Phosphorus Retention in Lakes
15
where the dimensionless quantity R, is termed the phosphorus retention of
the lake, or the ratio of P retained in the lake (S,) to P entering the lake (L,).
Equation (2.3) implies that predicting the response in terms of lake total P
concentration from a given dose in terms of P loading is equivalent to
predicting the phosphorus retention of the lake. It is, therefore, not surprising that much research effort has been focused on predicting R, for a
given lake from other, more easily measurable morphometric and hydraulic
parameters.
The annual average water renewal rate of a temperate lake will necessarily conceal substantial temporal variation; since much of the annual water
budget will be associated with the spring flood, throughflow will usually be
much less in other seasons. An annual average dilution rate of 0.01 day"1
would thus correspond to a much lower dilution rate experienced by the
summer plankton community. If we are mostly concerned with water exchange rates in the stratified season, then we must also take into account
water exchange between epi- and hypolimnion due to deepening of the
mixed layer (entrainment). If mixed layer depth doubles from start to end
of the stratified season (5-6 months in northern temperate areas), this will
be equivalent to an entraiment-driven dilution rate <0.0066 day-I. In other
words, dilution loss rates due to throughflow and entrainment are most
likely in the order of <1% day-I for plankton communities of stratified,
temperate lakes. As will be apparent from forthcoming chapters, such a
rate will be small compared to maximal specific rates for most biological
process in plankton communities.
2.2 Phosphorus Retention in Lakes
Apart from certain transient situations occurring after strong reductions of
P loading to highly eutrophic lakes, the phosphorus retention will generally
be positive. This means that over sufficiently long timescales, all lakes will
act as net sinks for phosphorus. Mass-balance studies in many lakes show
that phosphorus retentions over the whole domain 0 S; Rp:S 1 can be measured, making the predictive power of Eq. (2.3) essentially zero as long as Rp
is unknown for a particular lake. Vollenweider (1976) observed that phosphorus retention varied systematically with water residence time, so that an
increase in the water residence time would increase the probability of a
given P atom being trapped in the bottom sediments instead of leaving the
lake via the outflow. Several authors have since presented slightly different
functions fitted to empirical data sets for the purpose of predicting P
retention when water residence time or dilution rate is known (e.g.,
Oglesby 1977; Lee et al. 1978).
15
where the dimensionless quantity R, is termed the phosphorus retention of
the lake, or the ratio of P retained in the lake (S,) to P entering the lake (L,).
Equation (2.3) implies that predicting the response in terms of lake total P
concentration from a given dose in terms of P loading is equivalent to
predicting the phosphorus retention of the lake. It is, therefore, not surprising that much research effort has been focused on predicting R, for a
given lake from other, more easily measurable morphometric and hydraulic
parameters.
The annual average water renewal rate of a temperate lake will necessarily conceal substantial temporal variation; since much of the annual water
budget will be associated with the spring flood, throughflow will usually be
much less in other seasons. An annual average dilution rate of 0.01 day"1
would thus correspond to a much lower dilution rate experienced by the
summer plankton community. If we are mostly concerned with water exchange rates in the stratified season, then we must also take into account
water exchange between epi- and hypolimnion due to deepening of the
mixed layer (entrainment). If mixed layer depth doubles from start to end
of the stratified season (5-6 months in northern temperate areas), this will
be equivalent to an entraiment-driven dilution rate <0.0066 day-I. In other
words, dilution loss rates due to throughflow and entrainment are most
likely in the order of <1% day-I for plankton communities of stratified,
temperate lakes. As will be apparent from forthcoming chapters, such a
rate will be small compared to maximal specific rates for most biological
process in plankton communities.
2.2 Phosphorus Retention in Lakes
Apart from certain transient situations occurring after strong reductions of
P loading to highly eutrophic lakes, the phosphorus retention will generally
be positive. This means that over sufficiently long timescales, all lakes will
act as net sinks for phosphorus. Mass-balance studies in many lakes show
that phosphorus retentions over the whole domain 0 S; Rp:S 1 can be measured, making the predictive power of Eq. (2.3) essentially zero as long as Rp
is unknown for a particular lake. Vollenweider (1976) observed that phosphorus retention varied systematically with water residence time, so that an
increase in the water residence time would increase the probability of a
given P atom being trapped in the bottom sediments instead of leaving the
lake via the outflow. Several authors have since presented slightly different
functions fitted to empirical data sets for the purpose of predicting P
retention when water residence time or dilution rate is known (e.g.,
Oglesby 1977; Lee et al. 1978).