20
The Biogeochemical Theatre - Phosphorus Cycling and Phosphorus Household in Lakes
One would expect motile members of the phytoplankton community to
suffer negligible sinking losses while alive. If nonpredatory mortality is
important in the dynamics of plankton algae, as suggested by some authors
(e.g., Jassby and Goldman 1974), dead cells of both motile and nonmotile
species of the same size would be expected to be lost from the mixed layer
at a similar rate. The effect of predatory mortality on the residence time of
algal material in the mixed layer is, among other things, dependent on the
characteristics of the grazer.
The fecal material from calanoid copepods and euphausids remains surrounded by a peritrophic membrane after egestion, constituting discrete
units called fecal pellets. The grazing activity by marine macrozooplankton
is therefore expected to increase the nutrient losses from the plankton
community through the repackaging of smaller, both motile and nonmotile,
algal cells into larger, faster-sinking fecal pellets (Turner and Ferrante
1979). The peritrophic membranes of Daphnia and other freshwater cladocerans are disrupted before egestion so that feces are released as a flocculent
substance composed of very small, and therefore slowly sinking, particles
(Peters 1987). We can thus expect that in freshwater plankton communities,
the presence of cladoceran grazers will tend to increase the average residence
time of dead or nonmotile algal material in the mixed layer by shredding
and disrupting larger cells and aggregates into smaller particles.
Detritus will often be the major component of the suspended organic
matter - even in lakes that receive negligible amounts of allochtoneous
particle input. It seems that zooplankton feces, especially from cladoceran
grazers, can be a major source of autochtoneous detritus (Olsen et al.
1986a; Hessen et al. 1990). Feces will generally be impoverished in nutrients
compared to the food, as grazers are able to extract a major fraction which
can either be released as recycled nutrients or incorporated into grazer
tissue. Repeated reingestion of detritus will, as discussed by Hessen and
Andersen (1992), lead to a progressive reduction of its nutrient content.
While grazing indisputably can be a major process in the vertical carbon
flux in the oceans (Banse 1990) and in the production and destruction of
detritus carbon in lakes (Hessen et al. 1990), these carbon fluxes will not
necessarily be mirrored by nutrient fluxes of comparable magnitude - at
least not in lakes dominated by cladocerans.
If we interpret the inverse of the maximal sinking loss rate as the minimal
residence time in the mixed layer, we can expect dead algal cells and fecal
material on the average to remain suspended in the mixed layer for at least
5 days before being lost to the hypolimnion. During this time, suspended
detritus can be the victim of autolytic processes, microbial degradation, and
reingestion by grazers, all of which would tend to extract nutrients from the
detritus before it leaves the mixed layer. Some field evidence for such an
efficient reclamation of phosphorus from dead algal cells and fecal material is
given by Olsen et al. (l986b), who found by use of X-ray micrography analysis that suspended detritus particles have very low P content.
The Biogeochemical Theatre - Phosphorus Cycling and Phosphorus Household in Lakes
One would expect motile members of the phytoplankton community to
suffer negligible sinking losses while alive. If nonpredatory mortality is
important in the dynamics of plankton algae, as suggested by some authors
(e.g., Jassby and Goldman 1974), dead cells of both motile and nonmotile
species of the same size would be expected to be lost from the mixed layer
at a similar rate. The effect of predatory mortality on the residence time of
algal material in the mixed layer is, among other things, dependent on the
characteristics of the grazer.
The fecal material from calanoid copepods and euphausids remains surrounded by a peritrophic membrane after egestion, constituting discrete
units called fecal pellets. The grazing activity by marine macrozooplankton
is therefore expected to increase the nutrient losses from the plankton
community through the repackaging of smaller, both motile and nonmotile,
algal cells into larger, faster-sinking fecal pellets (Turner and Ferrante
1979). The peritrophic membranes of Daphnia and other freshwater cladocerans are disrupted before egestion so that feces are released as a flocculent
substance composed of very small, and therefore slowly sinking, particles
(Peters 1987). We can thus expect that in freshwater plankton communities,
the presence of cladoceran grazers will tend to increase the average residence
time of dead or nonmotile algal material in the mixed layer by shredding
and disrupting larger cells and aggregates into smaller particles.
Detritus will often be the major component of the suspended organic
matter - even in lakes that receive negligible amounts of allochtoneous
particle input. It seems that zooplankton feces, especially from cladoceran
grazers, can be a major source of autochtoneous detritus (Olsen et al.
1986a; Hessen et al. 1990). Feces will generally be impoverished in nutrients
compared to the food, as grazers are able to extract a major fraction which
can either be released as recycled nutrients or incorporated into grazer
tissue. Repeated reingestion of detritus will, as discussed by Hessen and
Andersen (1992), lead to a progressive reduction of its nutrient content.
While grazing indisputably can be a major process in the vertical carbon
flux in the oceans (Banse 1990) and in the production and destruction of
detritus carbon in lakes (Hessen et al. 1990), these carbon fluxes will not
necessarily be mirrored by nutrient fluxes of comparable magnitude - at
least not in lakes dominated by cladocerans.
If we interpret the inverse of the maximal sinking loss rate as the minimal
residence time in the mixed layer, we can expect dead algal cells and fecal
material on the average to remain suspended in the mixed layer for at least
5 days before being lost to the hypolimnion. During this time, suspended
detritus can be the victim of autolytic processes, microbial degradation, and
reingestion by grazers, all of which would tend to extract nutrients from the
detritus before it leaves the mixed layer. Some field evidence for such an
efficient reclamation of phosphorus from dead algal cells and fecal material is
given by Olsen et al. (l986b), who found by use of X-ray micrography analysis that suspended detritus particles have very low P content.