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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
>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.