18
The Biogeochemical Theatre - Phosphorus Cycling and Phosphorus Household in Lakes
observations in Fig. 2.2; thus constituting a crude 95% confidence band for
the individual observations in the data set. The envelope curves shown in
Fig. 2.2 correspond to a variation in RL from 0 to 0.35, and in Up from
0.00025 day' I to 0.008 day'l. In other words, the load decay can be negligible
in some lakes while constituting more than one third of the load in others,
while the net sedimentation loss rate up can vary by a factor of 32 among
lakes with similar hydrological properties.
2.3 Biologically Mediated Phosphorus Retention
Processes
Phosphorus losses from the water phase to the sediments will usually be
closely linked to the settling of suspended particles, although exceptions
might be found in shallow lakes with a large ratio of sediment surface area
to lake volume, where direct adsorption/desorption processes between
water and sediment might be of importance. In lakes with nonreducing
hypolimnia, the P flux associated with settling particles will largely be a net
loss process, as phosphorus will normally be tightly bound to inorganic
sediment particles or incorporated in the biota of the sediment. While
inorganic particles like calcite and iron hydroxides or allochtoneous
organic particles like humus aggregations can be an important part of the
sedimenting material flux in certain types of lakes, it seems reasonable to
assume that the majority of the settling particles will be organic and of
autochtoneous origin. This means that much of the variability in phosphorus
loss rates among lakes with similar hydrology should be related to the
partitioning of phosphorus between particles with different residence times
in the pelagic zone.
The steady-state loss rate of passively sinking particles from a homogeneous pelagial is given by 0' = u1z,., where u is the terminal sinking velocity
(m day'l) and Zm is the depth of the mixed layer (m). The terminal sinking
velocity of a spherical particle is determined by the balance between
viscous and gravitational forces, which can be expressed as a function of
particle diameter and density by Stokes law. For nonspherical particles the
Stokes equation needs to be adjusted by a form-resistance factor, accounting for the fact that, for example, elongated, needle-like particles have
much lower sinking velocities than spherical particles of equivalent volume
and density (Hutchinson 1967).
In addition to the morphologically determined form-resistance factor,
plankton organisms have several other adaptations to remaining suspended in the water column. Flagellated phytoplankton species, ciliates,
and all planktonic metazoans have the ability to counteract the force of
gravity by active motion. Several nonflagellated groups of plankton algae
The Biogeochemical Theatre - Phosphorus Cycling and Phosphorus Household in Lakes
observations in Fig. 2.2; thus constituting a crude 95% confidence band for
the individual observations in the data set. The envelope curves shown in
Fig. 2.2 correspond to a variation in RL from 0 to 0.35, and in Up from
0.00025 day' I to 0.008 day'l. In other words, the load decay can be negligible
in some lakes while constituting more than one third of the load in others,
while the net sedimentation loss rate up can vary by a factor of 32 among
lakes with similar hydrological properties.
2.3 Biologically Mediated Phosphorus Retention
Processes
Phosphorus losses from the water phase to the sediments will usually be
closely linked to the settling of suspended particles, although exceptions
might be found in shallow lakes with a large ratio of sediment surface area
to lake volume, where direct adsorption/desorption processes between
water and sediment might be of importance. In lakes with nonreducing
hypolimnia, the P flux associated with settling particles will largely be a net
loss process, as phosphorus will normally be tightly bound to inorganic
sediment particles or incorporated in the biota of the sediment. While
inorganic particles like calcite and iron hydroxides or allochtoneous
organic particles like humus aggregations can be an important part of the
sedimenting material flux in certain types of lakes, it seems reasonable to
assume that the majority of the settling particles will be organic and of
autochtoneous origin. This means that much of the variability in phosphorus
loss rates among lakes with similar hydrology should be related to the
partitioning of phosphorus between particles with different residence times
in the pelagic zone.
The steady-state loss rate of passively sinking particles from a homogeneous pelagial is given by 0' = u1z,., where u is the terminal sinking velocity
(m day'l) and Zm is the depth of the mixed layer (m). The terminal sinking
velocity of a spherical particle is determined by the balance between
viscous and gravitational forces, which can be expressed as a function of
particle diameter and density by Stokes law. For nonspherical particles the
Stokes equation needs to be adjusted by a form-resistance factor, accounting for the fact that, for example, elongated, needle-like particles have
much lower sinking velocities than spherical particles of equivalent volume
and density (Hutchinson 1967).
In addition to the morphologically determined form-resistance factor,
plankton organisms have several other adaptations to remaining suspended in the water column. Flagellated phytoplankton species, ciliates,
and all planktonic metazoans have the ability to counteract the force of
gravity by active motion. Several nonflagellated groups of plankton algae