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It's Only Natural

The Enduring Legacy of Professor Secchi

By Tamia Nelson
tamia@paddling.net

July 8, 2003

Dawn, or near enough as makes no difference. On a glassy Adirondack lake too small to warrant a name, a brother and sister are fishing. They watch their bobbers from opposite ends of a battered Grumman canoe, peering down into the tea-colored water from time to time, searching for the worm each knows is there, but which neither can see. A fish breaks the surface, splashes once, and submerges again. Distant bullfrogs add a bass note to the falsetto thrum of hundreds of mosquitoes. Overhead, a kingfisher rattles determinedly. "Water's dirty," says the boy, looking up. "Sure is," replies his sister. "Do you s'pose it's always like this?" The boy starts to speak, but just as he opens his mouth his bobber trembles, then dips below the surface and disappears. Now the boy is transformed into a hunter, with a hunter's contempt for unnecessary words. He raises his rod-tip high. A short struggle follows. In less than a minute a chubby rock bass lies in the bottom of the canoe.

I was the girl in the Grumman. The brief exchange between my brother and me was soon forgotten, but a seed of inquiry had lodged deep in my mind, only to germinate decades later on another Adirondack lake, known locally as Green Pond. When I first visited it, however, there was no hint of green in the colorless, transparent water. As I peered over the gunwale, I could see right down to the lake's white sand bottom, 15 feet below my keel. Waterlogged trunks of ancient cedars and pines rested where they had fallen. Drifts of birch and poplar leaves formed a ragged carpet, their once-brilliant colors bleached to a faded yellow-brown. Even a few old casting plugs still clung determinedly to the branches of drowned trees.

Turning my head from side to side, I searched for signs of life in and around the crystal-clear water. Nothing. No tangled mats of pondweed blanketed the bottom. No dragonflies patrolled for mosquitoes in the air above the silent surface. No leaches undulated through the shallows. No fat trout rose lazily to catch a hapless spider. No kingfisher rattled. Green Pond was lovely, still, and silent, a calendar image of an Adirondack lake. Yet it was dead. The teeming, murky water of my half-recalled childhood seemed very far away.

Acid rain, I thought. It was obvious to me then and there that clarity alone wasn't any guarantee that a lake was healthy.

And I was right. But clarity — the technical literature often uses the equivalent term "transparency" — is an important index of water quality, nonetheless. Dissolved minerals, microscopic algae, and suspended sediments all affect transparency. As their levels increase, transparency diminishes. The implications of this are sometimes hard to interpret, let alone forecast, but at least it's easy to measure the changes. Repeated measurements, carefully recorded from season to season and from one year to the next, can provide an early warning of potentially deleterious trends. It's a simple way to take the pulse of a body of water. And while a person's pulse may not be the best guide to her overall health, it's a pretty good place to start. The same is true for lakes and rivers.

So, how do you measure transparency? The easiest way is with a Secchi (pronounced SEK-key) disk. This wonderfully low-tech instrument has an intriguing history. Father Pietro Angelo Secchi was director of the Vatican Observatory and scientific advisor to Pope Pius IX. In this capacity he was once approached by Commander Cialdi, the senior officer in the Navy of the Papal States. Commander Cialdi described a newly-devised procedure for measuring water transparency. Would Father Angelo be good enough to test the new method? Commander Cialdi wondered. With pleasure, Father Angelo replied. And perhaps, continued Commander Cialdi, the professor could also suggest improvements? Father Angelo could, and he did. On April 20, 1865, the first Secchi disk was lowered over the side of the papal yacht l'Immacolata Concezione into the wine-dark waters of the Mediterranean. Father Angelo's pioneering work in astrophotography and spectral classification is now largely forgotten, but his name lives on in the pages of scientific supply catalogs, and in every department of limnology and oceanography in the world. To be sure, today's Secchi disk is somewhat changed from that of 1865: the modern freshwater disk bears a quadrant pattern developed in 1899 by George Whipple. Oceanographers, however, still use an all-white disk, just like the one Father Angelo took aboard l'Immacolata Concezione.

