> But this cannot work in theory. A population cannot grow if its only nutrient input comes from dead members.
That’s the point your missing. The nutrient input is coming first from the sunlight. The phytoplankton nutrient for the krill is limited by the available iron.
> Sunlight is a source of power; by assuming unlimited sunlight input we can rearrange as much material as we want, but we can't add or subtract any.
Photosynthesis creates nutrients at the base of all food webs (except deep sea geothermal vents) It’s able to allow plants to take carbon dioxide out of the air and fix it as a source of chemical energy. The sunlights radiative energy then is converted to chemical energy which then acts as a nutrient for other living things. The nutrient addition is literally arriving out of thin air. (C02 into sugars)
However, organisms and in this case the phytoplankton doing that photosynthesis, are limited by specific nutrients, otherwise known as regulating factors. [1] In this case sunlight, carbon dioxide, etc would allow for massive growth of phytoplankton in the ocean. However, even given unlimited amounts of those, the phytoplankton will hit a growth wall when it runs out of iron.
That iron, in this case, was locked up in vast amounts of krill. Krill from my knowledge can live up to 10 years. So that means that a balance will be reached between the krill and phytoplankton in terms of population densities and where that limiting nutrient of iron is stored.
Whales change that limiting nutrient balance. By eating the krill and releasing the iron, the phytoplankton is able to increase its growth rate.
Krill then, feeding on the phytoplankton blooms will reproduce quicker. Iron that was once locked up in krill for 10 years allows faster reproduction rates of new krill.
The energy input that allows both populations to grow in this scenario is the sunlight. Again, the iron is only a limiting factor. The limiting factor is not 100% of the energy input. The iron is not “used up” meaning it’s availability to allow increased photosynthesis remains.
The end result is that a significantly larger proportion of sunlight is captured and converted to living matter which allows a net positive gain in both populations. This is possible because the regulating factor for increased photosynthesis is maintained in the ocean at higher levels with whales.
If we arrange more material into phytoplankton, that would seem to leave less material available to be krill.
> It’s like fertilizing soil with the dead… it could in theory support ever growing populations.
But this cannot work in theory. A population cannot grow if its only nutrient input comes from dead members.