Geoscience Reference
In-Depth Information
(a)
(b)
10 m
Mussel density (No. m
-2
)
dry
<1
1-10
11-50
51-100
>100
Figure 10.3
Contour maps of mussel densities recorded at a site on the River Kerry, north-west Scotland (a) before,
and (b) after a major flood. Based on visible numbers of mussels per 1m
2
quadrats (n
=
445 and 434, respectively)
counted in 36 cross-river transects. From: Hastie
et al.
(2003a)
Ambio
32
: 40-6, with kind permission of Springer
Science
+
Business Media B.V.
clean and aerate the stable substrates making them
more suitable for juvenile recruitment. However,
during 1998, a 1 in 100-year return flood in the
River Kerry killed
50 000 mussels (
ca
5-10%
of the total population). There were other major
floods in other mussel rivers during this period, but
the effects were not quantified (Hastie
et al.,
2001).
Once again, of the 71 extant pearl mussel rivers
under consideration, 63 of these are relatively small
to moderate, shallow watercourses, which will be
susceptible to flooding. Many of these watercourses
have limited suitable habitat, and extreme floods
would probably have significant and detrimental
effects. However, a significant moderating factor
within the headwaters of a pearl mussel catchment
is the presence of a sizeable lake. These can
dramatically 'dampen down' or ameliorate the
effects of flood and spate events, by accumulating
and releasing water over greater lengths of time
than typical smaller 'spate' rivers, reducing the
downstream magnitude of such extreme events.
Sixty-three of the 71 extant populations (89%)
have lakes within the mid-upper catchment,
potentially benefitting pearl mussels present.
The eight largest Scottish pearl mussel river
catchments are likely to be big enough (with
significant areas of suitable habitat) to be able
to withstand predicted extreme flood events
