Secchi disk

Overview

A black-and-white disk lowered into the water until it disappears from view. The depth at which that happens is the Secchi depth, and it is a direct measure of water transparency — the presence of algae, suspended particles, and sediment.

Simple, immediate, requiring no power, calibration, or expertise.

What problem does this solve for our lake?

A Secchi disk costs $50 and produces the longest-running, most comparable dataset a lake can own. Records going back to the 1970s exist in ordinary lake associations, and they are frequently the most valuable environmental data those communities hold. The reason is standardisation. The method has not changed in a century and a half, requires no instrument that can drift out of calibration, and produces a number that means the same thing whoever collected it. A reading taken by a volunteer in 1981 is directly comparable to one taken this morning — which is true of almost nothing else in lake monitoring. Transparency itself is a real indicator, not just a proxy. It tracks algal abundance closely enough to be a first-order productivity measurement, and a declining multi-year Secchi trend is one of the clearest signals of eutrophication available. Every serious volunteer monitoring programme in the country is built on this disk. If your lake measures one thing, measure this.

What's included?

• Secchi disk with measured line
• Standard black-and-white quartered pattern
• Ready to use — no calibration, power, or setup
• Compatible with every historical Secchi record ever collected

What's not included

• Water chemistry of any kind. Transparency indicates productivity; it does not measure nutrients.
• Identification of what is reducing clarity — algae, sediment, or tannins all reduce Secchi depth differently
• A boat, which you will need to reach open water
• Automated or logged readings

Requirements

• A boat or dock over water deeper than the expected Secchi depth
• The same site, revisited — comparability is the entire value
• Consistent conditions: mid-morning to mid-afternoon, shaded side of the boat, no sunglasses
• Someone willing to take a reading every week or two through the season