The concentration of total phosphorus (TP) in a water-body
depends primarily on the external load of this nutrient, but to varying degrees
also on so-called “internal loading”, i.e. release of phosphorus from
sediments. This may be particularly expressed in very eutrophic water-bodies
with anaerobic water above the sediments. Usually, after substantial decrease
of external loading, P stored in the sediments is gradually leached and diluted
out of the system, and with time, internal loading also declines. In
water-bodies with low water exchange rates, this may take very long, and
sediment treatment may be adequate to speed up the process of trophic recovery.
For rough
orientation for assessing the risk of eutrophication from external loading, the
model by Vollenweider et al. (1980) may be applied. This is a purely empirical
model, in which internal TP-concentrations depend only on 3 parameters: the
concentration in the tributaries, hydraulic load (i.e. amount of water flowing
into the water-body per surface area), and the retention time of the water.
TP = 1,55 [Lp
/Qs /(1 +Ö Tw )] 0,82
Where
TP = total phosphorus
concentration in the water-body (µg / L)
Lp = annual load of TP (mg m-2 a-1)
(calculated from hydraulic load and TP concentration in the tributaries)
Qs = hydraulic load (m a-1)
Tw = retention
time of the water (a)
With this function and the
mean depth of the water-body, the critical load for a given water-body (LPcrit)
can be calculated in relation to the TP-concentration targeted for the
water-body:
Lcrit =
TPtarget (1 + Ö Tw) zm/Tw
where Zm = mean
depth of the water-body
Reference: Vollenweider R (1982) Eutrophication of Waters. Monitoring,
Assessment and Control. OECD, Paris, 154
pp
A good explanation is given by Persson G. and Jansson M. (eds., 1988):
Phosphorus in Freshwater Ecosystems. Kluwer Academic Publishers.
This rough estimate of target TP load for
achieving a targeted TP concentration is linked to high uncertainty, as
specific conditions within the water-body may lead to higher or lower
load/concentration relationships. For example, this relationship strongly
depends on the extent to which sediments act as source or sink for phosphorus.
(This may be estimated using the model SIMPL by Schauser et al.) Therefore, setting a target for the critical
or acceptable TP load may substantially improve estimates of the specific
relationship of load to concentration for a given water-body.