Shifting strengths of species interactions with ocean acidification in a guild of coralline algae

Background/Question/Methods

Anthropogenic impacts on the global carbon cycle pose a dual risk to ocean ecosystems. Aside from rising temperatures, the process of ocean acidification is causing coastal water chemistry to change dramatically. It is therefore important to assess the potential for biological response to seawater chemistry on both the species and community levels, with emphasis on natural communities. Much work to date has focused on isolated physiological responses of calcifying species to elevated CO₂, but few studies thus far have focused on the ecological consequences of those responses in a truly natural context.

In light of well-documented recent changes in ocean carbon chemistry in the NE Pacific, we looked for ecological changes compared to a historical baseline. We repeated experiments identical to those performed previously (1981-1999) to quantify crustose coralline algal interactions as a function of grazing. These experiments from 30 years ago established the community dynamics of competition for space in the low rocky intertidal zone within this guild.  

Results/Conclusions

When we compared the outcome of competitive bouts among crustose coralline algae over time, from 1981-1999 and from 2010-2012, we found that the competitive dominant historically, Pseudolithophyllum muricatum, has experienced a reversal in its competitive abilities, such that it now wins during competitive overgrowth only 35% of the time, compared to 99% of the time three decades ago in the natural presence of grazers. Other species also show shifts in their competitive abilities, responses to grazers, and abundance patterns. These shifts correspond to alterations of species-specific traits underlying these changes, such as maximum growth rates and the thickness and composition of skeletal tissue, which are consistent with expectations for acidified conditions based on coralline algal physiology.