Environmental Engineering Reference
In-Depth Information
of acid and/or base or by bubbling CO
2
-enriched air through the seawater
have different effects on the water bio-chemistry. According to Iglesias-
Rodriguez et al. (2008), the latter pH manipulation may be a more realistic
representation of the ocean response to anthrophogenic change: the relative
proportion of the carbonate species are controlled by decreasing pH at
the same time when concentration of DIC increases. However, as recently
reviewed by Andersson and Mackenzie (2012), addition of acid, base or
CO
2
gas to seawater can all be useful techniques to manipulate seawater
chemistry in ocean acidifi cation experiments.
Interactive effects on phytoplankton:
Ecophysiological responses of marine
phytoplankton to acidifi cation not only depend on community composition
but on acclimation and growth conditions as well. The interactive effects
of CO
2
rise and warming-related changes, i.e., shifts in mean irradiance
exposure, nutrient inputs, sinking rates and organic carbon exportation
from the euphotic zone, lead to large uncertainties related to phytoplankton
physiological assumptions (Feng et al. 2008, Tagliabue et al. 2011, Gao
et al. 2012). For instance, Feng et al. (2008) empirically demonstrated
that the combined effects of CO
2
levels, temperature and irradiance on
Emiliania huxleyi
enhanced photosynthesis by increasing both CO
2
and
temperature regardless of the irradiance regime, and calcifi cation decreased
by a combined effect of CO
2
and light. In contrast, natural phytoplankton
assemblages exposed to rising CO
2
and increased light (Gao et al. 2012)
reduced their growth and photosynthesis rates and the community shifted
away from diatoms. These recent works highlight the synergistic effects
of key modulating factors of phytoplankton primary production and
community composition, which can signifi cantly impact higher trophic
levels and carbon cycle in the ocean.
In natural environments, seawater pH directly affects the phytoplankton
growth rate and therefore the timing and abundance of coastal species
(Hinga 2002). Accordingly, direct CO
2
-related effects on
E. huxleyi
growth,
calcification and elemental stoichiometry of uptake and production
processes have been reported from outdoor mesocosms experiments with
eventual implications for the marine biogeochemistry (Engel et al. 2005).
The oceanic CO
2
enrichment and acidifi cation also perform indirect effects
at the base of the pelagic food webs through modifi cations in the elemental
composition of the marine water. On the one hand, a recent study in a coastal
system has shown that nitrifi cation rates were highest at low pH indicating
that nitrifying organisms tolerate a wide range of pH levels (Fulweiler et
al. 2011). The impact of pH range on nutrient cycles depends upon the
local environmental conditions and active biological processes (Hendriks
et al. 2010). For instance, under eutrophic conditions, productivity and C
sink are predicted to increase, and nuisance phytoplankton blooms may
