Chlorophylls and their Degradation in Nature - Photobiogeochemistry of Organic Matter - page 709

SCM is often observed in coastal seawater and subsequently decreases in the
offshore direction, whereas DCM is increased along the offshore direction with
its enhanced depth (Millán-Núñez et al. 1996 ; Hayward et al. 1995 ; Maranón et
al. 2004 ). Chl a values are quite high in SCM in coastal seawater, and decrease
much more rapidly in the offshore direction (Millán-Núñez et al. 1996 ; Echevin
et al. 2004 ). The occurrence of SCM in coastal seawater is possibly responsible
for the high contents of DOM and nutrients, which is a general phenomenon in
coastal environments. High contents of DOM can protect surface waters against
sunlight exposure (Laurion et al. 2000 ; Hayakawa and Sugiyama 2008 ), and the
photoproducts generated from photoinduced degradation of DOM and POM can
enhance photosynthesis. The consequence is SCM formation in surface waters.
For similar reasons, SCM is limited at the epilimnion (~0-5 m) in Lake Kinneret,
Lake Hongfeng (4-5 m), Lake Baihua (0-2 m); Lake Biwa (2.5-10 m) and Lake
Baikal (~0-30 m, with a peak at 10 m). DOC concentrations are quite higher
(258-485 μ M C) in Lake Kinneret compared to Lake Hongfeng (134-250 μ M C
at 0-25 m), Lake Baihua (157-330 μ M C at 0-25 m), Lake Biwa (76-135 μ M
C at 0-80 m), and Lake Baikal (88-142 at 0-1,620 m) (Table 1 ) (Fu et al. 2010 ;
Mostofa KMG et al. unpublished data; Satoh et al. 2006 ; Yacobi 2006 ; Berman
et al. 1995 ; Yoshioka et al. 2002 , 2007 ; Annual Report 2004 ; Sugiyama et al.
2004 ; Yuma et al. 2006 ; Mostofa et al. 2005 ). The overall penetration depth in
Lake Kinneret was on average 1.77 m, and the uppermost layer is supposed to be
representative of the entire euphotic zone (Yacobi 2006 ). Therefore, it is suggested
that DOM and mechanical perturbation of surface waters (e.g., by wind, waves
and storms), which also depends on the depth and size of the water ecosystem
(particularly for lakes), are the two key factors for SCM formation.
Lakes having high water temperature (WT) often exhibit the SCM in the
epilimnion, such as Lake Hongfeng (10.9-47.8 μ g L − 1 at 4-5 m and 15.3-
31.0 °C), Lake Baihua (15.0-65.5 μ g L − 1 at 0-2 m and 15.3-31.0 °C), Lake Biwa
(2.0-12.3 μ g L − 1 at 2.5-10 m and 11.6-28.7 °C), Lake Kinneret (95 % of Chl a at
0-5 m and 15-30 °C), Lake Baikal (0.7-5.8 μ g L − 1 at 0-30 m and 16.5-17.9 °C);
Lake Malawi (0.03-18.7 μ g L − 1 at upper mixing layer and ~40 °C); Lake Victoria
(4.7-78.5 μ g L − 1 at upper mixing layer and 25-29 °C) and Lakes of Experimental
Lakes Area (<311-327 μ g L − 1 at 5-7 m and 4-20 °C) (Table 1 ) (Fu et al. 2010 ;
Mostofa K et al. unpublished data; Guildford and Hecky 2000 ; Satoh et al. 2006 ;
Yacobi 2006 ; Fee 1976 ; Berman et al. 1995 ; Mostofa et al. 2005 ). Therefore, high
contents of DOM in surface water under high WT, driven by strong sunlight, can
photochemically decompose DOM and POM to produce high amounts of DIC,
CO 2 , and H 2 O 2 . These species are directly linked with occurrence of high pho-
tosynthesis and high primary production. Moreover, in mesocosm experiments
it has been observed that increasing DOM concentrations from ~10 mg C. L − 1
to ~20 mg C. L − 1 had a negative effect on total phytoplankton growth. The most
likely explanation is the reduction of irradiance because of radiation absorption by
DOM (Klug 2002 ).
DOC concentrations are relatively low in the offshore direction, which may afford
easy penetration of sunlight (UVR) that can reach the deeper layers. This issue may