26. Stoichiometric Analysis of Pelagic Ecosystems: The Biogeochemistry of Planktonic Food Webs
395
as recycling of potentially limiting nutrient elements from the food. Thus, in the work synthesized
by Sterner and Hessen (1994), imbalance between
herbivore requirements and food elemental composition is seen to have effects at a variety of levels
of ecological integration, from individual behavior
to ecosystem processes.
Our understanding of the importance of food
quality on zooplankton dynamics and the rest of the
pelagic ecosystem has been considerably refined
even in the brief period since the review of Sterner
and Hessen (1994) appeared (Sterner and Schulz
1998). Evidence for the role of certain essential
fatty acids (highly unsaturated fatty acids, or
HUFA) in constraining zooplankton growth when
feeding on moderately nutrient-stressed algae has
been produced (Muller-Navarra 1995) but Sundbom and Vrede (1997) have shown that these fatty
acid effects are most likely only under conditions
of moderate P stress when algal CIP is 150-300:1.
Above that ratio, direct mineral P limitation of
grazer growth is more likely. Furthermore, since the
fatty acid profiles vary considerably among various
algal species, potential HUFA effects are enhanced
by the mono specific conditions of laboratory studies but are less likely under field conditions, where
many algal species are on the menu. The first direct
evidence for mineral limitation of Daphnia growth
has recently been produced. Urabe et aI. (1997)
showed via an ingenious experimental design that
much of the reduced growth of Daphnia fed high
CIP algae (CIP ~ 1200) can be recovered by briefly
soaking animals in solutions with high concentrations of mineral phosphate (P04). Animals were apparently able to relieve their P-limitation by taking
up P0 4 -P through the gut (a phenomenon with the
discomfiting name of "anal drinking"!). While this
mechanism is clearly not relevant in natural situations, this experiment does provide direct evidence
that reduced growth of Daphnia when fed high CIP
algae reflects a real mineral limitation by P. These
data come from laboratory studies. However, there
is accumulating evidence that the deleterious effects of poor food quality associated with Plimitation of algal growth may be widespread in
natural systems (DeMott and Gulati 1999; MacKay
and Elser 1998). It now seems clear that food of
high CIP ratio is manifestly unsuitable for many
grazers, especially animals with high body P contents that must acquire disproportionately more P
from their diet in order to construct their Pintensive biomass.
The observation of characteristic differences in
elemental composition among various zooplankton
species has inspired considerable speculation and,
more recently, tests assessing the physiological,
cellular, and evolutionary bases of those differences. These include efforts to interpret the strong
variation in body phosphorus content that has been
documented. Why do animals such as daphnids and
calanoid copepods, organisms of similar body size,
similar habitat, and reasonably close taxonomic affiliation, have such drastically different body phosphorus contents (around 1.2 to 1.8% P in Daphnia,
~0.3 to 0.5% P in adult copepods)? Hessen and
Lyche (1991) were first to speculate that these differences might be associated with differences between these groups in body RNA content, arguing
that RNA is a P-rich molecule known to vary considerably among organisms. This idea was echoed
by Sterner and Hessen (1994), who suggested that
if the RNA hypothesis were supported, it would be
"possible to see a clear thread extending all the way
from molecular biology to ecosystem function."
That thread is beginning to be woven into a more
substantive fabric. Elser et al. (1996) delved in considerable detail into the elemental composition of
biomolecules and cellular structures, showing that
major biomolecules (especially proteins and nucleic acids) differ little in terms of percentage of N
but strongly in terms of percentage of P. This result
that makes it easier to understand the relative uniformity of N composition of cellular structures and
whole organisms along with the strong variation
observed in percentage of P. Elser et al. (1996) established that the ribosome, the cellular structure of
protein synthesis and thus animal growth, is the
most P-rich of major cellular structures considered,
and thus argued that animal elemental composition
(especially percentage of P but also NIP) was
strongly linked to body growth rate due to the high
demand for ribosomes and P-rich RNA during
rapid growth. They also produced evidence from
literature studies showing negative correlations between body-specific growth rate and body NIP ratio
in Daphnia and Drosophila. New data specific to
this hypothesis have recently been produced by
Main et al. (1997), who showed in studies of five
cladoceran species that percentage of N changed
little with body growth rate but percentage of P
395
as recycling of potentially limiting nutrient elements from the food. Thus, in the work synthesized
by Sterner and Hessen (1994), imbalance between
herbivore requirements and food elemental composition is seen to have effects at a variety of levels
of ecological integration, from individual behavior
to ecosystem processes.
