A dry spring raised hopes for good Bay water quality this summer, but an unusually wet summer appears to have washed away those hopes.
Monitoring data collected by Maryland and Virginia agencies through August show that 22.1 percent of the Bay suffered from low oxygen, or hypoxic, conditions this summer. That was slightly worse than the 29-year average of 21.9 percent.
That was also a little worse than expected, as scientists this spring had predicted slightly better-than-average oxygen conditions, though the actual figures were still within the margin of error for the forecast.
That forecast was based in large part on lower-than-average spring flows from the Susquehanna River.
Susquehanna River flows have a big impact on dissolved oxygen levels in deep areas of the Bay. High flows contribute to strong stratification between oxygen-rich surface waters and low-oxygen bottom waters, preventing them from mixing. That, in turn, contributes to the
formation of oxygen-starved dead zones that often plague deep waters of the Bay during summer months. On the other hand, low flows typically mean less stratification and better mixing.
But lower-than-average spring flows were followed by a rainy summer, which likely flushed more nutrients off the land and into the Chesapeake Bay.
U.S. Geological Survey river monitoring data show that during June, average freshwater flows into the Bay were 83,200 cubic feet per second, compared with the long-term average of 64,300 cfs. In July, it was 79,000 cfs compared with the long-term average of 37,700 cfs.
“The higher observed [hypoxic] volume than projected may be due to higher nutrient loadings to the Bay during late spring/early summer rains,” according to a summary posted on the Maryland Department of Natural Resources Eyes on the Bay website.
Nutrients spur algae growth in the Bay. When the algae die, they decompose in a process the depletes the water of oxygen.
Overall, the average hypoxic volume was 4.7 cubic kilometers, compared with the predicted 4.5 km. That ranked this summer 16th out of the 29 years of data from the Bay Program’s monitoring program, or just slightly worse than average. Hypoxia is defined as water with less than 2 milligrams of oxygen per liter.
For information about the monitoring results, visit www.eyesonthebay.net.
For information about the summer forecast, which is produced annually by scientists from the University of Maryland, University of Michigan and the National Oceanic and Atmospheric Administration, http://ian.umces.edu/ecocheck/forecast/chesapeake-bay/2013/.