The Main Points
- Sedimentary phosphorus occurs in several forms but when released into the water body can comprise a substantial portion of the total phosphorus load.
- Sedimentary phosphorus can have long-term effects on eutrophication and the growth of algae and plants.
- Measuring sedimentary phosphorus is an important step in the management of the internal load of phosphorus.
Phosphorus entering lakes and rivers can be broadly divided into two pathways. The external load originates in stormwater runoff, septic tank or sewer seepages, aerial deposition and other external sources. The internal load originates from the phosphorus trapped in the lake sediments. The internal load is further divided into organic and inorganic phosphorus. Inorganic forms include the water-soluble bio-available phosphorus as well as several insoluble forms. The organic phosphorus is the material that is incorporated into the tissues of plants and animals. Leaves and other detritus that sinks to the bottom of the lake are the source of this material.
Phosphorous trapped in the sediments may have little or no effect on the day-to-day growth of algae and plants, but it can have profound long-term effects. For example, when dissolved oxygen levels are low, inorganic phosphorus can be released from the internal load. Sediment disturbance (i.e. wave action, fish nesting and zooplankton migration) can also result in a release of phosphorus. Even if the entire external load could somehow be controlled, the internal load would continue to release phosphorus for many decades.
For example, in one New Jersey lake, each square meter of sediment was found to be releasing of six milligrams of phosphorus every day. Over the course of a year, the release added up to several hundred kilograms of phosphorus and comprised a substantial portion of the total load.
Measuring the sedimentary phosphorus provides much useful data. It can help to anticipate releases from the internal load, identify phosphorus hot spots for remediation, provide an overview of phosphorus distributions, and help select the most effective remediation measures.