The Secchi disk - how to measure water transparency

Methods guide · June 11, 2026

There is something captivating about the simplest of measurements. A white disk on a line, the calm surface of a lake in the morning, someone leaning over the side of a boat and staring into the water - and that’s it. No electronics, no calibration. And yet this single gesture, repeated since 1865, builds some of the longest and most valuable data series on the state of our waters.

It is done by students on field classes, by community members looking after their lake, by science clubs, ecologists and citizen-science volunteers. Often it is precisely they - the people who regularly return to the same water - who first notice that something is changing: that the water “blooms” earlier than it used to, that it gets murkier after heavy rain. A Secchi disk reading turns that attentiveness into a number you can compare - over time, between water bodies, between people. It is a solid, meaningful piece of work, available to literally anyone.

What this measurement actually is

The Secchi disk measures water transparency - the depth at which the white disk disappears from sight. The deeper you can see the disk, the clearer the water.

It is a simple indicator, but surprisingly rich: transparency drops because of three things at once - algae (phytoplankton), suspended matter (soil and sediment particles) and dissolved organic matter that colours the water. That is why the Secchi depth is one of the classic signals of a lake’s trophic state - how “fertile” it is and how prone to blooms.

What you will need

• A Secchi disk - a round plate about 20-30 cm in diameter. In lake studies it is most often black and white (four alternating quarters, better contrast); the oceanographic version is sometimes all white.
• A line marked every 10 cm (or a measuring tape attached to the line) and a weight, so the disk descends vertically.
• Access to the water column - a boat, jetty or footbridge. Take the measurement over a spot deep enough for the disk to disappear before it touches the bottom.

How to take the measurement - step by step

1. Choose a fixed spot (the same on every visit) and a time close to midday (roughly 10:00-14:00), when the sun is high.
2. Measure on the shaded side of the boat/jetty and take off your sunglasses - they cut glare but distort the assessment.
3. Slowly lower the disk until you can no longer see it. Note that depth.
4. Slowly raise it until it reappears. Note the second depth.
5. The result = the average of those two values, given in metres (e.g. 2.3 m).

The whole charm of the method lies in repeatability: the same disk, the same spot, the same time - and readings from years ago talk to today’s.

What the result means

Secchi depth is an indicative picture of how fertile the water is. The thresholds below are not arbitrary - they follow directly from Carlson’s formula TSI(SD) = 60 − 14.41 × ln(SD), so every trophic class boundary corresponds to a specific depth:

The Carlson TSI calculator returns the same figures, so you can convert a disc reading into the index straight away. Caveat: the conversion assumes that transparency is limited mainly by phytoplankton. In waters turbid with mineral suspension (rivers after heavy rain, reservoirs) or strongly humic-stained, the disc will show low transparency without high fertility - TSI computed from Secchi then overstates the trophic state and needs to be checked against chlorophyll or phosphorus.

A single reading says little - the real value is in the trend. Transparency falling year on year, or ever earlier spring collapses, are signals that only a series reveals.

The most common mistakes

• Variable time and weather - a reading in full sun and one at dusk are two different numbers. Stick to consistent conditions.
• Waves and ripples - in wind the reading “runs away”. It is best to measure on a calm surface.
• Different observers - every eye sees a little differently. Note who measured; over a long series this helps.
• Too shallow - if the disk touches the bottom before disappearing, the measurement is invalid (it only tells you “the bottom is visible”).

How often to measure

In the growing season (roughly spring to autumn) a rhythm of every 1-2 weeks is valuable - then you catch blooms and their decline. Outside the season, less often is enough. What matters most is regularity and the same conditions, not the number of measurements.

Sources

• The TSI formula and scale: Carlson R.E. (1977), A trophic state index for lakes, “Limnology and Oceanography” 22(2): 361-369, doi:10.4319/lo.1977.22.2.0361. The class boundaries in the table above come from there - converted from TSI into metres.
• Wider context of the index: Carlson TSI - putting trophic state into a number (including what to do when the three components of the index disagree).

⚠️ A note on the method itself: the disc measures transparency as seen by the observer, so the result depends on eyesight, cloud cover and the state of the water surface. It is a comparative method - strong in a series from one station, weaker for comparisons between different observers.

🧮 Tool: Carlson TSI calculator - enter transparency, chlorophyll-a or total phosphorus and see the index and trophic status according to the table above at once. No sign-up.

How to make it easier

The measurement itself is trivially simple - its value only grows when the readings form a continuous, ordered series. And that is usually where the trouble begins: notes in a backpack, photos of jottings, spreadsheets nobody later merges.

LimnoLog was built so that this part disappears from your mind:

• you record the reading on your phone right by the water - even with no signal (the data syncs to the cloud when you are back in range);
• every measurement goes to a station and a session, so “the same spot, the same time” takes care of itself;
• a trend chart appears from the series right away - you can see whether transparency is rising or falling;
• you can share the results via a public link (great for groups and lake communities) or export them to Excel.

And since LimnoLog is not only about the Secchi disk - with the same system you record oxygen, pH, temperature or any other indicator you happen to be tracking. One place for all your field work.

Early users get free access for now and keep their features for good. If you run your own measurements - take a look and try it.