Professor Secchi's Disk.
It's Only Natural
The Enduring Legacy of Professor Secchi
By Tamia Nelson
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 piece 1/4" exterior-grade plywood
- 1 threaded eyebolt (stainless steel is best)
- 3 nuts
- 2 washers
- 1 2-lb weight (cast-iron sash weights work well)
- 65 feet (20 meters) of non-floating line
- White and black paint (exterior matte finish)
The Secchi Disk in Depth
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:
- 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,
- Mark the depth at which the disk disappears. A spring clothespin
works well for this. Call this depth D for "depth disk DISAPPEARS."
- 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."
- 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
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