Biologically Mediated Phosphorus Retention Processes
19
have acquired the ability to modulate their sinking velocity through buoyancy regulation. This ability is most highly evolved in colonial cyanobacteria, where the regulation of density by means of gas vesicles enables this
group to perform active diurnal migration (Walsby and Reynolds 1980).
More limited changes in sinking velocity related to growth conditions have
also been found in other, nonflagellated groups like diatoms (Titman and
Kilham 1976).
The basic principles of the Stokes relation apply equally to organisms
with positive and negative buoyancy, so that both terminal upward migration velocity and terminal sinking velocity will generally increase with cell
or colony size (Reynolds 1989). It is, therefore, only the largest phytoplankton units like colonial cyanobacteria and large dinoflagellates that are
able to perform significant diurnal vertical migration, enabling them to
exploit hypolimnetic nutrients resources that would be unavailable to
nonmigrating species. It is thus conceivable that in certain lakes, vertically
migrating species of algae can constitute a net source of phosphorus to the
pelagic zone.
Vertical migration is also well documented in the major groups of
macrozooplankton. Although a variety of theories have been presented to
explain this behavior (see, e.g., Mangel and Clark 1988), the current consensus seems to be that vertical migration is mainly the result of a tradeoff
between foraging and predator avoidance (Gliwicz 1986; Gliwicz and
Pijanowska 1989). Wright and Shapiro (1984) proposed that the reduction
in total phosphorus after biomanipulation in Round Lake was caused by
vertically migrating Daphnia that were excreting phosphorus in the hypolimnion at night, which was ingested in the epilimnion during the day.
From the close coupling between ingestion and release of phosphorus that
has been found in Daphnia (Olsen and 0stgaard 1985; Olsen et al. 1986b)
and the short gut passage times found in Daphnia (Geller 1975), it is
unlikely that Daphnia P release in the hypolimnion should be related to P
ingestion in the epilimnion for more that a short transient period (a few gut
evacuation periods) after migrating. Vertically migrating zooplankton
should therefore have only minor impact on the phosphorus budget of the
pelagic zone.
In a review of measured sinking rates in plankton algae, Heaney and
Butterwick (1985) indicate sinking velocities (j = 0.1 - 1 m day·· in large,
nonmotile plankton algae like diatoms and desmids. Such sinking velocities
would translate to loss rates from 0.01 to 0.2 day·· for organisms suspended
in a typical mixed layer of depth zm = 5-10 m, which is more than an order
of magnitude larger than the average sinking loss rate as estima:ed by
Prairie (1989) and also above the 95% confidence band in Fig. 2.2. This
indicates that the phosphorus economy of lakes is far more efficient than
would be the case if large, nonmotile plankton algae were a major compartment of total phosphorus.
19
have acquired the ability to modulate their sinking velocity through buoyancy regulation. This ability is most highly evolved in colonial cyanobacteria, where the regulation of density by means of gas vesicles enables this
group to perform active diurnal migration (Walsby and Reynolds 1980).
More limited changes in sinking velocity related to growth conditions have
also been found in other, nonflagellated groups like diatoms (Titman and
Kilham 1976).
The basic principles of the Stokes relation apply equally to organisms
with positive and negative buoyancy, so that both terminal upward migration velocity and terminal sinking velocity will generally increase with cell
or colony size (Reynolds 1989). It is, therefore, only the largest phytoplankton units like colonial cyanobacteria and large dinoflagellates that are
able to perform significant diurnal vertical migration, enabling them to
exploit hypolimnetic nutrients resources that would be unavailable to
nonmigrating species. It is thus conceivable that in certain lakes, vertically
migrating species of algae can constitute a net source of phosphorus to the
pelagic zone.
Vertical migration is also well documented in the major groups of
macrozooplankton. Although a variety of theories have been presented to
explain this behavior (see, e.g., Mangel and Clark 1988), the current consensus seems to be that vertical migration is mainly the result of a tradeoff
between foraging and predator avoidance (Gliwicz 1986; Gliwicz and
Pijanowska 1989). Wright and Shapiro (1984) proposed that the reduction
in total phosphorus after biomanipulation in Round Lake was caused by
vertically migrating Daphnia that were excreting phosphorus in the hypolimnion at night, which was ingested in the epilimnion during the day.
From the close coupling between ingestion and release of phosphorus that
has been found in Daphnia (Olsen and 0stgaard 1985; Olsen et al. 1986b)
and the short gut passage times found in Daphnia (Geller 1975), it is
unlikely that Daphnia P release in the hypolimnion should be related to P
ingestion in the epilimnion for more that a short transient period (a few gut
evacuation periods) after migrating. Vertically migrating zooplankton
should therefore have only minor impact on the phosphorus budget of the
pelagic zone.
In a review of measured sinking rates in plankton algae, Heaney and
Butterwick (1985) indicate sinking velocities (j = 0.1 - 1 m day·· in large,
nonmotile plankton algae like diatoms and desmids. Such sinking velocities
would translate to loss rates from 0.01 to 0.2 day·· for organisms suspended
in a typical mixed layer of depth zm = 5-10 m, which is more than an order
of magnitude larger than the average sinking loss rate as estima:ed by
Prairie (1989) and also above the 95% confidence band in Fig. 2.2. This
indicates that the phosphorus economy of lakes is far more efficient than
would be the case if large, nonmotile plankton algae were a major compartment of total phosphorus.