As for the whales’ role in nutrient recycling, the researchers calculated that prior to whaling losses, whale populations produced a staggering 12,000 tons of fecal iron each year, 10 times the amount whales currently recycle in the Southern Ocean.
These accounts indicate that at that time there were a lot of whales feeding on krill and there must have been a lot of krill to eat.
Savoca said the decline in krill numbers after many of their largest predators disappeared is known to scientists as the “krill paradox.” The decline in krill numbers is most evident in areas where whales are heavily fished, such as the Scotia Sea between the Southern Ocean and the Atlantic Ocean in southeastern South America.
“This decline is meaningless until you consider that the whales are operating as mobile krill processing plants,” Savoca said. “These are animals the size of a Boeing 737, that eat and defecate off the ground in a system that is very limited in the presence of iron in many places. These whales produce seeds in the open Southern Ocean, and once they are gone, there is little left to recycle that fertilizer.”
The problem is that a lot of krill that has not been eaten sinks to the bottom when it dies. There, the iron contained in phytoplankton, which in turn is the main food for krill and other zooplankton, is of no use.