The limitation is on biomass not numbers of krill. Replacing a X million with average weight 2g with a population that weighs 1.8g and numbers could increase even if biomass decreased. But even that’s assuming a uniform static population. Suppose in area X there is an abundance of Krill and iron, but in area Y they are heavily limited. Whales migrating through Y might cause a net migration of iron from X to Y even if they aren’t 100% efficient.
Another possibility is recycling. Suppose the rate of iron addition via dust seeding is A tons per day, the rate of iron lost when a krill dies of old age is 60% and the rate of iron lost when a krill is eaten by a whale is only 30%. If the average age of krill dropped by say 20% then the biomass of krill should significantly increase.
Another possibility is recycling. Suppose the rate of iron addition via dust seeding is A tons per day, the rate of iron lost when a krill dies of old age is 60% and the rate of iron lost when a krill is eaten by a whale is only 30%. If the average age of krill dropped by say 20% then the biomass of krill should significantly increase.