Environmental Engineering Reference
In-Depth Information
and finally reaches 40-70 m in August (Barbiero and Tuchman
2004
; Moll et al.
1984
; Fahnenstiel and Scavia
1987
). It has been shown that WT is relatively higher
(3-24 °C) in Lake Michigan than in Lake Superior (6-18 °C), and high WT along
with DOM contents may affect the DCM depth variation in Lake Michigan. Redfish
Lake and other Sawtooth Valley (Idaho) lakes had DCM with mean Chl
a
peaks
reaching 240-1,000 % of the mean epilimnetic Chl
a
concentrations. The DCM can
be present at low light levels and account for 36.72 % of the lake primary produc-
tion (Gross et al.
1997
). The Sawtooth Valley lakes have DCM Chl values that can
be up to 10 times higher in the metalimnia and hypolimnia than in the epilimnia
(Steinhart et al.
1994
; Budy et al.
1995
). The DCM in the Sawtooth Valley lakes
are located at depths where the light levels are near or below 1 % of surface light
(Gross et al.
1997
).
Seasonal changes in mixing and light intensity can produce a seasonal reset of Chl
distributions, which can alter the DCM or SCM formation and ablation as a regime
shift (Hense and Beckmann
2008
; Hamilton et al.
2010
; Abbott et al.
1984
; Vincent
1983
; Vincent et al.
1984
; Marshall and Peters
1989
; Bayley et al.
2007
; Carpenter
et al.
2003
). Three different 'regimes' can occur during the seasonal occurrence of a
DCM in Lake Tahoe, with transitions alternatively controlled by diffusion, nutrient
supply and light (Abbott et al.
1984
). Seasonal changes of DCM in the water column
can depend on the depth of light penetration, which can largely affect DCM depth
during the summer stratification period (Hamilton et al.
2010
). Seasonal variations in
the water-column attenuation coefficient of the photosynthetically available radiation
(PAR) can shift the 1 % sea-surface PAR depth from approximately 105 m in winter to
121 m in summer, in the North Pacific Subtropical Gyre (Letelier et al.
2004
). Such a
seasonal depth shift of isolumes (constant daily integrated photon flux strata) can also
be increased to 31 m due to the added effect of changes in sea-surface PAR (Letelier et
al.
2004
). Such a discrepancy can induce a significant deepening of the DCM during
the summer period, with a concomitant increase in Chl
a
(Letelier et al.
2004
).
The DCM phytoplankton contains higher amounts of phosphorus than for the
epilimnion, which is likely caused by the rapid photochemical degradation of
SCM phytoplankton in epilimnion. Nutrient-rich DCM might be useful as a food
source for grazers, including deep-living calanoid copepods that may have a sub-
stantial impact on total lake phytoplankton productivity (Barbiero and Tuchman
2004
; Moll et al.
1984
). The DCM also releases the new DOM and nutrients in
the hypolimnion under microbial assimilation (Rochelle-Newall and Fisher
2002
;
Maurin et al.
1997
; Yamashita and Tanoue
2008
). Phytoplankton from DCM do
not show marked differences from epilimnetic communities in taxonomy or nutri-
ent status, but can exhibit substantially higher photosynthetic impairment under
UVR exposure (Harrison and Smith
2011
). This suggests that epilimnetic phyto-
plankton can be acclimated to in situ light conditions in a spectrally-specific man-
ner, and that ultraviolet-A radiation may be a stronger stressor than ultraviolet-B
or photosynthetically active radiation in the mixed layers of lakes (Harrison and
Smith
2011
). DCM has varying characteristics that suggest multiple processes
contributing to its formation and maintenance in lakes and oceans (Anderson
1969
; Steele
1964
; Hobson and Lorenzen
1972
).
