Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 130

Light

Sea surface

Autotrophs:
produce organic
compounds

Larger marine animals

Heterotrophs:
consume organic
compounds

Nutrients

Lost or recycled
organic material

Seabed

Figure 5.1 A framework for this chapter. We will describe the production of organic
compounds and how those compounds can either reach larger marine animals or
be exported/recycled.

5.1 Primary production in the sea: photosynthesis and nutrients

...................................................................................
The photo-autotrophs, or phytoplankton, fix dissolved inorganic carbon (DIC) into the
organic components that the rest of the marine food chain requires. If sufficient DIC is
fixed in the surface water, reducing the concentration below the saturation value, 1 then
CO 2 is transferred across the sea surface from the atmosphere to the sea. Thus, by
fixing carbon and affecting the air-sea transfer of CO 2 , the phytoplankton also play a
fundamental role in the cycling of carbon within the Earth's climate system.
Growth of the phytoplankton, referred to as 'primary production', uses sunlight as
the energy source, and also requires various nutrients. Let us first look at the common
requirements of light and nutrients needed by all photo-autotrophs, and consider some
of the methods we use to measure autotrophic growth at sea. At the end of this section
we will also describe the main phytoplankton groups, some of their important

1 DICinseawaterismadeupofdissolved'aqueous'CO 2 , carbonate and bicarbonate. Dissolved CO 2 only
accounts for 1% of DIC. The chemistry of what happens to CO 2 as it is absorbed by the seawater is

beyond the scope of this topic (see Chapter 11 ofWilliams and Follows 2011), but there are two key points to
bear in mind. First, the amount of DIC that can be held by seawater is strongly temperature dependent,
with warmer water able to hold less DIC. Second, the flux of CO 2 across the sea surface depends on the
gradient between the atmospheric CO 2 and the aqueous CO 2 , rather than with the total DIC, and so the time
scale for DIC to reach equilibrium with the overlying atmosphere is much longer than, say, the time it
takes dissolved oxygen to equilibrate.

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