Discussion following Bill Dennison seminar on Chesapeake Bay health: What causes positive and negative trajectories?

Bill Dennison ·
30 March 2010
Environmental Report Cards | 

This blog post discusses the seminar given by Dr Bill Dennison, of the Integration and Application Network, at the IAN Seminar Series on January 28, 2010.

Several lines of inquiry were discussed as a way of further analyzing the Chesapeake Bay trajectories. The issue of watershed geomorphology (e.g., watershed size, topography, shoreline length) was one that could account for some of the basic patterns, but these were discounted as a way of affecting the trajectories, as they do not change substantially over time. The issue of factoring in the upgrades in sewage treatment and sewage overflows was considered and recognized as one in which there is quantitative data, with the caveat that sewage inputs are typically a small component of nutrient inputs into Chesapeake Bay. Another factor that would merit attention is the North Atlantic Oscillation (NAO), which, like the more well known Southern Oscillation/El Nino, accounts for interannual and decadal patterns and positions of large weather systems that affect local climate. The NAO could affect portions of the watershed differentially (e.g., wet regions vs. dry regions) which could change the loads that affect the Bay health index. The summer of 2009 was an example of regional patterns in rainfall (relatively dry in the Susquehanna basin vs. relatively wet in the James River), influencing the Bay health index.

The role of salt marshes and submerged aquatic vegetation (SAV) in filtering nutrients and sediments was also raised. The importance of these filters is tied to location, location, location; and the 'functional ecology of location' could mean that upper western shore tributaries are buffered due to abundant wetlands near the tributary mouths. In contrast, the lack of abundant wetlands near of the mouths of upper eastern shore tributaries could account for the poor bay health index scores.

A large gap in our ability to interpret changes in Bay health is the lack of widespread tracking of the implementation of various Best Management Practices. For example, the application of cover crops is not mapped so that the reduction in groundwater nitrate that could be attributed to cover crops can't be factored into the analysis. The issue of groundwater time lags was also discussed. If the groundwater time lags differ from region to region, the time scale of implementation would be manifested at different times due to the time lags. Patterns of agriculture practice also change over time and an intensification of agriculture (increased fertilizer use; increased animal feed lots) could account for the negative trajectories noted in some reporting regions and vice versa for a diminution of agriculture.

The Chesapeake Bay Program is developing a modeling tool called Scenario Builder that will be able to interrogate the watershed model so that changes in land use can generate different diffuse loadings. This modeling tool will allow analyses that could provide insights into the trajectories observed in the Bay health index.

About the author

Bill Dennison

Dr. Bill Dennison is a Professor of Marine Science and Vice President for Science Application at the University of Maryland Center for Environmental Science (UMCES). Dr. Dennison’s primary mission within UMCES is to coordinate the Integration and Application Network.

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