Why your pond algae keeps coming back (it's the bottom)

Where does the phosphorus come from?

Phosphorus is the nutrient that limits algae growth in many freshwater ponds and lakes. It washes in with stormwater, lawn fertilizer, grass clippings, leaves, pet waste, and eroded soil. Very little of it stays in the water for long. It settles to the bottom with dead algae and soil particles, and over the years the bottom becomes a phosphorus bank.

Scientists call release from that bank internal loading, to separate it from the external loading that washes in from the land. A widely cited review of shallow lakes found that how long a lake takes to recover after outside inputs are cut depends on how much phosphorus has built up in its sediment. Some lakes kept giving off more phosphorus than they took in for decades.

Why does the bottom release phosphorus in summer?

Much of the phosphorus in pond mud is held by iron. While there is oxygen at the surface of the mud, the iron holds on tightly. When the oxygen runs out, the iron chemistry changes and phosphorus dissolves into the water just above the bottom.

Low oxygen at the bottom is common in warm months. Warm water holds less oxygen, and microbes breaking down dead algae and leaves use it up faster. On hot, still days a warm layer can sit on top of cooler bottom water, so the bottom never gets fresh oxygen. Even shallow ponds can layer like this for hours or days at a time.

Classic lake research found that lakes whose bottom water goes without oxygen hold on to far less phosphorus than lakes whose bottoms stay oxygenated, and linked the difference to release from the oxygen-free mud. When wind, a storm, or a cold front mixes the pond, that phosphorus reaches the sunlit surface, and the next bloom follows.

Why do algaecides make the problem come back?

Copper and other algaecides kill algae cells. They do not remove phosphorus. The dead cells sink and decompose, and the nutrients they held go back into the water or onto the bottom, ready for the next round of growth.

Decomposition also uses oxygen. A large die-off can pull oxygen down near the bottom, which is exactly the condition that releases more phosphorus. In some ponds this sets up a cycle: treat, die-off, low oxygen, release, regrowth, treat again.

Algaecides can have a place, for example short-term relief before an event. But when the source is the bottom, they become a recurring cost rather than a fix.

Why one water test can mislead you

The usual next step is a water sample. A water sample tells you what is in the water at that moment. It does not tell you where the nutrients came from or what the bottom is doing, and bottom release can swing from hour to hour and from day to night as oxygen and temperature change.

Our team's peer-reviewed study measured nutrient exchange between a stormwater pond's bottom and its water continuously, then compared that record with what single snapshots would have shown. One water sample misread what the pond bottom was doing 79% of the time. In 48% of cases a snapshot got the direction wrong, calling the bottom a source when it was soaking nutrients up, or the reverse. And 31% were off by more than half.

That doesn't make water tests useless. It means one sample can't answer the question that matters most: where is the algae's food coming from?

What should you do instead?

Start with a diagnosis. Is the phosphorus coming mostly from the watershed, mostly from the bottom, or both? The answer decides the fix.

• Mostly the watershed: the work happens upstream. Shoreline buffer plantings, fertilizer-free zones, keeping clippings and leaves out of storm drains, and fixing erosion. In-pond treatments wear out quickly if new phosphorus keeps arriving.
• Mostly the bottom: options target the sediment, such as phosphorus binders like alum or lanthanum-modified bentonite, oxygen management, or in some cases dredging. Each fits different ponds, and binders tend to last longer once outside inputs are under control.
• Both: the most common case. Bring the watershed under control first, then address the bottom, or the bottom simply refills.

Telling these apart means measuring the water and the sediment over time, not once. That includes how much phosphorus is stored in the mud and how much of it can move (see sediment phosphorus testing), oxygen and temperature near the bottom through the warm season, and the timing of runoff.

This is the work we do at WaterDx. We are independent: we guide you through working with the right vendor and take no referral fees, so the recommendation follows the evidence. We measure water and sediment, read the water body over time, and combine satellite and lab data, in freshwater ponds as well as saltwater and brackish water.

For a starting point, get a free read of your pond from satellite data, no account needed (water about 1 acre and up). To measure your water and your pond bottom, Our $179 Pond Health Check is a mail-in kit, ordered by phone or email. If you'd rather talk it through, talk to a scientist: 573-WATERDX.