Oyster Bay Estuary Water Quality, a project of Friend of the Bay.

Dissolved Oxygen (DO) levels in water are often used to gauge the overall health of the aquatic environment. DO refers to the amount of oxygen that is present in a given quantity of water. We measure it as a concentration using units of mg/L (i.e., the milligrams of oxygen dissolved in a liter of water).

When DO levels in the bottom water layer of the Sound are low (a condition called hypoxia) to nonexistent (anoxia), the survival, reproduction, or use of an area by marine life is impaired. This can affect commercially valuable marine species by depleting their food sources or impairing their development due to stress caused by inadequate oxygen concentrations.

From mid July through September, Long Island Sound, its embayments (like the Oyster Bay/Cold Spring Harbor Complex), and many of its aquatic inhabitants suffer from hypoxia. During this period, oxygen levels in the bottom waters fall to levels inadequate to support healthy populations of aquatic life. Hypoxia is a symptom of a larger problem, the over fertilization of the Sound with nutrients, primarily nitrogen.

To date, research shows that the most severe effects (such as mortality) occur when dissolved oxygen levels fall below 2.0 mg/L at any time and below 3.5 mg/L in the short term (i.e., 4 days), but that there are probably mild effects of hypoxia when dissolved oxygen levels fall below 5 mg/L.

Friends of the Bay uses a Quanta Hydrolab Water Quality Monitoring System to determine the dissolved oxygen levels within the estuary. Dissolved oxygen readings, which are measured in milligrams per Liter (mg/L), are taken at 0.5 meter above the bay bottom, at 1.0 meter below the water's surface, and at 0.5 meter below the water's surface.

The next item we monitor is >>> Nitrogen.