About 6 months ago, I wrote a post about aquatic insects and water quality that highlighted the differences in the diversity of species in a polluted river compared to a clean mountain stream in Arizona. Considering how much I enjoy this subject, it’s been far too long since I wrote about it! It’s time to do something about this sorry state of affairs. My next few Monday posts are thus going to be about how insects are used as indicators of water quality in streams and lakes.
Aquatic insects are very useful for making environmental policy decisions and in deciding when managers need to step in and actively manage a body of water. I think it’s useful to know how this works! But first I should introduce the concept of macroinvertebrates. If you ever delve into the insects-as-indicators-of-water-quality literature, you’ll see this term over and over again. Although I tend to talk about insects more than the other invertebrates in streams and lakes, not all invertebrates that live in freshwater habitats are insects. There are lots of other inverts, including crustaceans (crayfish, shrimp, and their relatives), aquatic worms (earthworms, tube worms, leeches, etc), flatworms, mites, snails, and clams. The inverts are divided into the microinvertebrates and the macroinvertebrates. Essentially, anything that you can see with the naked eye is a macroinvertebrate. Everything else is a microinvertebrate. I personally don’t find this definition very satisfying because one person’s macroinvertebrate might be another person’s microinvertebrate (e.g. my macroinvertebrate is very small). Any one person’s cutoff for what makes a macroinvertebrate can change over time and as they gain experience too. Still, water resource managers love the term macroinvertebrate and everyone uses it, including me.
In my first post on using aquatic insects as indicators of water quality, I focused on the changes in diversity that you see along the clean to polluted water continuum and I’ll talk about it again in a future post. The number of species of macroinvertebrates in a stream is a quick and dirty way to compare bodies of water and determine the relative amount of pollution or impairment because clean streams and lakes tends to have more species in them than highly polluted bodies of water. It isn’t precise though. A fairly dirty stream can have almost as many macroinvertebrate species in it as a clean stream under the right conditions. In this situation, it becomes important to consider the specific species that are found in a body of water. This is where tolerance values come in handy.
Tolerance values tell you how tolerant any given species is to pollution in its habitat (go figure). The scale most commonly used goes from 0 to 10. Things with low numbers are very sensitive to pollution. Things with tolerance value numbers closer to 10 tolerate a lot more pollution in their habitats and can live in some pretty nasty water. And, just to make everything confusing, sometimes you find super tolerant species with scores that go right off the top of the regular scale. (They’re like Nigel’s amplifier in Spinal Tap – they go to 11!)
Considering how often tolerance values are used in aquatic research and how valuable they are to water resource agencies and managers, I think it is worthwhile to know where tolerance values come from. It takes a lot of time and effort, and often a lot of money, to calculate tolerance values for macroinvertebrates, but the concept is very simple. First, someone (often a water manager for a state’s environmental protection department or a scientist) will take measurements of pollution or other impairments in many different bodies of water. These could be simple physico-chemical measurements (such as pH, dissolved oxygen, temperature), measurements of embeddedness (how far down into the silt/sand the rocks and pebbles are buried) or periphyton (the algae growing on the surfaces of rocks, soil, and plants in the water), or full water chemistry analysis. Which measurements are taken will depend on the region, the group doing the work, the funding, and the time available to put toward the project. After measurements are made, bodies of water are grouped according to the level of pollution/impairment they exhibit, such as pristine, impaired, and polluted.
Next, the researchers send out a hoard of samplers to pull out every invert they can find from as many bodies of water as possible. Some poor group of technicians then “picks” the samples (separates the inverts from the massive amount of junk that you get in aquatic samples such as leaves, sand, silt, twigs, trash, etc) and passes the inverts off to the identification guru to identify. After all the water measurements and invert ID work is done, then the researchers compare the species present in each water body to its pollution classification and use statistics and other mathematical tools to look for overall trends.
Inverts that are found only in pristine lakes and streams and never in impaired or polluted waters have narrow pollution tolerances and are assigned low pollution tolerance values, usually 3 or less. Inverts found in impaired and pristine waters but not highly polluted waters have a wider tolerance for pollution. These inverts prefer clean water, but they can tolerate some pollution in their habitats and are usually assigned mid-range values around 5 or 6. Things commonly found in highly polluted waters get high scores, between 8 and 10, though they are sometimes found in clean water systems too. And those things with scores of 11? Well, they can live in some of the filthiest water you can imagine! I don’t know about you, but I can imagine (and have worked in) some pretty nasty water, and there are insects living in nearly all of them.
It is important to note that macroinvertebrate pollution tolerance values vary from region to region. Here in Arizona, we can’t use the pollution tolerance values calculated for inverts on the east coast, even when the species are the same, because our waters and the inverts living in them behave differently than those on the coast. Thus, every region develops their own pollution tolerance values. When I’ve done water quality studies using insects as indicators of pollution/impairment in the past, I’ve used a list of tolerance values developed within Arizona that was given to me by the Arizona Department of Environmental Quality. The tolerance values therefore accurately reflect how inverts in Arizona react to pollution/impairment that occur in Arizona. The list doesn’t have every species, but you can often use what you know about which waters you find them in and published records of their presence to fill in the gaps.
Next Monday I will go through an example of how scientists and water managers use tolerance values by discussing a project I was involved in a few years ago looking at the insects in Arizona’s effluent dominated streams. Tolerance values played a huge role in the analysis of our results, and it was an interesting (but disgusting) project. Until then, have a great week – and don’t forget to enter my latest contest!
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