Different Types of Phosphorus

Phosphorus is a common pollutant in surface waters. When phosphorus concentrations are too high, it causes excessive growth of algae. But different forms of phosphorus have different impacts on water quality in Lake Erie.

What is dissolved phosphorus?

Dissolved phosphorus is the phosphorus that remains in water after that water has been filtered to remove particulate matter. Phosphorus attached to the particulate matter that remains on the filter is called particulate phosphorus. Together these two forms of phosphorus make up the total phosphorus concentration in a water sample. In laboratories, water samples are typically analyzed for total phosphorus and dissolved phosphorus. Particulate phosphorus is calculated by subtracting dissolved phosphorus from total phosphorus

Why is dissolved reactive phosphorus such a problem for Lake Erie?

Dissolved phosphorus is a special problem because:

  1. it is highly “bioavailable” to algae (e.g., it supports rapid algal growth and reproduction),
  2. the amounts or loads of dissolved phosphorus entering Lake Erie have been increasing dramatically in recent years, and
  3. dissolved phosphorus remains in the water while particulate P settles to stream and lake bottoms where it may no longer be available to algae.

For example, during storm runoff events from the Maumee River, large volumes of river water move into the western basin of Lake Erie in a short amount of time, carrying eroded soil and nutrients with it. Sediment and particulate phosphorus settle to the bottom rather quickly as runoff waters enter the lower river and Maumee Bay. Dissolved phosphorus, by contrast, remains in the water column, where it supports the development of algal blooms.

About 95% of dissolved phosphorus is bioavailable to algae, while only about 30% of the particulate phosphorus attached to eroded sediment is bioavailable. Even though particulate phosphorus dominates total phosphorus loading to Lake Erie from the Maumee and other Northwest Ohio rivers, dissolved phosphorus contributes more bioavailable phosphorus. While loads of particulate phosphorus have been trending downward in recent years, loads of dissolved phosphorus are increasing. Learn more about the dissolved reactive phosphorus problem (PDF).

Different kinds of phosphorus need different kinds of management

The characteristics of particulate phosphorus and dissolved phosphorus are so different that they should undergo separate management programs, rather than being linked as part of a total phosphorus management program. Particulate phosphorus and dissolved phosphorus tend to come from different sources; respond differently to rainfall and stream flow; and require different interventions to control them at their sources. Learn more about phosphorus management at Heidelberg University’s NCWQR (PDF).

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