Nutrients, especially nitrogen and phosphorus, are key indicators of water quality in estuaries. Plants require many nutrients (e.g., carbon, nitrogen, phosphorus, oxygen, silica, magnesium, potassium, calcium, iron, zinc, copper) to grow and reproduce. Of these, nitrogen and phosphorus are the most essential for aquatic plants.
Nitrogen and phosphorus naturally enter estuarine waters when freshwater runoff passes over geologic formations rich in phosphate or nitrate, or when decomposing organic matter and wildlife waste get flushed into rivers and streams. Manmade sources of nutrients entering estuaries include sewage treatment plants, leaky septic tanks, industrial wastewater, acid rain, and fertilizer runoff from agricultural, residential and urban areas. Too much nitrogen and phosphorus acts as a pollutant in the water. This leads to explosive blooms in algae that cloud the water and deplete it of the oxygen that is critical for aquatic animals. This is called eutrophication.
Excessive nutrient concentrations have been linked to hypoxic (very low oxygen) conditions in more than 50 percent of U.S. estuaries. Under the worst conditions, the waters of an estuary can become anoxic (having no oxygen). High nutrient concentrations have also been linked to algal blooms such as red and brown tides, some of which produce harmful toxins. Nutrients are also believed to cause the growth of the potentially toxic organism Pfiesteria (USGS, 1999). Red and brown algal tides and Pfiesteria have been linked to fish and shellfish kills, and may be harmful to human health. The image below illustrates what an estuary with a healthy input of nutrients looks like—lush and vibrant.