10.5. Environmental monitoring using aquatic insects
Aquatic insects form assemblages that vary with their geographical location, according to historical biogeographic and ecological processes. Within a more restricted area, such as a single lake or river drainage, the community structure derived from within this pool of locally available organisms is constrained largely by physico-chemical factors of the environment. Amongst the important factors that govern which species live in a particular waterbody, variations in oxygen availability obviously lead to different insect communities. For example, in low-oxygen conditions, perhaps caused by oxygen-demanding sewage pollution, the community is typically species-poor and differs in composition from a comparable well-oxygenated system, as might be found upstream of a pollution site. Similar changes in community structure can be seen in relation to other physico-chemical factors such as temperature, sediment, and substrate type and, of increasing concern, pollutants such as pesticides, acidic materials, and heavy metals.
All of these factors, which generally are subsumed under the term “water quality”, can be measured physico-chemically. However, physico-chemical monitoring requires:
- knowledge of which of the hundreds of substances to monitor;
- understanding of the synergistic effects when two or more pollutants interact (which often exacerbates or multiplies the effects of any compound alone);
- continuous monitoring to detect pollutants that may be intermittent, such as nocturnal release of industrial waste products.
The problem is that we often do not know in advance which of the many substances released into water- ways are significant biologically; even with such knowledge, continuous monitoring of more than a few is difficult and expensive. If these impediments could be overcome, the important question remains: what are the biological effects of pollutants? Organisms and communities that are exposed to aquatic pollutants integrate multiple present and immediate-past environmental effects. Increasingly, insects are used in the description and classification of aquatic ecosystems and in the detection of deleterious effects of human activities. For the latter purpose, aquatic insect communities (or a subset of the animals that comprise an aquatic community) are used as surrogates for humans: their observed responses give early warning of damaging changes.
In this biological monitoring of aquatic environments, the advantages of using insects include:
- ability to select amongst the many insect taxa in any aquatic system, according to the resolution required;
- availability of many ubiquitous or widely distributed taxa, allowing elimination of non-ecological reasons why a taxon might be missing from an area;
- functional importance of insects in aquatic ecosystems, ranging from secondary producers to top predators;
- ease and lack of ethical constraints in sampling aquatic insects, giving sufficient numbers of individuals and taxa to be informative, and yet still be able to be handled;
- ability to identify most aquatic insects to a meaningful level;
- predictability and ease of detection of responses of many aquatic insects to disturbances, such as particular types of pollution.
Typical responses observed when aquatic insect communities are disturbed include:
- increased abundance of certain mayflies, such as Caenidae with protected abdominal gills, and caddis-flies including filter-feeders such as Hydropsychidae, as particulate material (including sediment) increases;
- increase in numbers of hemoglobin-possessing bloodworms (Chironomidae) as dissolved oxygen is reduced;
- loss of stonefly nymphs (Plecoptera) as water temperature increases;
- substantial reduction in diversity with pesticide run-off;
- increased abundance of a few species but general loss of diversity with elevated nutrient levels (organic enrichment, or eutrophication).
More subtle community changes can be observed in response to less overt pollution sources, but it can be difficult to separate environmentally induced changes from natural variations in community structure.