But what if your "yacht" is a canoe or kayak? You can still follow in Father Angelo's wake, using his simple instrument to track changes in the transparency of your favorite body of water. Why not begin right now? You'll need a Secchi disk, of course. The standard limnological (freshwater) disk is 8" in diameter. (That's approximately 20cm, but this dimension isn't usually critical.) The upper surface is divided into four quadrants, alternating black and white, and the disk is suspended on a low-stretch, non-floating line. A weight hung below the disk helps sink it through the water column. Commercially-made Secchi disks can be purchased from most biological and environmental supply companies, but if you think you can find a better use for $50, it's easy to make your own. A hand or power drill and a keyhole or saber saw are the only tools you'll need. If you're like most boaters, you probably have the necessary materials on hand already. (Look in that pile of scrap in the garage you've been planning to clean out for the last five years.)

Here's the materials list…

  1. 1 piece 1/4" exterior-grade plywood
  2. 1 threaded eyebolt (stainless steel is best)
  3. 3 nuts
  4. 2 washers
  5. 1 2-lb weight (cast-iron sash weights work well)
  6. 65 feet (20 meters) of non-floating line
  7. White and black paint (exterior matte finish)

A Do-It-Yourself Secchi Disk

Professor Secchi's Disk.

The rest is pretty much self-explanatory. Cut out the disk and drill a hole through its center. Scribe the quadrants and paint them. (Paint the underside and edges of the disk white.) Using two nuts and two washers, secure the eyebolt through the hole you've just drilled, with the eye on the upper side of the disk. Knot (or splice) the line to the eye. (You can mark the line in feet or meters, if you wish, but it's probably better to measure the submerged depth with a separate tape or yardstick.) Now suspend the weight from the underside of the disk. (This will require a slip-on hanger — not on the materials list — or some carefully-executed lashing.)

That's it. You're done. Once you have your disk, you're ready to make your debut as an amateur limnologist. Here's how:

  1. Proceed to your first sampling point, or "station." Once there, lower your disk until it disappears from sight. If glare makes keeping the disk in view difficult, try to change your viewing angle. (Don't wear sunglasses, though.)

  2. Mark the depth at which the disk disappears. A spring clothespin works well for this. Call this depth D for "depth disk DISAPPEARS."

  3. Lower the disk a couple of feet more, then slowly raise it. Mark the depth where it reappears. Call this R for "depth disk REAPPEARS."

  4. Add the two depths together. Call their TOTAL T, and divide it by two. This final number is a measure of the transparency at the station. Call it S for (you guessed it) "SECCHI depth."

    Now repeat the procedure at each station.

Piece of cake, eh? If you like formulas, here they are:

T= D + R

and

S= T/2

A Secchi Disk in Use

The Secchi Disk in Depth

That's all there is to it. Almost. To make sure your measurements are comparable from season to season and from one year to the next, always follow a standard procedure. Take all readings at about the same time of day — noon is best — being careful not to stir up bottom sediments in the process. Remove your sunglasses, and lower your disk over the shaded side of the boat. Locate your sample stations accurately by careful compass triangulation or GPS. Give each a unique name, and annotate your map or chart accordingly. Then, as soon as you get your data, record it in your field notebook, jotting down date, time, station name, and weather conditions (cloud cover, temperature, barometric pressure). Note any recent storms, too. High winds often stir up sediments, reducing transparency. And don't forget the common-sense precautions that are second-nature on your purely recreational outings. Wear your PFD, and be sure to dress for the water temperature.

Once you're back home, copy your data in another notebook or enter it in a computer file. This is your "office" record. It's also a backup should you lose your field notebook. As you acquire more and more data, you'll start to notice trends. Ask yourself why this is. If transparency is declining, is there new construction or logging going on nearby? Is fertilizer running off from farm fields in the watershed? Is increasing jet-ski or other powerboat traffic stirring up bottom sediments?

On the other hand, if your home waters are growing more transparent, why is that? Has a local farm been abandoned, and are once intensively-cultivated fields now lying fallow? Have leaky shoreline septic tanks been repaired or replaced by a central sewer? Or is the change driven by some long-term regional trend?

The possibilities are endless. Ask questions. Make connections. You may even want to pool your data with others. In that case, consider getting in touch with the folks at Kent State University (Ohio, USA) who organize the annual Secchi Dip-In. This event attracts volunteers from around the world, and the program serves as a clearinghouse for collecting and disseminating information about global water-quality trends. The 2003 Dip-In continues until July 13th, so it's not too late. To learn more, check out the Dip-In's website.

Whether you choose to participate in the Dip-In or not, however, take a minute to reflect on the contribution that Father Angelo made more than 135 years ago to the study of the earth's waters. After all, it's only natural!

Copyright 2003 by Verloren Hoop Productions. All rights reserved.









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