Geographic Variation in Growth Response of Ulva spp.to Ocean Acidification and Productivity in High Carbon, Low Oxygen Conditions

The effects of ocean acidification on macroalgae are thought to be regulated by a combination of physiological characters (calcification, carbon uptake mechanism, morphology) interacting with habitat conditions of temperature, light availability and water flow. Generally, growth rates of most macroalgae are unaffected or slightly elevated in short term experiments in pCO₂expected for midcentury and negative effects on relative growth rates have been observed at pCO₂expected by the end of the century. To test for geographic variation in tolerance of macroalgae to elevated pCO₂, we collected Ulva spp.of the sheet morphology from Bodega, CA, Malibu, CA and Swans Island, ME. The sites all have episodic low pH, and are geographically distant and isolated from each other. When grown in common garden experiments at pCO₂ from ~300 to >1100 µatm, growth dynamics over time differed between populations. In all cases, the absolute value of the pCO₂ effect on growth rate increased over time. Positive effects of pCO₂>800 µatm were evident for the population at Malibu, CA each week of the experiment, whereas the populations at Bodega Bay, CA and Swans Island, ME showed negative effects of pCO₂>800 µatm on growth within 1-2 weeks. Geographic location of source populations had a large effect on the impact of ocean acidification for this common, bloom forming green seaweed. These results have implications for biogeographic shifts and evolution of macroalgal populations over the coming century.