thiamin, biotin, and B 12 (a protein-bound form of cobalamin), are present in small
amounts in seawater, sometimes becoming reduced to limiting levels.
(^) The kinetics of phytoplankton growth and uptake rates as a function of nutrient
concentration are hyperbolic functions of various forms (Box 3.2). The response of
phytoplankton growth to a range of concentrations of a single limiting nutrient (other
nutrients in excess) is illustrated (Fig. 1.7) for the case of nitrate. Growth rate
increases rapidly over a small range of low values, then levels off. Once the nutrient
requirement is met by adequate concentrations, something else becomes limiting
(light or physiological potential at the prevailing temperature). In much of the current
literature, Ks values are used to characterize the phytoplankton growth responses to
nutrient availability (Table 3.3). Small Ks values indicate rapid response of the growth
rate to increased nutrient availability. Large Ks indicates that relatively high
concentration is required to achieve a near-maximal growth rate.
Table 3.3 Half-saturation constants (Ks) ± 95% confidence intervals for nitrate uptake
for eight clones of phytoplankton.
(After Carpenter & Guillard 1971.)
SPECIES AND CLONESOURCEKS (μM)
Cyclotella nana
3-H Estuarine 1.87 ± 0.48
7-15 Shelf 1.19 ± 0.44
13-1 Oceanic 0.38 ± 0.17
Fragilaria pinnata
0-12 Estuarine 1.64 ± 0.59
13-3 Oceanic 0.62 ± 0.17
Bellerochia sp.
Say-7 Estuarine 6.87 ± 1.38
675D Shelf 0.12 ± 0.08
SD Oceanic 0.25 ± 0.18
(^) Determination of the responses of phytoplankton species and phytoplankton
assemblages to variation in nutrient availability took much of the effort in
phytoplankton ecology from 1970 to 1990. The data are complex and various. Note
that the results in Fig. 1.7 are presented as doublings/day, a specific growth rate
(growth/abundance/time). That is the ideal form for information on nutrient responses.
However, many of the data available are as nutrient uptake rates combined with one
determination of the maximum growth rate at high nutrient concentration to produce a
synthetic μ vs. [S] curve.
(^) Equality of the uptake and growth curves is ensured (forced) in many studies by use
of chemostats for determination of uptake rates. A chemostat is a container of well-