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Hypoxia:The Causes and Cures

Oxygen enters the waters of the Sound when it is stirred in from the atmosphere by wind and wave action or produced by plants carrying out photosynthesis. It is used up by the respiration of fish, shellfish, plants, and microorganisms that live in the water. The ideal, healthy dissolved oxygen concentration varies for different species of fish or shellfish. Under conditions believed to be minimally acceptable for the general health of marine life, oxygen would be uniformly mixed into the water at a concentration of 5 milligrams per liter (mg/L). (At 22°C, or 72°F, the maximum oxygen content is about 7.5 milligrams per liter.)

Hypoxia can occur when dissolved oxygen is consumed faster than new dissolved oxygen is supplied. It is not necessarily a manmade condition. Estuaries are especially prone to low dissolved oxygen concentrations in the summer for two reasons. First, the solubility of oxygen in water is lower at high temperatures. Second, during summer the water is calm. In the absence of mixing by wave action, the water is not aerated and, furthermore, can separate into two distinct layers. On the top floats warmer, less salty, less dense water. On the bottom lies colder, more salty, more dense water. Just like oil and water, the water layers of different densities do not mix.

The critical result is that oxygen can't cross the boundary between the two layers (called the pycnocline), meaning that as dissolved oxygen in the bottom layer is used up by the decomposers that break down organic matter, the oxygen can't be replenished. And the more organic matter available for decomposition, the faster the decomposers use up oxygen.

Hypoxia is usually initiated by a lush growth of algae in the surface waters - an algal bloom - fueled by warm temperatures and ample supplies of nutrients. Algae are not very long-lived. As dead algae sink to the bottom, they become food for oxygen-consuming decomposers in both layers of the water column and then on the bottom. The thick mat of algae on the surface also contributes to hypoxia by slowing down the wave action that mixes in oxygen. As human activity around the Sound has expanded, summertime hypoxia has grown worse.

The LISS studies have identified nitrogen as the nutrient that controls algal blooms in the Sound. Hypoxic periods are longer and more frequent when nitrogen levels are elevated. Every summer since 1985, when the LISS began, hypoxia has been observed at the bottom of the western Sound.

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