Aquatic Insects and Water Quality

water sampling

The Dragonfly Woman recording data during a sampling trip to an effluent stream. Photo by Dave Walker.

I’ve decided to put off talking about my damselfly research another month or two so the post is closer to the paper release date, so I’m going to talk about another subject today: aquatic insects and water quality.  In my second year of grad school, a professor for a class I was taking recommended that I apply for a job.  A month later, I had a second job in an aquatic ecology lab and was working as an aquatic sampler and insect identification guru.  The project I was hired for originally was for the Environmental Protection Agency and focused on the impacts of effluent (treated wastewater) on the insect populations downstream of wastewater treatment plants in Arizona.  I can’t say that sampling for the project was pleasant and I can attest to the fact that wearing chest waders in poorly treated, reeking wastewater when it is 110 degrees in the shade is quite awful.  However, the project taught me firsthand just how much of an impact water quality has on aquatic insect populations and it is a subject I find fascinating.  In fact, my work in my second job has helped direct my plans for my research program once I have completed my doctorate.  I haven’t talked much about my second job yet in my blog and it’s time to rectify this!  Today I’m going to introduce the subject of aquatic insects and water quality.

Arizona  has a lot of problems with its water.  There are huge demands placed on the little water that is available and streams and rivers have been sucked dry by farmers and growing cities over the last hundred years.  This means that in some sections of several rivers there would be no flow at all if it weren’t for wastewater treatment plants releasing effluent.  So just how to insects respond to wastewater?  The short answer is this: not well.  Aquatic insects are typically adapted to a particular range of conditions.  If those conditions change, such as when effluent is dumped into a stream, the insects must often move to a different habitat or die.  Other insects, things that are very tolerant to polluted waters, may move into the area in their place.  You can therefore see a huge shift in the types of insects living in a clean, relatively pristine stream relative to an effluent dominated stream (a stream nearly completely or completely made up of effluent).  Many types of insects simply can’t live in environments with low water quality – and those that can tell you a lot about how terrible the water quality really is.  In fact, aquatic scientists often use aquatic insects as biological indicators of water quality.  That was exactly what we were doing in the effluent project, using insects to tell us about the quality of water in effluent dominated streams.  I’m not going to go into detail about that project today.  Instead, I’ll illustrate the differences in the insect populations between two streams in the Tucson area, one effluent dominated stream (the Santa Cruz River) and one mostly clean water stream (Sabino Creek in Sabino Canyon) so you can see the shift from dirty water insect populations to clean water populations.

Let’s take a look at an effluent stream first, a dirty stream.  The Santa Cruz River is one of the major “rivers” in Tucson and it is dry most of the year along most of its length.  However, it always flows downstream of the wastewater treatment plants.  The Santa Cruz River is therefore 100% effluent for most of the year.  Effluent, even when it is very well treated, has all kinds of bad things in it.  In my area, the nitrogen levels of the waters released from treatment plants are very high.  Scientists have also discovered many compounds that humans secrete in our wastes, such as pharmaceuticals, flame retardants that are on our clothing, triclosan (an antibiotic used in antibacterial soaps), and many other nasty chemicals.  These don’t get cleaned out of the water with current treatment techniques, so all those chemicals end up in the streams when water is released from treatment plants.  In essence, the biota living in the streams below wastewater treatment plants are bathing in a stew of antibiotics, birth control chemicals, detergents, etc.  You can imagine why this might be a problem.  My labmates have found many of these chemicals in the Santa Cruz River water.  So what kinds of insects do you find there?  We recently took a reporter for the Arizona Daily Star to sample from the river as part of a story about the wastewater treatment plant.  These are the insects we found:

blood worms

Bloodworms, larvae of the non-biting midges (a type of fly). This photo made the front page of the newspaper the day the article ran!

Ah, the lovely bloodworm.  And notice that ALL of the insects in this image are bloodworms.  Not all bloodworms are a sign of troubled waters so you can’t simply say that bloodworms = low water quality.  However, if you find tons of bloodworms and nothing else in a stream, that’s usually a bad sign.  Bloodworms get their name from their red coloration (it has been mostly broken down in the image due to the preservatives used – they’re flaming red when they’re alive) and that red coloration comes from a chemical bloodworms have that almost no other insects have: a hemoglobin-like compound.  If you want to read more about bloodworms and their hemoglobin, please read my post on aquatic insect respiration.  For now all you need to know is that the hemoglobin-like compound allows these insects to live in very low oxygen environments.  Thus, the sheer abundance of these insects and the lack of other insect species tell you something important about this stream: there is hardly any oxygen in the water at least some of the time.  The chemicals in the water probably contribute to the overall inhospitability of the river for insects as well.  Thus, the insects in the stream tell you that this stream has poor water quality.  We found this to be the case at all of the effluent streams we sampled during the EPA study, but this particular wastewater treatment plant had the fewest species of aquatic insects downstream of the plant of all of the streams we tested.

