Background

Hypoxia and the models used to forecast it

“Dead zones”—areas with too little oxygen for most marine life—can occur when nutrients from fertilizers, sewage, and other sources feed algal blooms. When these algae die and sink, their decomposition takes up dissolved oxygen from the water. Scientists refer to these oxygen-poor waters as anoxic, and to water with no measurable dissolved oxygen as hypoxic. Hypoxia is typically most pronounced in deeper waters, and during summer. Learn more about how and why dead zones form.

Low oxygen levels impact marine organisms. Click for larger view.Because even small decreases in dissolved oxygen can significantly impact fish and other marine animals, researchers at the Virginia Institute of Marine Science and elsewhere are working to develop and refine computer models to accurately predict when and where hypoxia may develop. Their goal is to help anglers and other Bay enthusiasts better plan their day-to-day activities.

The Chesapeake Bay Hypoxia Forecast uses a three-dimensional, numerical model to simulate dissolved-oxygen levels in the bottom waters of Chesapeake Bay's main stem. The 3-D model is a component of NOAA's Coastal and Ocean Modeling Testbed, which is itself part of the U.S. Integrated Ocean Observing System (IOOS). The forecast model simulates 3 conditions:

  1. Nowcast: present-day levels of dissolved oxygen in Chesapeake Bay,
  2. 2-Day Forecast: levels of dissolved oxygen in the Bay 2 days from now, and
  3. Forecast Trend: difference between nowcast and forecast (% change over 2 days)

The 3-D model is forced by winds provided by NOAA and river input provided by stream gauges operated by the U.S. Geological Survey (USGS). NOAA forecasts of other Chesapeake Bay parameters are available here. The 3-D model has 20 layers in the vertical and a horizontal resolution of approximately 1 km (0.6 miles). A hydrodynamic component calculates temperature, salinity, and water transport. The model sets dissolved-oxygen levels at the surface to a temperature-based saturation value. It removes dissolved oxygen below the surface by assuming a constant rate of oxygen respiration associated with the decomposition of organic matter.

For more information on the model and its development, contact VIMS professor [[v|marjy,Marjy Friedrichs]], the Principal Investigator of the Estuarine Hypoxia component of the Coastal and Ocean Modeling Testbed.

Both the nowcast and 2-day forecast use light green for high-oxygen, healthy waters and red for low-oxygen, hypoxic waters. The forecast trend shows areas of increasing dissolved oxygen in blue and areas with decreasing levels in red. Units are in percent change per day.