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Unformatted text preview: hus indicate more accurately those
groups of organisms which may be affected under actual use conditions. It is also possible that
or ganisms may be less susceptible under field conditions than in the labora tory because of
double jeopardy factors existing in the field but not in the laboratory. Recovery of a group o f
organisms and particularly the time taken for this after a toxic insult is also useful information
from a regulatory point of view and can only be observed in the field or under field conditions.
184.108.40.206.2 Structure, diversity, and function Effects noted in numbers of organisms (structure) may also be reflected in measurements of their
diversity and in the function of the ecosystem. These effects will usually be observed in a strict
hierarchy, that is, changes in numbers must precede changes in diversity and these usually
precede changes in function. Ecosystem function, which is usually expressed in terms of
productivity, for example, photosynthesis, is strongly conserved in the face of a toxic insult
because of redundancy built into most ecosystem functions. However, a long -term change in
diver sity may have indirect effects, such as those caused by the re placement of one type of food
organism by another. While most preda tor or herbivore organisms would not be affected under
these condi tions, others may have specific food requirements and feel the impact.
220.127.116.11.3 Ecosystem productivity Ecosystem productivity in water is usually measured in terms of photosynthesis or the ratio of
photosynthesis to respiration. Because this is an integrative measurement, it is subject to a lack
of sensitivity result ing from redundancy and homeo static mechanisms in the system.
18.104.22.168.4 Assessment of impact In terms of impact assessment, studies in enclosures and microcosms have allowed the
demonstration of statistically significant differences between single spe cies or taxa in a set of
treated systems compared to a set of untreated reference systems. These differ ences are more
obvious for those compounds which act rapidly, cause acute toxicity and diminish as numbers of
the target species recover to levels comparable to refer ence systems (). Differences at the
community level are less obvious and more difficult to d emonstrate as one moves from individ ual species to larger and larger taxa. One species will often increase in numbers while another,
susceptible to the substance in question, will de crease. If one ascends further up the scale of
generalization to the l evel of the ecosystem, it again becomes more and more difficult to
demonstrate changes due to the action of a toxicant. Gross community parameters such as pro ductivity are the most insensitive to perturbation. In limnocorral enclosures, significant cha nges
in phytoplankton species appear to have less overall effect on production or photosynthesis.
Similarly, the zooplankton dependent on these primary pro ducers seem to be able to adjust their
diet and, for the most part, do not...
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