Canopy macroalgae influence understorey corallines’ metabolic control of near-surface pH and oxygen concentration

Understorey macroalgae can alter pH at their surface via metabolic activity within the concentration boundary layer (CBL), but it is unknown to what degree the presence of larger macroalgal canopies can modify the pH micro-environment of understorey species. We examined whether flow reduction by a canopy-forming macroalga could alter the thickness of the CBL at the surface of understorey crustose coralline macroalgae (CCA). This could lead to a greater metabolic influence of macroalgae on the pH and oxygen environment at the coralline’s surface. Three experimental treatments were examined in a re-circulating flume: (1) a full canopy (consisting of <i>Carpophyllum maschalocarpum</i>) and understorey (<i>Corallina officinalis</i> and CCA), (2) a mimic (plastic/silk) canopy plus understorey, and (3) an understorey only. Profiles of seawater velocity and pH/O₂ concentration gradients were measured at 3 bulk seawater velocities (2, 4 and 8 cm s^-1) above the CCA in both the light and dark. Canopy macroalgae altered the pH and O₂ environment encountered by understorey coralline algae via their physical presence rather than by directly altering bulk seawater chemistry through their metabolism. Reduced seawater velocities beneath <i>Carpophyllum</i> and mimic canopies resulted in increased CBL thicknesses, higher pH (up to 8.9) and O₂ concentrations in the light, and lower pH (down to 7.74) and O₂ concentrations in the dark. The ability of canopies to facilitate greater metabolic changes in pH at the surface of understorey species highlights a previously unrecorded species interaction that could play an important role in influencing the physiology and ecology of understorey assemblages.