Productivity, Nutrient Limitation and Ecosystem Stoichiometry

Light, water and nutrients can limit production. A limiting nutrient has the lowest usable supply relative to organism demand, so it caps new biomass. Requirements are fixed in this model. Light provides energy; it does not supply missing nutrient atoms. More light alone therefore cannot remove a nutrient shortage.

Much phosphorus is stored in rocks and sediments. Weathering can release phosphate into usable supplies slowly compared with organism demand. This can make phosphorus limiting. Its cycle lacks a major gaseous reservoir, though dust carries phosphorus through air.

For a nutrient addition, calculate the biomass limit before and after it. Add the dose to the existing usable supply. Divide each updated nutrient supply by demand per biomass unit. The smaller result caps production. Compare this with the initial limit and identify which nutrient now sets it.

Decomposers retain nutrients for growth and release surplus in mineral form. With enough carbon and energy, compare usable dead-material nutrients with microbial demand. If all phosphorus is retained, multiply its amount by the required N:P ratio to find nitrogen retained. Subtract retained nitrogen from supplied nitrogen to find surplus. Material rich in nitrogen relative to microbial demand can favour nitrogen release, but ratios do not set a release order.

In the single-limiter model, adding a nonlimiting nutrient alone leaves the limiting nutrient's shortage in place. If phosphorus limits, nitrogen alone does not remove that limit. Where nitrogen limits and other resources are adequate, nitrogen runoff can raise algal growth. Oxygen-consuming microbial decay of extra algae can lower dissolved oxygen and create dead zones.