Biology Now, 2e

Here and Gone ■ 389

But correlation does not prove causation,

and much more work needs to be done to

support the idea that global warming caused

the decline. It is a logical hypothesis, however,

because as the planet warms, water in the

oceans mixes less, limiting the nutrients deliv-

ered to the surface from decomposers in the

deep sea. As a result, phytoplankton do not

receive the nutrients they need for growth

and reproduction. In 2014, Boyce published

experimental data supporting the hypothesis.

Working with marine scientists in Germany, he

found that warming the water in a controlled,

experimental ocean water enclosure led to

reduced phytoplankton biomass.

A 2014 study by a large, international group

of researchers predicted that an increase

in ocean temperature due to global warm-

ing would cause phytoplankton biomass to

decrease by 6 percent by the end of this century.

Consequences of continued phytoplankton

decline could include altering the carbon cycle

between the ocean and the atmosphere, chang-

ing heat distribution in the ocean, and causing

a decrease in the supply of food in the ocean.

Whatever the case, if phytoplankton popula-

tions are decreasing, says Boyce—and he is sure

they are—there will be profound effects, which

have only just begun.

McQuatters-Gollop. Still, they could not agree

on whether phytoplankton populations have

increased or decreased in the ocean. At the meet-

ing, the researchers agreed that the best thing to

do was to combine as many data sets as possible.

Heating Up

In 2015, the long-awaited satellite data came

in—and they weren’t good. Ocean color measure-

ments from two NASA satellites led scientists to

conclude that diatoms, the largest type of phyto-

plankton, had declined more than 1 percent

per year from 1998 to 2012 globally, with major

losses in the North Pacific, North Indian, and

Equatorial Indian Oceans.

It looks like Boyce, Worm, and Lewis may be

right, but the important question of why a decline

is occurring remains. As part of his research,

Boyce investigated possible causes of the down-

ward trend. In one study, he compared changes

in sea surface temperature to changes in chloro-

phyll levels and noted a strong correlation: over

the last 100 years, chlorophyll concentrations

declined and ocean temperatures increased,

in line with increases in global warning. This

correlation has been closely followed in recent

years (Figure 21.10).

2000 2005

Year

Change in chlorophyll concentrations

from the average

Change in temperature

from the average

Chlorophyll

Temperature

0.1

0

–0.1

0.2

0

–0.2

Figure 21.10

As ocean temperature increases, chlorophyll decreases

Between late 1997 and mid-2008, satellites observed that warmer-than-average temperatures (red line)

were correlated with below-average chlorophyll concentrations (green line).

Q1: In what years were chlorophyll levels the highest?

Q2: In what years were the temperature changes from the average the greatest?

Q3: How do you predict this graph will look 10 years from now?