Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 467

Wissinger, 1996). Insect herbivory on the areal portions of emergent plants
can be important (Wissinger, 1999). Top predators in the water are gener-
ally insects, crayfish, small fishes, and amphibians (Porter et al., 1999).
Vertebrate predators, such as raccoon and birds, rely on the larger animals
in the water.
Wetlands associated with rivers and streams are strongly affected by
floods. The flood pulse concept provides a view of the river as a dynamic
system that is connected with its flood plain, and it may provide valuable
ways to describe the riparian ecosystem (Lewis et al., 2000). In large rivers,
this pulsing is particularly important because such systems are character-
ized by seasonal flooding and associated connections with riparian wet-
lands. As discussed previously, flooding can be important in the biology of
large river fish (see Sidebar 21.1). The flood-riparian connection provides
riverine species with food resources and spawning habitat and riparian lake
and wetland species with an avenue for dispersal. The connection is also
important in material transport.
Floods inundate riparian area, which slows water velocity and allows
for settling of sediments. Dry, coarse organic
debris initially floats and can be moved from
the riparian zones into the large channels or
moved within the wetland (Molles et al.,
1998). Likewise, nutrients move from the
river into the side pools (Knowlton and
Jones, 1997). This process has been demon-
strated to be important in tropical flood-
plain rivers, in which lakes and wetlands in
the riparian zone become progressively less
productive during the dry season as nutri-
ents are taken up by organisms and are de-
posited into the sediments. Flooding then
provides a new pulse of nutrients that boost
productivity (Hamilton and Lewis, 1990).
Flooding in arid-zone rivers also provides
nutrients to the riparian zone. This can oc-
cur even in the absence of overland flow be-
cause the flooding can pulse water and nu-
trients into the hyporheic zone (Martí
et al., 2000).
Flood pulsing is an essential character-
istic of riparian wetland ecosystems. These
wetlands are highly endangered by river
channelization and modification. Addition
of dams and regulation of extreme flow can
severely alter their natural cycles. Restora-
tion of riparian wetlands requires manage-
ment of floods and connectivity to the main
river channel (Middleton, 1999). The Pan-
tanal is the largest wetland in the world and
is threatened by changes in the hydrological
regime (Sidebar 22.2).

Sidebar 22.2.
The Pantanal, the World's Largest
Wetland Ecosystem Complex

The Pantanal is a vast complex of seasonally
flooded wetlands, lakes, and streams along the
Paraguay River in Brazil, Bolivia, and Paraguay.
The total area of wetlands and savannah in-
cludes 140,000 km 2 in Brazil and 100,000 km 2 in
Bolivia and Paraguay; the portion in Brazil is
larger than the state of New York. During the
wet season, about 80% of the area is flooded.
During the dry season, the grasses emerge,
and the area is used heavily for cattle grazing.
The wetland undergoes a strong seasonal
succession. During the wet season, several
meters of water cover all but the highest tree
islands. Massive growths of macrophytes oc-
cur and aquatic species disperse. During the
dry season, the ponds become isolated from
the main channel. Fish are trapped in these
ponds and waterbirds and caimans congre-
gate to feed on the trapped fishes (Fig. 22.13).
The ponds become eutrophic from nutrients
released from the fish carcasses and excretia
of the predators (Heckman, 1994).
The Pantanal is a wetland of international
importance for ecological conservation. It is
habitat for the endangered spotted jaguar
(Panthera onca), giant anteater (Myrme-
cophaga tridactyla), and giant river otters