Seaweed Responses to Ocean Acidification - Seaweed Biology - page 407

Sporolithon durum has DBLs that are up to 1.7 mm thick, and althoughDBL thickness
declines as seawater velocity increases, average DBL thickness at a mainstream
velocity of 10 cm s 1 ranges from ~0.2 to 0.4 mm. In wave-sheltered sites, or for
small, foliose or turf-forming algae in understory canopies, the seawater velocity at the
seaweed surface will be much slower than that of mainstream flows, and they may
encounter a wide range of pHs at their surface; this range is likely to vary on daily and
tidal cycles. Therefore, the possibility exists that at their surface, many seaweeds
naturally experience pH at night that are as low as those predicted for OA, and during
the day pH at their surface, will increase, perhaps ameliorating any detrimental low-
pH effects. Essentially, seaweeds in slow flows may be adapted to a wider range of pH
fluctuations than those in fast flows which has implications for differential suscepti-
bility of organisms to OA (Hurd et al. 2011 ). However, experiments examining the
effects of water motion on the susceptibility to OA have been over short timescales
(hours) and the question is whether over the long term (i.e., generations) seaweeds will
have the physiological flexibility to adapt to lower pH seawater.

19.7.2 Temperature and Regional Vulnerability

Due to the greater CO 2 solubility in cold waters, seawater is expected to acidify more
strongly making high latitude and polar regions more vulnerable to OA compared to
tropical regions (McNeil and Matear 2008 ). Likewise, carbonate ion concentrations
are highest in the tropics and decrease poleward in concert with decreasing tempera-
ture (see also Chap. 18 by Bartsch et al.). Carbonate ion concentrations will therefore
remain higher in warmer parts of the oceans, so calcification rates in these regions
should remain higher than in cooler regions. However, tropical coral reef ecosystems
will be affected in multiple ways by rising CO 2 , but most directly by the simultaneous
changes in temperature and in seawater chemistry. Differential responses among
populations of cosmopolitan species, or species with broad latitudinal distribution, to
OA and greenhouse conditions (high CO 2 and high temperature) will be an important
line of investigation. For example, comparisons could be made between tropical,
temperate, and arctic populations of cosmopolitan macroalgae (e.g., Acrosiphonia,
Ulva, Plocamium ), and different Macrocystis populations; this species is widely
distributed throughout the continental margins of the northern (Alaska to Mexico)
and southern (Peru to southernmost Chile) Pacific Ocean, and is also found in isolated
regions of South Africa and around most of the sub-Antarctic islands to 60 S
(Graham et al. 2007). A poleward shift in the geographic range of major subtidal
kelp species has been suggested for future climate scenarios in marine coastal
ecosystems (Merzouk and Johnson 2011 ).

19.7.3 pH, Chemical Speciation, and Nutrient Availability

The effect of OA on seawater chemistry goes beyond the carbonate system (see also
Chap. 4 by Gordillo). Chemical species that may undergo speciation under lower