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The great ocean fertilizers

Humpback whales feeding at a marine sanctuary off the coast of Massachusetts, USA

Elliott Hazen

Whales are the largest living beings on the planet and play an important role in recycling ocean nutrients. They consume a colossal amount of food and their feces fertilizes water close to the surface. It is difficult, however, to measure exactly how much large whales eat, because they hunt for their prey at depths of up to 100 meters. To better understand the consumption of these massive mammals (the biggest can reach 30 meters in length), a group led by Matthew Savoca, an ecologist from Stanford University, USA, tracked the movement of 321 whales from seven species between 2010 and 2019, using sonar to estimate the size and density of the schools of plankton and fish ingested. The group concluded that large whales consume three times more food per year than previously estimated. A blue whale (Balaenoptera musculus), for example, eats 16 tons of fish, squid, and small crustaceans per day (Nature, November 4). In the Southern Ocean alone, whales recycle 1,200 tons of iron per year, an essential nutrient for the reproduction of plankton, microscopic organisms at the base of the marine food chain that help remove carbon dioxide from the atmosphere. It is estimated that iron levels in the ocean were up to 10 times higher at the beginning of the twentieth century, before industrial hunting decimated several whale species.