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Re: Aquatic Plants Digest V3 #123

Frank I. Reiter wrote:

>Last night I watched a program about nitrate levels in seven freshwater 
>springs which combine to form a good sized river in Florida.  As the divers 
>swam down this river they reached  point where suddenly the grasses were 
>covered in brown algae.  They said it was because at that point the river 
>was joined by a spring which was producing water with a high nitrate level, 
>a new phenomenon.  "High" in this case was apparently less than 10 ppm.
>Here on the list I know people that keep their nitrate levels elevated to 
>avoid algae.
>Can anyone suggest why nitrate levels which are low (and beneficial) in a 
>planted aquarium cause algae problems in this river?

Plants and algae grow when there are enough of *all* the necessary 
nutrients available. If one nutrient is not available in sufficient 
quantity, growth slows or stops. The environment in which that occurs is 
considered to be limited by the unavailable nutrient, hence terms like 
"phosphorus limited" or "nitrogen limited" which occasionally are 
utilised in discussions on this list.

>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. It's hard to say what is going on in the 
river without knowing what the other nutrient levels including phosphorus 

Now, the big difference between streams and aquariums. A plant in a 
stream is bathed in a constantly changing flow of water. It doesn't 
matter that nutrient levels are lower than we have in an aquarium 
provided the water keeps flowing, bringing a fresh supply of nutrients 
every second. In a tank there is a fixed and limited supply of water with 
no new supply of nutrients except those added by fish waste and the 
aquarist. Assuming nothing is being added, a plant in a stream which came 
into contact with 1,000 gallons of water a day would have the same 
nutrient exposure as a plant in a 10 gallon aquarium when the nutrient 
levels were 1% of the levels in the aquarium (basically because it's 
exposed to 100 times the water as the plant in the aquarium). Others have 
already made this observation in response to the question that started 
this thread but I hope that putting it in numerical terms like this might 
help anyone who hasn't grasped the point to understand it.

That is also the reason why a 10ppm nitrogen level in a river has an 
astronomically greater impact than a 10ppm nitrogen level in an aquarium. 
The infinitely greater volume of water means that there is an infinitely 
greater amount of nitrogen available. Since plant levels (including 
algae) per volume of water are much lower in a river than in an aquarium 
(we tend to have lots of plants in a relatively small volume of water 
while nature goes for large volumes of water with relatively lower 
amounts of plant mass), the impact of a given nutrient level is much 
greater in nature than it is in an aquarium, all other things being equal.

David Aiken