SECCHI DISK

The Secchi Disk

• A Secchi disk is an 8-inch (20-cm) disk with black and white quadrants that alternate.
• It is dropped into a lake until it is no longer visible to the viewer.
• The Secchi depth, or depth of disappearance, is a measure of the transparency of the water.
• The color of the water, algae, and suspended sediments can all have an impact on transparency.
• Transparency diminishes with increasing color, suspended particles, or algae abundance.
• Decomposing plant waste frequently stains water yellow or brown.
• The brown stain can turn water in some lakes and ponds the color of strong tea.

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• Algae are tiny, green aquatic plants whose abundance is proportional to plant nutrients, particularly phosphorus and nitrogen.
• The amount of plant nutrients entering the lake from sources such as sewage treatment facilities, septic tanks, and lawn and agriculture fertilizer can therefore alter transparency.
• Suspended sediments are frequently derived from sources such as lake bottom recovery, building sites, agricultural fields, and urban storm runoff.
• Transparency measures the influence of human activities on the area around the lake.
• Transparency patterns can be detected when measured seasonally and year-to-year.
• Transparency can serve as an early signal if land-based activities are having an impact on a lake.

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