The basic principle behind the study of macroinvertebrates is that some species are more
sensitive to pollution than others. Therefore, if a stream site is inhabited by organisms that
can tolerate pollution, and the pollution-sensitive organisms are missing, a pollution
problem is likely.
For example, stonefly nymphs, which are very sensitive to most pollutants, cannot survive if
a stream's dissolved oxygen falls below a certain level. If a biosurvey shows that no
stoneflies are present in a stream that used to support them, a hypothesis might be that
dissolved oxygen has fallen to a point that keeps stoneflies from reproducing or has killed
them outright.
This brings up both the advantage and disadvantage of the biosurvey. The advantage of
the biosurvey is it tells us very clearly when the stream ecosystem is impaired, or "sick,"
due to pollution or habitat loss. It is not difficult to realize that a stream full of many kinds of
crawling and swimming "critters" is healthier than one without much life. Different macros
occupy different ecological niches within the aquatic environment, so diversity of species
generally means a healthy, balanced ecosystem. The disadvantage of the biosurvey, on
the other hand, is it cannot definitively tell us why certain types of creatures are present or
absent.
In this case, the absence of stoneflies might indeed be due to low dissolved oxygen. But is
the stream under-oxygenated because it flows too sluggishly, or because pollutants in the
stream are damaging water quality by using up the oxygen? The absence of stoneflies
might also be due to other pollutants discharged by factories or run off from farmland, water
temperatures that are too high, habitat degradation such as excess sand or silt on the
stream bottom has ruined stonefly sheltering areas, or other conditions. Thus a biosurvey
should be accompanied by an assessment of habitat and water quality conditions in order
to help explain biosurvey results.