Daily Archives: February 12, 2012

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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