Accuracy of summer forecasts variable
Below average 2009 winter-spring river flow and nutrient loads resulted in forecasts for fair dissolved oxygen and harmful algal bloom conditions. However, recent analysis of observed conditions shows that Bay health was worse than predicted. The average volume of anoxic waters in the Bay's mainstem during summer 2009 was 1.2 km3 and was 0.66 km3 more than the forecast volume. Average July mainstem Bay hypoxic volume was forecast to be 5.1 km3, however, the observed conditions was 6.2 km3, which is slightly more than the forecasted value but within the predicted range. One cause of this discrepancy may be the large amounts of rainfall that occurred in Maryland and Virginia that did not occur in Pennsylvania. The Susquehanna River flow and loads help to predict mainstem anoxia and hypoxia, but do not account for the above average flow from other tributaries that occurred in 2009. Dissolved oxygen in tributaries, such as the Patuxent, Patapsco, and Potomac River, was poorer than average this year, although still within the range seen in the last 20 years of monitoring. These localized effects are not accounted for in our mainstem forecasts.
It was predicted that 9% of routinely collected samples from Potomac River would have bloom levels of Microcystis. However, a Microcystis bloom did not occur in the Potomac River this year. The presence/absence of Prorocentrum minimum blooms in certain months was forecasted for five regions of the bay. The observed conditions matched the forecast in the lower Potomac River. In the Patapsco River, the P. minimum bloom appears to have started late; the first bloom was recorded in May, although it was predicted to start as early as March. While blooms were predicted for the mainstem stations (see map), no bloom was recorded.
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Acknowledgements:
The forecast and review have been produced in collaboration with members of the Chesapeake Pay Program's Monitoring and Analysis Subcommittee (MASC). The following individuals are acknowledged for their contributions: Richard Lacouture (Morgan State University), Harold Marshall (Old Dominion University), Dave Jasinski, Maggie Sexton (UMCES/HPL), Jon Anderson (Morgan State University), Mike Lane (ODU).