Turf algal epiphytes metabolically induce local pH increase, with implications for underlying coralline algae under ocean acidification

- The interaction between turf algae (TA) and CCA was explored under two CO2 regimes.
- The effect of TA on pH and O2 within the DBL above CCA crusts was measured.
- TA increased the diel amplitude of pH and O2 within the DBL under elevated CO2.
- CCA response to ocean acidification may change in the presence of epiphytes.
- Species interactions may modify individual response to environmental change.

The presence of epiphytic turf algae may modify the effects of ocean acidification on coralline algal calcification rates by altering seawater chemistry within the diffusive boundary layer (DBL) above coralline algal crusts. We used microelectrodes to measure the effects of turf algal epiphytes on seawater pH and the partial pressure of oxygen (pO2) within the DBL at the surface of Hydrolithoideae coralline algal crusts under ambient (36 Pa) CO2 and an ocean acidification scenario with elevated CO2 (200 Pa). Turf algae significantly increased the mean diel amplitude of pH and pO2, and this effect was more pronounced under elevated CO2. We suggest that increases in seawater CO2 under ocean acidification conditions may drive an increase in the abundance of epiphytic turf algae, consequently modifying the chemistry within the DBL. Thus, the effect of epiphytic turf algae on microscale pH is striking and will likely affect coralline algal response to ocean acidification, highlighting the importance of understanding the effects of environmental change on species interactions.

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