Direct and Indirect Effects of Herbivorous Zooplankton
7
1989, 1990) indicate that any extension to the simple loading models, for
the purpose of describing biological effects on the phosphorus retention
process, should incorporate at least the partitioning of phosphorus
between the major biological compartments of the plankton.
Biological Phosphorus Retention Processes. The results of Mazumder et al.
(1988, 1989, 1990) fit well with the significant reductions in total phosphorus observed after biomanipulation in, for example, Round Lake (Shapiro
and Wright 1984) and Haugatj0nna (Reinertsen et al. 1989). If the phosphorus loading stays the same after the biomanipulation, this must be the
result of an increased phosphorus retention, or a real oligotrophication of
the system sensu Hutchinson (1973). As these lakes were dominated by
cladoceran zooplankton, the increased loss of phosphorus was not likely to
be caused by "the fecal pellet express" associated with marine copepods
(Turner and Ferrante 1979). Wright and Shapiro (1984) suggested that
excretion from vertically migrating Daphnia could constitute a net transport of phosphorus into the hypolimnion, but both the short gut passage
time in Daphnia and the common observation that vertical migration
ceases when vertebrate predation is relaxed, speak against this explanation.
Although the elimination of fish through poisoning or fishing might remove a substantial fraction of the standing stock of phosphorus in a lake, a
change in the size of this pool would normally be undetectable in phosphorus budgets based on total phosphorus estimates from water samples
alone. If the fish biomass stands in some kind of long-term equilibrium
with the phosphorus loading and if the loading is unchanged after an episodic elimination of fish, the part of the phosphorus load supporting the fish
populations probably will be directed elsewhere in the food web. It is only if
fish populations are continually harvested that fishing will constitute a retention process in the long-term phosphorus dynamics of the lake.
One of the major effects of reduced vertebrate predation would be a
change in the ultimate fate of large Daphnia from a high probability of
being eaten by fish, to a likelihood of dying from senescence or starvation.
From the knowledge of growth efficiencies of planktivorous fish, it would
be expected that a large amount of the phosphorus contained in ingested
zooplankton would be recycled in the epilimnion (Olsen 1988). On the
other hand, zooplankters dying from nonpredatory mortality would sink
quickly out from the epilimnion with a velocity of 2 to 20 m h-l, according
to ·data reviewed by Hutchinson (1967), making any decomposition before
reaching the sediment surface very unlikely. This was observed in Haugatj0nna as a large increase in dead zooplankton collected by the sedimentation traps after the rotenone treatment (Reinertsen et al. 1989).
7
1989, 1990) indicate that any extension to the simple loading models, for
the purpose of describing biological effects on the phosphorus retention
process, should incorporate at least the partitioning of phosphorus
between the major biological compartments of the plankton.
Biological Phosphorus Retention Processes. The results of Mazumder et al.
(1988, 1989, 1990) fit well with the significant reductions in total phosphorus observed after biomanipulation in, for example, Round Lake (Shapiro
and Wright 1984) and Haugatj0nna (Reinertsen et al. 1989). If the phosphorus loading stays the same after the biomanipulation, this must be the
result of an increased phosphorus retention, or a real oligotrophication of
the system sensu Hutchinson (1973). As these lakes were dominated by
cladoceran zooplankton, the increased loss of phosphorus was not likely to
be caused by "the fecal pellet express" associated with marine copepods
(Turner and Ferrante 1979). Wright and Shapiro (1984) suggested that
excretion from vertically migrating Daphnia could constitute a net transport of phosphorus into the hypolimnion, but both the short gut passage
time in Daphnia and the common observation that vertical migration
ceases when vertebrate predation is relaxed, speak against this explanation.
Although the elimination of fish through poisoning or fishing might remove a substantial fraction of the standing stock of phosphorus in a lake, a
change in the size of this pool would normally be undetectable in phosphorus budgets based on total phosphorus estimates from water samples
alone. If the fish biomass stands in some kind of long-term equilibrium
with the phosphorus loading and if the loading is unchanged after an episodic elimination of fish, the part of the phosphorus load supporting the fish
populations probably will be directed elsewhere in the food web. It is only if
fish populations are continually harvested that fishing will constitute a retention process in the long-term phosphorus dynamics of the lake.
One of the major effects of reduced vertebrate predation would be a
change in the ultimate fate of large Daphnia from a high probability of
being eaten by fish, to a likelihood of dying from senescence or starvation.
From the knowledge of growth efficiencies of planktivorous fish, it would
be expected that a large amount of the phosphorus contained in ingested
zooplankton would be recycled in the epilimnion (Olsen 1988). On the
other hand, zooplankters dying from nonpredatory mortality would sink
quickly out from the epilimnion with a velocity of 2 to 20 m h-l, according
to ·data reviewed by Hutchinson (1967), making any decomposition before
reaching the sediment surface very unlikely. This was observed in Haugatj0nna as a large increase in dead zooplankton collected by the sedimentation traps after the rotenone treatment (Reinertsen et al. 1989).