Rewilding the Sea - Feral: Rewilding the Land, the Sea, and Human Life - page 236

power caused by movements of animals in the oceans is comparable
to that of the wind, waves and tides. 15 This, it says, is a conservative
estimate. When whales were more abundant, the effect would have
been still greater. Simply by plunging up and down through the water
column, the whales help to keep plankton circulating in the surface
waters. But their impacts extend far beyond that. They often feed at
depth and defecate at the surface, producing great plumes of iron-rich
manure that fertilize the plants in the photic zone, on which krill, fish
and other animal plankton feed. One paper estimates that, before
their population was reduced, whales recycled at least 12 per cent of
the total iron content of the southern ocean's surface waters. 16 More
whales meant more nutrient cycling, which gave rise to more plank-
ton, producing more fish and krill.
Another study, in the Gulf of Maine, estimates that whales and
seals, by defecating at the surface and recycling nutrients there, would,
before they were hunted, have been responsible for releasing three
times as much nitrogen into those waters as the sea absorbed directly
from the atmosphere. 17 Whales in the gulf typically dive to a hundred
metres or more to feed, bringing back the nutrients they harvest to the
surface. The volume of plant plankton has declined across most of
the oceanic regions in which it has been studied over the past century.
The principal reason is the rising temperature caused by man-made
climate change. 18 But according to the marine biologist Steve Nicol,
the decline has been steepest where whales and seals have been most
heavily hunted. 19 The fishermen who have insisted that the predators
of the species they hunt be killed might have been reducing, not
enhancing, their catch.
If the production of plankton declines, so does the transport of car-
bon to the deep ocean. By stimulating plankton blooms through
recycling iron, another study suggests, sperm whales in the southern
oceans cause the removal of around 400,000 tonnes of carbon from
the atmosphere every year. 20 The extra plants absorb carbon dioxide,
then, after being kicked around the surface waters a few times, sink
into the abyss, where the carbon remains for a very long time. The
whales also release around 200,000 tonnes of carbon through respir-
ation, which means that, on balance, roughly the same amount of
carbon is taken out of circulation.