Chesapeake Bay Hypoxia: Locating and Avoiding "Bad Water"
Current and Forecasted Low-Oxygen Conditions in Local Bay Waters
What is 'Bad Water'?
'Bad Water' is a local term for the low-oxygen, or hypoxic, conditions that develop in the Chesapeake Bay in the summer. It occurs when deep, oxygen-depleted water is pushed into shallower areas by wind or tides. For watermen, this shift means "dead water" - it can kill crabs in pots, fish in nets, and drive fish away from their traditional summer haunts. Learn more about hypoxia →
Where can I find current & forecasted 'Bad Water' in my area?
Visit the Virginia Institute of Marine Science (VIMS) "Depth to Low Oxygen" tool (opens in a new tab) for current and forecasted locations and depths of low oxygen or 'Bad Water'. This tool uses reported observations and computer models to predict current status and possible short-term changes. Example maps produced by VIMS are shown below:
How to Read These Maps:
- Legends: The depth to low oxygen ('Bad Water') appears on the legend to the right of each plot.
- Deployment: Use this depth information to avoid 'bad water' and help identify the best areas and depths to deploy fishing or crabbing gear.
- The 10-Foot Rule: If the depth to low oxygen is less than 10 feet (red areas), be aware that localized winds or currents can push the 'Bad Water' into much shallower areas, impacting fish, crabs, and oysters.
Add 'Depth to Low Oxygen' Tool to Your Home Screen
Save the VIMS tool to your phone for quick access on the water:
Android (Chrome)
- Go to the VIMS Tool
- Tap the three dots in the upper right corner
- Select 'Add to Home screen'
- Type a name for the shortcut (or keep the default)
- Tap 'Add', -or- touch and drag the widget to your home screen.
- Shortcut will now appear on your home screen like an app
iPhone/iPad (Safari)
- Go to the VIMS Tool
- Tap the Share button
- Tap 'Add to Home Screen'
- Type a name for the shortcut (or keep the default)
- Tap 'Add'
- Shortcut will now appear on your home screen like an app
Understanding How Low Oxygen (Hypoxic) Areas Set Up
What causes hypoxia to occur?
Low-oxygen (hypoxic) conditions are a seasonal occurrence in the Chesapeake Bay estuary. This happens through a process called stratification: as temperatures rise, a layer of warm, buoyant surface water settles over the cooler, denser water below. This stratification prevents oxygen in the surface waters from mixing into the lower water column. Meanwhile, natural decomposition on the Bay bottom waters continually consumes oxygen. Without a way to 'refill' the oxygen supply from the surface, these bottom waters quickly become hypoxic.
Human Impact: This natural cycle is intensified by excess nutrients from land runoff or wastewater. These nutrients act like fertilizer, triggering algal blooms. When the algae eventually die and sink to the bottom, they provide a massive feast for bacteria. This accelerated decomposition rapidly consumes oxygen, shrinking the livable habitat for fish, crabs, and oysters.
When does hypoxia occur?
Hypoxia usually occurs during warmer months (April - September) when Bay waters are stratified and low oxygen conditions are common.
What makes hypoxia worse?
- Periods of intense heat
- Recent heavy rains
- Algal blooms
- Sustained winds from a constant direction
What is Maryland doing to reduce hypoxia?
There are ongoing efforts by Maryland's Environmental Protection Agency's (EPA) Chesapeake Bay Program (CBP)(opens in a new tab) and neighboring Bay states (Virginia, Pennsylvania, West Virginia, New York, Delaware, and Washington D.C.) to reduce nitrogen and phosphorus pollution in our waters to decrease hypoxic conditions throughout the Chesapeake Bay.