Secchi Depth Calculators

0 calculators tagged with “Secchi Depth”

Secchi depth is a simple, low-cost measure of water transparency obtained by lowering a standardized disk (the Secchi disk — a 20 cm diameter plate with alternating black and white quadrants) until it just disappears from view and recording that depth. Secchi depth reflects the combined effect of all light-absorbing and light-scattering materials in the water — primarily phytoplankton (algal biomass), suspended sediments, and dissolved organic matter (colored water). In productive (eutrophic) lakes with dense algal blooms, Secchi depths may be < 0.5 m; in clear oligotrophic mountain lakes, > 10–40 m.

All Calculators

No calculators found for this topic.

Measurement Protocol

Lower a 20 cm Secchi disk on a calibrated line from the shaded side of a boat on a clear, bright day around solar noon. Record: disappearance depth (D₁) when disk first disappears from view; reappearance depth (D₂) when disk just becomes visible on the way up. Secchi depth = (D₁ + D₂) / 2.

Factors Affecting Secchi Depth

  • Algal biomass (most important in productive lakes): phytoplankton scatter and absorb light; Secchi depth inversely correlated with chlorophyll a concentration
  • Suspended sediments: inorganic turbidity from erosion, wind resuspension; particularly important in shallow or disturbed lakes
  • Dissolved organic matter (DOC): humic substances from watershed stain water brown/tea-colored (dystrophic lakes); absorb light but don't scatter
  • Weather: measure in calm, sunny conditions; wind chop and cloud cover increase variability

Secchi Depth and Trophic State

Carlson TSI formula: TSI(SD) = 60 − 14.41 × ln(Secchi depth in meters). Typical values: oligotrophic (SD > 4 m); mesotrophic (SD 2–4 m); eutrophic (SD 0.5–2 m); hypereutrophic (SD < 0.5 m).

Limitations

In dystrophic (brown-water) lakes: Secchi depth underestimates algal turbidity (DOC color reduces clarity independently of algae). Cannot distinguish causes of low Secchi depth — need chlorophyll a and turbidity measurements to diagnose.

Glossary

Secchi Depth
Water transparency measured as depth at which a 20 cm black-and-white disk disappears from view; Secchi depth = (disappearance + reappearance depth)/2; used in Carlson TSI; oligotrophic > 4 m; hypereutrophic < 0.5 m.
Carlson TSI (Secchi)
TSI(SD) = 60 − 14.41 × ln(Secchi depth in m); part of the Carlson Trophic State Index; TSI < 40 = oligotrophic; TSI 50–70 = eutrophic; TSI > 70 = hypereutrophic.
Photic Zone
The depth to which sufficient light penetrates for net photosynthesis; empirically ≈ 2 × Secchi depth; limits algae and SAV growth; shallower in eutrophic water.

Frequently Asked Questions

Secchi depth is a measure of water transparency: a standardized Secchi disk (20 cm diameter, alternating black and white quadrants) is lowered on a calibrated rope into the water until it disappears from view (depth D₁), then raised until it just reappears (depth D₂). Secchi depth = (D₁ + D₂)/2. Measurement conditions: measure from the shaded side of the boat; best on a clear, calm day around noon; avoid bright sunshine reflecting off the surface. Secchi depth is the simplest and most widely used water clarity measurement — inexpensive, requires no laboratory analysis, and is easily understood by the public and citizen science programs.

Secchi depth reflects trophic state and is directly used in the Carlson Trophic State Index: Oligotrophic: Secchi depth > 4 m; clear, blue water; low algal biomass; TSI < 40. Mesotrophic: Secchi depth 2–4 m; moderate clarity; TSI 40–50. Eutrophic: Secchi depth 0.5–2 m; reduced clarity; algal blooms possible; TSI 50–70. Hypereutrophic: Secchi depth < 0.5 m; turbid green or brown water; dense algal blooms; TSI > 70. Example values: Lake Tahoe (oligotrophic): Secchi depth 20–30 m. Lake Erie western basin (eutrophic): Secchi depth 1–2 m during summer algal blooms. Hypereutrophic ponds: Secchi depth < 0.3 m.

Three main factors reduce Secchi depth (reduce water transparency): (1) Phytoplankton (algae): the dominant factor in eutrophic lakes; phytoplankton cells scatter and absorb light; Secchi depth inversely correlates with chlorophyll a concentration; eutrophication is the most common cause of declining clarity. (2) Suspended inorganic particles: fine clay and silt from watershed erosion, agricultural runoff, wind resuspension of bottom sediments, or boat wake; particularly important in shallow lakes and reservoirs with high watershed sediment inputs. (3) Colored dissolved organic matter (DOC): humic and fulvic acids from decomposing plant material; creates brown, tea-colored water (dystrophic lakes); absorbs light but doesn't scatter — lake may be 'clear' but dark-colored. In most lakes, algal biomass is the primary target for management because it is nutrient-driven.

The photic zone (euphotic zone) is the depth to which sufficient light penetrates for net photosynthesis. Empirical relationship: photic zone depth ≈ 2 × Secchi depth. Rule of thumb: the 1% light level (compensation depth, where photosynthesis = respiration) occurs at approximately 2.5–3 × Secchi depth. This relationship varies with water color and dominant light-absorbing materials. Importance: photic zone depth determines where algae can grow; limits the depth of submerged aquatic vegetation (SAV); influences thermal stratification (light drives surface warming); in eutrophic lakes, very shallow photic zones restrict algal growth to the surface and contribute to stratification.