Science Sunday: Adult Dragonflies and Damselflies as Indicators of Water Quality

dragonfly

Aquatic insects and other invertebrates have been used as indicators of water quality for about 40 years.  Insects can be found in a huge variety of freshwater habitats year round and can tell scientists and water resource managers a great deal about the conditions within a body of water.  By simply scooping some insects out of a stream and identifying them (that is the hard part), you can assess the conditions of the stream’s water relatively cheaply, easily, and reliably.  I suspect that their use in water quality assessments drives most current research on aquatic insects – they’re that valuable.

However, using aquatic insects as indicators of water quality only works if you’re able to identify the insects that you pull out of a body of water and know something about the sorts of habitats the species you find typically inhabit.  What happens if you want to do water quality assessments in an area where aquatic insect habitats aren’t all that well-known, or a place where many species haven’t even been identified?  Such areas are at a disadvantage because they are unable to rely on a valuable insect tool for determining their water quality.  Unfortunately, the areas that would typically benefit most from a cheap, easy, and reliable insect-based assessment of water quality are also areas where the freshwater insects aren’t known well enough to make it work.

This is exactly the sort of situation that you find in Mexico and several South American countries.  One of the big problems is that no one’s really bothered to describe the aquatic insects.  If an insect lives on land, there’s at least a chance that someone somewhere has gone through the trouble of describing it, but in the water…  That’s a whole different matter.  What ends up happening is that certain groups of aquatic insects that are terrestrial as adults have been incompletely described and the immatures remain obscure.  In many common damselfly and dragonfly species, for example, people have no idea what the nymphs look like!  And, if you can’t identify the aquatic insect species in a region, you can’t use them to assess water quality, making things like odonates useless as indicators.

damselfly

This is the situation that researchers Daniel de paiva Silva, Paulo De Marco, and Daniela Chaves Resende faced in Brazil.  They were interested in using aquatic insects as indicators of water quality, but they are incompletely described in their area, especially the odonates.  So, they wondered: since odonates lay eggs in water, females choose males based on characteristics of their territories that are conducive to the survival of their offspring, and nymphs typically grow in the same areas where adult males patrol, is it possible to use the adults as a measure of water quality instead of the nymphs?  With all the habitat assessments odonates apparently do on their on as they choose where to lay their eggs, it just might work!

The team tested their idea along the Turvo Sujo River in southeastern Brazil.  The river is impacted by humans along a large part of its length, especially near the urban area of Viçosa.  Upstream of the city the banks of the river have been converted from the natural forest lands into pastures.  Donwstream of the city the river is surrounded by pastures as well, but is influenced by the city too, especially by sewage.  By comparing the odonate species and water quality at six sites upstream of the city and six downstream, the researchers hoped to determine whether it was possible to use odonate adults in place of nymphs to assess the quality of water.  They assumed that the downstream sites would be more polluted and would have fewer adult dragonflies relative to upstream.  If there was greater abundance and species diversity of odonates at the upstream sites, they could argue that they were useful as indicators of water quality.

Pachydiplax longipennis female

Their efforts were, unfortunately, largely unsuccessful due to one major issue in their design: both the upstream and the downstream sites were so impaired by land use, pollution, and other factors that there was less of a difference between the two areas than they expected and there was no real control.  They found that because the forest had been replaced by pasture, many perching species of odonates that would normally be found in the area were conspicuously absent both up and downstream.  The team measured several water quality parameters in the two areas and found that they were nearly identical apart from a slightly lower dissolved oxygen level downstream of the city.  The diversity of odonates was a little higher at the upstream region, but only in the wet season, and  a few species of damselflies dominated both areas in abundance.  All in all, the upstream and downstream areas were very similar.

So, the researchers didn’t get the answers they sought and were unable to determine whether adults odonates could be used instead of nymphs in water quality studies.  Instead, they learned that when humans, with their sophisticated water measuring tools, can barely tell the difference between the water quality of two areas, the odonates can’t seem to tell the difference either.  They also learned that because the riparian vegetation along the river had been destroyed, the rest of the water quality parameters no longer seemed to matter.  And, they found that most of the odonates along the river were exactly the sorts of species you expected to find in rivers that are highly impacted by humans. The river had been so completely altered that several pond species were even found mixed in with the river species, a clear sign of river impairment at both ends of the city.

Even though this particular study didn’t support using adult odonates in place of nymphs for water quality analyses, I think the authors have a really great idea that should be pursued by other researchers.  Their argument is sound: odonates are assessing water quality to ensure the survival of their offspring, so using adult odonates in place of immatures to make those cheap, easy, and reliable studies of water qualities might be possible.  And if it turns out that it works, well that would be an amazing thing!  Adult odonates are big, beautiful, showy insects that people pay attention to.  The adults are well described in many areas where the aquatic insects remain largely ignored.  If you could use well-described adults in place of undescribed nymphs, many more regions would be able to use aquatic insects as indicators of water quality.  I, for one, think that’s a very good thing.