Our understanding of the importance of food
quality on zooplankton dynamics and the rest of the
pelagic ecosystem has been considerably refined
even in the brief period since the review of Sterner
and Hessen (1994) appeared (Sterner and Schulz
1998). Evidence for the role of certain essential
fatty acids (highly unsaturated fatty acids, or
HUFA) in constraining zooplankton growth when
feeding on moderately nutrient-stressed algae has
been produced (Muller-Navarra 1995) but Sundbom and Vrede (1997) have shown that these fatty
acid effects are most likely only under conditions
of moderate P stress when algal CIP is 150-300:1.
Above that ratio, direct mineral P limitation of
grazer growth is more likely. Furthermore, since the
fatty acid profiles vary considerably among various
algal species, potential HUFA effects are enhanced
by the mono specific conditions of laboratory studies but are less likely under field conditions, where
many algal species are on the menu. The first direct
evidence for mineral limitation of Daphnia growth
has recently been produced. Urabe et aI. (1997)
showed via an ingenious experimental design that
much of the reduced growth of Daphnia fed high
CIP algae (CIP ~ 1200) can be recovered by briefly
soaking animals in solutions with high concentrations of mineral phosphate (P04). Animals were apparently able to relieve their P-limitation by taking
up P0 4 -P through the gut (a phenomenon with the
discomfiting name of "anal drinking"!). While this
mechanism is clearly not relevant in natural situations, this experiment does provide direct evidence
that reduced growth of Daphnia when fed high CIP
algae reflects a real mineral limitation by P. These
data come from laboratory studies. However, there
is accumulating evidence that the deleterious effects of poor food quality associated with Plimitation of algal growth may be widespread in
natural systems (DeMott and Gulati 1999; MacKay
and Elser 1998). It now seems clear that food of
high CIP ratio is manifestly unsuitable for many
grazers, especially animals with high body P contents that must acquire disproportionately more P
from their diet in order to construct their Pintensive biomass.
The observation of characteristic differences in
elemental composition among various zooplankton
species has inspired considerable speculation and,
more recently, tests assessing the physiological,
cellular, and evolutionary bases of those differences. These include efforts to interpret the strong
variation in body phosphorus content that has been
documented. Why do animals such as daphnids and
calanoid copepods, organisms of similar body size,
similar habitat, and reasonably close taxonomic affiliation, have such drastically different body phosphorus contents (around 1.2 to 1.8% P in Daphnia,
~0.3 to 0.5% P in adult copepods)? Hessen and
Lyche (1991) were first to speculate that these differences might be associated with differences between these groups in body RNA content, arguing
that RNA is a P-rich molecule known to vary considerably among organisms. This idea was echoed
by Sterner and Hessen (1994), who suggested that
if the RNA hypothesis were supported, it would be
"possible to see a clear thread extending all the way
from molecular biology to ecosystem function."
That thread is beginning to be woven into a more
substantive fabric. Elser et al. (1996) delved in considerable detail into the elemental composition of
biomolecules and cellular structures, showing that
major biomolecules (especially proteins and nucleic acids) differ little in terms of percentage of N
but strongly in terms of percentage of P. This result
that makes it easier to understand the relative uniformity of N composition of cellular structures and
whole organisms along with the strong variation
observed in percentage of P. Elser et al. (1996) established that the ribosome, the cellular structure of
protein synthesis and thus animal growth, is the
most P-rich of major cellular structures considered,
and thus argued that animal elemental composition
(especially percentage of P but also NIP) was
strongly linked to body growth rate due to the high
demand for ribosomes and P-rich RNA during
rapid growth. They also produced evidence from
literature studies showing negative correlations between body-specific growth rate and body NIP ratio
in Daphnia and Drosophila. New data specific to
this hypothesis have recently been produced by
Main et al. (1997), who showed in studies of five
cladoceran species that percentage of N changed
little with body growth rate but percentage of P