B. Sulzberger
W(A) is the spectral photon flux that hits the surface of a water body (in millieinstein cm- 2 S-I nm- I ) (also called incident light intensity), Wz(A) is the light intensity at depth z, and a(A) is the decadic absorption coefficient (in em -I if the unit of z
is em). Equation 3.2 is only valid under the assumption that sunlight enters a water
body perpendicular to the surface, that reflection, backscattering and internal scattering can be neglected, and that the water body is well mixed. Neglecting absorption
oflight by suspended matter (Miller and Zepp 1979) and by all the other light-absorbing constituents except CD OM, the decadic absorption coefficient, a(A), can then be
approximated as,
a(A) -[CDOM]"£(A)"
where "£(A)" is the decadic extinction coefficient of CDOM at wavelength A (in
I mg- I cm- I if the unit of [CDOM] is mg r l ). [Note, that CDOM is not a well-defined,
light-absorbing chemical compound but includes more than one chromophore.] The
depth of a water body at which the incident solar light intensity at a given wavelength
is attenuated by a factor of two, ZW(Il)/2' can then be calculated:
log 2
z
-----=::.-W(Il)/2 - [CDOMj" £(A)"
Hence, the higher the CDOM concentration is in a water body, the smaller the penetration depth of the sunlight is at a given wavelength.
UV absorption by CDOM (see Fig. 3.2) is the primary mechanism shielding aquatic
organisms from the harmful effects, especially of UV -B radiation (280-315 nm). However, UV radiation also destroys CD OM through photo oxidation, resulting in a decrease
in the CDOM concentration and thus in an increase in the penetration depth of UV
light. Increases in UV radiation, due to stratospheric ozone depletion, could therefore cause more damage to aquatic ecosystems than would be expected from direct effects alone.
3.3
Photooxidation of CDOM: Role for Carbon Bioavailability
Photo oxidation of CDOM leads to carbon gases including carbon monoxide (CO)
(Valentine and Zepp 1993; Tarr et al.1995), carbon dioxide (C02) (Miller and Zepp 1995),
carbonyl sulfide (COS) (Andreae and Ferek 1992; Zepp and Andreae 1994), and to low
molecular-weight compounds that are bioavailable, such as formaldehyde, acetaldehyde,glyoxylate and pyruvate (Kieber et al.1989; Mopper et al.1991). Kieber et al. (1989)
have demonstrated that biological uptake of pyruvate was highly correlated with its
rate of photochemical production in sea water. Furthermore, Bushaw et al. (1996) have
shown that ammonium is among the nitrogenous compounds produced upon CDOM
photooxidation. Most of the products are thought to result from the net oxidation of
CDOM by O 2 (Blough and Zepp 1995), the most abundant marine oxidant, where photooxidation of CDOM by O 2 can occur through different pathways (Table 3.1). Zafiriou
and collaborators (1994 pers. comm.) calculated the global marine O2 photochemical
W(A) is the spectral photon flux that hits the surface of a water body (in millieinstein cm- 2 S-I nm- I ) (also called incident light intensity), Wz(A) is the light intensity at depth z, and a(A) is the decadic absorption coefficient (in em -I if the unit of z
is em). Equation 3.2 is only valid under the assumption that sunlight enters a water
body perpendicular to the surface, that reflection, backscattering and internal scattering can be neglected, and that the water body is well mixed. Neglecting absorption
oflight by suspended matter (Miller and Zepp 1979) and by all the other light-absorbing constituents except CD OM, the decadic absorption coefficient, a(A), can then be
approximated as,
a(A) -[CDOM]"£(A)"
where "£(A)" is the decadic extinction coefficient of CDOM at wavelength A (in
I mg- I cm- I if the unit of [CDOM] is mg r l ). [Note, that CDOM is not a well-defined,
light-absorbing chemical compound but includes more than one chromophore.] The
depth of a water body at which the incident solar light intensity at a given wavelength
is attenuated by a factor of two, ZW(Il)/2' can then be calculated:
log 2
z
-----=::.-W(Il)/2 - [CDOMj" £(A)"
Hence, the higher the CDOM concentration is in a water body, the smaller the penetration depth of the sunlight is at a given wavelength.
UV absorption by CDOM (see Fig. 3.2) is the primary mechanism shielding aquatic
organisms from the harmful effects, especially of UV -B radiation (280-315 nm). However, UV radiation also destroys CD OM through photo oxidation, resulting in a decrease
in the CDOM concentration and thus in an increase in the penetration depth of UV
light. Increases in UV radiation, due to stratospheric ozone depletion, could therefore cause more damage to aquatic ecosystems than would be expected from direct effects alone.
3.3
Photooxidation of CDOM: Role for Carbon Bioavailability
Photo oxidation of CDOM leads to carbon gases including carbon monoxide (CO)
(Valentine and Zepp 1993; Tarr et al.1995), carbon dioxide (C02) (Miller and Zepp 1995),
carbonyl sulfide (COS) (Andreae and Ferek 1992; Zepp and Andreae 1994), and to low
molecular-weight compounds that are bioavailable, such as formaldehyde, acetaldehyde,glyoxylate and pyruvate (Kieber et al.1989; Mopper et al.1991). Kieber et al. (1989)
have demonstrated that biological uptake of pyruvate was highly correlated with its
rate of photochemical production in sea water. Furthermore, Bushaw et al. (1996) have
shown that ammonium is among the nitrogenous compounds produced upon CDOM
photooxidation. Most of the products are thought to result from the net oxidation of
CDOM by O 2 (Blough and Zepp 1995), the most abundant marine oxidant, where photooxidation of CDOM by O 2 can occur through different pathways (Table 3.1). Zafiriou
and collaborators (1994 pers. comm.) calculated the global marine O2 photochemical