function:
(^) that has been applied by many field workers (e.g. Welschmeyer 1993). The parameter
β is the intensity at onset of photoinhibition. This function applies to “normalized”
photosynthetic rates, that is, rates per unit phytoplankton biomass. In fieldwork, the
usual normalizing variable is chlorophyll-a concentration, and the ratio of the
photosynthetic rate (carbon uptake) per unit volume to chlorophyll concentration is
termed an assimilation number.
(^) Different algal groups have significantly different light requirements for growth and
photosynthesis (Richardson et al. 1983). Dinoflagellates and cyanobacteria
photosynthesize and grow best at low light intensities. Diatoms can utilize low light
intensities, but are much more tolerant of high light than most other groups. There are
numerous reports and summaries of the photosynthetic parameters for P vs. E curves
for most algal groups and many species of phytoplankton. We show some
representative parameters in Table 3.1. Diatoms have the highest maximum rate of
photosynthesis per unit chlorophyll (PmaxChl), cyanobacteria are intermediate and the
nanoflagellates (primarily prymnesiophytes) have the lowest PmaxChl. Diatoms have
the steepest initial slope for the rate of photosynthesis per unit chlorophyll (αChl),
nanoflagellates have an intermediate slope, and cyanobacteria have the lowest slope.
Table 3.1 Photosynthetic characteristics of dominant phytoplankton groups. αChl is
the initial slope of the P vs. E curve with units of mg C (mg Chl-a)−1 h−1 (μmol
quanta m−2 −1). PmaxChl is the maximum rate of photosynthesis normalized to Chl-a
with units of mg C (mg Chl-a)−1 h−1. The photosynthetic parameters (αChl and
PmaxChl) are calculated from a model of primary production calibrated with field data
on specific phytoplankton pigments, irradiance as a function of depth, and specific
absorption coefficients (Uitz et al. 2008)
(^) GROUP ΑChl PmaxChl
Diatoms 0.032 ± 0.0074.26 ± 0.45
Prymnesiophytes0.026 ± 0.0052.94 ± 0.43
Cyanobacteria 0.007 ± 0.0033.75 ± 0.37
(^) In addition to genotypic differences, termed photoadaptation, in the ability of
phytoplankton to utilize light, all phytoplankton exhibit short-term phenotypic
adjustments, termed photoacclimation, in response to variations in light intensity
(MacIntyre et al. 2002). The amount of photosynthetically active pigment may
increase at low intensities (more chlorophyll-a, more P700 in the Chlamydomonas
and Phaeodactylum, causing an increase in the slope of the P vs. I curves in short-