The Dynamics of the Pelagic Phosphorus Cycle
25
Hessen et al. (1992) estimated the zooplankton contribution to particulate phosphorus in 45 Norwegian lakes, based on zooplankton biomass
measurements and species-specific conversion factors from dry weight to
P. As shown in Fig. 2.5, zooplankton phosphorus constituted from 3 to 49%
of particulate P, with a median of 14%. The distribution was more skewed
than the distributions of total P fractions (Fig. 2.4), and could be reasonably well represented by a lognormal distribution.
The phytoplankton contribution to particulate P is more difficult to
quantify, but a rough maximum estimate is possible from phytoplankton
biovolume measurements and the assumption that the algae contain C and
P in Redfield proportions (atomic C:P = 106:1). The Redfield ratio is generally thought to be characteristic of algae growing at a rate close to their
innate capacity (e.g., Goldman et al. 1979), so that slower-growing algae
would be expected to contain less P than this per unit biomass. The resulting sum of zoo- and phytoplankton contributions ranged from 18 to
>100% of particulate P, with a median of 41% (Fig. 2.5). The presence of
two cases where this maximum estimate exceeded the measured particulate
P is not unexpected, when taking into account the accumulated measurement uncertainties involved in this calculation, though it might also be
noticed that these two lakes are among those that had the highest yield of
the chlorophyll a per unit total P in Fig. 2.3, indicating that algal growth
rates might have been appreciably below their potential.
A main feature of Fig. 2.5 is that even when applying a maximum estimate for the algal P:C ratio, the compound contribution from phyto- and
zooplankton amounted to <60% of particulate P in 75% of the investigated
lakes. Although the remainder may have some contribution from nonliving
particles (like clay or humus), it is most likely that a significant fraction of
it is located in bacteria (cf. Vadstein et al. 1988). It thus seems reasonable to
conclude that, while a significant fraction of particulate P is allocated to
phytoplankton, it is often not the major phosphorus pool in the plankton
community. On the other hand, both bacteria and zooplankton seem to
contain more conspicuous fractions of particulate P than would be
expected from their contributions to plankton biomass.
2.5 The Dynamics of the Pelagic Phosphorus Cycle
We can expect different components of the plankton to suffer widely different sinking losses, making the actual sedimentary loss of phosphorus in
a given lake crucially dependent on both the partitioning of phosphorus
between dissolved and particulate fractions, and among different groups of
plankton organisms. The distribution of phosphorus within the particulate
fraction will depend on both the biomass proportions among different
plankton components and their specific phosphorus contents. The biomass
25
Hessen et al. (1992) estimated the zooplankton contribution to particulate phosphorus in 45 Norwegian lakes, based on zooplankton biomass
measurements and species-specific conversion factors from dry weight to
P. As shown in Fig. 2.5, zooplankton phosphorus constituted from 3 to 49%
of particulate P, with a median of 14%. The distribution was more skewed
than the distributions of total P fractions (Fig. 2.4), and could be reasonably well represented by a lognormal distribution.
The phytoplankton contribution to particulate P is more difficult to
quantify, but a rough maximum estimate is possible from phytoplankton
biovolume measurements and the assumption that the algae contain C and
P in Redfield proportions (atomic C:P = 106:1). The Redfield ratio is generally thought to be characteristic of algae growing at a rate close to their
innate capacity (e.g., Goldman et al. 1979), so that slower-growing algae
would be expected to contain less P than this per unit biomass. The resulting sum of zoo- and phytoplankton contributions ranged from 18 to
>100% of particulate P, with a median of 41% (Fig. 2.5). The presence of
two cases where this maximum estimate exceeded the measured particulate
P is not unexpected, when taking into account the accumulated measurement uncertainties involved in this calculation, though it might also be
noticed that these two lakes are among those that had the highest yield of
the chlorophyll a per unit total P in Fig. 2.3, indicating that algal growth
rates might have been appreciably below their potential.
A main feature of Fig. 2.5 is that even when applying a maximum estimate for the algal P:C ratio, the compound contribution from phyto- and
zooplankton amounted to <60% of particulate P in 75% of the investigated
lakes. Although the remainder may have some contribution from nonliving
particles (like clay or humus), it is most likely that a significant fraction of
it is located in bacteria (cf. Vadstein et al. 1988). It thus seems reasonable to
conclude that, while a significant fraction of particulate P is allocated to
phytoplankton, it is often not the major phosphorus pool in the plankton
community. On the other hand, both bacteria and zooplankton seem to
contain more conspicuous fractions of particulate P than would be
expected from their contributions to plankton biomass.
2.5 The Dynamics of the Pelagic Phosphorus Cycle
We can expect different components of the plankton to suffer widely different sinking losses, making the actual sedimentary loss of phosphorus in
a given lake crucially dependent on both the partitioning of phosphorus
between dissolved and particulate fractions, and among different groups of
plankton organisms. The distribution of phosphorus within the particulate
fraction will depend on both the biomass proportions among different
plankton components and their specific phosphorus contents. The biomass