43
In waters (mostly open ocean) consisting of very low phytoplankton abundances, most visible light is scattered by the
water molecules. Water selectively scatters and absorbs certain wavelengths of visible light (Pope and Fry 1997). Longer
wavelengths are quickly absorbed from water while shorter
wavelengths penetrate deeper, which gives the deep open
oceans their characteristic blue color (Fig. 2, blue spectra).
In coastal waters (influenced by terrestrial runoff) with
higher proportions of dissolved and particulate matter, both
absorption and scattering increases, making them appear
green (Fig. 2, green spectra) or brown depending on its constituents (Morel and Prieur 1977). Detailed and accurate
understanding of the water constituents and their interaction
with light is essential in studies of radiative transfer (Chang
and Dickey 2004).
Light Penetration and Euphotic Depth
Only the surface layer of the ocean receives sufficient light to
allow phytoplankton growth through primary production.
Sunlight entering the ocean may travel up to 1000 m deep but
there is barely any significant light beyond 200 m. Based on
light availability, water columns are divided into 3 different
zones. The upper 150–200 m layer of the ocean is called the
‘sunlit’ or the ‘euphotic’ zone. The extent of this layer is
determined by the depth at which the Photosynthetically
Active Radiation (PAR) reduces to 1% of its surface value. In
bio-optical literature, PAR values are given in units of mol
photons s
−1
m
−2
or einst s
−1
m
−2
, where one einstein is one
mole of photons (6.023 × 10
23
photons). PAR is a broadband
quantity, often estimated using only the visible wavelengths,
400–700 nm (Mobley 1994). Beyond approximately 200 m
depth, the intensity of light decreases rapidly with increasing
depth and is insufficient to support any photosynthetic activity. From about 200–1000 m the zone is referred to as ‘twilight’ or ‘dysphotic’. Below 1000 m the zone is known as
‘aphotic’ or ‘midnight’ zone and is entirely dark.
The depth of the euphotic zone (Zeu) depends highly on
the turbidity of the water column caused by varying concentrations of organic and inorganic optically active constituents
(OACs) present either in dissolved form or in suspension.
Phytoplankton populations, dead organic matter, CDOM, and
inorganic sediments diminish the amount of light available
for photosynthetic activity causing the depth of light penetration to differ dramatically between oceanic and coastal waters
(Fig. 3a). In open ocean waters with relatively low phytoplankton, the blue-green wavelengths penetrate deeper in the
water column. In contrast, high concentrations of both suspended particulate (phytoplankton and sediments) and dissolved matter strongly absorb the blue-green wavelengths in
coastal waters thereby restricting penetration in deeper
waters. The longer red wavelengths, however, are quickly
absorbed by water molecules in near surface waters irrespective of the water optical type (Fig. 3b). In estuarine and fjordal
ecosystems, with different fresh and saltwater mixing zones,
the euphotic depth reduces gradually with increase in turbidity from the outer (downstream) to inner region (upstream)
(Mascarenhas et al. 2017). It is in the euphotic zone, that the
majority of primary production takes place.
Fig. 2 Spectral reflectance in case-1 (Pacific Ocean) and case-2 waters
(Norwegian Fjord). Case-1 waters consisting of very low optically
active constituents (OACs), reflect light in the blue region. Case-2
waters with high concentrations of OACs (here chlorophyll a in phytoplankton), reflect strongly the green wavelengths. (Data: Daniela Voss,
ICBM, University of Oldenburg)
Marine Optics and Ocean Color Remote Sensing
In waters (mostly open ocean) consisting of very low phytoplankton abundances, most visible light is scattered by the
water molecules. Water selectively scatters and absorbs certain wavelengths of visible light (Pope and Fry 1997). Longer
wavelengths are quickly absorbed from water while shorter
wavelengths penetrate deeper, which gives the deep open
oceans their characteristic blue color (Fig. 2, blue spectra).
In coastal waters (influenced by terrestrial runoff) with
higher proportions of dissolved and particulate matter, both
absorption and scattering increases, making them appear
green (Fig. 2, green spectra) or brown depending on its constituents (Morel and Prieur 1977). Detailed and accurate
understanding of the water constituents and their interaction
with light is essential in studies of radiative transfer (Chang
and Dickey 2004).
Light Penetration and Euphotic Depth
Only the surface layer of the ocean receives sufficient light to
allow phytoplankton growth through primary production.
Sunlight entering the ocean may travel up to 1000 m deep but
there is barely any significant light beyond 200 m. Based on
light availability, water columns are divided into 3 different
zones. The upper 150–200 m layer of the ocean is called the
‘sunlit’ or the ‘euphotic’ zone. The extent of this layer is
determined by the depth at which the Photosynthetically
Active Radiation (PAR) reduces to 1% of its surface value. In
bio-optical literature, PAR values are given in units of mol
photons s
−1
m
−2
or einst s
−1
m
−2
, where one einstein is one
mole of photons (6.023 × 10
23
photons). PAR is a broadband
quantity, often estimated using only the visible wavelengths,
400–700 nm (Mobley 1994). Beyond approximately 200 m
depth, the intensity of light decreases rapidly with increasing
depth and is insufficient to support any photosynthetic activity. From about 200–1000 m the zone is referred to as ‘twilight’ or ‘dysphotic’. Below 1000 m the zone is known as
‘aphotic’ or ‘midnight’ zone and is entirely dark.
The depth of the euphotic zone (Zeu) depends highly on
the turbidity of the water column caused by varying concentrations of organic and inorganic optically active constituents
(OACs) present either in dissolved form or in suspension.
Phytoplankton populations, dead organic matter, CDOM, and
inorganic sediments diminish the amount of light available
for photosynthetic activity causing the depth of light penetration to differ dramatically between oceanic and coastal waters
(Fig. 3a). In open ocean waters with relatively low phytoplankton, the blue-green wavelengths penetrate deeper in the
water column. In contrast, high concentrations of both suspended particulate (phytoplankton and sediments) and dissolved matter strongly absorb the blue-green wavelengths in
coastal waters thereby restricting penetration in deeper
waters. The longer red wavelengths, however, are quickly
absorbed by water molecules in near surface waters irrespective of the water optical type (Fig. 3b). In estuarine and fjordal
ecosystems, with different fresh and saltwater mixing zones,
the euphotic depth reduces gradually with increase in turbidity from the outer (downstream) to inner region (upstream)
(Mascarenhas et al. 2017). It is in the euphotic zone, that the
majority of primary production takes place.
Fig. 2 Spectral reflectance in case-1 (Pacific Ocean) and case-2 waters
(Norwegian Fjord). Case-1 waters consisting of very low optically
active constituents (OACs), reflect light in the blue region. Case-2
waters with high concentrations of OACs (here chlorophyll a in phytoplankton), reflect strongly the green wavelengths. (Data: Daniela Voss,
ICBM, University of Oldenburg)
Marine Optics and Ocean Color Remote Sensing