THE ROLE OF MATHEMATICAL MODELING IN DEVELOPING A NUTRIENT TMDL FOR LONG ISLAND SOUND

According to the USEPA/NOAA-sponsored Long Island Sound Study (LISS), between 20 and 40% of the bottom area of Long Island Sound is “mar ginal to unusable” many marine aquatic species during the summer months due to the occurrence of low dissolved oxygen or hypoxia. Hypoxia usually occurs during the late summer, generally from July through September, and varies in duration from year to year (Figure 1). Extensive monitoring and modeling have linked hypoxia to an overabundance of nitrogen together with the naturally occurring density stratification of the water column. While nitrogen is essential to drive primary production and provide for a productive ecosystem, too much nitrogen can stimulate the excessive growth of phytoplankton. When phytoplankton die, they settle to the lower layers of the water column and to the sediment bed, where they are consumed by bacteria. The microbial decay of these phytoplankton in the lower layers of the water column and in the bottom sediments of the sound use up the available oxygen in the lower waters and in the sediment bed. If the water column is stratified and remains that way for an extended period of time and if the levels of organic matter are high enough, oxygen may fall to hypoxic or even anoxic levels.