Now let’s compare the low quality stream to one with high water quality, Sabino Creek.  Sabino Canyon is one of the most popular outdoor spaces in the Tucson area in part because it has a gorgeous clear stream that flows through most of the canyon.  We went to Sabino Creek to sample right after we sampled in the Santa Cruz, and these are just some of the insects we found in the creek:


Backswimmer (Family: Notonectidae)


Dragonfly nymph, a clubtail dragonfly (Family: Gomphidae)


Damselfly nymph, a spreadwing damselfly (Family: Lestidae)

Hellgrammite (Corydalus cornutus)

Hellgrammite (Family: Corydalidae)

Creeping water bug

Creeping water bug (Family: Naucoridae)


Dragonfly nymph, a skimmer (Family: Libellulidae)

Notice the difference between this stream and the effluent stream?  Look at how many more species there are!  And some of these, including the hellgrammite and the clubtail dragonfly, only live in pretty clean water and need a lot of oxygen.  Even if you didn’t know that though, you could tell that this is a fairly clean water stream simply by looking at the number of insect species living in it.  There is one caveat, however, when comparing the Santa Cruz River to Sabino Creek.  The river is in the Tucson valley and is located at a lower elevation than Sabino Creek, which means that the types of insects you find in the stream would likely be a bit different even if they had the same quality.  Still, if you compare other effluent streams at similar elevations, or even the Santa Cruz River below the wastewater treatment plant upstream of Tucson at Nogales, it is obvious that the section of the Santa Cruz flowing through Tucson is really nasty.  Sabino Creek is comparatively very clean.  And, the insects in the stream can tell you just how clean the water is because they are excellent indicators of water quality.

This trend, that clean water has much higher insect diversity than polluted water, seems to hold true throughout the world in the majority of aquatic habitats.  For this reason, insects have become very important in water quality studies.  By collecting insects and identifying them, a scientist can say some very profound things about the water quality in that environment even if he doesn’t take any other measurements.  I’ve personally done a lot of work using insects as indicators of water quality through my second job and this work has profoundly impacted how I think about aquatic systems and the insects that call them home.  I’ll definitely be revisiting the topic in the future.


Unless otherwise stated, all text, images, and video are copyright © 2010


9 thoughts on “Aquatic Insects and Water Quality

  1. Very interesting and putting my own observations at both bodies of water into context. At the SC in Marana I have seen some teneral Damsels. There are clearly more adult Dragonflies at the Santa Cruz in Amado, but I didn’t look for aquatic larvae.

    • The Santa Cruz downstream of the other wastewater treatment plant in Tucson, the part that runs through Marana, is much nicer than at the site downstream of the plant I featured. It’s no surprise to me that there would be tenerals there. There are tons of dragonflies and damselflies at the site I’ve featured too – but either they’re not laying their eggs in the water or the eggs aren’t surviving as I’ve never seen an odonate in the water on any of the sampling trips I’ve made there. All I’ve ever found in that section are bloodworms, drain fly larvae, and sludge worms. It’s such a terrible stretch of river for most aquatic insects! Almost no emergent vegetation, little shade, LOTS of sand in the riverbed, variable flow throughout the day that can get very low at times – all bad qualities for aquatic insects. The wastewater treatment plant in Nogales is superior to this section as well and there are lots of things living in that part of the river – giant water bugs, odonates, bloodworms, leeches, beetles, etc. What you see in Amado is actually not bad (though I sill wouldn’t want to get in the water). It’s amazing how big of a difference there is between the insect populations in what is technically the same river! And it’s all dependent on the water coming out of those treatment plants.

    • I haven’t done the post on the damselfly project yet. I was going to discuss it earlier, but I decided I should wait until the paper was actually published, and then I was knee deep in dragonfly swarm posts when it was released. It’s coming soon, though! In the next 2 weeks for sure.

  2. Hi Dragonfly woman, i am a student who has lots of interest in dragonflies. i am conducting a research on the diversities of dragonflies along some selected water systems in Ghana. I have found your information really helpful and would be happy if You can assist me with some ideas and advice. Please you can reach me through this address.

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