Seaweed Responses to Ocean Acidification - Seaweed Biology - page 405

in TA and DIC. On the incoming tide, the pH was lowered and DIC and TA
increased, effectively buffering the system. Such an upward shift in seawater pH
due to photosynthesis also affects the physiology of local organisms. For example,
higher pH (10.1), lower inorganic carbon (DIC

0.6 mM), and possibly a super-
saturated O 2 condition generated by Ulva intestinalis in temperate rockpools inhibit
the growth of Fucus vesiculosus and Chondrus crispus ; these species normally
coexist in the eulittoral where the prevailing natural open water chemistry lies at pH
8.2 and 2.2 mMDIC (Bjork et al. 2004 ). Conversely, biologically induced increases
in pH ( > 1 pH unit) in a tropical seagrass meadow caused higher calcification rates
of three seaweeds species in the community, Hydrolithon sp. and Mesophyllum sp.,
and Halimeda renschii (Semesi et al. 2009a ).
Spatial and seasonal variations in p CO 2 and DIC are also observed in sub-
Antarctic kelp beds (Delille et al. 2000 , 2009 ). Outside the kelp bed, the buffer
factor (

¼

) indicates that DIC dynamics are mainly influenced by air-sea exchange
and photosynthesis, but inside the kelp bed,

b

suggests that DIC dynamics are
controlled by calcification by the epiphytic community (Delille et al. 2000 ). Diel
variations in p CO 2 and DIC were prominent in spring and summer but absent in
winter. The seasonal drivers responsible for variations in p CO 2 and DIC are the
higher photosynthetic rates in spring and summer, and the decay of organic matter
in autumn which leads to a strong oversaturation of p CO 2 (Delille et al. 2009 ).
The above examples illustrate that seaweeds themselves modify the carbonate
chemistry of the surrounding seawater (in terms of total alkalinity, p CO 2 , DIC, and
pH) due to their metabolic activities. This ability of seaweeds to modify their local
environment needs to be taken into account in future experiments to test the effects
of OA on individual seaweeds and communities.

b

19.7 Physicochemical Coupling

19.7.1 Water Motion and Diffusion Boundary Layers

At the surface of all aquatic organisms that have metabolic exchange across their
outer surface (i.e., plankton, seaweed, and some invertebrates) is a diffusion
boundary layer (DBL), a region of stagnant water across which molecules and
ions move by molecular diffusion (Hurd et al. 2011 ). In seaweeds, the thickness of
the DBL depends on thallus morphology, seawater velocity, and the type of water
motion (waves vs. currents) (Hurd 2000 ; Hurd and Pilditch 2011 ). The traditional
view is that thick DBLs reduce seaweed growth rates because the flux of essential
nutrients (including inorganic carbon) is hampered, but there is little empirical data
to support this for temperate seaweeds (reviewed by Hurd and Pilditch 2011 ).
Metabolic processes at the seaweed surface cause the local pH to increase (photo-
synthesis) and decrease (respiration and calcification). Laboratory experiments
demonstrate that under slow flows that are typical of many subtidal habitats
( 5cms 1 ), the pH at the surface of the seaweed could be considerably higher

<