Hi all,
Do floating plants reduce the surface area of the water, thus reducing the potential dissolved oxygen levels? I'm not sure if this a stupid question or not
It is sensible question and the simple answer is
yes, they do to both questions.
You do need to go a little bit deeper, and make some attempt to quantify the different factors that reduce, or increase, dissolved oxygen levels.
When plants are actively photosynthesising they produce oxygen from:
carbon dioxide + water + light energy → glucose + oxygen + water
(6 CO2(gas) + 12 H2O(liquid) + photons → C6H12O6(aqueous) + 6 O2(gas) + 6 H2O(liquid).
In floating plants most of this oxygen will be lost to the atmosphere, although some will go into solution.
All plants will also take up NH3, NO2 and NO3, and any NH3 and NO2 that doesn't go into the nitrification (which is an oxygen intensive process) will help maintain dissolved oxygen levels.
The demand for oxygen of the water/fish/tank/microbes/plant system can be described by its Biochemical Oxygen Demand (BOD), but unfortunately we can't easily measure this, although we can estimate it.
As a general rule tanks with plants will maintain higher dissolved oxygen levels and better water quality even when the plants aren't photosynthesising (at night), until the plant biomass significantly reduces both flow and surface area.
Other than photosynthesis the major way that oxygen enters the water column is through diffusion at the gas exchange surfaces. Because diffusion occurs over a gradient, we need a water surface that is as large as possible.
We can make the active gas exchange surface of the tank larger by having a rapid turn-over of water, so that there isn't time for the surface layer to become saturated with oxygen before "new" water replaces it at the surface.
Ideally we would like to make the surface much, much larger, which is what a wet and dry trickle filter does. If we used this type of trickle filter, we could to a large degree ignore all the other variables.
As a rule of thumb, in a tank for rheophilic fish, that doesn't have a trickle filter, you need to make sure that the floating plants/leaves don't cover more than 1/2 the surface area in any tank.
As the BOD increases (larger fish load per tank volume, carnivorous fish with a high protein diet), we need to try and make both flow and gas exchange surface as large as possible, whilst also ideally maintaining a large plant biomass. These competing requirements may mean that some plants need to be in a separate sump, possibly with a reversed lighting regime.
This is covered in more detail in: <
http://plecoplanet.com/?page_id=829>
cheers Darrel