Water Quality

Internal Phosphorus Loading

The release of phosphorus stored in lake-bottom sediments back into the water column, often triggered by low oxygen conditions in deep water.

What It Means for Your Lake

Internal phosphorus loading is the process by which phosphorus that has accumulated in lake-bottom sediments over years or decades is released back into the overlying water column, fueling continued algae growth even after external nutrient sources have been reduced. This phenomenon is one of the most challenging obstacles in lake restoration and explains why many lakes remain impaired long after watershed management improvements. The primary mechanism driving internal loading is the change in sediment chemistry under anoxic (oxygen-free) conditions. In oxygenated water, iron compounds in the sediment bind phosphorus in an insoluble form, effectively locking it in the lake bottom. When dissolved oxygen in the hypolimnion drops below approximately 1 mg/L, a common occurrence during summer stratification in eutrophic lakes, the iron-phosphorus bond breaks, and dissolved reactive phosphorus diffuses out of the sediments into the water column. This phosphorus becomes available for algae growth when it reaches the photic zone, either through diffusion across the thermocline, wind-driven mixing events, or during fall turnover. In severely impacted lakes, internal loading can exceed external loading from the entire watershed, delivering hundreds or thousands of kilograms of phosphorus per year. Research on Twin Cities area lakes has shown that internal loading can sustain poor water quality for 10 to 30 years after external sources are controlled. The most common treatment for internal loading is aluminum sulfate (alum) application, where liquid alum is applied to the lake surface and settles to the bottom, forming a floc that permanently binds phosphorus in the sediments. Alum treatments have been used successfully on dozens of Minnesota and Wisconsin lakes, with typical benefits lasting 10 to 20 years before retreatment is needed.

Frequently Asked Questions