48
Algae and Nutrients: Uptake and Utilization of Limiting ...
3.6 Resource Competition Under Phosphorus Limitation
The r-K strategy tradeoff, a concept first introduced by MacArthur and Wilson
(1967) in terms of the parameters of the logistic growth equation, has been proposed as a useful classification for phytoplankton species with different competitive traits (Kilham and Ki1ham 1980; Sommer 1981; Reynolds 1984; Ki1ham
and Hecky 1988). Typical r-strategists are opportunist species with high maximal growth rates, and thus high capacities for exploiting and monopolizing
sudden pulses of resource abundance. By contrast, typical K-strategists are efficient exploiters of situations where resources are uniformly scarce and competition is high. For such a classification to be meaningful there must be a tradeoff
between the two traits so that no species can be competitively superior at all
levels of resource availability.
The quartile statistics of phytoplankton growth and uptake parameters
in Table 3.1 is a reasonable starting point for constructing realistic model
species exhibiting a tradeoff between r- and K-strategies. Such model species should be regarded as extremes of what could be encountered in
nature, rather than as mimicking any particular phytoplankton species. If
we assume an inverse relationship between the maximal growth rate (p"
and the maximal phosphorus uptake affinity [a~ I (mg er l day"I], we can
construct an r-K gradient composed of the three species listed in Table 3.2.
The phosphorus subsistence quota [Q'; (~g P) (mg en, and the ratios
Q"IQ' and p'lp" are identical in all three species, and equal to the median
values of Table 3.1. At this stage, we will for simplicity assume that there is
no efflux of phosphorus from any of the species (that is, the threshold level
for positive net P uptake, S', is zero). From these parameter values the corresponding Monod half-saturation parameter (K1, given by Eq. (3.11), will
range from 0.17 to 3.0 (~g P) r
l in the three species.
Table 3.2. Parameters describing three idealized phytoplankton species with a tradeoff
between T- and K-strategies. constructed from the quartile statistics in Table 3.1
Parameter
2
3
Unit
p"
Maximal growth rate
0.80
1.20
1.80
dat'
p'
Asymptotic growth rate
0.92
1.38
2.08
dat'
Q'
Subsistence P quota
3.8
3.8
3.8
(\1g P) (mg cr'
Q"
Maximal P quota
28.5
28.5
28.5
(\1g P) (mg cr'
a'
Maximal P uptake affinity
18.2
6.5
2.3
1 (mg Cr' dat'
K'
Monod parameter
0.17
0.70
3.0
(\1g P) r'
Algae and Nutrients: Uptake and Utilization of Limiting ...
3.6 Resource Competition Under Phosphorus Limitation
The r-K strategy tradeoff, a concept first introduced by MacArthur and Wilson
(1967) in terms of the parameters of the logistic growth equation, has been proposed as a useful classification for phytoplankton species with different competitive traits (Kilham and Ki1ham 1980; Sommer 1981; Reynolds 1984; Ki1ham
and Hecky 1988). Typical r-strategists are opportunist species with high maximal growth rates, and thus high capacities for exploiting and monopolizing
sudden pulses of resource abundance. By contrast, typical K-strategists are efficient exploiters of situations where resources are uniformly scarce and competition is high. For such a classification to be meaningful there must be a tradeoff
between the two traits so that no species can be competitively superior at all
levels of resource availability.
The quartile statistics of phytoplankton growth and uptake parameters
in Table 3.1 is a reasonable starting point for constructing realistic model
species exhibiting a tradeoff between r- and K-strategies. Such model species should be regarded as extremes of what could be encountered in
nature, rather than as mimicking any particular phytoplankton species. If
we assume an inverse relationship between the maximal growth rate (p"
and the maximal phosphorus uptake affinity [a~ I (mg er l day"I], we can
construct an r-K gradient composed of the three species listed in Table 3.2.
The phosphorus subsistence quota [Q'; (~g P) (mg en, and the ratios
Q"IQ' and p'lp" are identical in all three species, and equal to the median
values of Table 3.1. At this stage, we will for simplicity assume that there is
no efflux of phosphorus from any of the species (that is, the threshold level
for positive net P uptake, S', is zero). From these parameter values the corresponding Monod half-saturation parameter (K1, given by Eq. (3.11), will
range from 0.17 to 3.0 (~g P) r
l in the three species.
Table 3.2. Parameters describing three idealized phytoplankton species with a tradeoff
between T- and K-strategies. constructed from the quartile statistics in Table 3.1
Parameter
2
3
Unit
p"
Maximal growth rate
0.80
1.20
1.80
dat'
p'
Asymptotic growth rate
0.92
1.38
2.08
dat'
Q'
Subsistence P quota
3.8
3.8
3.8
(\1g P) (mg cr'
Q"
Maximal P quota
28.5
28.5
28.5
(\1g P) (mg cr'
a'
Maximal P uptake affinity
18.2
6.5
2.3
1 (mg Cr' dat'
K'
Monod parameter
0.17
0.70
3.0
(\1g P) r'