An Introduction to Environmental Chemistry

220 Chapter Six

Box 6.6 Oceanic primary productivity

The rate of growth of phytoplankton

(primary productivity) in the oceans is mainly

limited by the availability of light and the

rate of supply of limiting nutrients (usually

accepted to be nitrogen (N), phosphorus (P),

silicon (Si) and iron (Fe)).The need for light

confines productivity to the upper layers of

oceans. Also, in polar waters there will be

no phytoplankton growth during the dark

winter months.

In temperate oceans there is little winter

productivity because cooling of surface

waters destroys the thermal stratification and

winds mix waters and phytoplankton to

depths of hundreds of metres. This deep

mixing, which occurs in all temperate and

polar oceans, also means that vertical

gradients in nutrient concentrations are

temporarily eliminated, allowing a

corresponding rise in surface-water nutrient

concentrations. In spring, surface-water

stratification is re-established as the

waters warm and winds decrease. Once

enough light is available, vigorous plankton

growth begins. Nutrient concentrations

are high, having built up over the winter,

resulting in the ‘spring bloom’ of

phytoplankton. The bloom is generally later

0 ∞

20 ∞

40 ∞

60 ∞

80 ∞

20 ∞

40 ∞

60 ∞

80 ∞

North Atlantic

JFMAMJJASOND

Arctic

JFMAMJJASOND

Month

Change in

phytoplankton

biomass

Change in

zooplankton

biomass

North Pacific

JFMAMJJASOND

Tropical

JFMAMJJASOND

Month

Month

Month

120 ∞ 60 ∞^0 ∞ 60 ∞ 120 ∞

<150

mg C m–2 day–1

150–

250

>250

Fig. 1Seasonal cycles of productivity and global average rates of primary production. Reprinted from

Koblentz-Mishke et al.(1970), with permission from Scientific Exploration of the South Pacific, Courtesy of

the National Academy of Sciences, Washington, DC.

(continued)