Although both nitrogen and phosphorous are considered positive as nutrients in plant fertilizers and as ingredients in household cleaners such as dishwashing detergent, at higher concentrations they interfere with the ecology of natural water systems like lakes and streams. In fact, excess nitrogen and phosphorous runoff from lawns, farm fields and septic systems are some of the largest threats to overall lake and stream health. Both substances contribute to large algal blooms and unwanted plant growth, and phosphorus is often the limiting nutrient in aquatic systems. Excess nutrients encourage non-native, invasive aquatic plants such as Eurasian watermilfoil to grow and spread rapidly, not only choking out the pre-existing native plants but also creating a thick mat at the surface that blocks sunlight.
Nitrogen is the number one nutrient that promotes excessive macrophyte growth, although it does not directly decrease water clarity. Nitrogen concentrations of 250 ppb or lower are considered normal or background in lakes in many parts of the country. Phosphorous, on the other hand, does promote algae growth resulting in decreased water clarity when it reaches concentrations as low as 10 ppb.
Luckily, there are things that we can do to reduce the amount nitrogen and phosphorous entering our lakes. Having your septic system inspected and pumped each year is a great way to reduce both nitrogen and phosphorous pollution. Even simpler measures can be taken—some common household cleaning products like dishwashing detergent contain phosphorous, but switching to phosphate-free detergents and cleaners can make a big difference. Examples include Bi-O-Kleen®, EcoVer®, Mrs. Meyers®, and Seventh Generation®. Phosphate-free lawn fertilizers are also now widely available.
A ban on dishwashing detergents that contain more than 0.5 percent phosphates will go into effect in July 2010 in the state of Washington. Several other states are planning to or already have done the same, including Michigan, Minnesota, Pennsylvania, and New York.