26. Stoichiometric Analysis of Pelagic Ecosystems: The Biogeochemistry of Planktonic Food Webs
401
of the detrital material being metabolized (Tezuka
1989, 1990; Hessen 1994). For example, Tezuka
(1989) showed that mineralization ofN and P (i.e.,
release of N and P into dissolved forms) by bacteria
was strongly dependent on the elemental composition of the detrital material. When detritus was
derived from P-limited algae and thus had high CIP
and low CIN ratios, bacterial activity resulted in
release of inorganic N but no release of inorganic
P, as P was apparently retained by P-limited bacteria. Conversely, when N-limited algae with high
CIN and low CIP ratios were used as a source of
detrital material, bacteria freely mineralized P but
retained N. Field studies also support the key role
of stoichiometry in determining the role of bacteria
in nutrient cycling in pelagic systems. For example,
Elser et al. (1995a) showed that when the NIP ratio
of bacteria-sized particles « 1.0 )lm) was less than
20, bacteria immobilized inorganic N, but when
NIP was greater than 20, bacteria released N. Thus,
pelagic bacteria were sinks or sources of N in the
internal N cycle depending on their current physiological condition. In the same lakes, experiments
using naturally derived organic substrates from algae varying in their nutritional status showed that
the C, N, and P demands of the bacteria varied considerably during the growing season, closely tracking bacterial population development during
growth and stationary phases (Chrzanowski et al.
1997). These studies show that under natural conditions, bacterial demands for key elements are
highly variable and that the natural environment
rarely provides organic energy and important nutrient elements at rates and proportions best suited
for maximal bacterial growth.
The work just briefly outlined identifies similarities in key principles regulating ecological dynamics in the microscopic and macroscopic worlds.
That is, at the bottom of the microbial food web is
a food base, the bacteria, that (like algae in the conventional food chain) has a strongly varying elemental composition linked to its growth status. Analytical and theoretical approaches that ignore
potential variation in the elemental composition of
bacteria may lead investigators astray. Somewhat
unique to the microbial food web is that organisms
at the base, the bacteria, are heterotrophic. Nevertheless, bacterial activities also operate within a
stoichiometric framework, as illustrated by shifts in
source/sink function with changes in growth status
and substrate composition. Stoichiometric principles also hold during microbial trophic interactions,
as bacteria are fed upon by consumers whose rates
of nutrient release are a function of both food and
consumer elemental composition. Food quality effects on protozoan growth are, to my knowledge,
not yet documented but seem likely given the realities of mass balance and the possibility of poor
matches between protozoan demands and the composition of nutrient-stressed bacteria. The work
summarized above illustrates that stoichiometric
mechanisms are fully at play in the microbial realm.
Application of stoichiometric thinking may help integrate our approaches to microscopic and macroscopic ecology.
Methodological Issues
Progress in understanding pelagic ecosystem dynamics in stoichiometric terms has been relatively
rapid, pursued by a number of investigators internationally. Nevertheless, limitations on further progress remain. Some of these are conceptual and
theoretical while others are technological. With respect to the latter, progress in understanding the
stoichiometric relations of some key pelagic players under field conditions, such as bacteria, protozoa, and small metazoans, is limited by our ability
to make reliable determinations of their elemental
composition. Such limitations may be solved by
improved methods of isolating appreciable numbers of these taxa from field samples and by improved methods of elemental analysis capable of
providing accurate data on small sample sizes. Furthermore, we need better methods of characterizing
the elemental composition of all of the individual
particles (bacteria, various algal species, detritus,
protozoa) that contribute to suspended particulate
matter, the base of pelagic food webs. Some studies
have made initial attempts via X-ray microanalysis
(Olsen and others 1986) but currently such procedures are too laborious for routine application.
Analysis of materials isolated by flow cytometry of
live samples may be an alternative means, but this
technology is currently too expensive and cumbersome for routine application.
