Skip to main content

Articles

Justin Wolbert  positions the Ponar sampling box at a site on Fort Loudoun Reservoir

Getting to the Bottom

Tiny Creatures Are Clues to Reservoir Health

In the Tennessee River system swim monsters.

Or at least, monstrously large fish.

Catfish can top 100 pounds and lake sturgeon might grow to 200.

But that’s not what Lyn Williams, Tennessee Valley Authority fisheries and aquatic monitoring manager, and Justin Wolbert, fisheries biologist, were looking for on a sun-soaked day at Fort Loudoun Reservoir.

They were surveying something equally impressive: the life that powers those gentle giants – millions of tiny insect larvae called benthic macroinvertebrates.

Benthic means bottom-dwelling. The animals lurk in the muck layer at the bottom of reservoirs, where young catfish and massive sturgeon slurp them up.

Macro means these invertebrates, animals without a skeleton, are big enough to see without a microscope.

And these creatures aren’t only important as a food source for fish big and small – they also matter to Valley residents who rely on these waters for drinking water, recreation and navigation.

Scientists collect information about the type of animals, their numbers and their habitat. Over time, this data tells a story, because some species will only live in the cleanest waters.

“It all works together to give a pretty good picture of ecological health,” Wolbert said.

Justin Wolbert, TVA fisheries biologist, indicates the length of a burrowing mayfly at a sample site.

Justin Wolbert, TVA fisheries biologist, indicates the length of a burrowing mayfly at a sample site.  

Ponar Power

To capture Fort Loudoun’s vital signs, Wolbert and Williams launched a specialized TVA boat from the Louisville Point Park ramp and motored across the misty lake.

As they neared the far bank, Williams leaned in close to the GPS display. Wolbert unfurled a paper map to orient the boat to the shore.

The first sampling site was in shallow water. The biologists could have waved to someone having coffee in the gazebo onshore.

Wolbert swung a boom – a specially engineered crane arm built right into the boat by TVA mechanics – over the water and lowered a metal sampling scoop called a Ponar.

The Ponar disappeared under the water, jaws open. Wolbert waited for the taut rope to go limp when the scoop touched bottom.

“We’re looking at all those different strata, all those different depths,” Wolbert said, pointing across the reservoir.

“We also will assess the substrate, so what’s on the bottom,” he said. “Is it primarily silt? Is there gravel? Is there sand? We’ll tie that data in with water quality folks who are doing the same thing.”

The first site was one end of a transect – or line – stretching across the reservoir.

In all, biologists take 10 samples on each transect for a cross-section of life. They also compare what they find in one reservoir to others in the TVA region.

“You’ll see changes when you drop (the Ponar) at the bank, in the shallower water that’s a little more oxygenated, as opposed to potentially 50 up to 500 feet in Fontana (Reservoir),” Wolbert said.

Wolbert tugged on the winch rope to dislodge the spring-loaded pin. As he began to reel in the rope, the Ponar’s jaws snapped shut, swallowing a bottom sample of mud, debris and benthic macroinvertebrates. The whole boat – and the people in it – lurched left with the weight.

When the Ponar emerged, streaming water, Williams helped Wolbert guide it over the sample screen.

Wolbert opened its jaws. A silty pile of muck slumped out.

Williams moved a hose sprayer left to right slowly over the sample. The mud dissolved and ran through a tube in the bottom of the boat, back to the river below.

Left on the screen were tiny pebbles, leaves, shells and wiggling shimmers of benthic macroinvertebrates.

Some were thin and translucent, like rice noodles.

Some were red like delicate threads of saffron.

“One of the most common things we find are burrowing mayflies,” Wolbert said. “There’s one.”

Using curved tweezers, he pointed out a wiggling, 2-inch animal that resembled a miniature crayfish, dark gray at the abdomen, creamy tan at the head.

He gently scooped the larva into a sample jar, where it swept its tiny legs as if doing the breaststroke.

“Mayflies fall into that group we call EPT, which are our three taxonomic orders of insects that we really want to look out for: Ephemeroptera, Plecoptera and Trichoptera,” Wolbert said.

Those are mayflies, stoneflies and caddisflies, respectively.

“Those are what we’re looking for when we’re out doing our stream surveys,” he said. “And those tend to be the more sensitive, or intolerant, organisms.”

People look for mayflies, stoneflies and caddisflies as signs of clean, oxygenated water.

Sturgeon look for these insect larvae as a snack.

“Through previous efforts and studies, we found a lot of heads and pieces of mayflies in the stomachs of sturgeons,” Wolbert said. “So we know that they are eating them.”

Williams and Wolbert bent over the screen, sifting the sample and tallying the life they found.

The best tool for the job, they said jokingly, was a hotel key card, although any flat, rigid tool that wouldn’t harm the squishy bodies of aquatic creatures would do.

As they spotted macroinvertebrates, they measured target species, tallied them all and released them back into the reservoir.

“Most of what we find are aquatic worms and chironomids – (non-biting) midges,” Wolbert said. “We may find Sialidae – alderflies. Occasionally, dragonflies.”

All of these indicate healthy water in TVA reservoirs.

Lyn Williams, TVA fisheries and aquatic monitoring manager, prepares to release a reservoir-bottom sample that Justin Wolbert, TVA fisheries biologist, scooped up with the Ponar box.

Lyn Williams, TVA fisheries and aquatic monitoring manager, prepares to release a reservoir-bottom sample that Wolbert scooped up with the Ponar box. 

Sampling Success

After the biologists had identified the life in the sample, Williams sprayed the screen over the water and piloted the boat to the next sampling spot. Wolbert held the boom steady, ready to drop the Ponar again.

After 10 samples and several hours, they had filled their data sheet. The boat’s floor and their bibs were soaked – all satisfying signs of a productive day in the field.

Each sampling day is a snapshot that adds to a larger picture of aquatic health.

“Reservoirs can age over time,” Wolbert said. “You want to know what’s down there. You want to look at changes.

“This gives us an idea of how healthy our reservoirs are.”

Photo Gallery

Close up of a burrowing mayfly.

This burrowing mayfly larvae swims with its powerful front legs. It’s a food source for bottom-feeding sturgeon and sucker fish.

Justin Wolbert, TVA fisheries biologist, records the numbers of benthic (bottom-dwelling) aquatic insect larvae.

Wolbert records the numbers of bottom-dwelling aquatic insect larvae, as well as their species and length, to keep tabs on reservoir health.

Lyn Williams, TVA fisheries and aquatic monitoring manager, and Justin Wolbert, TVA fisheries biologist, navigate to a reservoir-bottom sample site on Fort Loudoun Reservoir.

Wolbert and Williams navigate to a reservoir-bottom sample site on Fort Loudoun Reservoir.

Biologists count and measure these insect larvae to monitor reservoir health.

Benthic macroinvertebrates are a food base for fish big and small in TVA reservoirs. Biologists count and measure these insect larvae to monitor reservoir health.

Lyn Williams, TVA fisheries and aquatic monitoring manager, and Justin Wolbert, TVA fisheries biologist, meticulously sift through a sample from the reservoir bottom in search of tiny, wriggling aquatic insect larvae called benthic macroinvertebrates.

Williams and Wolbert meticulously sift through a sample from the reservoir bottom in search of tiny, wriggling aquatic insect larvae called benthic macroinvertebrates.

Electric plug icon

Explore

Learn how TVA helps protect plant and animal species for future generations at the Biodiversity page.

Share this story: