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Fig. 19.6 Percentage cover (mean
SE) of ( a ) coralline crusts and ( b ) turfs recruited to
unoccupied substrates when exposed to crossed combination of CO 2 (control [CCO 2 ] vs. elevated
[ECO 2 ]) and light (shade vs. low UV vs. full sunlight; in black, gray, and white bars , respectively).
(From Russell et al. 2011 , with permission of Blackwell Publishing Ltd)
On the other hand, elevated CO 2 concentration (1,000 ppm) in synergy with
ultraviolet radiation is reported to systemically negatively impact the physiology
of an articulate coralline alga Corallina sessilis in terms of photosynthesis, photo-
synthetic and accessory pigments, growth, and calcification rates (Gao and Zheng
2010 ). Positive effects of elevated CO 2 in synergy with other environmental
stressors are also reported. Emersed net photosynthesis of Porphyra haitanensis
increased with higher CO 2 (700 ppm), temperature, and level of desiccation (Zou
and Gao 2002 ).
At the community level, the synergistic effect of global increase in CO 2 and a
local stressor such as eutrophication can select for resilient species and cause phase
shifts in coastal ecosystem. For example, the combined effect of high CO 2 and
nutrients accelerates the expansion of filamentous turfs at the expense of calcifying
algae (Russell et al. 2009 ). Likewise, the synergistic effect of high CO 2 and high
temperature favors the filamentous turfs over the kelps (Connell and Russell 2010 ).
Conversely, increased photosynthesis and growth of seagrasses under nutrient
replete and elevated CO 2 may benefit some calcifiers, as shown in the higher
calcification rate of Hydrolithon sp. in the proximity of seagrasses, due to the
drawdown of CO 2 from the water column (Semesi et al. 2009a ).
Light (including ultraviolet radiation) is known for its role in shaping the
subtidal ecology of photosynthetic organism (Bischof et al. 2006 ). However,
Russell et al. ( 2011 ) found that the forecasted increase in CO 2 levels can alter
how light
influences community structure. For example, under control CO 2
(pH
8.06-8.16) the effects of light on the percent cover of the two groups of
algae studied, the calcifying red crustose Lithophyllum sp. and the turf-forming
brown Feldmannia spp., corresponded to their growth habitat's light environments:
the subcanopy red coralline grew better under low UV while the highest cover of
the brown turf was observed under full sunlight (Fig. 19.6 ). However, under
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