Using a Survey of Aquatic Bugs to Estimate Stream Quality
As we click, flip, and scroll through hundreds of sites and channels, cruising for our daily dose of e-thrills, it’s easy to forget there’s a beautiful, wild, creative universe right in our backyards.
I had the chance to experience a tiny part of that universe on a recent Saturday afternoon, when a couple of friends, Yolanda and Monika, asked me if I wanted to join them to monitor the water quality of the stream that runs in back of our house.
Yolanda and Monika are part of a large grassroots network of volunteers who selflessly give their time to regularly monitor the quality of streams, lakes, and rivers across the country. Government agencies simply don’t have the resources to regularly collect environmental data on all of the myriad waterways in the U.S., so they depend on getting help from trained volunteers like Yolanda and Monika.
Benthic Invertebrates: Bellwethers of Water Quality
Yolanda and Monika regularly monitor benthic invertebrates counts in a small local stream called Woods Creek.
Benthic invertebrates are tiny animals with no internal skeleton that live on the bottom of lakes, ponds, rivers and streams. Many are insect larvae that spend their “childhood” in the water, until they evolve into adult insects like dragonflies and damselflies.
Some of these tiny critters are extremely fussy and can survive only in fresh, clean water (the Water Penny, the cute-as-button tiny beetle larva shown at right, is one example). Others—like sowbugs, leeches, and scuds—can thrive even in stagnant, polluted environments. For that reason, scientists often survey benthic invertebrates in a natural body of water to assess its ecological condition.
The day I tagged along, we randomly sampled a local stream at 3 locations, following the protocols of the organization Virginia Save Our Streams (VASOS).
As Yolanda loosened rocks that formed riffles in the stream, Monika used a finely meshed seign net to collect the invertebrate sample. Then we recorded the tallies for each type of macroinvertebrate on a data collection sheet (shown partially below) before releasing them back into the stream:
(The images not to scale, of course. Most of these critters are very tiny. You need a magnifying glass to see them clearly.)
Before explaining how their counts are used to estimate stream quality, I hope you’ll grant me a temporary license to digress. Because I’d like you to meet my favorite new BBIFF (best benthic invertebrate friend forever).
(If you’d rather get right down to the number crunching in Minitab Statistical Software, skip ahead to the Using Minitab Formulas…section.)
Mother Nature: The Ultimate Quality Engineer
Life ain’t easy. Especially if you’re a soft, gooey dab of flesh getting regularly pummeled by stream rapids and in constant danger of being gobbled up as a tasty afternoon snack. (And you thought you had it bad...)
But the Caddisfly Strong-Case Maker has developed a resourceful way of protecting itself from the rough-and-tumble of life in a stream bed. A close relative of the butterfly, the larva collects sand, pebbles, sticks, leaves—whatever material is readily available—and then spins silk as a mortar to piece together a surprisingly hard, crush-resistant case to protect its soft body (hence its name).
We found several strong casemakers like this in our recent stream survey—a very good sign! Because the strong casemaker generally requires clean, fresh water to survive. (As do the trout that love to eat them once they sprout wings and flutter above the stream!)
Next time, I’m hoping that we’ll find the casemaker that uses leaves for building materials:
Creative, isn’t it? Like a Japanese origami design for body armor. This leaf case must be a heckuva lot lighter than the all-rock version—for the casemaker who wants to stay light on his feet!
To see more cool pics of casemakers and their brilliantly engineered cases, check out this blog on aquatic insects by Bob Henricks, who kindly gave me his permission to use these pictures.
Then take a look at how a French artist enlists casemakers to create gorgeous custom jewelry, by providing them with gold flakes, turquoise, pearls, and other precious source materials.) You can even watch a video of a casemaker artiste doing his thing, using precious gems.
Now back to statistics…
Using Minitab Formulas to Calculate Stream Quality Index
Once the counts for each macroinvertebrate category are tallied, VASOS uses a series of formulas to calculate a multimetric index that indicates the water quality of the stream.
VASOS provides data sheets that show how to calculate these formulas “by hand.” But I think it’s much easier—and much less prone to error—if you let Minitab do the heavy lifting. To automate the calculations, you can assign formulas to columns in a Minitab worksheet (right-click a column and choose Formula > Assign formula to column).
To illustrate this, I recorded benthic invertebrate counts for some actual Virginia streams in columns C4-C26 of a Minitab worksheet.
Then I followed the steps below to assign formulas to estimate the water quality of a stream.
(If you want to follow along and practice adding formulas to a Minitab worksheet, click here to download the Minitab project file that contains the original data without the formulas. If you don't have Minitab, you can get a free 30-day trial here.)
Step 1: Assign a formula to add sums
In column C27, I assigned a formula to sum the total number of benthic invertebrates recorded in columns C4-C26.
Tip: Notice I hid columns C4-C26 in Minitab for this screenshot. Hiding columns can make it easier to get a birds-eye view of selected data in your worksheet. To hide columns, select the columns you want to hide, right-click, and choose Columns > Hide Selected Columns. To unhide columns, choose Editor>Columns>Hide/Unhide columns.
