Geoscience Reference
In-Depth Information
a proxy for water temperature change. Finally, it
is possible to obtain seasonal mean projections for
temperature and precipitation, but as this draws
predictions away from possible extremes, which are
more likely to affect pearl mussels, they have not
been used.
of this are that differential effects of extreme
temperatures may cause problems by decoupling
the timings of pearl mussel and host salmonid
reproduction.
Of the 71 extant pearl mussel rivers under
consideration, 63 of these are relatively small
to moderate, shallow watercourses, which
will be susceptible to significant rises in air,
and consequently water, temperature. Only
eight Scottish river catchments are likely to be
large enough (depth and volume of water) for
pearl mussels to be able to withstand predicted
detrimental extreme air temperature rises. This
means nearly 90% of extant Scottish pearl mussel
rivers are considered vulnerable or susceptible
to UKCP09 high temperature projections
Temperature
The high projection scenarios for 2070-2090
suggest that likely mean temperature change is
between 0-5
◦
C warmer for summer (with July data
used as a proxy for summer). This broadly reflects
predicted UK changes to warmer summers. Taking
the upper extreme of the projections, with mean
July air temperatures increased by up to 5
◦
Cand
associated precipitation (a proxy for flows), this is
likely to have complex and detrimental effects on
pearl mussels. However, the UKCP09 projections
do not cover extreme events such as storms (and
resultant flooding) and prolonged droughts in
sufficient detail to provide likely predictions.
During the 20th century, the European mean air
surface temperature warmed by
ca
0.8
◦
C(Beniston
and Tol, 1998). Unfortunately, it is not known
how the projected summer increase of between
0-5
◦
C will affect the hydrothermal regimes of
rivers, owing to a dearth of long-term freshwater
temperature monitoring. There is some evidence
that elevated water temperatures may enhance
juvenile recruitment in pearl mussel populations
(Hruska, 1992; Mackie and Roberts, 1995; Hastie
et al.,
2003a). Therefore, it is possible that a
modest elevation of mean summer temperatures
may actually benefit some Scottish pearl mussel
populations. However, increased temperatures
have the potential to cause adult mussels to grow
more quickly, live shorter lives and thus have a
lower number of reproductive episodes (Ziuganov
et al.
, 2000).
Most of these studies have shown potential
beneficial changes within current temperature
ranges but critical, upper (extreme) thermal limits
for the survival of this species are unknown. Ross
(1992) showed that pearl mussel reproduction may
vary by several months due to thermal effects and
Young and Williams (1984) showed that the timing
of reproduction in Scotland may be related to when
host salmonid fry are abundant. The implications
for
2070-2090.
Precipitation
Future changes in mean summer precipitation
are more difficult to predict under the high
scenario than likely temperature changes. There is
a chance summer precipitation (using July data)
will increase by up to 20% or decline by up
to 40%. Predicted winter change (using January
data) is also not clear, but precipitation may
perhaps decrease slightly or more likely increase
by anything up to 60% for the high scenario
by 2070-2090. This broadly reflects predicted UK
changes to drier summers and wetter winters.
During the 20th century, the west coast of
Scotland saw a 16% decrease in sunshine (as a
consequence of increased cloud cover) between
1941 and 1971 and between 1964 and 1993,
with resultant 15-20% increases in precipitation
levels (Mayes, 1996; Harrison, 1997). Some of
this precipitation was concentrated into greater
storm events (Black, 1996), which caused dramatic
changes to river-bed habitat structure and the
distribution and abundance of pearl mussels
(Figure 10.3).
During detailed work on the River Kerry (at
Gairloch) in west Scotland (Hastie
et al.,
2003a)
striking correlations between historical rainfall and
pearl mussel recruitment patterns were observed
between 1955 and 1995, with recruitment doing
better in wetter years (Figure 10.4). It is believed
that higher flows associated with wet years help to
