Editing Lake Secchi Depth (Water Clarity)

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{| class="wikitable" style="float:right; clear:right; margin:0 0 1em 1em; width:320px;"
|+ SIGNAL Earth Structured Data
|-
! Object type
| Damage Signal
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! SIGNAL Earth ID
| DS-00136
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! Observable type
| Lake Secchi depth (water clarity)
|-
! Unit
| m (m)
|-
! Temporal structure
| Seasonal / Period Avg
|-
! Monitoring backbone
| EPA National Lakes Assessment
|}
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{{SignalTerm|type=DS|id=DS-00136|label=Lake Secchi Depth (Water Clarity)}} Lake Secchi depth, commonly referred to as water clarity, is a key indicator of the optical properties and ecological condition of freshwater lakes. It is measured by lowering a Secchi disk into the water and recording the depth at which the disk is no longer visible. This measurement provides insight into the transparency of lake water, which is influenced by factors such as suspended sediments, phytoplankton, and dissolved organic matter.

Water clarity is an important parameter in limnology and freshwater ecology, as it affects light penetration, primary productivity, and habitat quality for aquatic organisms. Changes in Secchi depth can indicate shifts in lake trophic status, sediment inputs, or biological activity, making it a valuable state indicator for freshwater systems.

Within the context of global environmental monitoring, Lake Secchi depth serves as a standardized metric to assess the condition of lakes across diverse geographic regions. Its relevance extends to understanding eutrophication processes, sediment dynamics, and the impacts of land use and climate variability on freshwater ecosystems.

== Geographic / System Context ==
Lake Secchi depth measurements are applicable to lakes worldwide, encompassing a broad range of climatic zones, geologies, and anthropogenic influences. Freshwater lakes vary greatly in size, depth, and watershed characteristics, all of which affect water clarity. Globally, lakes serve as critical reservoirs of biodiversity, sources of freshwater, and components of regional hydrological cycles. The spatial heterogeneity of lakes requires consistent monitoring approaches to enable comparative assessments and to track environmental changes over time.

== Monitoring and Measurement ==
Monitoring of Lake Secchi depth is conducted through direct field observations using a Secchi disk, a circular plate typically 20 centimeters in diameter with alternating black and white quadrants. The disk is lowered into the lake water until it disappears from view, and the depth is recorded in meters. This method is widely used due to its simplicity, low cost, and ability to provide repeatable measurements.

In addition to in situ measurements, remote sensing techniques have been developed to estimate lake water clarity over large spatial scales. Satellite data, such as from Landsat, can be analyzed to infer Secchi depth by correlating surface reflectance with water transparency. These approaches complement traditional monitoring and allow for expanded temporal and geographic coverage.

Institutions such as the United States Environmental Protection Agency ([https://en.wikipedia.org/wiki/United_States_Environmental_Protection_Agency EPA]) through its National Lakes Assessment program and networks like the Global Lake Ecological Observatory Network (GLEON) contribute to systematic data collection and analysis of Secchi depth and related water quality parameters.

Within the SIGNAL system, Lake Secchi depth is treated as a defined environmental signal whose boundaries and measurement conventions are described below.

== Signal Definition ==
The Lake Secchi depth signal represents the measured depth in meters at which a Secchi disk is no longer visible when lowered vertically into a lake. It quantifies water clarity as a state condition within the Freshwater-State domain, reflecting the combined effects of suspended particles, algal biomass, and dissolved substances that influence light attenuation in the water column. This signal is derived from the observable type 'Lake Secchi depth (water clarity)' and is expressed as a seasonal or period average to capture temporal variability.

== Boundary Conditions ==
Boundary inclusions for this signal encompass all lentic freshwater bodies classified as lakes where Secchi depth measurements are feasible and representative of surface water clarity. This includes natural and artificial lakes of varying sizes and trophic statuses. Boundary exclusions apply to lotic systems such as rivers and streams, where flow dynamics alter optical conditions, and to saline or brackish water bodies where different optical properties prevail. Measurements affected by surface ice cover, extreme turbidity events, or methodological inconsistencies are also excluded to maintain data quality and comparability.

== Aggregation Semantics ==
Geographic aggregation of Lake Secchi depth data is performed at multiple spatial scales, ranging from individual lake basins to regional and global assessments, enabling evaluation of spatial patterns and trends. Temporal aggregation typically involves seasonal or annual averaging to account for natural variability and to smooth short-term fluctuations. Cross-signal aggregation may integrate Secchi depth with related indicators such as chlorophyll-a concentration and suspended sediment levels to provide a comprehensive understanding of lake water quality and ecological status. These aggregation approaches support multi-dimensional analyses within the SIGNAL framework.

== Observational Status ==
Lake Secchi depth is actively monitored through established programs including the EPA National Lakes Assessment and international networks like GLEON. Data availability varies geographically, with denser coverage in regions with extensive limnological research and monitoring infrastructure. Remote sensing advancements have enhanced the capacity to estimate water clarity over broader areas, supplementing in situ observations. Future SIGNAL releases may incorporate expanded datasets, improved temporal resolution, and integration with complementary environmental signals to refine assessments of freshwater ecosystem conditions.

== Related Signals ==
* Freshwater eutrophication index
* Freshwater suspended sediment concentration
* Lake chlorophyll-a concentration

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== Key Associated People ==
* '''Catherine O’Reilly''' — Steward-candidate (Illinois State University / GLEON) [Domain expert]
* '''David Schindler''' — Steward-candidate (University of Alberta) [Domain expert]
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== Sources ==
* [https://gleon.org/ GLEON (Global Lake Ecological Observatory Network)]
* [https://www.oecd.org/ OECD 1982 Eutrophication of Waters: Monitoring, Assessment and Control]
* [https://doi.org/10.1111/j.1752-1688.1977.tb04108.x Chl-a as eutrophication indicator (Carlson TSI 1977)]
* [https://doi.org/10.1038/s41586-021-03553-8 Global lake water clarity remote sensing (Landsat-based)]
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