Environmental Engineering Reference
In-Depth Information
dissolution of their carbonate structures. Some experiments have tested
this hypothesis and have obtained signifi cant results (Borowitzka and
Larkum 1976, Gao et al. 1993a, Gattuso et al. 1999, Orr et al. 2005). Studies
by means of controlled experiments have shown bleaching processes in the
crustose seaweed
Porolithon onkodes
and a consequent sharp decrease in
productivity with reductions in calcifi cation rate of up to 190% (Anthony et
al. 2008). On the other hand, Johnson et al. (2012) found a positive response
to seawater acidifi cation on calcifi ed brown algae
Padina
from temperate
and tropical regions despite the reduction in their content of CaCO
3
with
CO
2
increases. Contrary to other studies of calcifi ed seaweed, they found
that there was an increase in the abundance of the species of
Padina
spp.
under conditions of acidifi cation. This phenomenon could be explained by
reduced grazing pressure by sea urchins and the signifi cant increase in the
rates of photosynthesis (Johnson et al. 2012).
There is evidence that photosynthesis can stimulate calcification
in calcifi ed seaweeds, thus it is expected that the increase in rates of
photosynthesis due to the increased availability of CO
2
in the seawater could
also act as a buffer mechanism against the negative effects of acidifi cation
on the calcifi cation process of calcifi ed seaweed (Gatusso et al. 1999). Recent
studies have found that those seaweeds that secrete an external organic
layer between the skeleton and the seawater, such as Corallinaceae and
calcareous green algae, increase net calcifi cation under intermediate levels of
CO
2
and generally exhibit high resilience, which may suggest that the direct
use of CO
2
through photosynthesis can infl uence the response of seaweed
to calcifi cation. Despite the complexity of the relationships between the
processes of calcifi cation and photosynthesis, an increase in CO
2
in water
may increase the rate of photosynthesis (Bowes 1993, Iglesias-Rodriguez et
al. 2008) and potentially increase the amount of energy available to convert
HCO
3
to CO
3
2-
through pH regulation in the sites where calcifi cation occurs
(Ries et al. 2009). The studies of Hofmann et al. (2012) support the above
mentioned hypothesis since they found that
Corallina offi cinalis
decreased
its growth rate under high CO
2
levels, while the carbonic anhydrase activity
was stimulated and negatively correlated with the inorganic content of the
algae.
Connell and Russell (2009) studied the effect of elevated CO
2
levels on
non-calcareous seaweeds and tested the hypothesis of an increase in the
abundance of turfs under the combined conditions of elevated CO
2
and
temperature. The results showed an interaction between temperature and
CO
2
levels on turf coverage and a positive effect of both factors on turf
biomass. They also found that the loss of kelp canopies may be exacerbated
by these positive effects of temperature and CO
2
on species turfs, inhibiting
the recruitment of kelp propagules.
