Indicator Details

DO - anoxia

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Summer anoxic volume close to average

The volume of anoxic (dissolved oxygen ≤ 0.2 mg L^-1), or oxygen-deprived, water in the Bay's mainstem during summer 2009 was 1.2 km³ and is slightly lower than the long-term (1985-2009) average. As winter-spring nutrient loads were relatively small this year, it was forecasted that the volume of anoxic water (forecast volume = 0.54 ± 0.33 km³) would have been the 2^nd smallest on record. The difference between the observed and forecast volume may in part be due to a low anoxic volume during late July (see below).

 

Although the average summer anoxic area was 1.2 km³, the map of average summer anoxia shows little area where dissolved oxygen was at or below 0.2 mg l^-1. This is probably a result of methods to interpolate (i.e., estimate values between data points) dissolved oxygen values and differences in the ways that these two measures were averaged.

Average dissolved oxygen values at single sampling stations can be skewed above 0.2 mg l^-1 by relatively high measurements in early and late summer. Also, interpolation techniques use data from the four nearest stations to estimate values between sampling stations, weighing closer values higher. This could create an average value between stations that is higher than stations nearby. Analysis methods for calculating these averages will be revisited to reduce this discrepancy. 

 

While the average summer dissolved oxygen map does not adequately represent the anoxic conditions in the Bay in 2009, the late August cruise map shows a significant amount of anoxia. The map below shows the height of anoxia that was measured in the Bay this past summer. The contrast between the highest amount and the average anoxia illustrates the variability that can occur in the indicators used to assess Bay health and therefore, the complexity of assessing such a system. 

This is the fifth year that anoxic volume has been forecasted in Chesapeake Bay. Although the forecasts have not proved to be as accurate as initially thought, continued analysis of the methods will help determine how to improve the forecast in the future. Additionally, these forecasting efforts improve our understanding of what conditions affect the Bay and how they change from spring to summer.

Please note the difference in number and time of cruises in 2009 compared to previous years.

Although average conditions were observed, the month to month variation in anoxic volume did not follow the usual summer pattern. Typically, the volume of anoxic water in the Bay's mainstem gradually increases during June, reaches a maximum in late July, and declines again in August. For the second year in a row, anoxic volume increased in late June/early July, dropped off in July, and then increased again in August. In 2009, anoxic volume decreased significantly in early July. The volume dropped considerably, with only a small amount of anoxia observed. This reduction in anoxic water may have been due to a wind event that drove southern bay waters into the central portion of the Bay. By the next sampling time in late July, however, the effects of the event had dissipated, and the volume of anoxic water increased to average levels by the end of the season.

Acknowledgements:
Anoxia forecast courtesy of Jeni Keisman, using methodology originally developed by Dave Jasinki. 2009 dissolved oxygen data provided by Jeni Keisman (CBP/UMCES) and Bill Romano (MD DNR).