Lisa S. Alter
Nitrogen is the major cause of the Sounds hypoxia problems. Hypoxia is a condition where the dissolved oxygen level in the water is too low for an organisms survival. 5mg/liter of dissolved oxygen is considered good. Hypoxia can occur naturally in estuaries during the summer. In warmer water, the solubility of oxygen is lowered. In calm water, little mixing occurs and water separates into layers called pycnoclines. See appendix p. 1-2. Here the warmer, less dense water stays on top and the cooler, more salty water stays on the bottom. Thus oxygen can’t cross this boundary. Then the decomposers which break down oxygen use it up. Hypoxia usually starts during an algal bloom, fueled by warm temperatures, and a good supply of nutrients. Algae (which are short-lived) die, fall to the bottom. Here they become food for oxygen consuming decomposers. Human pollution, adding nutrients to the water has increased the hypoxia problem in the Sound. (Atkin, Bangser, Jacobson, Smith, Wade, Backer, 1991)
Nitrogen (in various forms) is the major nutrient that contributes to the hypoxia in the Sound. See appendix 00. In pre-colonial times, the bottom dissolved oxygen reading was 5 mg/l (see appendix p. 1-2). Since then, the total nitrogen compound load to the Sound has more then doubled. (EPA, 1993)
Nitrogen compounds in Long Island Sound, come from the following sources; 44 sewage treatment plants (see appendix p.4), stormwater runoff, acid rain/ precipitation, runoff from urban areas, septic systems, industrial wastes, agricultural lands, lawns and gardens (overfertilization), and livestock feedlots. (DEP, 1993) 1996)
Forms of Nitrogen delivered to the Sound include;
-
Organic nitrogen, incorporated into dead or living organisms.
-
Ammonia, a by-product of waste and bacterial decay.
-
Nitrate and nitrite, which are by-products of bacterial decay.
-
Nitrate from atmospheric deposition, originating from combustion of fossil fuels. (Beristain, 1989)
Human activities mostly responsible for the excessive nitrogen, include both point and nonpoint sources. They are; sewage treatment plants that discharge directly into the Sound, alteration of land cover by developers, and agricultural and atmospheric deposition.(Beristain, 1988)
93,600 tons of nitrogen compounds are estimated to be delivered to the Sound each year. Of this, 39,900 tons come from natural sources and 53,700 tons come from human activities. (Dept. Of Env. Protection, 1993). 8400 tons of this nitrogen comes from nonpoint pollution sources. (Beristain, 1989)