WATER QUALITY - Water Quality Engineering in Natural Systems - page 25

and adaptive community of organisms having a species
composition, diversity, and functional organization com-
parable with those of natural habitats within a region
(Karr and Dudley, 1981). Typical aquatic ecosystems
include rivers, lakes, wetlands, and estuaries.
The occurrence, condition, and numbers of types of
fish, insects, algae, plants, and other organisms are
biological indicators that provide direct information
about the health of specific bodies of water. using
these biological indicators as a way to evaluate the
health of a water body is called biological assessment .
Biological criteria or biocriteria describe the qualities
that must be present to support a desired condition
in a water body, and they serve as the benchmarks
against which biological indicator measurements are
compared. Biological criteria are narrative or numeric
expressions that describe the reference biological integ-
rity of aquatic communities inhabiting waters of a given
designated aquatic life use. Biocriteria are based on the
numbers and types of organisms present, and represent
reference conditions that are regarded as minimally
impaired.
Biological indicator species are unique environmen-
tal indicators, as they offer a signal of the biological
condition in a water body. Biological indicators can
serve as early-warning measures of pollution or degra-
dation in an ecosystem. The major groups of biological
indicators include (1) fish, (2) invertebrates, (3) periphy-
ton, and (4) macrophytes. Marine environments utilize
biological indicators that are different from those used
in freshwater bodies.
In marine and estuarine waters, benthic macroinver-
tebrates are good indicators of water quality, as
their response to pollutants is comparable with those in
freshwater systems. Polychaetes (commonly known as
worms) are one of the most tolerant marine organisms
to stressors (e.g., low oxygen, organic contamination
of sediment, and sewage pollution), so they are
typically used as biological indicators. In addition, mac-
roinvertebrates also have limited mobility and a
long enough life span to both avoid pollutants and
assess environmental stressors accurately. Typically, it is
much more difficult to assess marine-estuarine condi-
tions, as it is often difficult to evaluate reference
conditions in these ecosystems. Typical marine-estuarine
indicators include: (1) phytoplankton, (2) zooplankton,
(3) benthos, (4) submerged aquatic vegetation, and
(5) fish.
It is important to keep the differences between bio-
logical water-quality indicators and chemical water-
quality indicators in mind when considering the
application of biocriteria. Biological water-quality indi-
cators provide direct measures of the cumulative
response of the biological community to all sources of

stress. Therefore, biocriteria set the biological quality
goal, or target, to which water quality can be managed
rather than the maximum allowable level of a pollutant
or other water-quality condition in a water body. Physi-
cal and chemical water-quality criteria are designed to
protect the biological community in a water body from
different categories of stress, such as toxic levels of pol-
lutants and unhealthy physical conditions.
One of the most meaningful ways to assess the quality
of surface waters is to observe directly the communities
of plants and animals that live in them. Because aquatic
plants and animals are constantly exposed to the effects
of various stressors, these communities reflect not only
current conditions, but also stresses and changes in con-
ditions over time and their cumulative impacts. Bioas-
sessment data are invaluable for managing aquatic
resources and ecosystems. They can be used to set pro-
tection and restoration goals, to decide what to monitor
and how to interpret what is found, to identify stresses
to the water body and decide how the stresses should
be controlled, and to assess and report on the effective-
ness of various remedial actions.
Traditional chemical and physical water-quality
assessments cannot fully answer questions about the
ecological integrity of a water body or determine
whether aquatic resources are being protected. relying
on traditional chemistry alone may lead to situations in
which meeting chemical standards may not be enough
to fully protect the aquatic community, or, conversely,
to situations in which the community remains in satis-
factory condition despite a failure to attain standards. A
comprehensive suite of measures and indicators is typi-
cally desirable to characterize the ecological health of a
water body. The presence, condition, and numbers of
types of fish, invertebrates, amphibians, algae, and plants
are data that together provide direct information about
the health of specific bodies of water. Bioassessment
data can also help distinguish among potential stressors.
Establishing credible relationships between stressors
and impairments can help identify likely causes of prob-
lems. Bioassessment data also serve as a measure to
evaluate the effectiveness of management actions as
reflected in the responses and improved conditions of
biological communities.

PROBLEMS

2.1. Saltwater intrusion into a river has increased the
average chloride concentration to 3000 mg/l. If the
summer water temperature is around 25°C and
the winter temperature is around 15°C, compare
the saturated dissolved oxygen level in the summer
with the level in the winter.