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oxygen vital to the ecosystem). Light attenuation also has implications for the
reduction of visibility, which prevents fish and zooplankton from seeing prey and
predators (Aksnes et al. 2009). Light availability also has some considerable effects
on human perception of recreational water bodies, fishing, and even health. Water
transparency or clarity is therefore often used as an indicator of water quality, with
an important economic impact.
Transparency is commonly represented as the depth where a white disk lowered
into the water is no longer visible to an observer at the surface. That depth is named
the Secchi disk depth and the device used to measure it, accordingly, is called the
Secchi disk.
The Secchi disk method itself has been widely adopted due to its ease of use and
cost-effectiveness compared to other instrumentation. Millions of Secchi disk depth
readings in aquatic ecosystems, covering a time span of more than 150 years, are
accessible in global databases (Lee et al. 2015), and it can be assumed that many
more are hidden or lost in handwritten or printed documentation. The Secchi disk
method provides remarkably stable results, given the fact that sea state and meteorological conditions typically can vary widely, as does the observer technique itself.
For that reason, it has been adopted by various scientific and governmental programs, as well as citizen science initiatives (see, for instance, Secchi Dip-In, a volunteer program started in 1994: http://www.secchidipin.org). Hence, Secchi disk
depth can be considered a popular as well as a successful indicator of water quality
and ecosystem health (Arnone et al. 1985; Burns et al. 2005; Olmanson et al. 2008;
Flemming-Lehtinen and Laamanen 2012; Aas et al. 2014; Garaba et al. 2014).
To show the importance of Secchi disk data, consider a few global and regional
studies.
A global decline of phytoplankton (algae, the primary producer of biomass in
aquatic environments) in the world oceans was investigated by Boyce et al. (2010),
based on the relation of a global ocean dataset of Secchi disk depth data from 1899
to 2008 to marine surface chlorophyll, a common means to measure phytoplankton
abundance. They analyzed Secchi disk depth data from the United States National
Oceanographic Data Centre (NOAA-NODC), containing more than 400,000 Secchi
disk depth observations. Based on this long-term time series, they concluded that
global phytoplankton concentration has declined unequivocally over the past century, a process of great relevance to understand processes of climate change evolving from human influence.
Focusing on coastal and shelf seas, many Secchi disk depth readings are available around Japan, in the Mediterranean, around the Arctic, as well as in the North
Sea and Baltic Sea. A global dataset of marine Secchi disk depth information is
illustrated in Fig. 13.1, available from http://www.eyeonwater.org. The North Sea
and Baltic Sea have been the subjects of a study of Aarup (2002), who compiled
40,829 Secchi disk depth measurements dating back to 1902, with the bulk of them
measured after 1970. Dupont and Aksnes (2013) used this dataset to extract trends
for different water bodies, identifying a centennial shoaling of Secchi disk depth
between 3.2 to 5.8 m for the Baltic Sea and 1.8 to 5.2 m for the North Sea, depending on distance to coast as well as bottom depth. These optical water clarity shifts
O. Zielinski
oxygen vital to the ecosystem). Light attenuation also has implications for the
reduction of visibility, which prevents fish and zooplankton from seeing prey and
predators (Aksnes et al. 2009). Light availability also has some considerable effects
on human perception of recreational water bodies, fishing, and even health. Water
transparency or clarity is therefore often used as an indicator of water quality, with
an important economic impact.
Transparency is commonly represented as the depth where a white disk lowered
into the water is no longer visible to an observer at the surface. That depth is named
the Secchi disk depth and the device used to measure it, accordingly, is called the
Secchi disk.
The Secchi disk method itself has been widely adopted due to its ease of use and
cost-effectiveness compared to other instrumentation. Millions of Secchi disk depth
readings in aquatic ecosystems, covering a time span of more than 150 years, are
accessible in global databases (Lee et al. 2015), and it can be assumed that many
more are hidden or lost in handwritten or printed documentation. The Secchi disk
method provides remarkably stable results, given the fact that sea state and meteorological conditions typically can vary widely, as does the observer technique itself.
For that reason, it has been adopted by various scientific and governmental programs, as well as citizen science initiatives (see, for instance, Secchi Dip-In, a volunteer program started in 1994: http://www.secchidipin.org). Hence, Secchi disk
depth can be considered a popular as well as a successful indicator of water quality
and ecosystem health (Arnone et al. 1985; Burns et al. 2005; Olmanson et al. 2008;
Flemming-Lehtinen and Laamanen 2012; Aas et al. 2014; Garaba et al. 2014).
To show the importance of Secchi disk data, consider a few global and regional
studies.
A global decline of phytoplankton (algae, the primary producer of biomass in
aquatic environments) in the world oceans was investigated by Boyce et al. (2010),
based on the relation of a global ocean dataset of Secchi disk depth data from 1899
to 2008 to marine surface chlorophyll, a common means to measure phytoplankton
abundance. They analyzed Secchi disk depth data from the United States National
Oceanographic Data Centre (NOAA-NODC), containing more than 400,000 Secchi
disk depth observations. Based on this long-term time series, they concluded that
global phytoplankton concentration has declined unequivocally over the past century, a process of great relevance to understand processes of climate change evolving from human influence.
Focusing on coastal and shelf seas, many Secchi disk depth readings are available around Japan, in the Mediterranean, around the Arctic, as well as in the North
Sea and Baltic Sea. A global dataset of marine Secchi disk depth information is
illustrated in Fig. 13.1, available from http://www.eyeonwater.org. The North Sea
and Baltic Sea have been the subjects of a study of Aarup (2002), who compiled
40,829 Secchi disk depth measurements dating back to 1902, with the bulk of them
measured after 1970. Dupont and Aksnes (2013) used this dataset to extract trends
for different water bodies, identifying a centennial shoaling of Secchi disk depth
between 3.2 to 5.8 m for the Baltic Sea and 1.8 to 5.2 m for the North Sea, depending on distance to coast as well as bottom depth. These optical water clarity shifts
O. Zielinski