Literature Cited:

Silva, D., De Marco, P., & Resende, D. (2010). Adult odonate abundance and community assemblage measures as indicators of stream ecological integrity: A case study Ecological Indicators, 10 (3), 744-752 DOI: 10.1016/j.ecolind.2009.12.004

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9 responses to “Science Sunday: Adult Dragonflies and Damselflies as Indicators of Water Quality

  1. That’s one of those “Why didn’t I think of that?” ideas.

    By the way, my copy of An Introduction to the Aquatic Insects of North America arrived on Friday, and I think the keys will be WELL worth the price. I shall AMAZE my friends by making (probably incorrect) identifications below the family level!!

    • I know, right? It seems so obvious when you think about it for a moment.

      I’m so happy you think you’re going to get something out of your book! It really is a good book, even the older editions. There’s so much information in it!

  2. Hmm… great idea! It looks like you’re heading towards something significant for your own research – awesome! I really like your glossary.

    Unfortunately, I could only access the abstract and a handful of graphs that didn’t mean much without any text. I’m assuming (correct or not) that these folks from Brazil did their research in a single field year? I have a preference for long-term studies (10 or more years), rather than the 1-2 years it takes to publish something from a post-graduate education. Look at our odd year during the 2011-2012 winter in a La Niña season, certainly not typical. Since I haven’t read the actual pub, I hold some doubts that odonates can “assess” H2O quality. How far do the adult females fly to “determine” appropriateness of any location to lay eggs?

    Oh, I wanted to share another aquatic blog with you, but I’ve quit Twitter (too many marketing/XX sites sending me messages – total waste of time). So, here’s a retired Chinese Religions fellow from VA (ya, totally incongruous) that might be of interest to you, despite the distance: http://aquaticinsectsofcentralvirginia.blogspot.com/

    • They definitely did this work in one season. In fact, I think they even did it over a span of a few days in the wet and dry seasons. Not a long term study by any means and that was another flaw in the design. And they didn’t say anything about how the dragonflies are assessing the water, but dragonflies are clearly looking for some qualities in an area that give their offspring a reasonable chance to survive as both males and females tend to be attracted to the same spots. What they’re looking for specifically… That hasn’t been determined yet for many species. Some might be looking for a certain type of vegetation or water clarity, others might look for completely different things. However, in support of the idea that the dragonflies might assess something of the water quality (and who knows what that might be!), females of many species have been observed dipping their abdomens in the water before mating with the resident male. In some cases they may simply be laying eggs from a previous mating, but they COULD be assessing the water too. The whole idea of using adult odes to signal water quality hinges on the latter being true, otherwise it wouldn’t work. Clearly, there are still a lot of unknowns, and I’m not convinced it will ever work even, but it’s an intriguing idea!

      Thanks for the link to the other blog! I appreciate it.

  3. In Ohio (where I grew up) the Dept of Natural Resources had a great citizen science project measuring stream quality using kick-seining to identify aquatic macro-invertebrates. They used it to help identify areas of concern for non-point source pollution. I even did a high school science project on it! (That’s been… ummm… a couple decades ago…). I still remember identifying & counting the various ode & mayfly nymphs, water pennies & so on. It was a fabulous way to get people involved in “adopting a stream” and discovering the incredible life underwater.

    From what I remember, dragonfly & damselfly nymphs were always lumped together for one count – there wasn’t any need to identify species. It would be really interesting to know if there are ode species that can tolerate poor-quality water more than other species. I can certainly imagine people becoming more involved in ode counts (like they do bird counts) if the presence or absence of a certain species indicated something about habitat/water quality.

    Is there any evidence yet to make the assumption that if adult odes as a group are absent or not breeding at a water site that it is poor quality?

  4. It’s my understanding from papers I’ve read that adult dragonflies don’t really assess water quality per se, they assess habitat quality via vegetation cues. So the remark that “[since] riparian vegetation along the river had been destroyed, the rest of the water quality parameters no longer seemed to matter” hits the nail on the head.

    • That has certainly been my understanding too, but the possibility that the odes MIGHT be able to assess the quality of the water (maybe even by assessing the habitat as an indirect measure of quality, though a correlation between habitat quality and water impairment would have to be established for this to actually be useful) seems to be driving the entire idea of using the adult dragonflies as indicators. There are a ton of unknowns that haven’t been taken into account in this study and I remain unconvinced that it’s going to work, but I think the idea is intriguing.

  5. It is quite exciting to know that other people rather your friends are reading you research! If any of you want a copy of that article, please let me know.
    Daniel Paiva Silva
    daniel.paivasilva@gmail.com

    • Thank you so much for offering your paper to people who want to read it! I think your ideas are fantastic, so I am thrilled that you are willing to share the paper with others. I really hope your ideas pan out in future experiments too as I think it can be a huge help to water quality studies if it turns out you’re right. I look forward to reading more of your work!

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