Aquatic systems worldwide are changing because of increasing climate variability and human activities. Yet it is difficult to capture such changes without standardized long-term observations.
Water transparency or clarity is commonly represented as Secchi depth (ZSD) measured with a Secchi disk. Secchi depth is determined by lowering the disk into the water and recording the depth where it is no longer visible to an observer at the surface.
The measurement provides a first-order indicator of water quality and ecosystem health. Unlike other optical parameters, ZSD can be easily and cost-effectively measured in the field. It has been measured for more than a century in global seas and lakes (Figure 1). Furthermore, ZSD can be estimated from satellite radiometry.
By combining historical and continued ZSD with satellite products, scientists could produce a standardized global ZSD product. Such a product would represent a unique, century-long Earth system data record enabling researchers to assess changes in water clarity in global seas and lakes. With such data, scientists could improve their evaluations of changes to phytoplankton abundance, river sedimentation, fisheries productivity, agriculture’s effect on downstream ecosystems, and more.
Going in Depth on Secchi Depth
The observation and recording of water clarity dates back more than 300 years. In an effort to understand the phenomena behind changing water clarity, Europeans started systematically documenting water clarity in the early 18th century (Figure 2) with wooden boxes, colored plates, and white and red cloths. But it was the development of the Secchi disk by Pietro Angelo Secchi (1818–1878) [Secchi, 1864] that led to the standardized monitoring of water transparency from 1865 onward by scientists and interested citizens.
The standard Secchi disk (Figure 2) is a circular white disk with a diameter of 0.30 meter (disks with diameters of 0.5 or 1 meter were also used when the water was extremely clear). The design has remained largely unchanged since it was first developed, although black-and-white disks are commonly used in monitoring lakes.
Studies have found that Secchi disk observations are remarkably insensitive to variability in observer technique, local meteorological conditions, or the solid white versus black-and-white disks. As a result, roughly 1 million ZSD measurements across the world’s marine and inland water bodies have been collected over the past 150+ years, all with similar uncertainties (~10%–15%).
Field Programs to Measure ZSD
Because ZSD measurements are quite observer independent and are easy and cost-effective to deploy, various government-supported and citizen science programs have been developed to collect water quality data via Secchi disks. These have significantly increased the breadth and length of data records.
For instance, the state of Wisconsin has estimated that citizen-collected ZSD observations since 1986 have a value of ~$4 million. These data are freely accessible and have been used to provide water clarity trends of more than 8,000 lakes in the state. The data also help educate and engage the public on the health of their lakes.
Broader programs also exist. The Secchi Dip-In program administered by the North American Lake Management Society now boasts more than 41,000 records on more than 7,000 separate water bodies in the United States and Canada. In parallel, the Secchi Disk, EyeOnWater, and Seen-Beobachtung Program projects initiated in Europe have accumulated tens of thousands of ZSD observations from lakes to open oceans.
Remote Sensing Approaches and Platforms to Estimate ZSD
ZSD can also be estimated from airborne or satellite radiometry, thus greatly increasing the geographic and temporal availability of water clarity observations. An early effort to collect such estimates started with empirical regressions that were used to relate multispectral Landsat signals to ZSD in lakes.
Lower-resolution sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Medium Resolution Imaging Spectrometer (MERIS) have also been effective in estimating ZSD in estuaries and offshore marine waters. The combination of a series of Earth-observing satellite sensors, including the most recent Visible Infrared Imaging Radiometer Suite (VIIRS) and Sentinel-3, could enable the production of a near-daily record of global ocean color measurements now spanning nearly 2 decades .
Further, Landsat 8 and Sentinel-2 have significantly improved radiometric precision and spectral sampling relative to their predecessors, making it possible to retrieve accurate water color at high spatial resolution for coastal waters and lakes. Thus, the research community is well on its way to providing ZSD products at various spatial and temporal scales with sufficient accuracy through a combination of medium- and high-resolution satellite sensors, offering great potential for assessing water clarity changes on a routine basis at both the local and global scales.
However, to date, there is no standardized ZSD product from any of these satellite missions. The reason is, in part, a lack of community consensus on an appropriate algorithm to relate water color to ZSD on the global scale. But, recently, Lee et al.  developed a new theory to interpret ZSD. It has profound implications for ZSD remote sensing, as it eliminates the need for local tuning as encountered in earlier estimations from satellite measurements. Examples of satellite-based ZSD products are presented in Figure 3.
Examples of Applications
If a standardized global ZSD map at various time points could be created, opportunities for science studies would be manifold. Several realms could see significant scientific gains.
