The Fate of Zooplankton Egesta: Carbon Cycling and Chaos
189
The different biogeochemical cycles in an ecosystem are interrelated in
the sense that they constitute complementary views of the same system, as
pointed out by Reiners (1986). The carbon cycle in a phosphorus-limited
system can therefore not be represented independently of the phosphorus
cycle. The main differences between the carbon cycle in Fig. 6.15 and the
phosphorus cycle in Fig. 6.21 are the reversal of the flow between the dissolved pool and the phytoplankton compartment, and the absence of a
detrital pool of phosphorus. The assumption of no detrital P compartment
is justified by X-ray micrographic observations of natural detritus particles
(Olsen et al. 1986b). The opposite flows of dissolved organic carbon and
dissolved inorganic phosphorus with respect to the phytoplankton compartment reflect the dualism as both commensalists and competitors in the
interaction between algae and bacteria (Bratbak and Thingstad 1985).
Organic Carbon Release Processes. The release of extracellular products
from algal photosynthesis was first recognized by Fogg (1966). Although
the importance of this process was violently disputed for some time (e.g.,
Sharp 1977), extracellular release seems now to be generally accepted as a
source of high-quality carbon to bacteria (Cole et al. 1982; Riemann and
Sendergaard 1986). In laboratory studies, extracellular release has been
found to increase under growth limitation (Jensen 1984), suggesting an
inverse relationship where an increasing fraction of total photosynthesis is
directed into extracellular release with decreasing growth rate. Inverse
relationships between fraction extracellular release and total primary production or phytoplankton biomass have been found in several data sets
from single habitats, but a recent compilation of measurements from different plankton communities failed to reproduce this pattern (Baines and
Pace 1991).
In the 93 observations from lacustrine systems considered by Baines and
Pace (1991), the fraction of extracellular release of total primary production ranged from 0.02 to 0.73, with a median ofO.2l. A parsimonious model
might therefore be that algal extracellular carbon release is a constant fraction Ie = 0.21 of total primary production. If we denote algal specific
growth rate and biomass by II. (day"l) and C. [(mg C) rl), the total extracellular release rate from phytoplankton becomes Ie II. C •.
Hessen et al. (1989) found high assimilation efficiency in Daphnia on
bacteria labeled with radioactive nucleic acids, while assimilation efficiencies by the same species were much lower on bacteria labeled with an
amino acid mixture (Hessen et al. 1990). These fmdings are consistent if
zooplankton have a differential food utilization in favor of P-rich nucleic
acids compared to proteins when the bulk of available food is highly
P-deficient relative to the demands for balanced growth. This view is supported by the observation (Hessen and Andersen 1990) that zooplankton
are able to utilize bacterial P with high efficiency. Zooplankton assimilation
efficiencies might thus be more variable among different biochemical con-
189
The different biogeochemical cycles in an ecosystem are interrelated in
the sense that they constitute complementary views of the same system, as
pointed out by Reiners (1986). The carbon cycle in a phosphorus-limited
system can therefore not be represented independently of the phosphorus
cycle. The main differences between the carbon cycle in Fig. 6.15 and the
phosphorus cycle in Fig. 6.21 are the reversal of the flow between the dissolved pool and the phytoplankton compartment, and the absence of a
detrital pool of phosphorus. The assumption of no detrital P compartment
is justified by X-ray micrographic observations of natural detritus particles
(Olsen et al. 1986b). The opposite flows of dissolved organic carbon and
dissolved inorganic phosphorus with respect to the phytoplankton compartment reflect the dualism as both commensalists and competitors in the
interaction between algae and bacteria (Bratbak and Thingstad 1985).
Organic Carbon Release Processes. The release of extracellular products
from algal photosynthesis was first recognized by Fogg (1966). Although
the importance of this process was violently disputed for some time (e.g.,
Sharp 1977), extracellular release seems now to be generally accepted as a
source of high-quality carbon to bacteria (Cole et al. 1982; Riemann and
Sendergaard 1986). In laboratory studies, extracellular release has been
found to increase under growth limitation (Jensen 1984), suggesting an
inverse relationship where an increasing fraction of total photosynthesis is
directed into extracellular release with decreasing growth rate. Inverse
relationships between fraction extracellular release and total primary production or phytoplankton biomass have been found in several data sets
from single habitats, but a recent compilation of measurements from different plankton communities failed to reproduce this pattern (Baines and
Pace 1991).
In the 93 observations from lacustrine systems considered by Baines and
Pace (1991), the fraction of extracellular release of total primary production ranged from 0.02 to 0.73, with a median ofO.2l. A parsimonious model
might therefore be that algal extracellular carbon release is a constant fraction Ie = 0.21 of total primary production. If we denote algal specific
growth rate and biomass by II. (day"l) and C. [(mg C) rl), the total extracellular release rate from phytoplankton becomes Ie II. C •.
Hessen et al. (1989) found high assimilation efficiency in Daphnia on
bacteria labeled with radioactive nucleic acids, while assimilation efficiencies by the same species were much lower on bacteria labeled with an
amino acid mixture (Hessen et al. 1990). These fmdings are consistent if
zooplankton have a differential food utilization in favor of P-rich nucleic
acids compared to proteins when the bulk of available food is highly
P-deficient relative to the demands for balanced growth. This view is supported by the observation (Hessen and Andersen 1990) that zooplankton
are able to utilize bacterial P with high efficiency. Zooplankton assimilation
efficiencies might thus be more variable among different biochemical con-