Geoscience Reference
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The effects of drought on benthic invertebrates were investigated in a number
of acid-sensitive streams in Ontario, Canada (Gilbert et al . 2008). Relative
abundance of Ephemeroptera, Plecoptera and Trichoptera, collectively indicators
of healthy benthic communities, increased the first year following drought but
then decreased the second year indicating high resistance but poor resilience.
Chironomids, on the other hand, showed the opposite effects. Drought acts as a
disturbance mechanism that simplifies benthic community assemblages and thus
reduces biodiversity.
The studies of acid episodes under future scenarios of climate change and acid
deposition point to increased frequency and severity. Kroglund et al . (2008)
point out that this implies poorer conditions for populations of salmon. A similar
conclusion was reached by Kowalik et al . (2007) with respect to invertebrates in
British streams.
There are clear thresholds at which ecosystem structure and function show
dramatic changes. An obvious such threshold in response to increasing temperature
is the point at which a lake is no longer ice-covered in the winter and thus
subjected to circulation rather than thermal stratification. In acidified lakes, this
might mean that the acid snowmelt in the spring may be mixed with the entire
lake volume rather than be restricted to a 1-m thin layer beneath the ice. Such
acidic layers have been shown to be the limiting factor with respect to recruitment
of lake-spawning fish species, such as lake trout in Canada (Gunn & Keller 1984)
and brown trout in Norway (Barlaup et al . 1998). Other such thresholds may be
related to biogeochemical processes in the catchment, for example, a severe
drought might kill the vegetation, or milder climate might promote expansion of
insects and diseases with similar effect, and to species-specific events, for example,
in a high elevation lake, where warmer water may result in crossing of a
developmental threshold such that invertebrates can complete an entire life cycle
(Borgstrøm 2001) within the year.
Conclusions
Projections of the synergistic effects of acid deposition and climate change on
freshwater ecosystems are inherently fraught with the uncertainty that such
projections are for climatic conditions not currently experienced. For many of
the climate scenarios, the projected mean temperature in the future will be well
above that observed even in extreme years during the period of observation
(maximum 30 years for most ecosystems). The ecosystem responses are probably
not linear; thus, extrapolation from observations, even those spanning several
decades, entails going outside the range of observations.
It is acid deposition that is responsible for the widespread acidification of
surface waters in sensitive areas of Europe, eastern North America and elsewhere
in the world. This means that measures to reduce acidification problems can
continue to be focussed on reducing emissions of S and N compounds to the
atmosphere. Although reductions in emissions of S and N compounds have led
to dramatic improvements and recovery in water quality in acidified freshwater
ecosystems, biological recovery has lagged and the problem will remain in many
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