Internal phosphorus loading: a guide for Florida stormwater managers

How a nutrient trap can become a nutrient source

Wet detention ponds remove phosphorus mainly by letting particles settle and by biological uptake. Over years, that settled material builds up as a layer of organic, phosphorus-rich sediment. The phosphorus doesn't disappear. It's stored.

Some of that stored phosphorus is held on iron minerals that stay put only while there is oxygen at the sediment surface. When bottom water goes anoxic, those bonds break and phosphate dissolves back into the water. Decomposing organic matter in the sediment can release phosphorus too. Warm water speeds both processes up and holds less oxygen.

Shallow does not mean well mixed. In a study of Minnesota stormwater ponds, four of seven ponds sampled closely were strongly stratified with persistent bottom-water anoxia, even though their mean depths were under 2 meters. Across 98 ponds in the same study, nearly 40% had median summer total phosphorus above the average concentration of stormwater runoff, which suggests the ponds were adding phosphorus rather than only removing it.

Is there evidence from ponds like ours?

The direct evidence is growing, though much of it comes from outside Florida, so we present it as a warning sign, not a verdict on any one pond.

• In nine coastal residential stormwater ponds in South Carolina, the oldest ponds (over 15 years) had the highest phosphorus and nitrogen, and all of them behaved as phosphorus sources to the water. Sediment in nearly all ponds, regardless of age, released phosphorus in the authors' measurements.
• In coastal urban stormwater pond networks in northwest Florida, researchers documented extensive bottom mats of cyanobacteria and multiple toxin classes, including cylindrospermopsin above national recreational guidelines. They suggested the ponds may episodically export toxins to estuarine waters.
• In Lake Okeechobee, a classic Florida study found that sediment phosphorus release rose sharply under low-oxygen conditions and that internal loads were roughly equal to external loads.

The picture is not one-sided. A study of three mature Minnesota detention ponds found they still retained about half or more of incoming phosphorus over a year, despite concerns about internal loading. The events where ponds exported nutrients were driven mostly by hydrology, when there wasn't enough storage to hold the storm. That matters for diagnosis: a pond can have an internal loading problem in summer and still look acceptable on an annual average.

Why routine grab samples miss it

Internal loading is episodic. It switches on when the bottom goes anoxic and can switch off after a wind event or a storm mixes the pond. A quarterly or monthly surface sample usually lands in between.

In our team's peer-reviewed study comparing single snapshots with a continuous record, one water sample misread what the pond bottom was doing 79% of the time. In 48% of cases it got the direction wrong, calling a releasing bottom a retaining one or the reverse. Compliance-style sampling isn't designed to catch this, and it often doesn't.

How to assess a pond for internal loading

Matching the fix to the cause

Once you know where the phosphorus comes from and when, the options sort themselves out. Each fits a different situation.

• Sediment removal (dredging): removes the stored phosphorus outright and restores treatment volume. It's often the most expensive option and requires permitting and disposal planning, but it addresses the root cause in an old, filled-in pond.
• Phosphorus inactivation (alum or lanthanum-modified clay): binds mobile phosphorus at the sediment surface. The dose must come from measured mobile sediment phosphorus, not a rule of thumb, and pH, alkalinity and water color all affect performance.
• Aeration or oxygenation: keeps the sediment surface oxygenated, which can limit iron-bound phosphorus release. It does less for phosphorus released from decomposing organic matter, which the Minnesota sediment work found also contributes in stormwater ponds.
• Vegetation management: healthy littoral plants take up nutrients and stabilize sediment. Large die-offs, including plants and algae killed by treatments and left to rot, add organic matter and consume oxygen.
• External load control: none of the in-pond fixes last if phosphorus keeps arriving from fertilizer, grass clippings, soil erosion or failing septic systems upstream.

Why it matters for BMAPs, TMDLs and permits

In Florida, a Basin Management Action Plan (BMAP) is the state's framework for reaching the pollutant reductions set by a Total Maximum Daily Load (TMDL), and stormwater improvements are among the projects local governments use to get there. If a pond is counted on to remove phosphorus but releases it in summer, the real load reduction may be smaller than the number on paper.

Florida also updated its statewide stormwater rule in 2024, with performance-based design criteria for new systems and stronger operation, maintenance and inspection expectations. Check FDEP's guidance for what applies to your permits. Either way, knowing whether an existing pond is a sink or a source is a sound basis for deciding where maintenance and capital dollars go.

We work independently: we guide you through working with the right vendor and take no referral fees. We measure the water and the sediment over time, using satellite, logger and lab data, to show which ponds are actually releasing phosphorus and when. For a quick first look at a pond, get a free read, or talk to a scientist: 573-WATERDX about a portfolio of ponds.