| Hypoxia |
Fundamentals
Hypoxia is the technical term for low levels of oxygen in the water, specifically less than 3mg/L. The Long Island Sound often suffers from hypoxic conditions during the July through September period (e.g., summer 2002), when oxygen rates of bottom waters fall to levels threatening the health of aquatic life populations. Hypoxia is often closely linked to the over fertilization of the Sound with nutrients mainly coming from sewage treatment plants that discharge into its waters as well as runoff from rainfall that carries high nutrient levels from sources such as lawn fertization. While nitrogen is an essential nutrient for productive ecosystem, in great quantities, nitrogen can lead to the excessive growth of planktonic algae. When the algae die and settle to the bottom of the Sound, they are decomposed by bacteria, a process that uses up available oxygen. If the oxygen is not replenished, then the aquatic organisms, who need oxygen to breathe, may suffocate and die in extreme conditions while others simply flee hypoxic zones.
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Physical Factors
The main source of oxygen for the deeper water of the Sound is atmospheric oxygen that gets absorbed by the surface water. The oxygen typically reaches the bottom water through vertical mixing that occurs via wind mixing and wave breaking. During the late fall through early spring, surface wind forcing associated with weather disturbances are strong enough and frequent enough to keep the water well mixed in the vertical, thus providing a constant source of oxygen to the deep water. However, natural stratification of the Sound’s waters occurs during the summer when winds are lighter and storms are less frequent. During this period, warmer, fresher water "floats" on the top of cooler, saltier water that is more dense. The separation between the deep/heavier water and the surface/lighter water is known as the pycnocline. The pycnocline isolates the deep water so that it cannot mix with surface water which is its primary source of oxygen. Thus to understand the development and breakdown of hyxpoxic conditions, it is necessary to have information on the vertical structure of the Sound's water properties, the local atmospheric forcing conditions, and in fact the three-dimensional water circulation in the Sound. |
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