Phosphorus - Understanding Lake Data

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Phosphorus promotes excessive aquatic plant growth. In more than 80% of Wisconsin's lakes, phosphorus is the key nutrient affecting the amount of algae and weed growth.

Phosphorus originates from a variety of sources, many of which are related to human activities. Major sources include human and animal wastes, soil erosion, detergents, septic systems and runoff from farmland or lawns.

Phosphorus provokes complex reactions in lakes. An analysis of phosphorus often includes both soluble reactive phosphorus and total phosphorus.

Soluble reactive phosphorus dissolves in the water and readily aids plant growth. Its concentration varies widely in most lakes over short periods of time as plants take it up and release it.

Total phosphorus is considered a better indicator of a lake's nutrient status because its levels remain more stable than soluble reactive phosphorus. Total phosphorus includes soluble phosphorus and the phosphorus in plant and animal fragments suspended in lake water.

CONCENTRATION UNITS express the amount of a chemical dissolved in water. The most common ways chemical data is expressed is in milligrams per liter (mg/l) and micrograms per liter (ug/l). One milligram per liter is equal to one part per million (ppm). To convert micrograms per liter (ug/l) to milligrams per liter (mg/l), divide by 1000 (e.g., 30 ug/l = 0.03 mg/1). To convert milligrams per liter (mg/l) to micrograms per liter (ug/l), multiply by 1000 (e.g., 0.5 mg/l = 500 ug/l). Microequivalents per liter (ueq/l) is also sometimes used, especially for alkalinity. It is calculated by dividing the equivalent weight of the compound by 1000 and then dividing that number into the milligrams per liter.

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Ideally, soluble reactive phosphorus concentrations should be 10 ug/l (micrograms per liter) or less at spring turnover to prevent summer algae blooms. A concentration of 10 micrograms per liter is equal to 10 parts per billion (ppb) or 0.01 milligrams per liter (mg/l). A concentration of total phosphorus below 20 ug/l for lakes and 30 ug/l for impoundments should be maintained to prevent nuisance algal blooms (Figure 4).

FIGURE 4. Total phosphorus concentrations for Wisconsin's natural lakes and impoundments. (Adopted from Lillie and Mason, 1983.)(Exit DNR)

Figure 4 - Index scale Very Poor vs 150 ug/l Total Phos. etc.

Phosphorus does not dissolve easily in water. It forms insoluble precipitates (particles) with calcium, iron, and aluminum. In hard water areas of Wisconsin, where limestone is dissolved in the water, marl (calcium carbonate) precipitates and falls to the bottom. Marl formations absorb phosphorus, reducing its overall concentration as well as algae growth. Aquatic plants with roots in the marl bottom still get phosphorus from sediments. Hard water lakes often have clear water, but may be weedy.

Iron also forms sediment particles that store phosphorus-but only if oxygen is present. When lakes lose oxygen in winter or when the deep water (hypolimnion) loses oxygen in summer, iron and phosphorus again dissolve in water. Strong summer winds or spring and fall turnover may mix iron and phosphorus with surface water. For this reason, algae blooms may still appear in lakes for many years even if phosphorus inputs are controlled.

Figure 5 shows the increase in total phosphorus for stratified lakes following fall turnover. Since shallow and windswept lakes that stay mixed do not experience oxygen depletion, they have the highest total phosphorus levels in summer following spring turnover and early summer runoff.

FIGURE 5 Seasonal total phosphorus averages for six lake types by season. (Adapted from Lillie and Mason, 1983)(Exit DNR)

Figure 5 - Total Phosphorus ploted in vertical bar graph of 4 seasons by lake type

The amount of iron that might react with phosphorus varies widely in Wisconsin lakes. Lakes in the southern part of the state are often low in iron due to a higher pH and more sulfur, both of which limit iron solubility. This in turn affects whether phosphorus mixed into lakes during fall turnover precipitates or stays in solution during the winter.

Lakes with low iron and insufficient calcium to form marl are most likely to retain phosphorus in solution once it is released from sediments or brought in from external sources. These lakes are the most vulnerable to naturally occurring phosphorus or to phosphorus loading from human activities because the phosphorus remains dissolved in the water-not pulled down into the sediments. Such lakes often respond with greater algae problems.

Figure 5 also shows that impoundments have the highest phosphorus levels. Mixed drainage lakes sustain intermediate levels, while seepage and stratified drainage lakes have the lowest. Even with the potential for internal phosphorus cycling caused by oxygen depletion, deep stratified lakes tend to have lower phosphorus levels than their mixed counterparts.

Phosphorus control has been attempted in some lakes by using alum (aluminum sulfate) to precipitate phosphorus. Sewage treatment plants use the same process to remove phosphorus. Aluminum phosphate precipitate, unlike iron phosphate, is not redissolved when oxygen is depleted.

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212


Lakes Partnership | Watershed Management | Fish Wisconsin | Fisheries Management

Last Revised: Thursday January 25 2007