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between warm surface water and cooler deeper water. Vertical
water movements (upwelling), which bring nutrient-rich water
up from deeper layers to surface waters (where most of the
phytoplankton live), are reduced. Consequently, phytoplank-
ton receive less nutrients and are less productive, because
productivity requires nutrients. Upwelling can be stimulated
by mixing due to winds. Less wind means less mixing, fewer
nutrients for phytoplankton, and fewer phytoplankton to sus-
tain ish populations. Concurrent with climate change, the
annual primary production of the oceans has decreased since
the 1980s. Modest changes in temperature have altered trade
wind intensity in the Caribbean, reducing the supply of nutri-
ents to phytoplankton and ultimately causing the collapse of
some isheries. Since late 1995, monthly observations of physi-
cal factors, including nutrient and chlorophyll levels and mete-
orological readings, have been collected at a site off the coast
of Venezuela to establish a long-range record. The sea surface
temperature increased about 1
o
C (1.8
o
F) and winds decreased.
But the effect on marine life was dramatic: populations of phy-
toplankton dropped, along with the local harvest of sardines.
Changes in ocean currents caused by climate change could
lead to shifts in regional climate and weather patterns.
Why are coral reefs particularly vulnerable?
Among the most sensitive groups of organisms are seagrasses,
mangroves, salt marsh grasses, oysters, and corals, which all
create habitat for thousands of other species. Current and future
CO
2
levels will produce changes in ocean temperature and
chemistry beyond what corals have experienced. Some scien-
tists fear that conditions have already reached a “tipping point”
for corals, which now are less able to recover from additional
change. They are considered one of the most sensitive ecosys-
tems to climate change, like the canary in the coal mine. Coral
reefs have been in existence for over 500 million years, but their
continued persistence is uncertain. With increases in ocean
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