04-16-2010, 01:12 PM
| || | I went into this topic in Part 3 of the series on low-tech planted aquaria, but it is apropos to this discussion. First, a summary of respiration taken from Peter Hiscock since I can't say it better:
The process of respiration occurs in all complex organisms and takes place in all plant cells. Respiration helps to break down food sources and release energy into the cells. During the process, oxygen is used up and carbon dioxide is released as a by-product. The chemical equation for the process of respiration is the exact reverse of photosynthesis, except that sunlight energy is not involved. Unlike photosynthesis, respiration is a continual process that does not stop at night. Thus, photosynthesis stores food "energy," whereas respiration releases energy.In any 24-hour period, the plants release much more oxygen through photosynthesis than they use in respiration. Fish and bacteria are also using oxygen continuously, and more oxygen is used by bacteria than fish (assuming the aquarium is balanced); and this refers to the aerobic bacteria living in the substrate that are essential in the conversion of organics into nutrients for assimilation by the plant roots. And while it may be possible for oxygen to become depleted to critical levels during darkness, this is only going to occur in aquaria that are way over-stocked and not balanced or have supplemental CO2 systems. In a balanced low-tech aquarium, this is not going to be an issue. In high-tech planted tanks, it is much more of a concern which is why CO2 systems should be off during darkness. With respect to water movement, plants require some in order to bring nutrients to the leaves and roots, but too strong a current is detrimental to their assimilation of carbon via CO2. Excessive levels of oxygen are also problematic for plants, as it inhibits their ability to assimilate nutrients. This is why I advocate minimal water movement and surface disturbance in planted aquaria; going with the minimum necessary is less likely to create issues. Extraneous water movement is detrimental in two ways: CO2 which is extremely important for plant growth is driven out of the water faster, and oxygen is brought into the water at levels beyond what is good for the plants, which have more difficulty assimilating nutrients when the oxygen level increases. But the more significant aspect is the loss of CO2. Submerged plants have difficulty obtaining enough CO2 in nature and in the aquarium; this fact is believed by many to be the reason for the inherently slow growth and low productivity of aquatic plants over terrestrial. It also explains why floating plants always grow faster than full-submerged plants--the surface plants can use CO2 from the air. Further, freshwater emerged plants have been shown to be four times more productive that submerged plants. The reason is because CO2 diffuses so slowly in water as opposed to air, and this limits the underwater plant's uptake of CO2 because the CO2 molecules don't contact the leaves quick enough to meet the plant's needs. Aquatic plants have to use enzymes to rapidly capture the CO2. When the CO2 levels in the water become depleted, these enzymes sit idle, so to speak, but the plant still has to provide energy to them. This results in a reduction in photosynthetic efficiency and therefore growth of the plant because energy is being wasted. Thus, anything that removes CO2 in however small an amount will likely be detrimental to the plant's growth. And filtration causing fast water movement will contribute to this detrimental state.
Last edited by Byron; 04-16-2010 at 01:18 PM..