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Art said:

Good plant growth = algae limitation BECAUSE:

1) Good plant growth results in limiting a key nutrient for algae; or
2) Good plant growth produces some sort of allelochemical that limits algae?
There is a third possibility---Good nutrient levels make nutritious algae, and the animals that eat the algae thrive and multiply and eat it all up. Poor nutrient levels result in algae so low in nitrogen that herbovires get no food value by eating it. Also, when nutrient levels are low, the algae may be able to put more of its photosynthetic effort into producing toxins that protect it.

It is important to distinguish what kind of algae we are talking about. Green water, Cyanobacteria, Red algae, Oedogonium, Cladophora, Rhizoclonium, green spot algae, etc.
 

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I have seen that various forms of bluegreens (Cyanobacteria) get started when nitrogen is low. They are not very green and are not so noticable. Snails are on the decline while this is happening. If I give a good shot of nitrients, the bga suddenly becomes very visible as it greens up, but then the snails start perking up, growing and eating it. Somewhere, a long time ago, I remember seeing a paper that claimed that many types of algae have a very wide range of nitrogen content, depending on nitrogen availability, ranging from too low to be of food value to quite high in protein.
 

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I have seen that, when N and P are at very low or undetectible levels, various kinds of algae get going almost secretly because they are yillowish or brownish colored. When N and P are added the algae greens up and suddenly become much more visible. When N and P are very low, snails decline even when the soft, surface-coating types of algae are on the increase. When things green up, the snails start growing and multiplying again. I would like to advance the hypothesis that snails play an important part in the control of soft alage---not hair algae---and that when N and P are low, the algae is so low in nutritive value that the snails starve to death eating it. With N and P high, the algae is nutritious, allowing the snails to multiply and eventually eat it all up.
 
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