40
P. Tett
The relationship can be gotten in two ways: by observing y for many values of x;
or by developing a mathematical model that predicts y from x. In either case, there
will be some uncertainty in any prediction of y, and this is shown by the grey area
that surrounds the relationship line.
Regulators need to set thresholds for the impact indicators, exemplified for the
water column and the zone B scale by the CSTT’s threshold of 10 mg chlorophyll
m
−3 in summer. The greatest tolerable pressure is that which takes the y-axis variable up to, but not past, the threshold. This pressure is the assimilative capacity of
that particular water body for the waste responsible for the pressure, and it is a regulator’s or planner’s task to consent discharges only up to this capacity, taking
account of any natural contributions towards it. It may also be a farm manager’s
task (as a condition of a consent to discharge) to ensure that the zone A pressures,
and the contribution to zone B scale pressures, are controlled so that the impacts do
not exceed those allowed.
Note that an impact threshold is not an EQS, which is explicitly defined by the
WFD in terms of concentrations of pollutants. An impact threshold can, also, be
stated implicitly, by way of an Ecological Quality Objective or EcoQO, exemplified for the zone A benthos by the Scottish regulator’s requirement that there must
be at least 2 taxa of Polychaeta worms alive within a fish-farm AZE. Painting et al.
(2005) consider the utility of several larger-scale EcoQOs intended to prevent
eutrophication.
maximum tolerable
impact
management aims to reduce pressure
below ecosystem's 'assimilative capacity'
uncertainty in knowledge of relationship
'safe' region - impact
below threshold,
allowing
for uncertainty
maximum safe pressure
(assimilative capacity)
pressure indicator
e.g. waste loading per
unit area or unit
exchanged volume
functional relationship,
between pressure and
impact, obtained from
mathematical model
or local observations
impact indicator
e.g. AMBI, ITI, PCI,
maximum chlorophyll,
oxygen deficiency
Fig. 1.8 The estimation of assimilative capacity. Note that the impact indicators: “maximum
chlorophyll” and “oxygen deficiency” directly indicate impact; the AMBI, ITI and PCI are constructed so that their values are high for high ecological status, and thus, strictly, it is their inverse
that is an indicator of impact
P. Tett
The relationship can be gotten in two ways: by observing y for many values of x;
or by developing a mathematical model that predicts y from x. In either case, there
will be some uncertainty in any prediction of y, and this is shown by the grey area
that surrounds the relationship line.
Regulators need to set thresholds for the impact indicators, exemplified for the
water column and the zone B scale by the CSTT’s threshold of 10 mg chlorophyll
m
−3 in summer. The greatest tolerable pressure is that which takes the y-axis variable up to, but not past, the threshold. This pressure is the assimilative capacity of
that particular water body for the waste responsible for the pressure, and it is a regulator’s or planner’s task to consent discharges only up to this capacity, taking
account of any natural contributions towards it. It may also be a farm manager’s
task (as a condition of a consent to discharge) to ensure that the zone A pressures,
and the contribution to zone B scale pressures, are controlled so that the impacts do
not exceed those allowed.
Note that an impact threshold is not an EQS, which is explicitly defined by the
WFD in terms of concentrations of pollutants. An impact threshold can, also, be
stated implicitly, by way of an Ecological Quality Objective or EcoQO, exemplified for the zone A benthos by the Scottish regulator’s requirement that there must
be at least 2 taxa of Polychaeta worms alive within a fish-farm AZE. Painting et al.
(2005) consider the utility of several larger-scale EcoQOs intended to prevent
eutrophication.
maximum tolerable
impact
management aims to reduce pressure
below ecosystem's 'assimilative capacity'
uncertainty in knowledge of relationship
'safe' region - impact
below threshold,
allowing
for uncertainty
maximum safe pressure
(assimilative capacity)
pressure indicator
e.g. waste loading per
unit area or unit
exchanged volume
functional relationship,
between pressure and
impact, obtained from
mathematical model
or local observations
impact indicator
e.g. AMBI, ITI, PCI,
maximum chlorophyll,
oxygen deficiency
Fig. 1.8 The estimation of assimilative capacity. Note that the impact indicators: “maximum
chlorophyll” and “oxygen deficiency” directly indicate impact; the AMBI, ITI and PCI are constructed so that their values are high for high ecological status, and thus, strictly, it is their inverse
that is an indicator of impact