River Ecology and Restoration - River Dynamics and Integrated River Management - page 631

(a) (b)

(c) (d)

Fig. 10.7 Aquatic plants: (a) moss on cobbles; (b) Potamogeton ; (c) Lemna minor L ; and (d) Phragmites communis trirn

Benthic invertebrates collectively facilitate the breakdown of organic material, such as leaf litter, that
enters the stream from external sources. Larger leaf litter is broken down into smaller particles by the
feeding activities of invertebrates known as shredders (insect larvae and amphipods). Other invertebrates
filter smaller organic material from the water, known as filters (blackfly larvae, some mayfly nymphs,
and some caddisfly larvae); scrape material off the surfaces of bedrock, boulders, and cobles, known as
scrapers (snails, limpets, and some caddisfly and mayfly nymphs); or feed on material deposited on the
substrate, known as collectors (dipteran larvae and some mayfly nymphs) (Moss, 1988). Some macro-
invertebrates are predatory, known as predators , such as dragonfly, which prey on small vertebrates.
Figure10.8 shows typical species of the five groups with different ecological functions.
Fish are the apex predator in the aquatic system. Many restoration projects aim at restoration of fish
habitat. From the headwaters to the estuaries the composition of fish species varies considerably due to
changes in many hydrologic and geomorphic factors which control temperature, dissolved oxygen, gradient,
current velocity, and substrate. The amount of different habitats in a given stream section is determined
by a combination of these factors. Fish species richness (diversity) tends to increase downstream as
gradient decreases and stream size increases. For small headwater streams, the gradient tends to be very
steep and the stream is small and environmental fluctuations occur with a greater intensity and frequency,
therefore, species richness is lowest (Hynes, 1970).
Some fish species are migratory and travel long distances to return to a certain site to spawn. They
have to swim against currents and go up over waterfalls, thus, showing great strength and endurance. When