Geoscience Reference
In-Depth Information
other instances, however, increased discharge in
warm, wet periods enhanced the negative effects
of episodic acidification (Kowalik
et al.
, 2007;
Ormerod and Durance, 2009). In combination,
all of these data illustrate how climate-mediated
effects on stream ecosystems are highly context-
specific.
For various riverine fish, long-term predictions
suggest that cold and cool water species will decline
substantially as a result of climate warming, with
habitat loss likely to be greater among those
with a more restricted distribution and in regions
with the greatest warming (Eaton and Scheller,
1996). Increasing problems with non-native species
establishment are also likely (Britton
et al.
, 2010).
Available trend data are limited, but sometimes
consistent with these predictions in revealing
changes in abundance and declining occurrence
in cooler-water taxa. For example, in the upper
Rh one since the 1970s, southern, thermophilic
fish species (e.g. chub (
Leuciscus cephalus
)and
barbel (
Barbus barbus
) have progressively replaced
northern, cold-water species such as dace (
Leuciscus
leuciscus
) (Daufresne
et al.
, 2004). Over longer
timescales, warming appears to have affected
the range of highly endangered species such as
the Atlantic sturgeon (
Acipenser sturio
) (Lassalle
et al.
, 2010). Adverse phenological effects are
also apparent through earlier emergence, for
example in the declining European grayling
(
Thymallus thymallus
) (Wedekind and Kung, 2010).
Elsewhere, prolonged drought effects during long-
term studies have reduced native fish abundance
while increasing threats from some invasive species
(Beche
et al.
, 2009). In the case of salmonids
- important commercially as well as through
their conservation status - northern species, such
as Arctic charr (
Salvelinus alpinus
), are liable to
be under particular threat. In riverine species,
however, and especially those that are long-
distance migrants, climatic effects are liable to be
extremely complex because of combined changes
during early life stages, during smoltification, on
migration and at sea (Jonsson and Jonsson, 2009;
Kennedy and Crozier, 2010). Emerging evidence
also suggests synergistic climatic effects between
temperature and discharge; hot, dry conditions
apparently
densities (Clews
et al.
, 2010). Northward retraction
of breeding ranges may well occur, and there is
some evidence of temperature increases already
reaching limits for successful breeding in some
currently occupied locations (Elliott and Elliott,
2010). However, despite thermal tolerances and
food requirements being sufficiently well known
in some salmonids for predictive models, better
field data are required on real trends and fitness
in wild populations in relation to temperature and
discharge (Weatherley
et al.
, 1991; Elliott, 2009).
This need for further field data extends also into
other taxonomic groups and processes. Beyond fish
and aquatic invertebrates, the literature on climate
change effects is still extremely sparse (Barlocher
et al.
, 2008).
Climate change effects on river
ecosystems: the bigger picture
Changes of the type described above are probably
a partial record of the trends now under way
because of changing climate. Moreover, there are
clearly data limitations and uncertainties over
predictions. Nevertheless, the trends described
earlier carry a range of important ramifications.
First, from a conservation perspective, they
indicate risks to species, such as Atlantic salmon,
which are not only important economically
but also figure in the notification of many
European rivers under the Habitats Directive
(Council of the European Communities, 1992).
Climate change also affects directly the wider
ecosystem character of river types that feature
in this European Directive as well as in national
policy, such as the UK Biodiversity Action
Plan (http://www.ukbap.org.uk/). Temperature
changes of the magnitude apparent not only
in upland headwaters, but also in lowland,
groundwater-fed chalk streams, might already
be having conservation effects (Durance and
Ormerod, 2007, 2009, 2010). For example, in
English chalk streams, winter temperatures during
the last 10 years have begun to approach the upper
developmental limit for the eggs of brown trout
(
Salmo trutta
)
and
Atlantic
salmon
(Elliott
and
matched
by
reductions
in
juvenile
Elliott, 2010).
