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Re: Nitrogen enrichment

>Date: Sat, 7 Mar 1998 06:59:04 +1000
>From: David Aiken <d.aiken at eis_net.au>
>Subject: Re: Aquatic Plants Digest V3 #123
>>From your description, the water in the river was obviously (as far as 
>the algae were concerned) nitrogen limited until they got the water from 
>the new stream coming in. Once that happened algae were able to really 
>get a go on - prior to that the plants had the upper hand in utilising 
>all of the nutrients to their advantage.
>Two major nutrients for plants and algae are phosphorus and nitrogen. The 
>experience or people on this list and elsewhere seems to be (gross 
>simplification) that phosphorus limitation is more important than 
>nitrogen limitation in avoiding algae. In other words, you can have 
>higher nitrate levels without algae if you have extremely low to 
>unmeasurable phosphorus levels. 

I would like to add a few things to David's nice explaination. 

Nitrogen limitation can occur in aquaria as in Rivers. An aquarium with a
lot a fast growing plants and a limited addition of fish food can become
nitrogen limited... this can mean 0 nitrates and not enough ammonia to
allow the plants to substantially multiply.  On the other hand, phosphates
are usually in sufficient supply because fish food supplies lots and
phosphates are available both directly from the water and from the
storehouse of the substrate (especially in substrates of tanks with
laterite, soil whose iron binds it and allows it to accumulate). Therefore,
extremely low or even non-detectable concentrations of phosphates are
sufficient to supply plants and feed algae. 

Algae are different from plants in that they depend exclusively on
phosphates in the water. So when the tank (or river) is nitrogen limited,
supplying nitrogen (assuming everything else is sufficient) speeds up the
growth rate of both plants and algae... In an aquarium, however, the more
quickly growing plants will soon pull enough phosphates out of the limited
amount of water to reduce the water column concentration .... which in turn
will starve the algae. Now the plants have the upper hand, they will
continue to suck phosphates and other nutrients, which help keep the algae
from reestablishing.

As David describes, this does not happen with nitrogen enrichment in the
nitrogen limited river whose moving water provides an effectively larger
supply of nutrients.....

Although what David says is true --
>a 10ppm nitrogen level in a river has an 
>astronomically greater impact than a 10ppm nitrogen level in an aquarium.....

...it may not be the larger AMOUNT of available nitrogen or the increased N
to P ratio, but instead a new ADEQUATE supply of nitrogen to the river.
This, in combination with the larger effective (essentially infinite)
supply of phosphates in the moving water (say, from the upstream sediment)
provide the necessary macronutrients to continue to feed the algae. 

Neil Frank, AGA