I’m actually curious about this – Because over-planting a small pond, and having the plants use up too much oxygen during the night to the point that the fish die or are gasping for air at the surface by morning because of it, is a not uncommon problem. Would you know why there is such a difference? Like do aquatic plants have higher oxygen usage in a lower oxygen environment, perhaps, or it’s a more enclosed space with much higher percentage of plants per area, or something else?
I have koi in outside fish ponds. At times during the long summer days there is an occurrence called an algae bloom where the water in the pond becomes filled with very small suspended algae. During the day there is no problem with the respiration of the koi that I have in the pond… but because the algae use up so much available oxygen during the night and do not add any O2 to the water…my koi in the very early morning hours before the sunlight starts photosynthesis of the algae run out of the amount of oxygen they need for respiration and are forced to breathe atmospheric O2 at the surface of the pond! They gasp for O2 out of the water from the atmosphere where there is enough available for them to survive. My point is in water ponds there is a semi closed environment where plants can use up so much oxygen at night that they force the fish to get their oxygen elsewhere. When days become shorter the algae bloom will naturally diminish if I wait it out and do not do massive water changes or resort to killing the floating algae with a chemical plant killer algaecide that will not kill my fish if used in the proper doses. Plants do use O2 at night and do not give off any O2 in darkness!
Plants, however, do not have lungs or a blood stream, so we cannot say that they breathe in the same way as animals.We also have to be careful when studying green plants because in the light the green parts of these plants carry out photosynthesis as well as respiration.Photosynthesis does the opposite of respiration.
A limiting factor is a factor that controls a process. Light intensity, temperature and carbon dioxide concentration are all factors which can control the rate of photosynthesis. Usually, only one of these factors will be the limiting factor in a plant at a certain time. This is the factor which is the furthest from its optimum level at a particular point in time. If we change the limiting factor the rate of photosynthesis will change but changes to the other factors will have no effect on the rate. If the levels of the limiting factor increase so that this factor is no longer the furthest from its optimum level, the limiting factor will change to the factor which is at that point in time, the furthest from its optimum level. For example, at night the limiting factor is likely to be the light intensity as this will be the furthest from its optimum level. During the day, the limiting factor is likely to switch to the temperature or the carbon dioxide concentration as the light intensity increases.
At night, when photosynthesis can’t take place, plants continue to consume oxygen but they don’t release any back into the room. Would that mean that plants really do compete with humans for oxygen?
Many of us are routinely advised by our dermatologists to put on our sunscreen before we go outside and to even add more sunscreen in the middle of the day when the sun’s UV rays are most intense. Wouldn’t it be nice if our bodies could sense how much UV is reaching our skin and then automatically adjust our natural sunscreen protection (in our case the pigment melanin) so as to avoid the negative effects of high UV exposure, such as skin cancer? Well, it seems that many green plants do such a thing on a regular basis. Plants have their own natural UV sunscreens (colorless pigments called flavonoids), which accumulate in their skin (epidermis), and these chemicals filter out much of the detrimental UV before it reaches sensitive photosynthetic tissue inside their leaves (Fig. 1.).