Macroalgae as ecosystem engineers and the implications for ocean acidification

Abstract

Ocean acidification (hereafter OA) is the process of increasing surface seawater CO2 and decreasing pH. OA is predicted to negatively impact many calcareous species, including coralline macroalgae. However, pH also varies in seawater surrounding macroalgae due to their metabolic activity. Research within this thesis examined how larger canopy-forming macroalgae (Carpophyllum maschalocarpum) and smaller articulate coralline macroalgae (ACA) altered the pH microenvironment at the surface of crustose coralline macroalgae (CCA). Larger canopy-forming macroalgae reduced the velocity of seawater through their canopies, which magnified changes in pH occurring at the surface of CCA (pH increasing in the light and decreasing in the dark) through increasing the concentration boundary layer (CBL) thickness. Smaller ACA Corallina officinalis also reduced seawater velocity through their canopies, but not to the same extent as Carpophyllum maschalocarpum, resulting in smaller changes in pH and thinner CBLs. Further research confirmed that these changes in pH also occur when pH in the bulk seawater is reduced to levels comparable to that expected to occur by 2100 due to OA. CCA under the ACA species Arthrocardia corymbosa increased pH at their surface under slow seawater velocities (0 – 1.5 cm s-1) up to ambient seawater pH in the light (8.00) when placed in seawater with a bulk pH of 7.65.

pH measured within a kelp (Macrocystis pyrifera) bed around Otago, New Zealand (the collection site of A. corymbosa) showed that pH variability within macroalgal beds can be high, with pH varying between 7.5 and 9.1 units. To investigate the impacts of OA and variability in pH on coralline macroalgae, A. corymbosa was grown under 4 different pH treatments for 40 days (2 means: pH 8.05 and 7.65; and two levels of variability: <0.01 and increasing by 0.40 units from the mean during the day and decreasing by 0.40 units from the mean at night). A. corymbosa growth was reduced by both decreases in mean pH and by increases in pH variability. However, the recruitment of juvenile coralline macroalgae, and all other measured aspects of the physiology of adults were not impacted by pH. During this experiment the CBL was reduced by high flow rates, but in a subsequent experiment the effects of the absence and presence of a thick CBL at pH 8.05 and 7.65 was investigated by modifying the flow conditions that A. corymbosa was grown under. Seawater velocity and pH interacted to influence the growth and calcification of A. corymbosa. At pH 8.05 growth and calcification rates was higher at fast velocities, but at pH 7.65 growth and calcification rates was significantly higher under slow seawater velocity. This means that the effects of OA could be ameliorated at slow flows for calcareous organisms that are capable of photosynthesizing, where the pH micro-environment is altered favouring higher calcification rates during the day.

The work in this thesis emphasizes that macroalgae are capable of acting as ecosystem engineers, altering both their chemical and physical environment in a way not previous examined. Also, their ability to alter their physical micro-environment has flow on effects for their chemical micro-environment. This alteration of their micro-environment has implications for both coralline macroalgae, and potentially other species that live within macroalgal beds. Organisms inhabiting macroalgal beds will encounter both flower flow rates and a more variable pH environment, which will likely modulate their response to OA.