Phosphorus and nitrogen

The six most important elements for life are carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus. The abundances of these elements in biomass approximately mirror what is found in the earth’s crust except for phosphorus which is about 6 times more abundant in biomass. However, the abundance of phosphorus in the ocean is the same as that found in biomass. This is no accident. Oceanographer Alfred Redfield found 70 years ago that the nitrogen to phosphorus ratio was 16:1 in both sea plankton and the ocean. He noted that this was not a coincidence but that the plankton was setting the ratio of the ocean. There is no intrinsic need for this ratio as plankton grown in laboratory conditions can exhibit a wide range.

The phosphorus in the ocean comes from the weathering of rocks on land. It is sequestered by oceanic life forms like plankton and then precipitates to the ocean floor when these organisms die. The availability of phosphorus sets the limit to how much life can be sustained by the ocean. This then sets the balance between oxygen and carbon dioxide in the oceans which in turn affects the balance of carbon dioxide in the atmosphere.

Now, some have argued that the increase of carbon dioxide in the atmosphere will lead to more vegetation which will counter the growth in CO2. However, more vegetation can grow only if it can acquire enough phosphorus and nitrogen. Although the atmosphere is 78% nitrogen, plants, other than legumes, cannot utilize it. They must obtain their nitrogen from the soil which mostly comes from animal waste or decaying biomass. Land animals that eat fish will transfer some nitrogen from the ocean back to the earth. The bottom line is that life on earth is precariously balanced and we really have no idea what will happen when we begin to perturb the system.

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