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.The idea that whale poo could help to fertilise the ocean has been floating around for a while. Basically, large whales in the Southern Ocean consume large quantities of krill, but because they are producing blubber rather than muscle they have no need for the iron in their diet. Consequently, this essential element passes straight through the digestive tract and exits the body in a plume of faecal material that acts like liquid manure.
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.
Whales could only play a role in recycling iron if there was a large amount of iron in their faeces. However, the difficulty of measuring the iron content of whale poo has held back research for a number of years. Luckily, in the Hobart area there is a critical mass of Southern Ocean scientists with a unique range of skills who were undaunted by the difficulties involved in testing this theory. As part of the non-lethal whale research program being conducted by the Australian Marine Mammal Centre at the Australian Antarctic Division, we had been using advanced molecular techniques to examine whale poo collected from around the world in a bid to to understand their diet. So we had a collection of poo samples and we knew what the whales had been eating. Scientists at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) had also been studying the role of iron in the Southern Ocean, so we also had specialised chemists and the sophisticated equipment required to measure the minute quantities of iron in seawater. Add some marine ecologists to this mix of expertise and we had the potential to address the question of whether whales could be involved in their own version of ocean fertilisation.
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.
The high concentrations of iron that we measured in the flesh of whales and krill in the Southern Ocean suggested that the role of larger animals in the cycling of nutrients had been overlooked in the past; so we did a few simple calculations. We had recent estimates of how much krill there are in the Southern Ocean; we knew the background concentration of iron in the seawater; and we could estimate the area in which krill were most abundant. So we were able to compare the total amount of iron in krill to the total amount of iron in the seawater where they live. We found that in 19 million square kilometres of the Southern Ocean krill appear to contain 24% of the iron. This iron is incorporated into their tissues and because krill are strong swimmers they can keep this significant quantity of iron suspended in the upper layer of the ocean, unlike smaller organisms or inorganic particles, which tend to sink.
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