Your lake can have high phosphorus and run clear all summer. Understanding why is most of lake science.
Ask anyone who has spent time around lake management what causes algae blooms and you will hear one word. Phosphorus. It is the right answer and it is incomplete in ways that matter enormously when you are deciding what to spend money on.
Why phosphorus at all
Algae need many things to grow — light, warmth, carbon, nitrogen, phosphorus, trace metals. Growth is limited by whichever necessary ingredient runs out first. In most fresh water, that is phosphorus, because it is scarce naturally and everything else is comparatively abundant.
This is why phosphorus gets the attention. Add a little and you get a lot of algae. Remove it and growth is capped regardless of how much sunlight and nitrogen are available. It is the throttle.
The distinction almost nobody makes
Total phosphorus is not the same as available phosphorus, and confusing them causes real errors.
Much of the phosphorus in a lake is bound — locked into sediment particles, incorporated into living tissue, chemically attached to iron. Algae cannot use it. What they can use is dissolved reactive phosphorus, sometimes called orthophosphate, and it is often a small fraction of the total.
So a lake can test high on total phosphorus and stay clear, because most of that phosphorus is unavailable. And a lake can test moderate and bloom hard, because a large share of what is there is immediately usable.
This is why the total number alone is a poor predictor, and why a lab report showing "high phosphorus" is the beginning of a question rather than the end of one.
The other things that decide
Timing. Phosphorus arriving in April, when the water is cold and mixed, behaves entirely differently from the same amount arriving in July into a warm, stratified surface layer. Same mass, different outcome.
Nitrogen ratio. When phosphorus is abundant relative to nitrogen, conditions favour cyanobacteria, which can fix nitrogen from the atmosphere while other algae cannot. This is the mechanism that turns a nutrient problem into a toxin problem.
Physical conditions. Warm, calm, sunny, stratified water is what blooms need. A cool windy summer can suppress a bloom in a lake with plenty of phosphorus, which is why one good year proves nothing.
Where it comes from. Phosphorus arriving from the watershed and phosphorus released from your own sediment are different problems requiring different responses.
Internal loading, which explains the disappointments
This is the part that catches communities out.
When the bottom layer of a stratified lake runs out of oxygen — which happens in most productive lakes by late summer — the chemistry changes. Iron compounds holding phosphorus in the sediment break down and release it back into the water.
The consequence: a lake can have decades of accumulated phosphorus in its sediment, and every summer that bottom layer goes anoxic, some of it comes back. Which is why a community can do everything right in the watershed — fix septics, buffer the shoreline, reduce fertiliser — and see no improvement for years. The lake is feeding itself from its own history.
What to actually do with this
Measure total and reactive phosphorus rather than total alone, so you know how much is available. Measure at more than one depth if your lake stratifies, because a surface sample says nothing about what the bottom is doing. Measure across seasons, since spring and August tell different stories.
And be sceptical of anyone who looks at a single phosphorus number and proposes a treatment. That number does not contain enough information to justify one.
