Why We Need Whales

"The second way in which animals can boost ocean productivity is by nutrient scavenging - feeding at depth and bringing nutrients back to the sunlit zone. Sperm whales, for instance, feed on squid and fish at great depths, and defecate at the surface. Models suggest that this recycling of deep material may well be significant for essential elements such as iron.

On a local scale, humpback whales in the Gulf of Maine have also been shown to scavenge nutrients. In fact, they release more nitrogen at the surface than flows in from all the rivers (PLoS One, vol 5, p e13255).

Many other species are known to feed in deep waters on occasion and return to the surface, including seals, penguins, turtles, seabirds and sunfish, so it is possible these animals also return significant amounts of nutrients to surface layers.

Surprisingly, even krill may play a part. While they were thought to live in the upper 200 metres of water, krill have recently been observed at much greater depths. There is footage of krill 3500 metres down on the sea floor, apparently feeding on material that has sunk to the bottom (Current Biology, vol 18, p 282). If krill regularly feed on the sea floor and return to the surface, this may be an important route for bringing nutrients from the sediments back to the surface.

Of course, the opposite process also occurs. Animals that feed near the surface at night and return to deeper waters take nutrients back down with them. We don't yet know on what scale nutrients are removed from and returned to the surface in this way or what the overall net effect is, so the importance of nutrient scavenging to the productivity of the oceans remains to be established."

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