Seaweed Responses to Ocean Acidification - Seaweed Biology - page 410

( 2011 ) showed that reproduction of some resilient seaweeds was not affected under
extremely high CO 2 conditions near the volcanic CO 2 vents.
Manipulative experiments (using the HCl method of pH manipulation) showed
that OA, at 2

7.91), is responsible for a 79% reduction in
crustose coralline algae (CCA) recruitment, 40% reduction in growth rate and 92%
reduction in cover, and a 52% increase of a mixed assemblage of noncalcifying algae
composed of macroalgal germlings, diatoms, and small filamentous algae (Kuffner
et al. 2008 ). Likewise, using multispecies Markov chain models, a method used to
link environmental change to species dynamics, Wootton et al. ( 2008 )showeda
strong link between in situ benthic species dynamics and variation in ocean pH. They
showed that the abundance of calcareous invertebrate species generally declined with
declining pH while fleshy algae ( Halosaccion glandiforme , ephemeral algae, fila-
mentous red algae, and foliose red algae) increased with declining pH.
Elevated CO 2 may potentially influence competitive exclusion of certain species
within a community. Quantification of carbon uptake mechanism (using carbon
isotope and pH drift methods) within a kelp forest community showed that the canopy
(Laminarians and Fucaleans) and dominant understory species (calcifying coralline
algae) all had functional CCMs, while the less dominant understory species,
consisting of mostly red noncalcifying species, relied on diffusive CO 2 uptake and
had no functional CCMs (Hepburn et al. 2011 ). Results of the above study suggest that
in the low-pH coastal waters of the future, the growth rates and competitive abilities of
noncalcifying and CO 2 -only seaweeds will increase, while those of calcifying CCM
seaweeds will decrease. This could cause major shifts in community structure, altering
functional diversity and near shore ecosystem functioning (Hepburn et al. 2011 ).
However, it is difficult to predict how ecosystems might change because some
HCO 3 -using species can switch to CO 2 use at lower pH 7.5, thereby increasing the
energy available for growth and reproduction (Cornwall et al. 2012 ).

present day (pH ¼

19.8.3 Interactive Effects of Ocean Acidification and Other

Stressors

The results of single-factor experiments may be different, or the physiological
response be of a lower magnitude, compared to the interactive effects of multiple
stressors that most seaweeds are exposed to in their natural habitats. For example,
calcification rate of CCA Lithophyllum cabiochae was not affected when indepen-
dently exposed to elevated CO 2 or temperature. However, when exposed to a
combination of elevated CO 2 and temperature, calcification rates decreased by
50% and an accelerated and extensive (~60%) tissue necrosis was observed (Martin
and Gattuso 2009 ). Higher sensitivity to OA under higher temperature is also
reported in the CCA Hydrolithon onkodes which sustained higher skeletal dissolu-
tion and concurrent increase in the abundance of their co-habiting endolithic algae
(Diaz-Pulido et al. 2012 ).