Total Phosphorus

Water Quality Measurement  | Chemical Parameter

Lake Pend Oreille | Pend Oreille River

Phosphorus is another essential plant nutrient that can also cause increased plant and algae growth, low dissolved oxygen (DO) levels, and increased temperature when its concentration in water becomes too high. Phosphorus sources include natural rock and mineral decomposition, stormwater runoff and snowmelt, soil erosion, detergents and cleaning fluids, industrial discharge, failing septic systems, fertilizers, animal waste, and agricultural runoff.

Phosphorus (P) is a nutrient that supports aquatic life by encouraging growth of aquatic plants, which in turn, increases the productivity of a water body. However, excess nutrients can cause excessive growth of aquatic plants, or cultural eutrophication. This excessive plant growth can reduce water quality when the plants eventually die. Bacteria are responsible for decomposing dead plants, during which they consume oxygen. This process reduces the dissolved oxygen in the water, possibly to the point where it is no longer capable of supporting aquatic life. Other deleterious effects of cultural eutrophication include reduced clarity, nuisance algal growth (some species which may produce toxins harmful to humans and wildlife), decreased species diversity, decreased food supply, and habitat destruction.

The “pure,” elemental form of P is rarely found in nature. Phosphorus most often exists within a phosphate molecule (PO4). Both inorganic and organic forms of phosphate can be found in aquatic systems. It is the inorganic form, orthophosphate (PO4 3- ) that is available to plants and animals. However, orthophosphate levels in water bodies fluctuate daily, because plants consume it quickly. Therefore, total phosphorus (TP) measurements are most commonly used. TP testing involves converting all forms of phosphorus that exist in a sample to the simpler inorganic form of orthophosphate. By doing this, the phosphorus contained in both organic and inorganic forms can be quantified (US EPA 2010).

We measure the total phosphorus of the water each time we collect data – once a month for five months out of the year (June – October). The bars represent the upper quartile (75% of the data taken at this site falls below this line), the median (or mid-point of the data) for each site, first quartile (25% of the data taken at this site falls below this line), and the box represents the middle 50% of the data. Please note that the data for this graph has been log-transformed for visual aid purposes.

Use our interactive graph to check out all the data we have collected since 2012. Hover over a data point to see more information or try clicking on the name of a month to see all the data for just one month! 

Some sources of excess phosphorus

  • Natural decomposition of rocks and minerals
  • Stormwater runoff and snow melt
  • Erosion and sedimentation
  • Atmospheric deposition
  • Direct input by animals/wildlife
  • Agricultural runoff
  • Detergents and cleaning fluids
  • Failing septic systems
  • Lake mixing (internal loading of phosphorus from sediment/water interface)
  • Wastewater treatment plants and permitted industrial discharge
  • Fertilizer

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