Nitrogen gas is found in large amounts in the atmosphere. In fact, it makes up 78 percent of air. However, most plants are unable to use nitrogen in this form. They must use nitrogen that has been "fixed" (converted from N2 gas to the forms of nitrate (NO3-) and ammonium (NH4). This is done by microorganisms such as blue-green algae (cyanobacteria) and bacteria. The series of conversions of nitrogen into various forms is called the nitrogen cycle.

People also fix nitrogen from the atmosphere when they make fertilizer in industrial plants using high-temperature combustion, and when they burn fossil fuels at high temperatures—for example in car engines, and at electric plants. This nitrogen moves from the atmosphere to the earth when it rains.

Nitrogen in various forms may also enter water bodies such as Greenwich Bay in discharge from sewage treatment plants, from boat sewage and home septic systems, and in fertilizer from lawns and agricultural fields. Animal wastes from farms may also contribute nitrogen to coastal waters.

In a saltwater environment, nitrogen is the "limiting" nutrient. Marine plants only grow when they have enough of both nitrogen and phosphorus. There is usually plenty of phosphorus, but not as much nitrogen. Therefore, the growth of a plant is limited by the amount of nitrogen in the water.

Phosphorus

Phosphorus (P) is an essential nutrient for all life forms. It is the eleventh-most abundant mineral in the earth's crust. Natural phosphorous deposits are usually found in the form of phosphate in the mineral apatite. Apatite is found in certain rocks and in guano deposits (piles of bird droppings). Phosphorus is also added from human sources, such as agriculture (rain washing fertilizer from fields), boat sewage, and sewage treatment plants (from phosphorus in human wastes, toothpaste, detergents, pharmaceuticals, and food-treating compounds).

Algae and Plankton Blooms

As nutrient levels rise, plants—both seaweeds and phytoplankton—may grow and reproduce more quickly, resulting in a "bloom." In Greenwich Bay, there are sometimes blooms of macroalgae. Toxic algae blooms have also been seen in coastal waters around the country. When these algae discolor the water, they are known as "red tide" or sometimes "brown tide."

Nitrogen is often the nutrient that is suspected to be a trigger for algal blooms or plankton blooms.

It is important to understand, however, that the effects of nutrients depend on a number of other factors, including how much fresh water is entering the estuary, how nutrients cycle through the system, and how much flushing there is in an estuary.