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Re:Nutrients vs. algae

Another reason not to limit nutrients, is that when there is low N 
and/or low P, soft, attached algae---the kind that ramshorn snails 
and pond snails eat---becomes much less palatable for the snails.  I 
have seen several times that I get a bluegreen algae build-up when N 
and P are limiting growth.  The snails don't seem to be able to eat 
the algae under these conditions, and they can actually die out and 
become extinct.  If I add N and P, there is a big rapid response in 
the algae, which has been sort of lurking.  But, there is also a big 
response in the snails.  They start growing rapidly and producing 
plenty of eggs. It takes some courage to keep N and P levels up after 
you see the response of the algae to the nutrients, but if you do, 
the snails will eventually eat up all of the algae.

Conclusions----Yes, adding N and P does stimulate the growth of 
soft-attached algae, but the algae becomes nutritious for the snails, 
and the snail population will get the algae under control.  The soft 
attached algae is able to get established when its nutritional 
content is so low that snails can't get any benefit eating it.  Also, 
the algae may become actually toxic for the snails.  BGA is known to 
produce a number of toxins, and these toxins may become much more 
concentrated in the algae when they are N and P limited.  Not only 
may the algae be more nutritions for the snails when N and P are 
higher, but also the toxin content may be quite a bit lower.

Disclaimer---I am only talking about soft, attached algae, not green 
water or hair algae that is too tough for snails to eat.

  Paul Krombholz in central Mississippi, having cool fall weather.