Seaweed Responses to Ocean Acidification - Seaweed Biology - page 412

elevated CO 2 (pH 7.84-8.12), the influence of light on the percentage cover of

understory crusts was substantially reduced (Fig. 19.6a ) which caused crusts to
grow less under all light treatments. Conversely, elevated CO 2 had the opposite
effect of positively influencing turf cover (Fig. 19.6b ), thereby reducing the struc-
turing effects of light and UVB. Hence, to predict the ecological consequences of
future CO 2 conditions, the role of contemporary processes (e.g., light and UVB)
cannot be assumed to produce similar effects relative to other processes, which will
change with a changing climate (Russell et al. 2011 ). Therefore, more studies are
needed on the synergistic effects of different stress factors at the organism and
community levels to better understand the biological and ecosystem impacts of OA.

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19.9 Experimental Considerations

The possible effects of OA have been studied for ~10 years and an enormous
volume of research has been produced during this time. Most studies have focused
on reducing pH and measuring response variables of individual algae or
communities. There are three methods available to lower SW pH: 1. addition of
HCl, 2. bubbling with CO 2 gas, or 3. HCl/NaOH additions (Hurd et al. 2009 ;
Riebesell et al. 2010 ). Methods 2 and 3 alter seawater carbonate chemistry in a
way that mimics OA (TA remains constant, DIC increases), while using HCl,
alkalinity decreases but there is no change in DIC. Importantly, two seawater
carbonate parameters must be measured during experiments so that the carbonate
speciation within experiments can be followed. Some first-generation OA
experiments on seaweeds suffered from pseudoreplication or lack of independent
replication (Hurd et al. 2009 ). However, with the design of new systems for
controlling and monitoring pH, many of these experimental problems have been
resolved (e.g., McGraw et al. 2010 ). Because seawater carbonate chemistry is
complex, and because algae themselves modify their local pH environment, it is
important that biologists and carbonate chemists collaborate on OA studies to avoid
the many pitfalls that will result in a nonrigorous experimental design.

19.10 Conclusions and Future Works

The responses of ecosystems to OA are difficult to predict because all algae,
calcifying and noncalcifying, could be affected. Most macroalgae are HCO 3
users, and the predicted increases in DIC may have little effect on growth rates
compared to seagrasses and seaweeds that rely on CO 2 . For calcifiers, several
studies have shown that calcification rates are depressed by increased levels of
CO 2 . This is one of the many indications of a negative biological response in the
marine environment. Predicting the responses of ocean chemistry to rising CO 2 is
reasonably straightforward (e.g., Caldeira and Wickett 2003 ), but with fairly