Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 23

compounds tend to persist and accumulate in the environment because they do not readily break down in
natural ecosystems. Some of the most toxic synthetic organics, DDT and PCBs, have been banned from
use in the United States for decades yet continue to cause problems in the aquatic ecosystems of many
streams. The chemical industry has produced many useful organic chemicals: plastics, paints and dyes, fuels,
pesticides, pharmaceuticals, and other items of modern life. These products and their associated wastes
and byproducts can interfere with the health of aquatic ecosystems.
Degradation —Synthetic organic compounds (SOC) can be transformed into a variety of degradation
products. Ultimate degradation, or mineralization, results in the oxidation of organic carbon to carbon
dioxide. Major transformation processes include photolysis, hydrolysis, and oxidation-reduction reactions.
The latter are commonly mediated by biological systems.
Photolysis refers to the destruction of a compound by the energy of light. The energy of light varies
inversely with its wavelength. Long-wave light lacks sufficient energy to break chemical bonds. Short
wave light (x-rays and gamma rays) is very destructive; fortunately for life on earth, this type of radiation
largely is removed by the upper atmosphere. Light near the visible spectrum reaches the earth's surface
and can break many of the bonds common in SOC.
Oxidation-reduction reactions are what fuels most metabolism in the biosphere. SOC are generally
considered as sources of reduced carbon. In such situations, what is needed for degradation is a metabolic
system with the appropriate enzymes for the oxidation of the compound. A sufficient supply of other
nutrients and a terminal electron acceptor are also required.
1.1.8 River Ecology
Ecology is the science of studying the relations among different organisms and the relations between
organisms and their environment.
Terrestrial ecosystems —The biological community of a stream corridor is determined by the
characteristics of both terrestrial and aquatic ecosystems. The terrestrial ecosystem is composed of
biological communities living or growing on land (the watershed). Terrestrial ecosystems are fundamentally
tied to processes within the soil. The ability of a soil to store and cycle nutrients and other elements
depends on the properties and microclimate (i.e., moisture and temperature) of the soil, and the soil's
community of organisms.
Terrestrial plant communities —These plant communities in the watershed are a valuable source of
energy for the biological communities, provide physical habitat, and moderate solar energy fluxes to and
from the surrounding aquatic and terrestrial ecosystems. Given adequate moisture, light, and temperature,
the vegetative community grows in an annual cycle of active growth/production, senescence, and relative
dormancy. The growth period is the product of incidental solar radiation, which drives the photosynthetic
process through which inorganic carbon is converted to organic plant materials.
The distribution and characteristics of vegetative communities are determined by climate, water
availability, topographic features, and the chemical and physical properties of the soil, including moisture
and nutrient content. The characteristics of the plant communities directly influence the diversity and
integrity of the faunal communities. Plant communities that cover a large area and that are diverse in their
vertical and horizontal structural characteristics can support far more diverse faunal communities than
relatively homogenous plant communities, such as meadows. As a result of the complex spatial and temporal
relation that exist between floral and faunal communities , current ecological characteristics of these
communities reflect the recent historical (100 years or less) physical conditions of the landscape.
The quantity of terrestrial vegetation, as well as its species composition, can directly affect stream
channel characteristics. Root systems in the streambank can bind bank sediments and moderate erosion
processes. Trees and smaller woody debris that fall into the stream can deflect flows and induce erosion