42
Algae and Nutrients: Uptake and Utilization of Limiting ...
As cell size is a major determinant of edibility to zooplankton (e.g.,
Sterner 1989, and references therein), one would expect small species to
need a higher maximal growth rate in order to compensate for higher grazing losses. The classification adopted here follows the current consensus
with regards to cladoceran feeding preferences as outlined by Sterner
(1989): grazing-resistant species are either large (equivalent spherical
cellular or colonial diameter> 20 !lm), or elongated (largest axial dimension > 100 J.lffi ), or have specialized morphological adaptions like long
spines or a resistant gelatinous sheath. Within this classification, the
grazing-resistant group has significantly lower mean growth rate than the
nongrazing-resistant group (t = 4.87, n = 71, P < 0.0001). The median
growth rates of the two groups (resistant: 0.9 day"\ nonresistant: 1.7 day"l)
are such that the lower and upper quartiles in Fig. 3.2 might also be taken
to represent algae with high and low resistance to zooplankton grazing.
The extremes within the two distributions are represented by diatoms
which have generally higher growth rates than most other grazing-resistant
algae, and cryptomonads which have some of the lowest growth rates
among the grazing-susceptible species. The relatively high growth rates of
large freshwater diatoms fit well with the pattern observed by Furnas
(1990) in marine plankton algae. Comparatively low growth rates in
cryptomonads are puzzling when taking into account the commonly observed dominance of this group under conditions of very heavy grazing.
Heaney and Sommer (1984) indicate that cryptomonads might not reach
their open-water growth rates when enclosed in bottles, and that cryptomonad growth rates therefore might be biased by some containment effect.
Minimum P Cell Quota (Q'). Shuter (1978) argued from a review of
published phosphorus subsistence quotas that there is an allometric relationship between cell size and the minimal amount of cellular P, so that
small cells would be expected to contain more P per unit carbon or volume
than large cells. In the light of the recent findings of very high P contents in
heterotrophic bacteria (e.g., Vadstein et a!. 1988), one might suspect the
conclusions of Shuter (1978) to be influenced by the inclusion of several
species of bacteria in his data set. If bacteria are excluded from the data set
of Shuter (1978), regression analysis on log-transformed cell volumes and
cellular P contents gives a slope of 0.92 ± 0.10, when corrected for bias by
the geometric means method recommended by Ricker (1973). As the slope
is not significantly different from 1, this reanalysis of the data set compiled
by Shuter (1978) does not support the idea of size-dependent phosphorus
subsistence quotas in autotrophic microorganisms.
Table AI0.2 lists the data used by Shuter (1978) recalculated to carbonspecific subsistence quotas together with several more recent accounts on
minimal P requirements in species of plankton algae. Analysis of variance on
the data in Table AI0.2 indicated no significant differences either between
prokaryote and eukaryote autotrophs (ANOV A: Fl.40 = 2.31, P = 0.137),
Algae and Nutrients: Uptake and Utilization of Limiting ...
As cell size is a major determinant of edibility to zooplankton (e.g.,
Sterner 1989, and references therein), one would expect small species to
need a higher maximal growth rate in order to compensate for higher grazing losses. The classification adopted here follows the current consensus
with regards to cladoceran feeding preferences as outlined by Sterner
(1989): grazing-resistant species are either large (equivalent spherical
cellular or colonial diameter> 20 !lm), or elongated (largest axial dimension > 100 J.lffi ), or have specialized morphological adaptions like long
spines or a resistant gelatinous sheath. Within this classification, the
grazing-resistant group has significantly lower mean growth rate than the
nongrazing-resistant group (t = 4.87, n = 71, P < 0.0001). The median
growth rates of the two groups (resistant: 0.9 day"\ nonresistant: 1.7 day"l)
are such that the lower and upper quartiles in Fig. 3.2 might also be taken
to represent algae with high and low resistance to zooplankton grazing.
The extremes within the two distributions are represented by diatoms
which have generally higher growth rates than most other grazing-resistant
algae, and cryptomonads which have some of the lowest growth rates
among the grazing-susceptible species. The relatively high growth rates of
large freshwater diatoms fit well with the pattern observed by Furnas
(1990) in marine plankton algae. Comparatively low growth rates in
cryptomonads are puzzling when taking into account the commonly observed dominance of this group under conditions of very heavy grazing.
Heaney and Sommer (1984) indicate that cryptomonads might not reach
their open-water growth rates when enclosed in bottles, and that cryptomonad growth rates therefore might be biased by some containment effect.
Minimum P Cell Quota (Q'). Shuter (1978) argued from a review of
published phosphorus subsistence quotas that there is an allometric relationship between cell size and the minimal amount of cellular P, so that
small cells would be expected to contain more P per unit carbon or volume
than large cells. In the light of the recent findings of very high P contents in
heterotrophic bacteria (e.g., Vadstein et a!. 1988), one might suspect the
conclusions of Shuter (1978) to be influenced by the inclusion of several
species of bacteria in his data set. If bacteria are excluded from the data set
of Shuter (1978), regression analysis on log-transformed cell volumes and
cellular P contents gives a slope of 0.92 ± 0.10, when corrected for bias by
the geometric means method recommended by Ricker (1973). As the slope
is not significantly different from 1, this reanalysis of the data set compiled
by Shuter (1978) does not support the idea of size-dependent phosphorus
subsistence quotas in autotrophic microorganisms.
Table AI0.2 lists the data used by Shuter (1978) recalculated to carbonspecific subsistence quotas together with several more recent accounts on
minimal P requirements in species of plankton algae. Analysis of variance on
the data in Table AI0.2 indicated no significant differences either between
prokaryote and eukaryote autotrophs (ANOV A: Fl.40 = 2.31, P = 0.137),