four regions: Grand Banks (showing ± 1 Standard Error), ice algae, Arctic open water,
and Sargasso Sea.
(^) (After Lewis et al. 1985.)
There is a remarkable triple complementarity among the spectra of sunlight, the
light transmissivity of water, and the absorption bands of various algal pigments.
Absorption of light by water has a minimum with respect to wavelength, a window of
clarity (Yentsch 1980), right at the peak range of wavelengths of solar irradiance (Fig.
1.5), the wavelengths of visible and photosynthetically active light. The absorption
peaks of phytoplankton pigments (Fig. 3.2b) tend to be centered over the deep trough
in the absorption coefficient, k, spectrum of water (Fig. 1.5b). Most importantly, the
absorbance peak for “antenna pigments” and chlorophyll-a acting together centers
close to the deepest trough near 465 nm. This is not an accidental coincidence, but
represents tuning by natural selection of the photosynthetic system to the properties of
water.
(^) The factors that control the overall rates of photosynthesis, primary productivity, in
the sea are: (i) those that control the rates of reaction of the photosystems (PSI and
PSII), and (ii) those that control the rates of the light-independent reactions. The
former are light intensity and availability of water and carbon dioxide. Water and
carbon dioxide are in abundant supply in seawater, although reaction rates can be
forced somewhat by carbon dioxide loading. The latter include temperature and
availability of nutrients. Nutrients are “fixed” nitrogen, phosphate, various metal ions,
silicic acid (for diatoms and chrysophytes), and sometimes vitamins. Roles of a
number of these are considered here.
Estimation of Primary Production
(^) Measurement of productivity is most successfully done at the primary-producer level
in pelagic habitats, because in some cases it is easy to separate phytoplankton from
their herbivores and observe the rate of accumulation of phytoplankton cells. We have