If we now add a slow upward movement of the water, to simulate the process of deep circulation, we have a basic, first-order model of the oxygen minimum in the oceans.
As we can see, there is a close relationship between the action spectrum and absorption spectrum of photosynthesis. There are many different types of photosynthetic pigments which will absorb light best at different wavelengths. However the most abundant photosynthetic pigment in plants is chlorophyll and therefore the rate of photosynthesis will be the greatest at wavelengths of light best absorbed by chlorophyll (400nm-525nm corresponding to violet-blue light). Very little light is absorbed by chlorophyll at wavelengths of light between 525nm and 625 (green-yellow light) so the rate of photosynthesis will be the least within this range. However, there are other pigments that are able to absorb green-yellow light such as carotene. Even though these are present in small amounts they allow a low rate of photosynthesis to occur at wavelengths of light that chlorophyll cannot absorb.
The answer is that as long as the upward motion and the pumping action are balanced � that is, as long as the nutrients coming up from below are sent back down quickly enough by phytoplankton to remove them from the surface layer � there is no effect.
This is a way of pumping nutrients and carbon down, against the upward movement of upwelling, and hence the term "biological pump."
The biological pump in effect puts some of the carbon into a hidden , where the atmosphere cannot reach it.
If the pump were operating at maximum capacity (that is, if all the ocean�s nutrients were used up) atmospheric CO2 would drop to a low of 140 ppm.
However, the biologic activity in the surface layer (aided by sunlight) keeps removing the nutrients and causing them to settle back down, together with the appropriate amount of carbon (determined by the Redfield Ratio).
So how can these factors have an effect on the rate of photosynthesis? Lets start off with the light intensity. When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. When the carbon dioxide concentration is low, the amount of glycerate 3-phosphate produced is limited as carbon dioxide is needed for its production and therefore the rate of photosynthesis is affected. Finally, many enzymes are involved during the process of photosynthesis. At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected.
This sequence of numbers is called the "Redfield Ratio" after , the oceanographer who first determined and explained its importance in understanding the of the ocean.
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.
(1) Carbon dioxide is fixed by photosynthesis, (2) this organic matter sinks into deeper waters, (3) bacterial decay releases carbon dioxide and other nutrients, making them available to be used again by , until (4) ultimately deposition locks away the carbon in ocean sediments.
At this point, if we draw a depth profile of the concentrations of nutrients in the ocean waters, we should find practically nothing in the warm layer, a maximum below the warm layer, where bacteria have re-mineralized many of the particles received from above, and an exponential decay with depth, as there is less and less left for the bacteria to remineralize and as the settling organic becomes selected for those types which are hard to oxidize.
At some point in our thought experiment, the recycling becomes negligible because all the nutrients have been exported to the cold layers below and nothing can grow anymore.