# UsuallyP,but sometime N



## BruceF (Aug 5, 2011)

Sextant Home

01/15/02

The Chemical Properties of Water
http://www.marietta.edu/~mcshaffd/aquatic/sextant/chemistry.htm
Phosphorous (P) and nitrogen (N) are critical to plant growth, and they (usually P, but sometimes N) are often limiting factors to plant growth. Before you object, recalling that dissolved nitrogen is common in water, remember that it is dissolved nitrogen gas, N2, which is inert and cannot be used by most plants. The exception here are the cyanobacteria, which can fix N2 in the heterocysts, which provide a local anoxic environment for the nitrogen-fixing enzymes (and bacteria in anoxic root nodules of legumes and other anoxic places in the soil). Bodies of water with a low N/P ratio are thus prone to blooms of cyanobacteria.

Phosphorous is often the limiting factor for plant growth in aquatic systems. In many aquatic habitats, it is impossible to measure any free phosphate in the water when heavy algal growth (blooms) are in progress, mostly because the algae use up the phosphate as soon as it becomes available. Many algae store inclusions of phosphorous (when available) in their cells as a "hedge" against later phosphate shortages. Tremendous algal blooms are a symptom of eutrophication, a natural process that occurs as lakes age and accumulate phosphorous. Such huge blooms of algae can cause problems when they die and decompose, or when, particularly on hot, still summer nights, their respiration uses up all the O2 in the water.


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## Zapins (Jul 28, 2004)

Nice looking website with a lot of well explained ideas.

While the bit about phosphorous may be true in local ponds this doesn't mean there is anything special about phosphorous that triggers an algae bloom. It just so happens that phosphorous is the limiting nutrient in nature and when it is replaced the plants and algae are no longer restricted by it.

The same exact algae bloom could be triggered if the waters were nitrogen deficient, or iron deficient etc... the algae could only grow as fast as the limiting nutrient is provided.

What complicates the story for me is that when all nutrients are provided in unlimited supply you don't see all species of algae growing at the same time in an aquarium. Even stranger is that when you culture one species of algae in the lab you provide the same unlimited nutrient conditions that we do in our tanks and the algae grows just fine, so algae clearly likes those conditions. What exactly causes a species of algae to bloom and others not to is the mystery that we still (unfortunately) don't know.


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## BruceF (Aug 5, 2011)

Yes I found it well written, My interest has to do with the fact that I have repeatedly found that adding P will cause green spot algae to disappear and improve the tank conditions. 

I can't tell you why that is. I just know that when I see this algae starting on the glass I often will increase the P and things will improve. I am talking about adding 1 whole teaspoon of Fleet to a 10 gallon tank. 

The other part about the cyano I think is also true. Though in that case it might help to add both of them. N and P. 

Anyway it is a good page for the basic review.


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