Climate Change and the Hydrology and Morphology of Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 73

Current, streambed composition and sediment transport are strongly linked
(Gordon et al . 2004). Simultaneously, an increase in discharge often results in an
increase in bed-sediment movement and more resuspension of bottom sediments or
soil eroded from banks (Wood & Armitage 1997). Erosion, transport and
sedimentation thus result in local changes within the streambed. Short-term
equilibria typically persist between disturbances, but spates, especially with
anthropogenic disturbances (canalization, agriculture in the watershed), can lead to
changes in stream slope and mean velocity, sediment particle size distribution,
sediment discharge and the planform of the stream (e.g. cut-off meanders). Climate-
change-induced extreme spates will reshape the gross form of the channel.
Maintenance of that form and its smaller-scale features, such as substrate patterns,
may be better related to more frequent, less-extreme discharge events. Streambed
and bank stability determine the influence of high discharges on the channel form.
For example, a high density of plant roots in the bank reduces erosion. The discharge
which just fills the stream to its banks, termed the bankfull discharge, is often
assumed to control the form of streams (Allan 1995; Gordon et al . 2004).
In a stream reach, different current velocities occur along meanders, around
obstacles like debris and in pool-riffle sequences. Spates erode, establish or
maintain these structures. Stream organisms respond differently to changes in
current extremes, substrate composition and stability. The patchy distribution of
aquatic macrophytes is related to variation in disturbance frequency and intensity
brought about by changes in discharge and by colonization success and growth
rates (Sand-Jensen & Madsen 1992). Climate change will cause changes, probably
increases, because of lower summer flows, in macrophyte abundances (Rørslett
et al . 1989; Walker et al . 1994). A more variable discharge pattern may reduce
macroinvertebrate species richness and standing crop within the streambed
(Layzer et al . 1989; Munn & Brusven 1991) but increase plant species diversity.
Cobb et al . (1992) show that a further increase in current velocity variability will
lower macroinvertebrate diversity due to substrate instability.
Fish are often adapted to a certain level of hydrological variability (Poff &
Allan 1995), but changes in variability may have negative effects on fish populations
(Ficke et al . 2007). With a change from predictable to unpredictable hydraulic
conditions (Poff & Allan 1995), the fish community will be prone to move from
'environmental specialists' to a community with only generalist species, that is,
those that are able to exploit a wide variety of resources and changing environmental
conditions (Fausch & Bestgen 1997). Also, if climate change results in more lake-
like conditions in summer with low to zero velocities along with macrophyte
growth, then pest species, such as mosquitoes, could become highly abundant (De
Moor 1986). Under such conditions, more tolerant, ubiquitous fish species may
become dominant (Pusey et al . 1993).
Effects of climate change on hydraulic conditions and channel
morphology at the habitat scale
At the habitat scale, subtle variations in current, near-bed velocities, bottom
roughness, grain sizes and distribution together influence the distribution and
abundance of particular species of plants and animals. The primary need of