Biology Reference
In-Depth Information
guineensis
,
Lobophora variegata
, and
Laurencia papillosa
collected at 3-5 m
depths, and
Turbinaria turbinata
collected at 10 m (Holbrook et al.
1988
). But if
in situ photosynthetic rates of deep macroalgal species are light limited, they may
essentially be DIC saturated under present-day CO
2
levels (Johnston et al.
1992
,
Surif and Raven
1989
). Under elevated CO
2
, downregulation of CCMs among
sublittoral macroalgal species growing under light limited conditions will require
less light energy for HCO
3
transport; this energy saving may stimulate growth rate
(Hepburn et al.
2011
).
19.5.2 Calcifying Seaweeds
Calcified macroalgae, represented by members of phylogenetically diverse brown,
green, and red species, are distributed from polar to tropical latitudes and inhabit
shores ranging from the mid-high intertidal to the deepest reaches of the euphotic
zone. These macroalgae produce marine sediments for reef accretion, provide
three-dimensional habitat structure functioning as autogenic ecosystem engineers,
contribute structural frameworks for coral reef ecosystems, and play critical eco-
logical roles for invertebrate recruitment processes (reviewed by Nelson
2009
, Ries
2009
,
2010
).
Regardless of the increased DIC availability, the severe consequences of the
predicted lower pH and lower carbonate concentration to calcifying macroalgae are
relatively well documented. The most studied calcified macroalgal group in relation
to OA belongs to the red algal order Corallinales. Calcification in
Corallina
pilulifera
is inhibited at pH 7.6 compared to 8.2 (Gao et al.
1993a
). A 20% decrease
in calcification was also observed in
Hydrolithon
sp. when incubated at 900 ppm
CO
2
(pH
7.8), while the photosynthetic rate was enhanced by 13% (Semesi et al.
2009b
). For the green seaweed
Halimeda tuna
from the Great Barrier Reef, calcifi-
cation declined when exposed to a pH of 7.5 compared to 8.0 (Borowitzka and
Larkum
1986
).
The fundamental changes in carbonate chemistry of seawater due to increasing
atmospheric CO
2
levels may enhance the competitive advantage of noncalcifying
over calcifying species (e.g., Fig.
19.5
, Hall-Spencer et al.
2008
; Kuffner et al.
2008
). Moreover, its interaction with increasing sea surface temperatures may have
serious impacts on calcifying macroalgae, associated biota, and coastal ecosystems
as a whole.
ΒΌ
19.6 Natural Fluctuations in Seawater Carbonate Chemistry
Most projections on how seawater carbonate chemistry and pH will change in the
coming decades are based on the open ocean, where pH is relatively stable and only
tends to vary on annual cycles. In coastal oceans, however, local-scale processes
