Barometers of Water Quality
By Michael J Caduto
Dragonflies are nature’s aeronautical marvels. Whether darting over water at 35 miles per hour or hovering in midair, they are a spectacle of color in motion—from a delicate blue-green translucency to a Jack-o’-Lantern black and orange.
It’s no surprise that they’ve entered the lore of writers and cultures. Henry David Thoreau once called them “devil’s needles,” though he certainly knew they were harmless. Centuries ago, Samurai warriors of Japan incorporated them into family crests that signified victory over enemies. The Diné (Navajo) of the American Southwest offer a more positive spin, believing dragonflies are a symbol of pure water. When reading Philip Pullman’s The Amber Spyglass I became enamored of the Gallivespians—fairy-sized spies in the service of the powerful Lord Asriel who sport poisonous spurs and ride airborne dragonfly steeds. Their Jurassic-sized mounts are fitted with a harness of “spider-silk reins, stirrups of titanium, a saddle of hummingbird skin.”
About 450 species of dragonflies and damselflies live in North America. Delicate damselfly adults rest with wings folded together over their back. Dragonflies have thicker bodies. Their wings beat twice as fast as a damselfly’s and are spread open at rest. Dragonflies can hover, fly sideways and backwards, mate on the wing and turn 90-degrees on an aerial dime. Sighting their prey, they calculate a trajectory and fly directly to that point of interception.
Eggs hatch into voracious aquatic nymphs that feed on other insects and virtually anything small enough to swallow. Growing nymphs molt their skin roughly a dozen times. Damselfly nymphs have three feathery gills at the end of the body that they also use to swim. The gills of dragonfly nymphs line a rectal chamber from which they shoot water for jet propulsion.
After living 2 to 4 years underwater, nymphs crawl up onto stems, rocks or leaves at night or in the early morning during the warm season and squeeze out of a split in the skin on the back. They inflate their nascent adult bodies with air and pump up the wings hydraulically.
When it comes to water quality, the Diné are on the right track. Immature nymphs—which abound in ponds, marshes and lakeshores—are barometers of healthy waters and shoreline environments. Biologists rate aquatic insects on a scale of 0 to 10. Those associated with low numbers require clean water. Insects higher on the scale tolerate low dissolved oxygen and moderate-to-high organic pollution—nutrient runoff from fertilizers or manure that feeds algal blooms. As algae die, the decomposer bacteria consume much of the available oxygen.
Nymphs of the Blackwinged Damselfly or Ebony Jewelwing (Calopteryx maculata) are found in clean streams. Cool, flowing stream water blends in oxygen and holds more dissolved gas, just as cold soft drinks are fizzy but become “flat” as they warm up. These nymphs hatch into elegant adults that flutter on the wing.
Many nymphs live in ponds, marshes and lakeshores with medium levels of dissolved oxygen and moderate organic pollution. One hatches into the aptly named Dragonhunter (Hagenius brevistylus). Watch the forest edge for its striking green eyes and jet-black body with yellow racing stripes.
Pollution-tolerant nymphs include those of the Familiar Bluet damselfly (Enallagma annexum) and dragonflies like the Common Green Darner (Anax junius), Twelve-spotted Skimmer (Libellula pulchella) and Common Whitetail (Plathemis lydia).
Adult male Whitetails patrol clearings on black-banded wings and have a stout, powder blue abdomen with white spots down the sides. “Twelve-spots” have a blue-gray abdomen and 3 black spots on each wing that alternate with white down to the base. They hunt over meadows and fields. Male Green Darners have a bright blue abdomen, emerald green thorax and a bulls-eye mark on top of the head. Females are reddish-brown. These 3-inch dragonflies are among our largest.
Biologists with the Connecticut River Conservancy monitor dragonflies to gauge water quality. The New Hampshire Dragonfly Survey—a venture between the New Hampshire Audubon Society, New Hampshire Fish and Game and UNH Cooperative Extension Service—gathered data on dragonfly locations, biology, conservation and related environmental issues.
Vermont Agency of Natural Resource studies indicate that habitat along the shores of lakes and ponds is critical. Emerging nymphs grasp the first object they reach above water and start to shed their skin. It takes at least 30 minutes for the adult wings and body to form and harden. If the shoreline has been disturbed and lacks tall objects to crawl onto, such as logs, rocks or plants, vulnerable young adults may emerge too close to the water line and be damaged by the smallest waves and wakes.
Dragonfly researchers and aficionados are protective and passionate about these aerial acrobats and their nymphs. And even though facts may not be as romantic as folklore, dragonflies do reveal important clues about the wellness of their surroundings.
Female Ebony Jewelwing or Black-winged Damselfly. Nymphs of this species are not tolerant of pollution, needing the kind of cool, clear waters also required by brook trout. (Photo: Sara Hollerich, USFWS.)
WHAT YOU CAN DO:
- Identify some of the dragonflies and damselflies flying around a local stream or pond as indicators of water quality.
- Search online under “macroinvertebrates water quality chart” to find some keys to identifying aquatic insects and the water quality they’re associated with. Gather a shallow white tray, a pair of tweezers, a large tea strainer, a hand lens and some old sneakers. Go down to your local pond or stream and search through the muck, or under the rocks, to gather aquatic insects. Gently place them in some water in the tray. Identify the insects on the chart to see what they indicate about the water quality.