The work reviewed above with reference to algal
CIP ratios and the light/nutrient hypothesis suggests
close coupling between hydrodynamic processes
401
of the detrital material being metabolized (Tezuka
1989, 1990; Hessen 1994). For example, Tezuka
(1989) showed that mineralization ofN and P (i.e.,
release of N and P into dissolved forms) by bacteria
was strongly dependent on the elemental composition of the detrital material. When detritus was
derived from P-limited algae and thus had high CIP
and low CIN ratios, bacterial activity resulted in
release of inorganic N but no release of inorganic
P, as P was apparently retained by P-limited bacteria. Conversely, when N-limited algae with high
CIN and low CIP ratios were used as a source of
detrital material, bacteria freely mineralized P but
retained N. Field studies also support the key role
of stoichiometry in determining the role of bacteria
in nutrient cycling in pelagic systems. For example,
Elser et al. (1995a) showed that when the NIP ratio
of bacteria-sized particles « 1.0 )lm) was less than
20, bacteria immobilized inorganic N, but when
NIP was greater than 20, bacteria released N. Thus,
pelagic bacteria were sinks or sources of N in the
internal N cycle depending on their current physiological condition. In the same lakes, experiments
using naturally derived organic substrates from algae varying in their nutritional status showed that
the C, N, and P demands of the bacteria varied considerably during the growing season, closely tracking bacterial population development during
growth and stationary phases (Chrzanowski et al.
1997). These studies show that under natural conditions, bacterial demands for key elements are
highly variable and that the natural environment
rarely provides organic energy and important nutrient elements at rates and proportions best suited
for maximal bacterial growth.
The work just briefly outlined identifies similarities in key principles regulating ecological dynamics in the microscopic and macroscopic worlds.
That is, at the bottom of the microbial food web is
a food base, the bacteria, that (like algae in the conventional food chain) has a strongly varying elemental composition linked to its growth status. Analytical and theoretical approaches that ignore
potential variation in the elemental composition of
bacteria may lead investigators astray. Somewhat
unique to the microbial food web is that organisms
at the base, the bacteria, are heterotrophic. Nevertheless, bacterial activities also operate within a
stoichiometric framework, as illustrated by shifts in
source/sink function with changes in growth status
and substrate composition. Stoichiometric principles also hold during microbial trophic interactions,
as bacteria are fed upon by consumers whose rates
of nutrient release are a function of both food and
consumer elemental composition. Food quality effects on protozoan growth are, to my knowledge,
not yet documented but seem likely given the realities of mass balance and the possibility of poor
matches between protozoan demands and the composition of nutrient-stressed bacteria. The work
summarized above illustrates that stoichiometric
mechanisms are fully at play in the microbial realm.
Application of stoichiometric thinking may help integrate our approaches to microscopic and macroscopic ecology.
Methodological Issues
Progress in understanding pelagic ecosystem dynamics in stoichiometric terms has been relatively
rapid, pursued by a number of investigators internationally. Nevertheless, limitations on further progress remain. Some of these are conceptual and
theoretical while others are technological. With respect to the latter, progress in understanding the
stoichiometric relations of some key pelagic players under field conditions, such as bacteria, protozoa, and small metazoans, is limited by our ability
to make reliable determinations of their elemental
composition. Such limitations may be solved by
improved methods of isolating appreciable numbers of these taxa from field samples and by improved methods of elemental analysis capable of
providing accurate data on small sample sizes. Furthermore, we need better methods of characterizing
the elemental composition of all of the individual
particles (bacteria, various algal species, detritus,
protozoa) that contribute to suspended particulate
matter, the base of pelagic food webs. Some studies
have made initial attempts via X-ray microanalysis
(Olsen and others 1986) but currently such procedures are too laborious for routine application.
Analysis of materials isolated by flow cytometry of
live samples may be an alternative means, but this
technology is currently too expensive and cumbersome for routine application.
The work reviewed above with reference to algal
CIP ratios and the light/nutrient hypothesis suggests
close coupling between hydrodynamic processes