 2011 anoxia conditions were poor, with a significant amount of anoxia already in the mainstem during the first monitoring cruise of the summer (early June). The cruise-by-cruise anoxia volumes show a usual pattern of increasing anoxia throughout the early cruises. However, this year's anoxia was completely gone by late August because winds from Hurricane Irene (August 27-28th) helped mix the water column. 



The largest amount of anoxia this summer occurred in late July and was the third highest amount of anoxia recorded during late July since monitoring began. The map below shows the bottom minimum dissolved oxygen conditions for late July. While there was no anoxia by late August, the total average amount of anoxia for the 2011 summer was still above the long-term average. 






Early summer anoxia

The figures below show the 2011 and long-term anoxic volumes, as calculated using a unique statistical interpolation technique (Murphy et al. 2010). In June 2011, scientists forecasted that the 2011 early summer anoxic volume would be 2.4 ± 0.9 km³, however the observed volume was actually 4.1 km³. This observed amount is the 2^nd highest seen since monitoring began in 1985 and is much higher than last year. 

The amount of anoxia in early summer was driven by the large flow entering Chesapeake Bay (and therefore also nutrients) during the spring. Stratification (layering of freshwater on top of salty water) set up early due to high spring flows, high summertime temperatures, and low summer flows.  

 

Late summer anoxia

In June 2011, scientists forecasted that there would be 2.4 ± 0.4 km³ of anoxia in late summer (mid-July to September). If the forecast had held true, late summer 2011 would have had the 5^th largest anoxic volume in 26 years. However, the actual observed amount of anoxia was 1.9 km³, which was lower than the forecasted volume and just outside the predicted range.





One of the reasons that the late summer anoxia was lower than forecasted was the occurance of Hurricane Irene in late August and Tropical Storm Lee in early September. The winds associated with these storms mixed the DO-rich surface waters with the DO-poor bottom waters of the Bay, causing the anoxic conditions to disappear after these events. 

It's important to note, however, that the average anoxic volume for late summer 2011 is still above the long-term average, even with the early disappearance. This shows that low dissolved oxygen conditions in the mainstem continue to be a problem, despite natural weather conditions affecting the anoxic volume this year.

 

Reference:

Murphy, R.R., W.M. Kemp, and W.P. Ball. 2010. Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading. Submitted to Estuaries and Coasts.

Acknowledgements:
Anoxia forecast and review courtesy of Rebecca Murphy (Johns Hopkins University), in collaboration with Bill Ball (Johns Hopkins University), Malcolm Scully (Old Dominion University), Michael Kemp (UMCES-HPL), Jeremy Testa (UMCES-HPL), and Jeni Keisman (UMCES-CBP).