Whales drive the iron-cycle in the Oceans
Baleen whales are the largest known animals that have ever lived. They feed on minuscule prey by filtering seawater through plates of frayed, bristle-like combs, termed baleen, that are fixed to their upper jaws. Savoca et al. recently reported in their Nature article (1) on the feeding patterns of the baleen whales that all seven species studied consumed up to three times more prey biomass than expected from previous estimates. They were found to eat even more than 30 percent of their own body weight per day.
By eating iron-rich krill and discharging iron-rich fecal plumes in the surface layer, whales were substantially enhancing phytoplankton growth (see also the insert), boosting the availability of food for krill. Thus with their iron-rich droppings, baleen whales indirectly promoted the growth of their own food (krill) in Antarctic waters. A perfect cycle. The krill biomass consumed by whales alone is estimated to have been 190 million tonnes annually, an amount substantially greater than the entire annual world fish catch in modern times.
Krill started declining in large quantities after the decimation of the whales, with the last large-scale surface swarms having been recorded in early 1980. This ‘paradoxical’ decline in krill is consistent with a model in which whale-aided iron cycling supported the growth of krill populations. Thus, the hugely productive ocean pastures dominated by diatoms, described in the past, have since reverted to the classic iron-limited water that is now characteristic of the degraded large areas of the ocean’s surface
In his comments on the article, Victor Smetaceks argued that humans have in their power the means to mimic the iron fertilization mediated by whales to create diatom blooms, to feed the krill, and thereby to feed the whales and perhaps restore their original numbers.
(1) Savoca, M. S. et al. Nature 599, 85–90 (2021)