(^) Physiological acclimation to variations in irradiance serves to minimize variations
in growth rate when light varies. For phytoplankton, this entails a balance between the
light and light-independent reactions of photosynthesis. At low irradiance,
photosynthesis is limited by the rate of light absorption and photochemical energy
conversion. At high irradiance, photosynthesis is limited by the rate of electron
transport, level of RuBisCO, or supply of ribulose 1,5 bisphosphate for the fixation of
carbon.
Models for Photosynthesis
(^) The basic P vs. E curve suggests directly how photosynthetic rates per unit biomass of
phytoplankton should vary downward in the sea. In the layers closest to the surface,
productivity will either be at Pmax or photoinhibited. As measures are made deeper in
the water column, they can be expected to increase to Pmax as irradiance decreases
below photoinhibiting levels, then remain at Pmax for some distance downward as
light absorption by water and particles (including phytoplankton) progressively
reduces available irradiance. From some depth not very far down, irradiance will be in
the sloped part of the P vs. E curve, so that photosynthetic output gets progressively
less. Since irradiance declines exponentially with depth (according to Beer’s Law, Ez
= E 0 e−kz, where k is the extinction coefficient and z is depth), the P vs. z relation
decreases exponentially downward from the depth where irradiance is less than that
forcing photoinhibition, as shown by vertical profiles (Fig. 3.8a) of ^14 C-uptake data.
Of course, the primary production per unit volume is also a function of the amount of
phytoplankton present, usually characterized by the chlorophyll profile. In temperate
and high latitudes (Fig. 3.8b), that is maximal and fairly even through the upper
mixing layers, then tapers off.
Fig. 3.8 (a) May (•) and September () profiles of primary productivity m−3 in the Gulf
of Alaska (50°N, 145°W). (b) Chlorophyll profiles from the same site at various times
are always maximal near the surface, and then taper off below 50 m.
(After Welschmeyer 1993.)