Secchi Depth and Water Clarity

Secchi depth is the standard measurement of water clarity: how far down a plain white disk stays visible from the surface. It's directly related to chlorophyll concentration, which means Chlorofishy can estimate it from satellite data without a disk in the water.

The formula

Chlorofishy estimates Secchi depth from chlorophyll concentration using Secchi depth (m) = 8.69 × chlorophyll (mg/m³)^-0.47, an empirical relationship: the constant 8.69 derives from Tyler's contrast theory of underwater visibility, and the -0.47 exponent is a fit to published open-ocean datasets. Higher chlorophyll means lower visibility, and the relationship is not linear: small changes at low chlorophyll values move visibility much more than the same change at high values.

Where it applies

This estimate is built for open, offshore (“Case 1”) water where phytoplankton is the dominant thing affecting clarity. It's a best-case ceiling, not a guarantee: sediment, river runoff, and other non-phytoplankton particles can reduce real visibility well below the chlorophyll-only estimate, especially in coastal and nearshore water.

Read the full treatment

This page is a short reference. For the full derivation, Tyler's contrast theory, the validity domain, and where the model breaks down, see the long-form article: Estimating Dive Visibility from Satellite Chlorophyll Data. For the quick how-to on the map itself, see estimating dive and water visibility.

Check today's estimated visibility on the ocean water clarity map. Definition only? Glossary entry. Back to Learn.