The overfertilization of the aquatic environment is a global problem. When nutrients reach a waterbody with excessive amounts of nutrients, certain plant species will often proliferate (called eutrophication), and can cause several ecological problems. The nutrients in question are phosphorus for freshwaters, and nitrogen for salt waters. Here, we are concentrating mainly on the marine environment, but there are a number of eutrophic freshwater lakes, rivers, and streams within the watershed. For Rhode Island saltwaters, the greatest impacts are in the upper half of Narragansett Bay, from perhaps around the Wickford-Quonset area up to the Providence-Seekonk tidal rivers.

The response of plants to excess nutrients in the upper half of Narragansett Bay include large dense blooms of microscopic species known as phytoplankton as well as intertidal blankets of nuisance seaweeds (macroalgae) like sealettuce (Ulva lactuca). When these organisms die, they are decomposed by bacteria, robbing the water of dissolved oxygen and causing a low oxygen condition known as hypoxia, a physiologically-stressful condition. This condition is most common and most severe during periods when the water is "stratified" or layered, with little mixing between the top and bottom waters. Calm, hot summer periods, especially during "quarter moon" phases when the tidal currents are weakest (neap tides) tend to be most susceptible periods. If the decaying bloom is large enough, loss of all oxygen (anoxia) may occur, causing fish kills. Low oxygen is most prevalent in bottom waters deeper than approximately 10-12 feet (3-4 m). The ecological impacts of hypoxia include fish kills, fish avoidance of hypoxic areas, and shifts in sessile (nonmobile) bottom species, with the elimination of sensitive species. Such species can include blue mussels (Mytilus edulis) and scallops (Argopecten irradians). The soft shell clam ("steamers", Mya arenaria) are somewhat resistant, but will die if oxygen levels get too low, while the hardy quahog (hardshell clam, Mercenaria mercenaria) is quite resistant to extremely low oxygen levels.

In 1998, the NBEP considered such impacts of excess nutrients to be a critical issue for further investigation in the upper Narragansett Bay area, and funded a workshop on nutrient treatment options: "Nutrients and Narragansett Bay: A workshop on nutrient removal from wastewater treatment facilities" through the RISeaGrant.