Environmental Engineering Reference
In-Depth Information
Table 10.3 List of selected metrics considered in the new index, their response to disturbance
( > : direct relation; < : indirect relation; >< : unimodal relation) and their importance in the
definition of the index value, expressed as weight
Biological quality
elements
Metrics
Response to
disturbance
Weights
1. Chl a concentration ( m gl 1 )
Phytoplankton
>
0.14
Macroalgae
2. Percent cover of opportunistic species
(%)
>
0.152
3. Biomass (gWW m -2 )
><
0.168
Seagrasses
4. Presence (yes-no)
0.097
<
Benthic
invertebrates
5. Shannon diversity (H')
0.145
<
Fish fauna
6. Total number of fish species
0.152
<
7. Number of estuarine resident
species
0.145
<
and unlikely to be affected by errors. Unfortunately, finding metrics with both features
is a challenging task. Single metrics often display ambiguous responses to human
disturbance, especially in transitional waters where the border between natural and
human-induced disturbance is really fuzzy. Furthermore, different biological elements
may respond in different ways to a disturbance. Therefore, an ecologically robust
approach could be a combination of different biological elements and characteristics
of these elements into a single classification tool, which can provide the integrated
response of the community.
First, a careful survey of relevant literature on ecological indices for transitional
waters was performed. The survey highlighted a set of seven metrics indicated as
ecologically relevant by many authors (Table 10.3 ). They were used as input
variables. Membership functions were drawn for all variables, trying to preserve
the personal judgement of different experts. Figure 10.2 illustrates how thresholds
indicated by different authors have been integrated and utilized to design member-
ship functions for the metric “total number of fish species”. For all variables, the
characteristics associated with bad ecological status and the characteristics asso-
ciated to high ecological status have been clearly identified. All seven metrics do
not have the same importance for ecological quality assessment: they have been
weighted as shown in the last column of Table 10.3 .
The rule-base has been developed by setting the combination of all variables in
their worst characteristics to bad ecological status: if (total number of fish species is
low, and Chl a concentration is high , and seagrasses are absent , and
.), then
ecological status is bad .Conversely, high ecological status has been assigned to the
combination of all the variables' best characteristics: if (total number of fish species is
high, and Chl a concentration is low, and seagrasses are present, and
...
.), then
ecological status is high . Intermediate combinations have been associated to the
other three classes of ecological status, good, moderate, poor . The metrics weighting
and the rules assessment have been performed by means of an automatic procedure
explained inMarchini et al. (2009). The following mathematical operations have been
used to calculate the output: algebraic product for conjunction ( and ) and for implica-
tion ( if ... then ), algebraic sum for disjunction ( or ). They have been chosen as they
...
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