The flood pulse--
The flood pulse refers to the processes of lateral movement of water and
materials on and off the floodplain (Junk et al. 1989). As this occurs,
a moving littoral (or moving shoreline) advances and retreats over the
floodplain according to the season. In consequence, the floodplain experiences
a terrestrial phase during the dry season, and an inundated aquatic phase
during the flood season. The extent and duration of each phase depends
on the magnitude of the rise and fall in river levels, and can vary among
years. For this reason, material flows on floodplains are extremely complex.
During
the dry season, animal dung accumulates along with decaying terrestrial
vegetation that died as the floodplain dried. When river levels rise,
nutrients contained in this material are released and sustain the autochthonous
production during the flood phase. At the same time, the floods deposit
nutrient-rich silts onto the floodplain. Nutrients from the flooded terrestrial
material allow a large biomass of aquatic plants to build up. Some of
these nutrients are also washed into the main channel, where they may
increase phytoplankton production. Flood-adapted grasses and sedges thrive
on the inundated floodplain, and their underwater portions provide surface
for the development of a biofilm that can be exploited by grazing invertebrates.
Fishes migrate onto the floodplain and exploit the variety of foods available.
As the floods subside, the aquatic macrophytes die and decay, providing
nutrients that support dry season vegetation.
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Multi-temporal RADARSAT image of the Great Lake/Tonle Sap floodplain in Cambodia |
The situation is complicated by the fact that most river
floodplains are far from their natural state, and large expanses have
been converted to agriculture. This results in the addition of allochthonous
nutrients derived from organic and inorganic fertilizers. Nutrient release
from dead plants is accelerated by the practice of burning. However, some
plant biomass is removed from the floodplain to serve as food for humans
and livestock. This may reduce the input of allochthonous terrestrial
material compared to floodplains in their natural state.
Putting things together: nutrient
spiraling--
Streams and rivers are flow-through systems, receiving all material passing
through them from the surrounding landscape and storing it, exporting
it downstream or mineralising some organic inputs. Members of the functional
feeding groups can be viewed as interrelated temporary storage bins for
organic compounds that are eventually converted to carbon dioxide and
component nutrients. In other words, these organisms consume plant parts
in the form of detritus, derived primarily from the surrounding land,
supplemented by material resulting from autochthonous primary production,
and bring about their transformation and temporary storage as animal tissues
or conversion – via respiration – into carbon dioxide.
The spiralling concept assumes that nutrients or carbon molecules exist
in three possible states:
• free in the water, in which case they drift downstream;
• as part of the body of an organism;
• buried in the sediments deposited on the riverbed.
Because of directional water movement, nutrients and
carbon molecules released at one point will be carried downstream before
they are taken up again. Upon subsequent release, they again travel downstream
before being taken up once more. The alteration between the free (or buried)
state and being part of an organism can be considered as a spiral. The
number of loops in the spiral before the molecule reaches the sea is a
measure of the biological activity of the system (Wallace et al., 1977;
Newbold et al., 1979). The more loops there are, the more efficiently
the aquatic community captures and retains material, slowing the rate
of transport to the sea.
Spiralling is well illustrated by detritus processing (see previous Section
'Organic matter processing'). After fragmentation of CPOM by shredders,
the continued cycle of colonisation and recolonization of detritus particles
by microorganisms, and their repeated ingestion and defaecation by collectors,
can be seen as improving the efficiency with which organic matter is utilised.
Filter-feeders are especially important in capturing and retaining organic
particles, and they have a wide variety of adaptations that allow them
to collect a range of different particle sizes (Wallace and Merritt 1980).
As detritus processing proceeds, more and more of the nutrients and carbon
in the original CPOM becomes 'locked' in the bodies of consumers. The
efficiency with which carbon and nutrients are transferred between organisms,
and from prey to predator, have an important influence on the length of
the spirals, and hence downstream loss of organic matter and the 'leakiness'
of a river section. Greater efficiency results in tighter spirals.
