Free oxygen enters the water through two main sources: through aquatic plants’ photosynthesis, and through contact with the atmosphere. While the concentration of oxygen in the air remains relatively constant at 20%, its concentration in water varies with several chemical, physical, and biological factors, which create an added dimension for the animals living in the Hudson River. Most aquatic Hudson River animals require dissolved oxygen levels between 5-11 mg/L (milligrams per liter) for healthy growth.
For a general introduction to oxygen and the role it plays in the Hudson River, click here for the previous article in this section An Introduction to Oxygen.
Seasonal changes in temperature and precipitation affect the maximum amount of oxygen that the water in the Hudson River can hold. The maximum amount of oxygen that a given amount of water can hold, or its solubility, is directly affected by both the temperature and salinity of the water. Oxygen is more soluble in cold water, which means that colder water can hold more oxygen than warmer water. Similarly, oxygen solubility decreases as salinity increases, which means that fresh water can hold more oxygen than brackish or salt water. The cooler, less salty water found in the Hudson River during early spring, can hold more oxygen than the warmer, saltier water of summer.
The turbidity, or clarity of the water, can impact the amount of dissolved oxygen in the Hudson. When the water is more turbid (cloudier) less light can pass through the water. This reduces the ability of the plants to photosynthesize, and less oxygen is added back to the water. It is natural for there to be some turbidity or cloudiness in the Hudson, but higher-than-normal amounts can occur when storms bring more runoff into the river, carrying large amounts of sediment and other pollutants.
Large storm events can also create sewage overflows, resulting in raw sewage and polluting stormwater runoff being deposited into the river. Sewage overflows can deposit large amounts of nitrogen and phosphorous in the river. Algae populations bloom as they feed on the extra nutrients. When algae die, oxygen is consumed during their decomposition. This process reduces the amount of oxygen in the water, possibly, to levels below those required for fish to be healthy and survive.
As you can see, the amount of oxygen that is found in the Hudson River is a function of many interrelated factors. There is a balance between natural variations in factors like temperature and salinity and human-induced changes caused by increased nutrients and sediments that are introduced into the river. It is important to maintain healthy levels of dissolved oxygen in the Hudson to preserve the aquatic life which it contains. You can monitor the health of our precious Hudson River by keeping an eye on the oxygen levels recorded by Beczak’s water monitoring equipment, whenever you visit our center, or from your own computer!