There is some evidence that seagrasses may do well at higher CO2. Richard Zimmerman and others have found positive responses to CO2 enrichment in seagrasses, consistent the response of other higher plants. Here is an article that describes further evidence (provided by Dr Richard K.F. Unsworth).
Research from the Chinese Academy of Sciences published in the Journal of Integrative Plant Biology (http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7909.2010.00991.x/pdf ) has found that the abundant Indo-Pacific seagrass species Thalassia hemprichii may actually prosper under conditions of Ocean Acidification. There has been much debate as to whether seagrass under conditions of ocean acidification will be released from present day carbon limitation, however until now most evidence for this has come from the study of the temperate seagrass Zostera marina.
The research by Zhi Jian Jiang et al. finds Thalassia hemprichii to have higher photosynthetic productivity and a lower saturating irradiance under conditions of elevated aqueous CO2 (and reduced pH). Although the conditions studied are mostly those expected in the next few centuries, the analysis does include one treatment at a pH of 7.75 that represents a potential condition in 2100.
Having a lower saturating irradiance is critically important, as the majority of seagrass loss over the last century has been the result of poor water quality reducing light availability (see Waycott et al 2009 –http://www.pnas.org/content/106/30/12377.abstract). If seagrasses under high CO2 can be more productive under lower light conditions, this indicates the potential for at least one tropical seagrass to be one of the ‘winners’ in a future ocean environment. Although these are credible findings, how such elevated productivity interacts with elevated temperature and more extreme weather events remains poorly understood. Understanding the potential viability of different species to future environments is important for setting realistic long-term conservation objectives for marine ecosystems.
Picture: Thalassia hemprichii in Guam (From Guamreeflife.com)