208
Grazers as Sources and Sinks for Nutrients:Conclusions, Limitations, and Speculations
net sink in the total nutrient mass balance of a lake. The actual quantitative
importance of zooplankton as a sink in the total nutrient budget will depend
on both the amount of nutrient located in zooplankton and on the dominating fate of individual organisms.
All the models considered in this work, as well as a recent regional survey
(Hessen et a!. 1992), suggest that the fraction of total nutrients located in
zooplankton biomass will increase to a maximum at some intermediate
nutrient loading rate, and then decrease with further enrichment. Since loss
rates from natural zooplankton mortality appear to be high compared to
observed net loss rates of total phosphorus, it is predicted that phosphorus
retention should also follow a unimodal pattern with a maximum of
zooplankton-mediated nutrient losses at some intermediate loading level.
Such a mechanism might possibly explain a substantial part of the orderof-magnitude variability in phosphorus retention that is found among
lakes with the same water renewal rate (Fig. 2.2).
In the presence of zooplankton predators, one would expect a reduced
importance of zooplankton mortality in total nutrient losses, since nutrients in dead zooplankters should then, to a larger extent, either be recycled
or incorporated into predator biomass, instead of being lost from the
pelagic zone. Changes in zooplankton-mediated retention offer an explanation to the observed reductions in total phosphorus after successful biomanipulation treatments (Wright and Shapiro 1984; Reinertsen et al. 1989;
Sanni and Wrervigen 1990). The decreasing importance of zooplankton
mortality as a retention process at very high nutrient loadings, predicted by
the present model (Fig. 5.16), might explain the contrasting observations of
Jeppesen et al. (1990), who found that biomanipulation did not reduce total
phosphorus in several hypereutrophic Danish lakes.
When seen from the perspective of the whole phytoplankton community,
grazers will be both a sink in the sense that they constitute a loss process in
algal mass budget, and a source in the sense that they supply the algae with
regenerated nutrients. On a sufficiently long-term scale, zooplankton will
nevertheless be a net sink of nutrients from the phytoplankton as a whole.
Comparison of the range of phosphorus subsistence quotas and storage
capacities in phytoplankton (Figs. 3.3 and 3.4) with typical zooplankton
element ratios (Andersen and Hessen 1991) indicates that P-limited food
algae should often be phosphorus-deficient relative to the requirements for
balanced grazer growth. If zooplankton were able to reclaim all metabolic
wastes when element composition of the food particles is inadequate for
balanced growth, grazers could in certain situations also constitute a gross
sink for phosphorus (i.e. no source at all). The analysis of P-release rates in
Section 4.5 suggests that the phosphorus economy of Daphnia is less efficient
than this extreme, and that we thus should expect some P to be released from
grazers, at least as long as the food P content remains nonzero.
In a simple two-level food chain, such as the minimal P-cycling model
described in Chapter 5, we should expect an efficient channeling of nutrient
Grazers as Sources and Sinks for Nutrients:Conclusions, Limitations, and Speculations
net sink in the total nutrient mass balance of a lake. The actual quantitative
importance of zooplankton as a sink in the total nutrient budget will depend
on both the amount of nutrient located in zooplankton and on the dominating fate of individual organisms.
All the models considered in this work, as well as a recent regional survey
(Hessen et a!. 1992), suggest that the fraction of total nutrients located in
zooplankton biomass will increase to a maximum at some intermediate
nutrient loading rate, and then decrease with further enrichment. Since loss
rates from natural zooplankton mortality appear to be high compared to
observed net loss rates of total phosphorus, it is predicted that phosphorus
retention should also follow a unimodal pattern with a maximum of
zooplankton-mediated nutrient losses at some intermediate loading level.
Such a mechanism might possibly explain a substantial part of the orderof-magnitude variability in phosphorus retention that is found among
lakes with the same water renewal rate (Fig. 2.2).
In the presence of zooplankton predators, one would expect a reduced
importance of zooplankton mortality in total nutrient losses, since nutrients in dead zooplankters should then, to a larger extent, either be recycled
or incorporated into predator biomass, instead of being lost from the
pelagic zone. Changes in zooplankton-mediated retention offer an explanation to the observed reductions in total phosphorus after successful biomanipulation treatments (Wright and Shapiro 1984; Reinertsen et al. 1989;
Sanni and Wrervigen 1990). The decreasing importance of zooplankton
mortality as a retention process at very high nutrient loadings, predicted by
the present model (Fig. 5.16), might explain the contrasting observations of
Jeppesen et al. (1990), who found that biomanipulation did not reduce total
phosphorus in several hypereutrophic Danish lakes.
When seen from the perspective of the whole phytoplankton community,
grazers will be both a sink in the sense that they constitute a loss process in
algal mass budget, and a source in the sense that they supply the algae with
regenerated nutrients. On a sufficiently long-term scale, zooplankton will
nevertheless be a net sink of nutrients from the phytoplankton as a whole.
Comparison of the range of phosphorus subsistence quotas and storage
capacities in phytoplankton (Figs. 3.3 and 3.4) with typical zooplankton
element ratios (Andersen and Hessen 1991) indicates that P-limited food
algae should often be phosphorus-deficient relative to the requirements for
balanced grazer growth. If zooplankton were able to reclaim all metabolic
wastes when element composition of the food particles is inadequate for
balanced growth, grazers could in certain situations also constitute a gross
sink for phosphorus (i.e. no source at all). The analysis of P-release rates in
Section 4.5 suggests that the phosphorus economy of Daphnia is less efficient
than this extreme, and that we thus should expect some P to be released from
grazers, at least as long as the food P content remains nonzero.
In a simple two-level food chain, such as the minimal P-cycling model
described in Chapter 5, we should expect an efficient channeling of nutrient