Whale poo fertilises oceans
Australian Antarctic scientists recently tested their hypothesis that whale poo can act as a fertiliser in the ocean. Their results suggest that the recovery of whale populations to pre-whaling numbers could actually increase the productivity of the ecosystem.
Photo: J. Brokowski
A number of studies have shown that the growth of microscopic algae in the Southern Ocean is limited by the iron concentration in the water. Iron enters the ocean either through wind-blown dust from the Antarctic continent or through the upwelling of water from the ocean depths. The deep water is richer in nutrients because particulate matter, including dead algae and detritus, sinks out of the sunlit layer to the ocean’s interior, where microbial processes break it down into its constituent chemicals. Any process that keeps the nutrients in circulation in the surface layer, rather than sinking, should ensure that algal growth is sustained. This is where the notion of whales as fertilisers of the ocean comes in.
Photo: Sarah Robinson
After analysing 28 samples from four different species of whale, we were surprised to discover that baleen whale poo contains 10 million times more iron than an equivalent weight of seawater. From our molecular studies we knew exactly what the whales had been eating and we were able to confirm that the krill in their diet was the source of the iron we were measuring. Finally, we wanted to see how much iron whales had stored in their bodies, so we measured the concentration in muscle samples from stranded blue and fin whales, and again we got very high values.
Photo: Mike Double
Although individual whales contain large amounts of iron, their relative scarcity means that their main role seems to be in converting the iron reservoir in the bodies of krill into liquid manure. This fertilising role is likely to work on a small scale today, but in the pre-whaling era when there were millions of great whales in the waters around Antarctica in summer, their effect on iron recycling was likely to be far greater. Interestingly, there are suggestions that when there were more whales, there was also more krill and that algal productivity would have had to be higher to support all these animals. If whales and krill are vital for recycling iron in the ocean, then this explains how an ecosystem with more animals in it can also be a more productive ecosystem. Could it be that allowing whales to recover to their former numbers would actually enhance fisheries production, rather than detract from it?
This research was conducted by Simon Jarman, Klaus Meiners and Steve Nicol from the Australian Antarctic Division and Andrew Bowie, Delphine Lannuzel and Pier van der Merwe from the ACE CRC.
Program Leader, Southern Ocean Ecosystems