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of plankton abundance in the oceans. Recently, Secchi disc measure-
ments made throughout the world oceans for the past century have
been analysed. It was an enormous task—but it was worth it. The
measurements fluctuated from year to year and from place to place,
as might be expected—but a clear overall trend appeared. Over the
past century global ocean waters have become clearer and hence less
plankton-rich. 104 The total decline is 14 per cent—a surprisingly large
figure given that the world's warming is only just beginning. Plankton
makes up the base of the food chain, and so the totality of open-ocean
life (regardless of what form it takes, and how humans might affect its
proportions by fishing) has already declined significantly. As the
oceans continue to warm and to become more stratified over the
coming decades and centuries, one might expect open-water ecosys-
tems to shrink further. This is food for thought, not least in a world
where a growing human population will doubtless seek to take yet
more fish from the seas.
The Oxygen Deficit
As the oceans become more stratified, and it becomes harder to
extract nutrients from the depths to the sunlit surface levels, so it will
also become harder to mix in oxygen from the surface into deeper
levels, especially into the oxygen minimum zones that are present at
intermediate depths, above those waters that are stirred and supplied
with oxygen by deep-water current systems (see Chapter 5). Reduc-
tions in oxygen content have already been observed, and are predicted
to spread farther as the world continues to warm. This has serious
implications, for fish need a good oxygen supply; as this declines,
one predicted response is that fish will simply get smaller. 105 There
are precedents for such change in the coincidence of ocean anoxia
with greenhouse states of the geological past (see Chapter 5). So far,
the effect is subtle. Closer to shore, less subtle and more peculiarly
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