221
13.3 The Future of Secchi’s Disk
From today’s perspective, Secchi’s scientific distillation of the white disk method
can be considered as the first cornerstone of the discipline of ocean optics. This
discipline is driven nowadays by sophisticated optical sensors operated from
research vessels, ships of opportunity, and onboard autonomous vehicles (Moore
et al. 2009). Beyond these advances in submersible instrumentation, spaceborne
satellite radiometry has identified the Secchi disk depth as a particularly useful
product that can be derived on global scales from spectral radiometric observations.
It is easy to communicate to the broader public, as it is linked to human perception
of water clarity; and it can be linked to a database of Secchi disk depth readings that
span more than a century, thus enabling one to address global and regional changes
on relevant timescales (Kratzer et al. 2003; Morel et al. 2007a, b; Sørensen et al.
2007; Lee et al. 2016).
With the advent of mobile devices (such as smartphones) incorporating cameras,
positioning information, computing power, and telemetry options, Secchi disk depth
observations have entered a new era. Citizen science – the involvement of members
of the general public, typically in collaboration with professional scientists, to participate in data gathering and/or analysis – is taking hold in marine sciences especially in coastal areas, thanks to smartphone software applications (Garcia-Soto
et al. 2017).
Secchi’s method has transitioned into “app-space,” most prominently with the
www.secchidisk.org initiative. This provides a structured software environment to
record Secchi disk depth readings, along with other marine observables. The www.
eyeonwater.org website offers a digitalized version of the historic Forel-Ule color
comparison scale (Wernand et al. 2013; Busch et al. 2016) along with the possibility
of recording one’s own Secchi disk depth readings and allows one to place both of
these in the context of historical datasets reaching back to the late nineteenth century (Fig. 13.1). It is noteworthy that this app has a slightly larger userbase in the
limnological community (the science of rivers and lakes) and is integrated into coordinated sampling initiatives (similar to the abovementioned Secchi Dip-In).
A peculiar yet good example for the combination of water transparency observations by nonprofessionals with satellite oceanography is Fowler’s Sneaker Depth
index of water clarity (Crooke et al. 2017), which originated in 1988. Citizens with
white sneakers wade into a section of the Patuxent River (Chesapeake Bay, USA)
and measure the depth of the water where the sneakers are no longer visible, analogous to Secchi’s clarity measurements with a white disk. There are obvious shortcomings of this method, such as its limited depth range and the potential influence
of stirred up sediment, yet it is another successful example of how to involve and
engage the general public. Its easy-to-understand concept along with its strength in
communicating long-term trends in water clarity is the very asset of Secchi’s method
and in dire need for today’s global climate challenges.
Secchi’s scientific investigation of the visibility of a white disk lowered into the
water, published in 1865, laid the foundation for a fundamental method to assess
13 The History and Future of the Secchi Disk
13.3 The Future of Secchi’s Disk
From today’s perspective, Secchi’s scientific distillation of the white disk method
can be considered as the first cornerstone of the discipline of ocean optics. This
discipline is driven nowadays by sophisticated optical sensors operated from
research vessels, ships of opportunity, and onboard autonomous vehicles (Moore
et al. 2009). Beyond these advances in submersible instrumentation, spaceborne
satellite radiometry has identified the Secchi disk depth as a particularly useful
product that can be derived on global scales from spectral radiometric observations.
It is easy to communicate to the broader public, as it is linked to human perception
of water clarity; and it can be linked to a database of Secchi disk depth readings that
span more than a century, thus enabling one to address global and regional changes
on relevant timescales (Kratzer et al. 2003; Morel et al. 2007a, b; Sørensen et al.
2007; Lee et al. 2016).
With the advent of mobile devices (such as smartphones) incorporating cameras,
positioning information, computing power, and telemetry options, Secchi disk depth
observations have entered a new era. Citizen science – the involvement of members
of the general public, typically in collaboration with professional scientists, to participate in data gathering and/or analysis – is taking hold in marine sciences especially in coastal areas, thanks to smartphone software applications (Garcia-Soto
et al. 2017).
Secchi’s method has transitioned into “app-space,” most prominently with the
www.secchidisk.org initiative. This provides a structured software environment to
record Secchi disk depth readings, along with other marine observables. The www.
eyeonwater.org website offers a digitalized version of the historic Forel-Ule color
comparison scale (Wernand et al. 2013; Busch et al. 2016) along with the possibility
of recording one’s own Secchi disk depth readings and allows one to place both of
these in the context of historical datasets reaching back to the late nineteenth century (Fig. 13.1). It is noteworthy that this app has a slightly larger userbase in the
limnological community (the science of rivers and lakes) and is integrated into coordinated sampling initiatives (similar to the abovementioned Secchi Dip-In).
A peculiar yet good example for the combination of water transparency observations by nonprofessionals with satellite oceanography is Fowler’s Sneaker Depth
index of water clarity (Crooke et al. 2017), which originated in 1988. Citizens with
white sneakers wade into a section of the Patuxent River (Chesapeake Bay, USA)
and measure the depth of the water where the sneakers are no longer visible, analogous to Secchi’s clarity measurements with a white disk. There are obvious shortcomings of this method, such as its limited depth range and the potential influence
of stirred up sediment, yet it is another successful example of how to involve and
engage the general public. Its easy-to-understand concept along with its strength in
communicating long-term trends in water clarity is the very asset of Secchi’s method
and in dire need for today’s global climate challenges.
Secchi’s scientific investigation of the visibility of a white disk lowered into the
water, published in 1865, laid the foundation for a fundamental method to assess
13 The History and Future of the Secchi Disk