LancsRick said:
Where my plants have grown within about 4 inches of the surface, I'm now seeing hair algae develop on their leaves. The rest of the tank is fine.
Am I right in assuming that this is due to the PAR at that depth of water becoming such that algae is thriving as the leaves at that level cannot react fast enough?
Yes, I partially concur with this analysis. Photon flux varies according to the inverse square rule so that, for example, if a bottom leaf is twice as far away from the bulb than a top leaf, then that bottom leaf is exposed to 4 times
less PAR than the top leaf. It is therefore much easier for the bottom leaf to deal with less light than for the top leaf, which will need to process a proportionally higher amount of CO2 in order to avoid being damaged by that 4X higher level of radiation. So it is not so much that the top leaves cannot react fast enough, but that you do not have enough CO2 at the top of the plant to satisfy the amount that is demanded by those leaves based on the radiation flux.
LancsRick said:
If so, I'm confused as to why it's so localised, since I'd expect the plant to react more as a whole instead of differently at levels?
If you suffer an injury on your left hand, do you feel it on your right hand? Light does two things to plants; it provides energy for food production and at the same time causes radiation damage to the tissues. If there is enough CO2 and nutrition available then the plant can produce enough food to repair itself and add higher levels of protection to overcome the damage caused by the radiation. If there is not enough nutrition the rate of damage exceeds the rate of food production. The lower leaves must produce enough food to keep themselves healthy as well as to produce food and oxygen for transport to the roots down below. They cannot spare enough resources to send to the leaves above. Those leaves must fend for themselves. If not, then they are sacrificed because the overall health of the plant is more important. If you see the algae on all levels of the plant then that means there is poor CO2 at all levels, not just at the top.
This is why there is so much failure with planted tanks, because people automatically assume that light is the only important thing and that it makes plants grow healthy. A plant does not grow because of light. It grows and is healthy because of food. The light is an oven or a stove top that powers the reactions that make the food, but at the same time it is dangerous and if it is not fed with the other important compounds necessary to complete the process of food production it will be out of control and will annihilate the plant. This is why few plants can grow in a desert, because there is too much light and not enough water. For aquatic plants which have an abundance of water, there is too much light and not enough CO2.
Cheers,
. Will that suffice or is it purely the PAR I'll need to worry about?