46
Algae and Nutrients: Uptake and Utilization of Limiting ...
resistant species, although not significant (t = 1.19, n = 64, P = 0.24). This
result is perhaps not so surprising when taking into account the variety of
experimental conditions and analytical methods used by the different
authors. The half-saturation parameter of the Monod model (K~ is dependent on accurate inorganic P measurements at concentrations close to
or below the detection limit of standard chemical methods. This means that
the presence of a threshold concentration (S~ as in Eq. (3.10) would easily
be undetected, leading to an overestimation of K', thus tending to underestimate a'lQ'. On the other hand, direct measurement of uptake parameters
can also be seriously biased if, for example, the possible existence of multiple uptake systems is neglected. Olsen (1989) found that in the green algae
Staurastrum luetkemuellerii P affinity measurements at low concentrations
[< 4 (Jig P) rl] were at least twice as high as measurements made at concentrations more commonly used in P uptake experiments, due to the presence of a high-affinity uptake system in this species.
Even if we attnbute a major part of the variance in Fig. 3.5 to methodological
aspects, it still seems reasonable to conclude that the biological variability in the
relative uptake affinity is large compared to the variability in other model
parameters like the P subsistence quota. For the median P subsistence quota
[Q' = 3.8 (Jig P) (mg C)'I] a typical value d = 6.51 (mg C),I day,l is suggested,
while values corresponding to lower and upper quartiles of the distribution
[2.3 and 18.21 (mg Cr l day"l] might be considered representative for species
with particularly low and high inorganic P affinity.
Threshold Concentration for Positive Net Uptake (S'). Compared to the relative
abundance of net or gross nutrient influx measurements, very few studies
have attempted to measure nutrient effluxes from phytoplankton. Due to the
close coupling of the influx and efflux processes, such measurements can
be made only indirectly through tracer kinetics (Lean and Nalewajko 1976) or
mass-balance calculations (Olsen 1989). Only the data set of Olsen (1989)
contains simultaneous measurements of biomass-specific P efflux rates
[v. ; (J.18 P) (mg C),I day'l] and inorganic P uptake affinities, such that the
threshold concentration for positive net uptake (S, can be calculated. From
the net uptake equation [Eq. (3.4)] we observe that the the efflux rate can be
written as v. =a S', and that this relationship can be used to calculate S' when
a and v. are known. From the data presented in Olsen (1989), we
can calculate that S' = 0.07- 0.60 (J.18 P) r l for Microcystis aeruginosa and 0.070.35 (J.18 P) r l in Staurastrum luetkemuellerii, when corrected for the presence
of a high-affinity uptake system. Although admittedly based on limited data,
one might conclude that the threshold concentration for positive net uptake
(S, is in the order of a few tenths of a Jig P per liter, and thus located safely
below the detection limit of standard chemical methods. It must nevertheless
be emphasized that the smallness in magnitude of this parameter does not
necessarily reflect its significance in determining the outcome of algal
nutrient competition (Olsen 1989; Olsen et aI. 1989).
Algae and Nutrients: Uptake and Utilization of Limiting ...
resistant species, although not significant (t = 1.19, n = 64, P = 0.24). This
result is perhaps not so surprising when taking into account the variety of
experimental conditions and analytical methods used by the different
authors. The half-saturation parameter of the Monod model (K~ is dependent on accurate inorganic P measurements at concentrations close to
or below the detection limit of standard chemical methods. This means that
the presence of a threshold concentration (S~ as in Eq. (3.10) would easily
be undetected, leading to an overestimation of K', thus tending to underestimate a'lQ'. On the other hand, direct measurement of uptake parameters
can also be seriously biased if, for example, the possible existence of multiple uptake systems is neglected. Olsen (1989) found that in the green algae
Staurastrum luetkemuellerii P affinity measurements at low concentrations
[< 4 (Jig P) rl] were at least twice as high as measurements made at concentrations more commonly used in P uptake experiments, due to the presence of a high-affinity uptake system in this species.
Even if we attnbute a major part of the variance in Fig. 3.5 to methodological
aspects, it still seems reasonable to conclude that the biological variability in the
relative uptake affinity is large compared to the variability in other model
parameters like the P subsistence quota. For the median P subsistence quota
[Q' = 3.8 (Jig P) (mg C)'I] a typical value d = 6.51 (mg C),I day,l is suggested,
while values corresponding to lower and upper quartiles of the distribution
[2.3 and 18.21 (mg Cr l day"l] might be considered representative for species
with particularly low and high inorganic P affinity.
Threshold Concentration for Positive Net Uptake (S'). Compared to the relative
abundance of net or gross nutrient influx measurements, very few studies
have attempted to measure nutrient effluxes from phytoplankton. Due to the
close coupling of the influx and efflux processes, such measurements can
be made only indirectly through tracer kinetics (Lean and Nalewajko 1976) or
mass-balance calculations (Olsen 1989). Only the data set of Olsen (1989)
contains simultaneous measurements of biomass-specific P efflux rates
[v. ; (J.18 P) (mg C),I day'l] and inorganic P uptake affinities, such that the
threshold concentration for positive net uptake (S, can be calculated. From
the net uptake equation [Eq. (3.4)] we observe that the the efflux rate can be
written as v. =a S', and that this relationship can be used to calculate S' when
a and v. are known. From the data presented in Olsen (1989), we
can calculate that S' = 0.07- 0.60 (J.18 P) r l for Microcystis aeruginosa and 0.070.35 (J.18 P) r l in Staurastrum luetkemuellerii, when corrected for the presence
of a high-affinity uptake system. Although admittedly based on limited data,
one might conclude that the threshold concentration for positive net uptake
(S, is in the order of a few tenths of a Jig P per liter, and thus located safely
below the detection limit of standard chemical methods. It must nevertheless
be emphasized that the smallness in magnitude of this parameter does not
necessarily reflect its significance in determining the outcome of algal
nutrient competition (Olsen 1989; Olsen et aI. 1989).