Posted by Bill Dennison.
Bob Hirsch and co-workers at the U.S. Geological Survey have developed a new method of analyzing long-term trends in nutrients that enter Chesapeake Bay from the rivers or tributaries that flow into the Bay. This method accounts for seasonal changes and year-to-year variation in flow, so that one can see the “forest for the trees.” The difficulty in analyzing flow and nutrients that enter Chesapeake Bay is the high degree of variability (or noise) in the data, making it difficult to discern trends, particularly long-term trends. Bob was able to use daily water flow data over a 31-year period, stretching from 1978 until 2008 from nine sites around Chesapeake Bay. Bob took these 100,000+ daily streamflow measurements and combined them with 13,000+ nutrient measurements to come up with daily nutrient loading estimates at each site. Then he was able to calculate a “flow-normalized daily flux,” which takes out the variations in flow due to weather. In this way, the long-term trends could be distinguished from the short-term variations.
The most dramatic result was that of the Patuxent River. In the Patuxent River, the total phosphorus levels went from over a half a ton of phosphorus per day in the late 1970s to less than a quarter a ton of phosphorus per day by the 1990s. This more than halving of the total phosphorus loads can be attributed to upgrades in sewage treatment and represents one of the early success stories in Chesapeake Bay management. Unfortunately, this was the only data set that exhibited a substantial phosphorus reduction. The other rivers, Pamunkey, Mattaponi, Choptank, Rappahannock, Appomattox, Susquehanna, Potomac and James Rivers were either not improving or slightly degrading in terms of long term phosphorus loads.
The same approach was used for nitrate levels and once again the Patuxent River had a reduction in nitrate that occurred in the 1980s. In contrast, the Choptank River has had steadily increasing nitrate levels throughout the 31 year record. This contrast between the Patuxent River on the Western Shore with sewage treatment upgrades vs. the Choptank River on the Eastern Shore with agricultural runoff is marked. Any progress that we have made in reducing nutrients through sewage treatment upgrades is being offset by increases in agricultural runoff.
These nutrient analysis trends are consistent with the trends in the Bay Health Index, comprised of different water quality parameters (water clarity, dissolved oxygen, chlorophyll concentrations) and biotic parameters (aquatic grasses, benthic animals, phytoplankton), which are used in the annual Chesapeake Bay report card released by Eco-Check. In the long-term analysis of the Bay Health Index, stretching back into the mid 1980s, we can see degrading conditions on the Eastern Shore, contrasted with improving conditions on the upper Western Shore. Various positive and negative feedbacks have been implicated in these different trajectories (see May 3, 2010 blog post “Chesapeake Bay Restoration: Are we headed in the right direction?”, but what Bob Hirsch’s analysis provides is the underlying causes of these trends—nutrient loading. The bottom line is this: In spite of all of our collective efforts in reducing nutrient inputs into Chesapeake Bay, we are still not making the progress that is needed.
View Bob Hirsch’s seminar and discussion following.