Limiting Factors on Productivity

Every ecosystem needs a list of ingredients to run photosynthesis and grow: sunlight, water, and nutrients like nitrogen, phosphorus, and iron. The Law of the Minimum says productivity is set by whichever ingredient is scarcest relative to need, not by the average abundance of everything else. A cornfield drowning in sunlight still can't grow if nitrogen runs out. The ocean is full of sunlight and water but often starved for iron or nitrogen, so productivity stalls until that one scarce nutrient shows up.

This is why dumping iron into open ocean water triggers a bloom almost overnight: phytoplankton had everything else they needed and were just waiting on the missing piece. Once the limiting factor is supplied, growth surges until some other factor becomes scarce and takes over as the new cap. This also explains why pouring extra light onto a desert plot barely helps. Desert plants already have more sunlight than they can use; water is what's scarce, so water is what controls growth. Pile on a non-limiting resource and you get nothing.

This idea connects directly to the energy and productivity numbers from earlier in the unit. Gross primary productivity, GPP, is all the energy plants capture; net primary productivity, NPP, is what's left after plants spend some on their own respiration. Limiting factors control how high GPP can climb in the first place, before any of that 90 percent energy loss between trophic levels even happens. Upwelling zones push this to the extreme: cold, nutrient-rich deep water rises to meet sunlit surface water, removing the usual ocean limiting factor and letting productivity rival a rainforest.