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Return of ‘The Blob’? Low oxygen levels in Cape Cod Bay could endanger marine life





Scientists monitor conditions in the southern region of the bay for signs of danger for lobsters, crabs and some fish.

Researchers have detected low levels of dissolved oxygen in several parts of Cape Cod Bay. Massachusetts Department of Marine Fisheries

Lobsters, crabs and other marine creatures that inhabit Cape Cod Bay may be at great risk due to the low levels of oxygen detected in those waters.

Recent data collected by the Massachusetts Department of Marine Fisheries (DMF) indicates that the southern half of Cape Cod Bay is experiencing low levels of dissolved oxygen, an important indicator of water quality.

Dissolved oxygen is simply the amount of oxygen in the water. All aquatic animals depend on dissolved oxygen for respiration, according to the Environmental Protection Agency. In the absence of enough oxygen, these creatures can suffer from hypoxia – a dangerous lack of oxygen.

Hypoxic conditions, according to the DMF, form when the surface water temperature rises, resulting in different levels of water temperature and density between different regions of the ocean. The coldest and denser waters are isolated in the lower regions. This prevents oxygen from mixing at the surface all the way to the bottom water. Decaying organic matter on the ocean floor consumes dissolved oxygen, and hypoxic conditions arise when surface waters do not mix with lower waters. Storms are a common remedy for hypoxic conditions, where different levels of water help to mix.

In 2019, an “acute hypoxia event” killed hundreds of pounds of lobsters, crabs and finfish in the same area. Scientists began using the new monitoring technology, which allowed them to detect a mass of water in Cape Cod Bay that contains especially low levels of oxygen. The researchers nicknamed it “The Point”.

This deadly mass is moving around the southern part of Cape Cod Bay, pushed and pulled by wind and sea conditions. In September 2019, strong winds from the north pushed The Plop into deeper waters, killing lobsters and other creatures there. “The Blob” was dismantled due to natural conditions shortly after, but could return to the waters off Sandwich and Barnstable.

The DMF said many animals will move in order to avoid hypoxic conditions, but can stumble and die if exposed to the conditions for more than two hours. Officials are urging fishermen working in the area to look for signs of hypoxia, such as unusual amounts of dormant or dead lobsters, crabs, or finfish in traps. They should check the traps frequently, and consider moving their equipment out of the affected area to prevent the animals from being trapped in the hypoxic water.


“Prolonged periods of severe hypoxia have not been detected so far this summer, but the trends of lower dissolved oxygen that we observed are concerning,” the DMF said in a statement.

The latest information collected by scientists shows that levels of dissolved oxygen near the sea floor are less than 4 milligrams per liter at multiple locations. Values ​​less than 4 milligrams per liter are considered moderately hypoxic, and values ​​less than 2 milligrams per liter are considered severely hypoxic and a threat to marine life.

Officials note that dissolved oxygen levels can change rapidly based on weather conditions, and it is difficult to predict the timing and location of hypoxic conditions.

It is normal for Cape Cod Bay to experience a seasonal decrease in dissolved oxygen levels, particularly in its lower waters, during late summer and early fall. This is thought to be due in part to plankton blooms that increase the amount of decomposing organic matter, which in turn takes up more oxygen. Scientists are studying why levels of dissolved oxygen are lower in some years than in others.


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