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Fig. 19.4 pH fluctuations at the surface of the coralline seaweed Sporolithon durum measured for
1 h in the light and 1 h in the dark at an initial seawater pH of ~7.9 ( a , c ) and ~7.5 ( b, d )at
mainstream velocities of 1.5 cm s 1 ( a , b ) and 6.3 cm s 1 ( c , d ). Data are also plotted against
modeled pH of surface waters (right-hand side) projected on centennial timescales, i.e., for the
period 1750-2215 (Caldeira and Wickett 2003 ). Symbols represent the mean of three replicates
( 1 s.e.m.). (Modified from Hurd et al. 2011 , with permission of Blackwell Publishing Ltd)
or lower than that of the bulk/mainstream seawater (De Beer and Larkum 2001 ;
Hurd et al. 2011 ). For example, in the light, at a mainstream flow of 1.5 cm s 1 ,pH
at the surface of Sporolithon durum increases to ~8.4 and declines to ~7.65 in the
dark (Fig. 19.4a , Hurd et al. 2011 ). As seawater velocity increases, the range of
these surface pH fluctuations declines because the DBL is thinner (Fig. 19.4c ).
When the mainstream seawater pH is decreased to 7.5 (worst-case scenario for
2215), S. durum is still able to raise the surface pH by 0.2 units in the light at flows
of 6.3 cm s 1 (Fig. 19.4d ) illustrating an ability to biologically modify the local pH
environment, at least in the short term.
The ubiquitous nature of DBLs has implications for how seaweed communities
might respond to OA. Seaweeds growing on wave-impacted shores or in fast
flowing currents will experience a pH at their surface that will be similar to that
of the mainstream seawater because DBLs will be thin: there are exceptions to this,
however, because some seaweeds such as the giant kelp Macrocystis pyrifera have
small (
1 mm) morphological features that trap seawater and cause thick DBLs
(0.67 mm) even under fast flows. Similarly, at flows of 1.5 cm s 1 , the coralline red
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