While many aquarist are enamored with stating "plants out compete algae for nutrients", there is very little evidence that healthy growing plants are able to exist at the concentration of nutrients required to limit the growth of virtually any species of algae. Algae have extremely low demands for all nutrients and CO2 compensation, and this is also true for light in many cases (BBA for example does well at very low light). Additionally, allelopathy has been ruled out for aquariums (See Ole and my own comments there) and is easy to test for aquarist to see if there is any effect.
Why then, do aquarist not consider plant- plant competition when it's clear that they are far more similar in terms of the limiting factors? They have far more biomass, can affect light, CO2 and nutrients dramatically. This biomass can and does change dramatically between prunings in higher light CO2 enriched system, less so, but it still does occur in lower light non CO2 or Excel enriched system.
It seems that aquarist often forget that as plants grow through time, they have more biomass that demands more nutrients and CO2, light etc. If the aquarist has just enough CO2 to grow a set of species of plants with a certain total biomass, then doubles that biomass over 1-2 weeks, then the CO2 demand will be much greater.
The same is true with nutrients, with some variation and adaptive abilities from species to species, and with light capture, as many species behave very differently. Some sit and grow slower, some grow taller and thinner and then form a canopy at the surface(Hydrilla, H polysperma etc, see references below).
I have to wonder why aquarist ignore this aspect and harp on algae out competing plants, or plants out competing algae so much? Much of the complaints on the forums tend to be specific to the tips or nutrients, or CO2 related issues, with less attention paid to plant- plant interactions.
I also find it interesting that for non CO2 aquarist, have by and large, ignored this, suggesting that some plants just do not grow for some unknown reason(we know it's not alleopathy that's significant). If CO2 is very limiting, a plant even with a small advantage with respect to CO2 compensation points translates into a really large significant different in which plant will dominate and which will slowly fade out.
Here's some references:
ScienceDirect - Aquatic Botany : Interference competition between Ludwigia repens and Hygrophila polysperma: two morphologically similar aquatic plant species
This is an old reference, but shows differences between light and CO2 and CO2 max photosynthetic rates:
http://www.apms.org/japm/vol23/v23p7.pdf
Comparison of the Photosynthetic Characteristics of Three Submersed Aquatic Plants -- Van et al. 58 (6): 761 -- PLANT PHYSIOLOGY
Hydrilla can photosynthesize in the eraly morning at a lower light level, thus acquire all the low levels of CO2 first, before the other species.
The first paper also shows how temperature influences growth rates and CO2 demand.
For us, it tends to increase it in the 20-30C ranges.
Cooler temps seems to slow it down.
Some plants likely do not show this same temp relationship also.
So that can play a role also.
This suggest that there are many other mechanism than these old tired parroted web site aquarium plant hobby myths. We can observe and see that it's not just a function of the CO2, light and nutrients, or some unknown allelopathic chemical.
We should try and rule those out as best as we reasonably can, then look at these other factors influencing growth and plant-plant interactions.
We can often observe that some plants do very well, whereas a certain species or a few of them do poorly. These references could explain this, and differential abilities to acquire CO2 and light, metabolic rates based on temperature and other species to species factors likely play a much more significant role.
Regards,
Tom Barr
Why then, do aquarist not consider plant- plant competition when it's clear that they are far more similar in terms of the limiting factors? They have far more biomass, can affect light, CO2 and nutrients dramatically. This biomass can and does change dramatically between prunings in higher light CO2 enriched system, less so, but it still does occur in lower light non CO2 or Excel enriched system.
It seems that aquarist often forget that as plants grow through time, they have more biomass that demands more nutrients and CO2, light etc. If the aquarist has just enough CO2 to grow a set of species of plants with a certain total biomass, then doubles that biomass over 1-2 weeks, then the CO2 demand will be much greater.
The same is true with nutrients, with some variation and adaptive abilities from species to species, and with light capture, as many species behave very differently. Some sit and grow slower, some grow taller and thinner and then form a canopy at the surface(Hydrilla, H polysperma etc, see references below).
I have to wonder why aquarist ignore this aspect and harp on algae out competing plants, or plants out competing algae so much? Much of the complaints on the forums tend to be specific to the tips or nutrients, or CO2 related issues, with less attention paid to plant- plant interactions.
I also find it interesting that for non CO2 aquarist, have by and large, ignored this, suggesting that some plants just do not grow for some unknown reason(we know it's not alleopathy that's significant). If CO2 is very limiting, a plant even with a small advantage with respect to CO2 compensation points translates into a really large significant different in which plant will dominate and which will slowly fade out.
Here's some references:
ScienceDirect - Aquatic Botany : Interference competition between Ludwigia repens and Hygrophila polysperma: two morphologically similar aquatic plant species
This is an old reference, but shows differences between light and CO2 and CO2 max photosynthetic rates:
http://www.apms.org/japm/vol23/v23p7.pdf
Comparison of the Photosynthetic Characteristics of Three Submersed Aquatic Plants -- Van et al. 58 (6): 761 -- PLANT PHYSIOLOGY
Hydrilla can photosynthesize in the eraly morning at a lower light level, thus acquire all the low levels of CO2 first, before the other species.
The first paper also shows how temperature influences growth rates and CO2 demand.
For us, it tends to increase it in the 20-30C ranges.
Cooler temps seems to slow it down.
Some plants likely do not show this same temp relationship also.
So that can play a role also.
This suggest that there are many other mechanism than these old tired parroted web site aquarium plant hobby myths. We can observe and see that it's not just a function of the CO2, light and nutrients, or some unknown allelopathic chemical.
We should try and rule those out as best as we reasonably can, then look at these other factors influencing growth and plant-plant interactions.
We can often observe that some plants do very well, whereas a certain species or a few of them do poorly. These references could explain this, and differential abilities to acquire CO2 and light, metabolic rates based on temperature and other species to species factors likely play a much more significant role.
Regards,
Tom Barr