Effect of ocean acidification and pH fluctuations on the growth and development of coralline algal recruits, and an associated benthic algal assemblage

Abstract

Coralline algae are susceptible to the changes in the seawater carbonate system associatedwith ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmentalstress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4× preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatmentsof daily ∼x = 8:05 (daytime pH = 8.45, night-time pH = 7.65) and daily ∼x = 7:65 (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in alltreatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults' response, except that adults had zero growth underfluctuating pH 7.65. The%dry weight of MgCO3 in calcite of the juveniles was reduced from 10%at pH 8.05 to 8%at pH 7.65, but there was no effect of pH fluctuation. A wide range offleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was noeffect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshymacroalgae, whose high metabolic activity may raise the pH of the local microhabitat.