Evaluation of Long-Term Changes in Phytoplankton. A prerequisite to understanding the state of the world’s waterways is obtaining standardized long-term measurements. For example, studies report that 40 to 60 years of standardized observations are required to reliably estimate time trends in phytoplankton concentration, and a lack of such data may be responsible for conflicting reports of time-dependent changes in phytoplankton biomass. Coastal seas and inland waters often show a decline in water clarity, with some of these changes driven by pollution or climate change, whereas other areas show increases in transparency due to remediation. Scientists could achieve increased confidence in long-term changes in water clarity with standardized ZSD products because of their direct representation and long-term data record (1865 to present).
Ecosystem Monitoring. Scientists have found that ZSD data are very valuable for consistent and systematic monitoring of ecosystems. Not only is ZSD a good indicator of suspended particulate matter, but it has been used as a trophic indicator for coral reef and overall estuarine ecosystem status. Lake managers have used the metric to assess the status of lakes, where the lake’s trophic state and algal biomass are related to ZSD. Also, euphotic zone depth, an important indicator of ecosystem state, can be predicted from ZSD .
Fisheries. Phytoplankton biomass production sets the carrying capacity of marine ecosystems and is highly correlated with commercial fishery landings, making ZSD a valuable indicator of a fishery’s productivity. Over most of the global ocean, where phytoplankton plays a dominant role in modulating water clarity, phytoplankton chlorophyll or primary production can be reasonably estimated from ZSD measurements. Using such an approach, ZSD observations could be used to explore changes in ocean ecosystems to support fisheries and to anticipate critical transitions in fish productivity. Separately, water clarity affects the predation dynamics within aquatic ecosystems, particularly for highly visual predators, which may impact ecosystem structure and fisheries.
Other Users. Secchi disk and ZSD products are also valuable for many other applications (see Figure 4), such as coastal recreation and valuation of waterfront properties. In addition, the U.S. Navy has used ZSD products for many applications, including diver visibility and water mass classification.
Decades ago, a Secchi depth atlas was initially developed for the world’s coastlines for waters less than 500 meters deep [Arnone et al., 1985] as an initial effort to define coastal water optics for hydrographic charting systems that are constrained by water clarity. With the advancement in both remote sensing algorithms and satellite sensors, a significantly revised atlas with improved spatial coverage, resolution, and accuracy will better meet many users’ needs.
More broadly, variations in the attenuation of visible radiation in the upper ocean, which directly relates to changes in ZSD, alter local heating and, consequently, have an effect on the thermal and fluid dynamical processes for the ocean-atmosphere system. Thus, a global, time-varying ZSD product would provide a convenient way to measure local heating for inclusion in weather and climate models.
ZSD in the Future
Understanding how aquatic environments will respond to ongoing human- and climate-related changes requires long-term time series. Although satellite measurements are spatially extensive, their coverage across time is limited (currently 1978–1986 and 1997–2017), making it difficult to differentiate trends driven by natural variability from those that are directed. The archive of ZSD over the past century provides one of the longest-running (1865 to present) and spatially extensive (global) standardized records to monitor Earth’s aquatic environment.
Through careful calibration, historical ZSD observations can and have been combined with new measurements and satellite products to greatly increase the spatial and temporal scales at which ocean changes are considered [e.g., Boyce et al., 2017]. This database can also serve a critical need: to gauge the response of aquatic environments to a changing climate.
To date, however, ZSD has not been a standard product of satellite missions. Although expanding the product suite of ocean color satellite missions to include ZSD might be relatively easy since these contemporary sensors are well characterized and monitored, dedicated efforts are required to systematically evaluate ZSD products from Landsat 8– and Sentinel-2-type land-targeted sensors to achieve sound monitoring on finer spatial scales.
Bridging Past and Future
Because of the deployment of advanced optical-electronic systems to measure inherent optical properties of waters, there is a tendency for reduced field measurements of ZSD in contemporary oceanic surveys. Although the optical data provide more details about water constituents, the measurement systems are not yet mature, and the collections will never match the time span of the historical data record of ZSD.
It is therefore of critical importance to continue monitoring ZSD in both marine and freshwater environments to bridge the past and the future. With 150+ years of global shipboard ZSD records and synoptic ZSD products of past (Coastal Zone Color Scanner (CZCS)) and future satellite sensors, the research community will not only obtain a clear picture of the current status of water clarity of the world’s water bodies but also develop a more complete understanding of how and why water clarity is changing globally.
We dedicate this article to our coauthor Dr. Marcel Wernand, who recently passed away. Marcel was a pioneering researcher on the color and clarity of the sea. We are indebted to all scientists and citizens who provided the long record of ZSD for community use, and we thank Xianfu Liu (South China Sea Marine Engineering and Environment Institute) for making Figure 1 and Yonghong Li (Xiamen University) for making Figure 3. The views expressed in this paper are those of the authors and do not represent the views or policies of the U.S. Environmental Protection Agency, the National Oceanic and Atmospheric Administration, or NASA.