The Moriches Bay Index: Dissolved Oxygen Dips to Stressed Level Daily

Moriches Bay
Moriches Bay. Photo credit: Brendan J. O'Reilly

This week, the Moriches Bay Index (TMBI) has continued to gather data on salinity, temperature and dissolved oxygen. This data is helping our team and the public to understand the quality of water in Moriches Bay. We will continue to send out information, so stay tuned!

Salinity in Moriches Bay
Salinity is a measurement of the mass of dissolved salts in water, typically reported in parts per thousands (ppt). It is important because it affects water density, which is one of the driving forces behind circulation. It also influences which plants and animals can survive. Bays (which are technically called estuaries) have a mixture of freshwater and salt water and support the greatest diversity of plants and animals. The typical salinity for bays is 22–26 ppt. Ocean water is much saltier  and has a salinity of 32‐35 ppt. Fresh water has very low salinity, it is normally below .5 ppt.

Salinity drops over this week
The graph below illustrates a decrease in salinity in Moniebogue Creek at the Stevens Lane Bridge.


Temperature remains in upper range for productivity
The graph below illustrates how temperature has changed over the course of one week. When we are in the green zone, our temperature is at an appropriate level to sustain life.


Dissolved Oxygen—What is it?
D.O. tells us how much oxygen is in the water. Oxygen is necessary for all aquatic life to breath. Healthy waters have high levels of D.O., ranging from 4-11 mg/L of D.O. Levels between 2-3mg/L can support very few animals, and anything below 2mg/L will kill most fish. Therefore, the more oxygen, the better the water quality is.


Estuary – a body of water with direct connection to the ocean that is measurably diluted with freshwater from upland sources.

Quahog – Native American name for a hard clam (Mercenaria mercenaria)  

Dissolved Oxygen dips down to stressed levels daily

When D.O. is in the green zone, it is at an appropriate level to sustain life. If it enters the yellow zone, then aquatic life is stressed. The red zone means D.O. levels are very low and cannot sustain life. 

Dissolved Oxygen is cyclic throughout the day as illustrated in the graph below. D.O. levels are mainly in the green zone, but enter the yellow zone daily, and occasionally enter the red zone. That means that every day the tide is bringing in fresh water, and when the tide goes out, our oxygen levels deplete. Therefore, when it is high tide, we have a lot of oxygen and when it is low tide, we have very little oxygen. 


The Moriches Bay Index is the first continuously operated and comprehensive water quality monitoring project in Moriches Bay, including Moniebogue and Quantuck Bays!

Chloe Rouhandeh is a summer intern deploying sensors at the water monitoring site in Moniebogue Creek at the Stevens Lane Bridge.

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