Information about this map
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Here are some helpful tips to explore the data:
- Click the arrow on the left side of the map to change the timeframe of the data.
- Use the three buttons at the top right of the map to zoom back to Cape Cod Bay, open the map legend, or change the basemap.
- Click on a point to see more information in a pop up. Click on the pop up header to show or hide the point's data. Click on the page numbers or arrow at the top right of the pop up to move through the different intervals of data for that point.
The data in this map represents a larger dataset that was aggregated into 6-hour averages per data logger per day. The time displayed in the pop up is the start time of the 6 hour interval. (ex. 2021-09-09 06:00AM represents data 6 a.m.–noon on 09/09/2021).
Data Source: Cape Cod Bay Study Fleet
Levels of dissolved oxygen:
- Normal: 6 mg/L or higher
- Low: 4–6 mg/L
- Very low: 2–4 mg/L
- Critical: 2 mg/L or lower
What is dissolved oxygen?
Dissolved oxygen describes how much oxygen is present in the water. Fish and crustaceans (like lobsters and crabs) breathe dissolved oxygen. When dissolved oxygen levels get too low, some animals may move from the area. Other animals may get trapped in these low-oxygen areas and can die. Hypoxic water has a low level of dissolved oxygen. Anoxic water has no dissolved oxygen.
How do hypoxic conditions form?
Warming surface waters can cause a steep temperature and density gradient in the water column. The gradient isolates the cooler, more dense bottom waters. This isolation prevents oxygen from the surface mixing down to the bottom water. The natural process of decaying organic materials on the bottom uses up dissolved oxygen. When surface waters can’t mix down to replenish it, hypoxia will occur. Storms help surface water mix with bottom water to replenish dissolved oxygen levels.
Dissolved oxygen in Cape Cod Bay
In September 2019, we recorded severe hypoxic conditions for the first time in southern Cape Cod Bay. This surprise event killed hundreds of pounds of lobsters caught in the traps of unsuspecting fishers in the area. After the 2019 event, we started a monitoring program with the Cape Cod Bay Study Fleet—a group of commercial lobster fishers who place water quality data loggers in their traps. Fixed-gear fishers can use the monitoring system to know when dissolved oxygen may be in decline. When they see the dissolved oxygen level decline, the fishers know to move their gear from the problem area. This saves lobsters and other animals from being caught in traps during hypoxic or anoxic events.