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| Secchi disk | |
|---|---|
| Name | Secchi disk |
| Caption | Illustration of a Secchi disk in use |
| Invented by | Pietro Angelo Secchi |
| Year | 1865 |
| Country | Papal States |
| Field | Oceanography; Limnology |
Secchi disk is a simple, standardized optical instrument used to estimate water transparency by lowering a circular disk into a water column until it disappears from view. Developed in the 19th century, it remains a widely used tool in observational programs run by organizations such as United Nations Environment Programme, National Oceanic and Atmospheric Administration, and volunteer networks coordinated by Citizen Science initiatives. The method provides rapid, low-cost measurements that complement instrumented techniques employed by research institutions like Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, and university programs in University of Liverpool and University of Washington.
The device was introduced by Italian astronomer and priest Pietro Angelo Secchi in the 1860s while serving in Rome and engaged with colleagues in Observatory of the Vatican. Early adopters included researchers at institutions such as Royal Society, French Academy of Sciences, and the German Marine Observatory as they sought standardized oceanographic observations. Throughout the late 19th and early 20th centuries, the Secchi disk was incorporated into expeditions led by figures and organizations like Charles Darwin-inspired voyages, the Challenger expedition, and naval research vessels of the British Admiralty. In the 20th century, agencies such as United States Geological Survey and International Council for the Exploration of the Sea promoted protocols that integrated the disk into long-term monitoring alongside technologies developed by entities like Rosenstiel School of Marine and Atmospheric Science and Lamont–Doherty Earth Observatory.
A standard Secchi disk is a circular plate, typically 20 to 30 centimetres in diameter, painted with alternating quadrants of white and black to maximize contrast. Variants include all-white models and modified disks marked with concentric rings used in studies by groups such as National Oceanic and Atmospheric Administration and research teams at Plymouth Marine Laboratory. Construction materials range from galvanized steel and marine-grade aluminum to rigid plastic used by programs at Environmental Protection Agency and university field stations like Gulf Coast Research Laboratory. The disk is attached to a non-elastic line with graduated markings; lines are often calibrated against standards maintained by laboratories at Smithsonian Institution and regional water-quality agencies. Specifications may be adapted in freshwater surveys coordinated by organizations like United States Geological Survey and international freshwater programs under World Health Organization guidelines.
The observer conducts a Secchi reading by lowering the disk from the shaded side of a vessel until the disk is no longer visible, recording the depth on the marked line. If the disk is lowered until disappearance and then raised until it reappears, the mean of the two depths is sometimes used, a refinement adopted in protocols by National Oceanic and Atmospheric Administration, European Environment Agency, and volunteer networks like Chesapeake Bay Program. Readings are best performed under standardized conditions—midday, low wind, and away from surface glare—per guidance from institutions such as International Oceanographic Commission and university-led training at Bigelow Laboratory for Ocean Sciences. Data are logged with metadata including position from systems like Global Positioning System, time coordinated with standards from International Atomic Time, and ancillary measures such as surface temperature, a practice followed in monitoring programs at NOAA National Estuarine Research Reserve and regional agencies.
Secchi depth is influenced by constituents that alter light attenuation, including phytoplankton pigments measured by laboratories like Marine Biological Laboratory and dissolved substances studied by researchers at Max Planck Institute for Marine Microbiology. Suspended sediments from rivers such as the Amazon River or events like Mount Pinatubo eruption-related runoff change turbidity and reduce Secchi depth. Colored dissolved organic matter originating in catchments like the Baltic Sea drainage or from wetlands studied by teams at Duke University affects light absorption. Surface conditions—sun position relative to the observer, sky cloud cover influenced by systems monitored by European Centre for Medium-Range Weather Forecasts, and wind-driven surface roughness observed by National Aeronautics and Space Administration sensors—also modify readings. Human activities overseen by entities such as United Nations Environment Programme and local authorities can alter Secchi depth via eutrophication, dredging, or discharge regulated under agreements like the Convention on Biological Diversity.
Secchi disk measurements serve as long-term indicators of water clarity in studies conducted by Long Term Ecological Research Network, International Council for the Exploration of the Sea, and national monitoring networks including Environment Agency (England). They provide input for assessments of primary productivity in systems studied by Woods Hole Oceanographic Institution and for calibration of optical sensors deployed by projects like Argo (oceanography). Citizen science campaigns organized by Ocean Conservancy and regional programs such as Chesapeake Bay Program leverage Secchi readings for lake and coastal monitoring. Historical Secchi datasets from archival efforts at institutions like Royal Netherlands Institute for Sea Research and Scripps Institution of Oceanography have supported climate-related analyses and model validation used by groups like Intergovernmental Panel on Climate Change contributors.
Critiques of the Secchi disk highlight subjectivity and observer variability documented in intercomparison studies by International Oceanographic Commission and academic groups at University of Maine and University of British Columbia. The method provides an integrated measure influenced by spectral light attenuation but lacks spectral resolution offered by instruments from manufacturers used by laboratories such as Sea-Bird Scientific and research groups at Lamont–Doherty Earth Observatory. In turbid, stratified, or highly colored waters common in regions around Ganges River deltas or boreal lakes monitored by Finnish Environment Institute, Secchi measurements can be insensitive to subsurface scattering layers. Despite limitations, standardized protocols promoted by institutions like United Nations Environment Programme and enhanced training through partnerships with universities continue to support its role as an accessible observational tool.
Category:Oceanography instruments