Chapter 3
Habitat determinants of primary production in the
sea
In the trophic–dynamic approach to ecosystems, we try to measure the production at
each link in the food chain or web, at each trophic level. “Production” is incorporation
of new organic matter into cellular material, that is, an increase in biomass. For
phytoplankton, this is done by photosynthesis, the dominant type of primary
production. (Chemosynthesis is important in some environments.) Gross primary
production is total photosynthate generated, and net primary production (i.e. growth)
is gross production less respiration. Net production is available to herbivores. For
herbivores, an increase in biomass can be expressed as a difference:
(^) As will be shown eventually, secondary and higher-level rates are much harder to
measure than primary productivity. Production rates (biomass elaborated per unit
time) are often termed productivities. Oceanographers speak of “the primary
productivity”, meaning the rate of phytoplankton production, usually measured as
carbon newly incorporated in organic matter per unit area (or volume) per unit time.
(^) The light-dependent reactions of photosynthesis (Fig. 3.1) comprise absorption of
light energy; transfer of electrons through the photosynthetic reaction centers coupled
with reduction of water to oxygen and production of adenosine triphosphate (ATP)
and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH). The
NADPH produced is then used as reducing power for the biosynthetic reactions in the
Calvin cycle of photosynthesis. The light-independent reactions (formerly called
“dark” reactions) of photosynthesis fix CO 2 into carbohydrates. Thus, photosynthesis
is often described in terms of the biochemical elaboration of carbohydrate (sugars and
their polymers). We will follow that tradition, but keep in mind that most unicellular
algae direct more than half of their reduced carbon to protein synthesis, and that the
principal store of high-energy molecules is often lipid. The overall photosynthetic
reaction producing carbohydrates is:
Fig. 3.1 Principal components and electron-flow pathways for the light reactions of