Climate Change and the Hydrology and Morphology of Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 71

The effects of climate change on hydraulics and morphology
at the reach scale
The pattern and variation of current velocity within a stream reach has a major
influence on longitudinal and transverse channel morphology, species diversity,
food-web structure and ecological processes (Jowett & Duncan 1990; Poff et al .
1997). It is often assumed that monthly or daily means, which are often readily
available, are sufficient to characterize flow regime (e.g. Clausen & Biggs 2000;
Olden & Poff 2003). However, even single events can cause substantial changes
in the physical habitat and can affect ecological functioning (Schlosser 1995;
Arndt et al . 2002). Gore et al . (2001) listed five major hydraulic conditions that
most affect the distribution and ecological success of stream biota: suspended
load, bed-load movement, water-column effects, such as turbulence and velocity
profile, and substratum interactions (near-bed hydraulics). However, stream
organisms are generally adapted to a wide variability in stream discharge (Allan
1995; Petts et al . 2000) and can accommodate large changes.
A study carried out in the River Lambourn assessed how hydraulic changes at the
reach scale affected benthic macroinvertebrate community composition. The results
suggested considerable resilience in stream macroinvertebrate communities to
future changes in discharge regimes. Changes in the cover of the five main
mesohabitats ( Berula , Ranunculus and Callitriche plant stands, gravel and silt
(Fig. 4.3)) were recorded over 9 years. The macroinvertebrate community associated
with each mesohabitat was also sampled (Wright et al . 2003). Berula and Ranunculus
stands were more abundant in early summers preceded by high-discharge winters,
while gravel habitats tended to be more prominent after winters with lower
discharge. The plant and mineral mesohabitats proved to be distinct from each
other, with the caddis fly families, Glossosomatidae and Goeridae, being relatively
more abundant in gravel, the bivalves, Sphaeriidae, favouring silt patches and the
mayfly families, Ephemerellidae and Caenidae, preferring the plants. However,
taxa did not strongly associate with just one particular mesohabitat; they tended to
be equally abundant on two or more mesohabitats. Perhaps because of the lack of
strong obligate relationships between taxa and specific mesohabitats, the early
summer density of most taxa was not correlated with previous winter discharge
levels. But distribution across a range of available habitats means that the
macroinvertebrate community is buffered against yearly variations in discharge
regime, as long as there are no extreme spates resulting in strong currents. These
can cause catastrophic downstream drift due to increased shear stress (Layzer et al .
1989), especially for small animals (De Jalon et al . 1994).
The increase in discharge extremes (both spates and droughts) and the decrease
in discharge predictability expected with changing climate may increase the relative
importance of abiotic parameters, both hydraulic conditions and physical habitat
features, in shaping future stream communities (Poff & Ward 1989). Spates have
substantial effects. Some organisms may move or be moved to habitats with less
disturbance (e.g. stream margins, the hyporheic zone (Boulton et al . 1998) or
other patches protected from high flow) and, thus, will seek refuges (Townsend &
Hildrew 1994; Fritz & Dodds 2004). Others may be able to remain in or on