Dead Zone Update

Earlier this summer the National Oceanic and Atmospheric Administration (NOAA), United States Geological Survey (USGS), University of Maryland Center for Environmental Science (UMCES) and University of Michigan scientists predicted a large hypoxic volume for the Chesapeake Bay in 2019 due to higher river flows last fall and this spring. That prediction has come to fruition. 

River flow volume is linked to increased size of the dead zone because the heavy rains and snow melt that create the river flow carry excess nutrients of nitrogen and phosphorus from agriculture, septic systems, overflows from sewage treatment plants and runoff from lawns, gardens and paved surfaces. These nutrients fuel the out of control grow of the phytoplankton that overwhelms the natural system. The decomposing phytoplankton, combined with higher water temperatures, can cause large areas of the deepest parts of the Chesapeake Bay's deep channel which is the ancient Susquehanna riverbed to have little or no oxygen to support marine life.

from VIMS

At various times each summer the Maryland Department of Natural Resources measures the dissolved oxygen in the Maryland portion of the Chesapeake Bay main stem and the size of the Dead Zone. While the Virginia Institute of Marine Science (VIMS), Anchor QEA and collaborators at UMCES, operate a real-time three-dimensional hypoxia forecast model using input of that predicts daily dissolved oxygen concentrations throughout the Bay (www.vims.edu/hypoxia) using the National Weather Service wind monitoring data.

In a wedge estuary such as Chesapeake Bay where the layers of fresh and salt water are not well mixed. The most important source of oxygen is the atmosphere. At sea level, air contains about 21% oxygen, while the Bay’s waters contain only a small fraction of a percent. This large difference between the amount of oxygen results in oxygen naturally dissolving into the water. This process is further enhanced by the wind, which mixes the surface of the water which is why Virginia Institure of Marine Science uses the wind monitoring data to predict the oxygen; and why storms reduce the size of the Dead Zone.

The peak of oxygen depletion typically occurs in July or August. Water temperatures are highest during these months and the days are longest accelerating the growth of phytoplankton that ultimately consumes all the dissolved oxygen. The dead zone is typically gone by late fall. Cooler air temperatures at that time of year chill the surface waters, while the deeper water remains warm and allows more mixing of the layers during storms. Cooler water also will hold more oxygen. The size and shape of the dead zone is variable from month to month during the summer.

forcast for today VIMS

The Maryland DNR monitoring data “showed that dissolved oxygen conditions in the Maryland portion of the Chesapeake Bay main stem were larger than average in early August. The hypoxic water volume (areas with less than 2 mg/l oxygen) was 1.77 cubic miles in early August, down from the 2.01 cubic miles seen in late July, but significantly higher than the 1985-2018 early August average of 1.19 cubic miles.” 

VIMS Daily Snapshot

These snap shots of the condition of the Chesapeake Bay are difficult to interpret. The severity of the Dead Zone is affected by both the size of the hypoxic zone and the duration. The Virginia Institute of Marine Science model also creates a total hypoxic area (size times duration) for the summer. As you can see below this summer total Dead Zone volume is larger than the last several years. Though it is anticipate that the winds and cooler air associated with Tropical Storm Dorian as it reached the Virginia coast broke up the Dead Zone; nonetheless, the total Dead Zone for the year broke recent records. 

from VIMS