Pollutants & Sources

Phosphorus Ammonia & Nitrite Combined Sewer Overflows Mercury & Other Industrial Wastes Sediment Loading: Tully Valley Mudboils Salinity Nonpoint Source Pollution

Phosphorus from Metro and other sources stimulates the excessive growth of algae in Onondaga Lake, thereby decreasing water clarity and reducing dissolved oxygen in the lake's bottom waters.

What is Phosphorus?

Phosphorus is an element that is important to plant nutrition. It is commonly found in high concentrations in fertilizers and sewage. The amount of phosphorus received by a lake is an important factor in determining water quality because phosphorus stimulates the growth of algae. Algae are microscopic green plants that live in lakes. Phosphorus input to a lake is described as phosphorus loading. Algae Blooms and Their Effects Phosphorus loading causes excess growth of algae of algae (referred to as algae blooms) in Onondaga Lake throughout most of the summer. The algae give the water a cloudy, green appearance and decrease the lake's water clarity. Eventually, the algae die and sink to the bottom of the lake where they are broken down by bacteria. This process uses up dissolved oxygen necessary for fish survival in the deeper waters of the lake.

Phosphorus Pollution and Water Clarity

One way to see the effects of phosphorus pollution in Onondaga Lake is to measure the lake's water clarity. Scientists routinely measure the clarity of Onondaga Lake by recording the depth at which a black and white Secchi disk can no longer be seen when lowered into the water.

The New York State Health Department standard for opening a beach on Onondaga Lake would require that the lake's clarity be 4 feet or greater throughout the summer. This is a matter of public safety so that people can be seen in the water. From 1970 to 1986 the lake's water clarity exceeded 4 feet only 1 day of 5. Currently, the clarity of Onondaga Lake exceeds 4 feet 1 day of 2. This improvement reflects the reductions in phosphorus loading from Metro and increased consumption of algae by small aquatic animals called zooplankton. However, further reductions in phosphorus will be necessary to routinely meet the 4-foot clarity requirement for recreational swimming safety.

Algae and Dissolved Oxygen

Most of the algae produced in the upper layers of Onondaga Lake ultimately settle to the bottom where they are decomposed by bacteria and fungi. Oxygen is used up as part of the decomposition process. The greater the algae production in the upper layers, the greater the loss of dissolved oxygen from the lower layers of the lake. Presently, the oxygen in the lake's lower waters is gone by July. See The Importance of Oxygen to learn more about dissolved oxygen in Onondaga Lake.

Phosphorus Load Sources

Phosphorus loads fall into two categories: point and nonpoint sources. Treated domestic and industrial wastes discharged from a pipe at Metro directly into Onondaga Lake represented a point source input. In 1990 and 1997, Metro contributed 65% of the total phosphorus loading into Onondaga Lake.

The remaining 35% of phosphorus entered the lake from its natural tributaries. Some of the phosphorus enters the tributaries from nonpoint sources such as runoff carried from the surrounding land. Other sources of phosphorus to the tributaries are from combined sewer overflows (CSOs) which discharge untreated sewage and stormwater to Onondaga Creek, Ley Creek and Harbor Brook during intense rainfall. See Sewage Indicators and CSOs to learn more about combined sewer overflows.

The present condition plateau indicates that the reductions in phosphorus loading achieved during the past 20 years have not caused major improvements in the water clarity of Onondaga Lake. Significant reductions of phosphorus loading, such as the reduction of phosphorus discharged from Metro, would result in a major improvement in the lake's overall water clarity.