Geoscience Reference
In-Depth Information
35
Diatoms
Macrophytes
30
25
20
15
10
5
0
0
500
1000
1500
2000
Discharge (mm)
Figure 11.2 Mean number of diatoms and macrophyte species at different discharge categories (based on catchment
rainfall runoff to that site). Vertical lines indicate standard error for each mean value. A linear regression for both the
diatoms and macrophytes is shown as a dotted line. The diatom regression line is significant at P = 0.036, and the
macrophyte regression line is significant at P
=
0.017. The lines have significantly different slopes at P
=
0.01.
(Seele et al ., 2000) helps to produce a more general
measure of ecological quality when they are used
in conjunction, making it useful for the WFD;
the highly fluctuating diatom metric is effectively
stabilized
The future of biological
indices?
Regulatory agencies may hope that biological
indices developed for the WFD provide an
integrated way to assess all the ecological
problems a river may be subjected to, their
causes, and management solutions. However,
biological monitoring in the WFD gives a subjective
estimate of ecological degradation based on a few
parameters that are often subject to high natural
variation. Ecological systems are not composed
of separate physical, chemical, macrophyte,
invertebrate, fish and phytobenthos components,
but are complex highly interconnected and
dynamic systems. The development of new indices,
as well as the specification of monitoring methods,
should reflect the information requirements,
and thus there is plenty of scope for developing
more specialized indices outside the scope of the
WFD, for example to diagnose specific impacts.
In such cases, combining information from
all the different biological elements (as well
as hydromorphological, riparian and chemical
components) in a more structured way that reflects
their ecological interactions may produce a more
by
the
longer
macrophyte
response
time.
The present method of combining indices from
biological elements in the WFD is a 'one-out,
all-out' method in which the status of the
worst biological element determines the overall
ecological quality. This approach has been criticized
repeatedly for being over-sensitive (Moss et al. ,
2003; Søndergaard et al. , 2005; Johnson et al. ,
2006; Dodkins and Rippey, 2008; Moss, 2008;
Noges et al. , 2009; Borja and Rodrıguez, 2010). The
alternative of averaging metrics is also problematic
due to eclipsing, i.e. a failing metric being ignored
owing to the dominance of other metrics reporting
high status values (Suter, 1993; Dodkins and
Rippey, 2008). By combining macrophytes and
diatoms, which compete for similar resources of
light and nutrients, a more balanced measure of
the ecological status is produced without the same
problems of eclipsing, since low scoring species,
whether diatoms or macrophytes, will still have an
influence on the final ecological status.
Search WWH ::




Custom Search