Environmental Engineering Reference
In-Depth Information
UV-A does not enhance carbon fixation in pelagic (or oligotrophic) water
because the picophytoplankton-dominated assemblages do not efficiently produce
UV-absorbing compounds (Fig.
4
) (Li et al.
2011
; Garcia-Pichel
1994
; Raven
1991
). The UV-A related inhibition of carbon fixation increases from the coastal
to pelagic waters, whereas UV-B impacts uniformly over time and space (Fig.
4
d).
Under reduced levels of solar radiation with heavy overcast, UV-A radiation
enhances photosynthetic carbon fixation by up to 25 % in coastal waters where
microplankton is abundant, but such a positive impact is not observed in offshore
waters where piconanoplankton prevails (Li et al.
2011
).
Water temperature, driven by solar radiation, is one of the crucial
physical factors regulating photosynthesis in natural waters (Baulch et al.
2005
;
Mortain-Bertrand et al.
1988
; Doyle et al.
2005
; Yoshiyama and Sharp
2006
).
Primary production (approximately 67 % of variability) is mainly controlled
by light availability and temperature. High nutrient concentrations do not stim-
ulate primary production in estuary (Yoshiyama and Sharp
2006
). Even the
slight oceanic warming during the interglacials would result in increased affin-
ity of active transport by algae and bacteria for nutrients (nitrate, phosphate
and silicate) and would effectively increase the available pools of such nutri-
ents in the oceans (Nedwell
1999
). This increase in availability of nutrients
with higher temperature would be predicted to enhance oceanic primary pro-
duction and CO
2
drawdown during the interglacials (Nedwell
1999
). Such a
scenario is consistent with the data from the profiles of
δ
13
C isotopic ratios
in benthic foraminiferan in Southern Ocean sediment cores. Such data sug-
gest in fact increased interglacial oceanic production (Broecker and Peng
1993
;
Neori and Holm-Hansen
1982
). It is also shown that the highest NH
4
concentra-
tions are detected in the colder months when temperature and daily irradiance
are lower, but primary production does not increase linearly with ammonium
(Yoshiyama and Sharp
2006
). Global warming may lengthen the summer sea-
son and enhance the water column transparency with modification of the depth
of the mixing layer or euphotic zone. Such processes would influence the
doses of UV radiation and PAR received by the phytoplankton cells (Malkin
et al.
2008
; Scully and Lean
1994
; Morris et al.
1995
; Morris and Hargreaves
1997
). The consequence could be a photoinhibition of the cells within the upper
layer in sunny days. Enhanced photosynthetic rates of polar phytoplankton are
observed in response to increasing temperatures (Broecker and Peng
1993
; Neori
and Holm-Hansen
1982
; Reay et al.
2001
; Jacques
1983
). This effect ultimately
causes an increase in photosynthesis in natural waters, in particular in the deeper
layers, because of an enhancement in its duration.
Global warming can also induce an increase in DOM contents in natural waters
because of enhanced DOM leaching from terrestrial soils connected to high soil
respiration following elevated atmospheric CO
2
concentrations (Porcal et al.
2009
). Global warming can enhance the photosynthesis of terrestrial plants bec-
uase of higher atmospheric CO
2
levels, which results in high primary production.
The parallel increase of atmospheric temperature would also increase the soil res-
piration (Porcal et al.
2009
; Freeman et al.
2001
,
2004
; Tranvik and Jasson
2002
;
