Partitioning of Phosphorus in the Plankton Community
21
Compared with nonmotile algal cells, dead zooplankton organisms sink
very fast; dead or narcotized Daphnia have sinking velocities of 50 to 500 m
day"1 according to data reviewed by Hutchinson (1967), corresponding to
an average residence time of at most 5 hours in the mixed layer after death.
It thus seems much more likely that the nutrients in dead algal cells could
be salvaged by autolysis and microbial degradation before leaving the mixed
layer, than in dead zooplankton material. The fate of zooplankton-bound
nutrients depends on the dominant cause of zooplankton mortality. If
predatory mortality is high, then nutrients will most likely be retained in
the pelagic zone either in the form of predator tissue or as nutrients recycled to the plankton community. If nonpredatory mortality due starvation
or senescence is dominating, the majority of zooplankton-bound nutrients
will eventually be lost from the plankton community. The potential importance of the latter process is emphasized by the observation that in lakes
with low zooplankton predation pressure, zooplankton can constitute a significant part of the vertical flux of phosphorus (Reinertsen et al. 1989;
Mazumder et al. 1990).
2.4 Partitioning of Phosphorus in the Plankton
Community
In lake management, the prediction of lake total phosphorus from loading
and retention is normally just a vehicle for predicting algal biomass, which
again is closely related to common water quality criteria like turbidity and
transparency. The general tendency for algal biomass (usually measured as
chlorophyll a concentration) to increase with total phosphorus is well
established, although most total P/chla regressions are found to contain
considerable residual variance. Figure 2.3 shows that while a 1:1 relationship between chla and total p forms a reasonable upper bound to potential
algal yield at given total P level (delimiting 99.1% ofthe observations), the
variance in the actual chla yield is more than one order of magnitude.
Many attempts have been made to reduce the prediction variance of algal
biomass in water quality models, most of them based on the assumption
that the variability in algal yield at a given total P level is mainly caused by
qualitative differences in the phosphorus supplied to different lakes. Much
research effort has therefore been invested in the quantification of the biologically available part of the phosphorus supply (cf. reviews by Cembella
et al. 1984a,b), although no generally accepted way to define bioavailable P
seems to have resulted from this. Much of the resulting confusion might be
rooted in the problems of defining a relevant timescale for bioavailability.
From the results of short-term bioassays, it has been argued that only the
pools of inorganic and easily hydrolyzable/desorbable phosphorus that are
21
Compared with nonmotile algal cells, dead zooplankton organisms sink
very fast; dead or narcotized Daphnia have sinking velocities of 50 to 500 m
day"1 according to data reviewed by Hutchinson (1967), corresponding to
an average residence time of at most 5 hours in the mixed layer after death.
It thus seems much more likely that the nutrients in dead algal cells could
be salvaged by autolysis and microbial degradation before leaving the mixed
layer, than in dead zooplankton material. The fate of zooplankton-bound
nutrients depends on the dominant cause of zooplankton mortality. If
predatory mortality is high, then nutrients will most likely be retained in
the pelagic zone either in the form of predator tissue or as nutrients recycled to the plankton community. If nonpredatory mortality due starvation
or senescence is dominating, the majority of zooplankton-bound nutrients
will eventually be lost from the plankton community. The potential importance of the latter process is emphasized by the observation that in lakes
with low zooplankton predation pressure, zooplankton can constitute a significant part of the vertical flux of phosphorus (Reinertsen et al. 1989;
Mazumder et al. 1990).
2.4 Partitioning of Phosphorus in the Plankton
Community
In lake management, the prediction of lake total phosphorus from loading
and retention is normally just a vehicle for predicting algal biomass, which
again is closely related to common water quality criteria like turbidity and
transparency. The general tendency for algal biomass (usually measured as
chlorophyll a concentration) to increase with total phosphorus is well
established, although most total P/chla regressions are found to contain
considerable residual variance. Figure 2.3 shows that while a 1:1 relationship between chla and total p forms a reasonable upper bound to potential
algal yield at given total P level (delimiting 99.1% ofthe observations), the
variance in the actual chla yield is more than one order of magnitude.
Many attempts have been made to reduce the prediction variance of algal
biomass in water quality models, most of them based on the assumption
that the variability in algal yield at a given total P level is mainly caused by
qualitative differences in the phosphorus supplied to different lakes. Much
research effort has therefore been invested in the quantification of the biologically available part of the phosphorus supply (cf. reviews by Cembella
et al. 1984a,b), although no generally accepted way to define bioavailable P
seems to have resulted from this. Much of the resulting confusion might be
rooted in the problems of defining a relevant timescale for bioavailability.
From the results of short-term bioassays, it has been argued that only the
pools of inorganic and easily hydrolyzable/desorbable phosphorus that are