Environmental Engineering Reference
In-Depth Information
increase the photosynthetic layer depth in the water column, i.e. increase the DCM
depth. A significant contribution to Chl
a
may come from phytoplankton in deeper
layers, in the case of a low-DOC lake water when UV attenuation increases with
Chl
a
concentrations (Laurion et al.
2000
). Moreover, the mixing depth can play an
important role in SCM or DCM formation in lakes or oceans. A low mixing depth
can often induce SCM formation, whilst high mixing depth can cause DCM forma-
tion. For example, SCM formation (~0-15 m) occurs when mixed-layer depth is low
(3-15 m), whilst DCM formation (~65 m) takes place when the mixed-layer depth is
high (e.g. 33 m in East China Sea) (Hung et al.
2000
).
The Chl
a
concentrations in Lake Biwa are several times (~15-24) higher at
the epilimnion (0-10 m), compared to those of deeper layers (40 and 70 m) during
the summer stratification period (Mostofa KMG et al. unpublished data). SCM is
often observed during autumn, e.g. November in 1999 and October in 2000 at the
epilimnion. Chl concentration largely fluctuates and it is lower during the sum-
mer stratification period compared to early spring and autumn seasons (Fig.
3
a)
(Mostofa KMG et al. unpublished data). The low Chl
a
contents in SCM and its
fluctuation during the summer stratification period is presumably caused by pho-
todegradation induced by strong sunlight, coupled with high WT (maximum
28.7 °C). However, an early bloom in 2000 compared to 1999 was probably caused
by a longer summer period. WT was 26.8 to 21.9 °C during September-October
in 1999, which is lower compared to 2000 (WT: 23.6 to 19.5 °C at the same time).
Moreover, reduction of water clarity through eutrophication can cause a shift in
phytoplankton distributions, from a DCM in spring or summer to a SCM within the
surface mixed layer. This may happen when the depth of the euphotic zone is con-
sistently shallower than the depth of the surface mixed layer (Hamilton et al.
2010
).
Such a SCM, which is susceptible to occur because of high primary production in
spring or summer, is initially caused by the photoinduced generation of photoprod-
ucts in waters. Simultaneously, the decrease of primary production because of pho-
toinduced degradation does not predominantly occur during that period. Therefore,
the new primary production may prevail over photoinduced degradation processes.
The DOM that is generated as a consequence of the high primary production can
substantially absorb sunlight and cause the depth of the euphotic zone to be shallow.
These results may suggest that two important phenomena account for the
occurrence of SCM and DCM in natural waters: First, waters with high contents of
DOM and POM can have intense solar radiation in the surface layer. In contrast,
photoproducts (DIC, CO
2
, H
2
O
2
, and so on) are generated photochemically under
high WT (caused by strong sunlight) from DOM and POM. They are responsible
for the occurrence of high photosynthesis, with the consequence that high primary
production can form SCM in surface waters. The second phenomenon is that water
with low contents of DOM and POM lets sunlight to penetrate in the deeper water
layer. Photoinduced or microbial products (DIC, CO
2
, H
2
O
2
, nutrients, and so on)
are generated from DOM and POM and are responsible for occurrence of photo-
synthesis. As a consequence, enhanced primary production at depth can produce
DCM in deep water. The two described phenomena are extensively discussed in
the next sections.
