Impacts of climate change on the primary productivity of marine macrophytes: Effects of pH and temperature on net productivity

Background/Question/Methods: The increase in atmospheric CO₂ leading to ocean acidification is expected to impact the physiology of many calcifying organisms. Although the impacts of acidification may be widespread, the regions likely to observe the greatest pH changes are cooler temperate waters. Furthermore, regions of strong up-welling may experience even greater pH change due to heterotrophic respiration in deeper waters. In the cool temperate waters of the Oregon coast, one of the dominant calcifiers of rocky reefs is coralline algae (primarily Corallina vancouveriensis), an important facilitator of many algal and invertebrate species. The effects of pH change on the growth and abundance of coralline algae could have wide-reaching consequences in near-shore ecosystems, including large changes in the recruitment and productivity of macroalgal assemblages. Here we examine the potential impacts of ocean acidification in a region regularly exposed to pulses of very low pH water using laboratory mesocosms.

Results/Conclusions: Maximum productivity decreased at high temperatures (20%) and low pH (50%) in coralline dominated assemblages. In contrast, only temperature caused an effect on productivity in two dominant non-calcifying macrophyte assemblages, the surfgrass Phyllospadix scouleri and the intertidal kelp Saccharina sessilis (25% reduction in each). In fact, decreasing pH had a positive effect on primary productivity in Saccharina (45% rise in productivity, with a pH drop of 0.4). These results show that complex assemblages have the potential to respond differently to any species studied in isolation, and indicate the potential importance of light intensity on assemblage responses. The combined effects of pH and temperature on coralline assemblages indicated an exacerbated effect of decreasing pH at higher temperatures, showing that multiple stressors have the potential to combine synergistically at further detriment to primary productivity. Differential responses of algal assemblages to ocean acidification could result in dynamic changes in community composition, with the potential for positive or negative feedback loops between canopy and subcanopy species.