Step 2: Assign a formula to calculate a percentage
Once Minitab calculates the total organisms in column C27, that value can be used to calculate the percentage of each type of invertebrate. For example, the following formula in column C28 calculates the % of invertebrates that are mayflies, caddiflies, or stoneflies:
Using similar formulas to calculate percentages of Gompihdea (dragonflies), beetles, and invertebrates tolerant of pollution, respectively, gives the percentage values in columns C28-C31 shown below:
Step 3: Assign a formula to assign an index value
The next step requires using a logical function. Whenever you want to assign numerical index scores or categorical text values based on multiple conditions, use the IF(general) function in the Minitab Calculator.
For the invertebrate data, the guidelines require assigning a score of 6, 3, or 0 depending on the percentage of each category of organisms in the sample.
For example, Metric 1 assigns a value of 6, 3, or 0 depending on the total percentage of mayflies, caddisflies, and stoneflies in the sample.
- If the percentage is greater than 4.6, the Metric 1 score is 6.
- If the percentage is greater than 1.6 but less than 4.6, the Metric 1 score is 3.
- If the percentage is less than 1.6, the Metric 1 score is 0.
Using IF(general) to define a formula
Here’s what the IF(general) function in the Calculator looks like before you enter any values: IF(test,value_if_true,...,test,value_if_true,[value_if_false]).
In test, enter a condition. After the comma that follows the condition, enter the value you want Minitab to return if the condition is true. For example, column C28 contains the percentage of mayflies, caddisflies, and stoneflies in the sample. IF(C28 >4.6,6…) tells Minitab to return a value of 6 if the percentage of mayflies, caddisflies, and stoneflies is greater than 4.6.
Now add the next condition. In this example, if the percentage is greater than 1.6 (but less than 4.6), we want Minitab to assign a value of 3. In the Expression field, enter IF(C28 >4.6,6,C28>1.6,3…)
Continue to enter conditions and the value to return if each condition is true, as needed. Minitab works from left to right, and returns the value for the first true condition, then the second true condition, and so on. You can also add an optional value at the very end of the expression to return if none of the conditions hold.
In this example, I’ll use this optional value to return a score of 0 for Metric 1 if the percentage is not greater than 1.6. So the complete IF(general) formula for Metric 1 looks like this: IF(C28 >4.6,6,C28 >1.6,3,0)
Metrics 2, 3, and 4 assign index values based on the percentages of gomphidea, beetles, and pollution-tolerant organisms, respectively. Each metric uses a different range of percentages to define the index values of 6, 3, and 0, so each one requires a different IF (general) formula assigned to the columns.
Step 4: Create an overall index to rate the stream
We’re almost there!
The last step is simply to add all four metrics together to calculate the Multimetric Index Score in column C36. Then, the ecological condition of the stream (acceptable, partially acceptable, or nonacceptable) can be determined by assigning a formula that uses—you guessed it—the IF(general) function, like this:
IF('Multimetric Index Score'>14,"Acceptable",'Multimetric Index Score'>=8,"Partially Acceptable","Unacceptable")
Note: Enclose any text values you want Minitab to return based on the conditions in double quotes.
Presto! An Instant Estimate of Stream Quality
Setting the formulas up in the worksheet initially requires some time and some basic knowledge of calculator functions. But after that, you’re good to go!
Now, just plop in the counts from each new survey (in C4-C26) and you can instantly estimate the ecological condition of the stream, without having to do any calculations.
Note: Not all water-monitoring organizations use these exact formulas and categories to estimate water quality based on benthic invertebrate counts. The formulas and categories can vary—but the general concept is the same.
For example, the Wadeable Streams Assessment Study was a large-scale statistical survey of streams throughout the U.S. that also used benthic invertebrate counts to assess stream quality. The 2004 study included nearly 1400 random sites chosen to represent the conditions of all U.S. streams. It was a great collaborative effort by state and federal agencies, Native-American tribes, universities, hundreds of dedicated volunteers like Yolanda and Monika.
Here's the national report card on the water quality in our wadeable streams:
The pie chart speaks for itself.
But here's the good news. The first step in improving quality is measuring it. You can't fix what you don't know needs fixing.
Earth Day is just one week away...
Looking for a way to feel more connected to the real world around you? Consider joining the thousands of trained volunteers like Monika and Yolanda who monitor local lakes, streams, and rivers in the U.S. (including urban environments). It’s an excellent way to meet some wonderful people and do some tangible good for the planet.
Here are a few organizations that depend on volunteer monitoring programs to help them regularly assess water quality:
- Save Our Streams (Izaak Walton League)
- U.S. Environmental Protection Agency Volunteer Water Monitoring
- Cooperative Extension Volunteer Water Quality Monitoring Programs
(And by the way, if you don’t like bugs there are many other methods of water monitoring you can do besides the benthic invertebrate survey! Trust me—it's fun.)