Increasing absorption of CO2 by the world’s oceans is lowering seawater pH and may have severe consequences for marine calcifying organisms. Understanding the ecological consequences of anthropogenic CO2 emissions will require examination of how calcifying organisms and their associated communities respond to natural variation in CO2 concentration. Many macroalgae may respond positively or neutrally to ocean acidification, but calcifying species such as coralline algae are predicted to be some of the most susceptible organisms to changing CO2. Here I test the impacts of temperature and pH variation on important photosynthetic metrics of macroalgal assemblages composed of coralline turf, Corallina vancouveriensis and the associated canopy-forming kelp, Saccharina sessilis using in situ photorespirometry and laboratory mesocosms. In situ photorespirometry was done at two locations on the Oregon (USA) coast, an area with variable upwelling of high CO2, low pH water. To complement in situ measurements, a series of laboratory mesocosms were used to disentangle the effects of pH and temperature on photosynthetic parameters across a light gradient. The acute effects of low pH were also tested across a temperature gradient, revealing an exacerbated effect of short duration, low pH events on respiration rates at increasing temperature. NPP (net primary productivity) was reduced by 10–20% within in situ coralline assemblages across a natural gradient of pH (8.1–7.9), but there was a mostly neutral effect of low pH on NPP of coralline–kelp assemblages. These results indicate varied responses of coralline and coralline–kelp assemblages to temperature and pH gradients, but under limiting light conditions primary production and growth of corallines are likely to decrease under modest scenarios of CO2 increase. Assemblage composition could play an important role in modulating the impacts of ocean acidification on calcifying organisms, and results from this study suggest that canopy and sub-canopy interactions could determine the response of susceptible species to changing climatic parameters.

Tait L. W., 2014. Impacts of natural and manipulated variations in temperature, pH and light on photosynthetic parameters of coralline–kelp assemblages. Journal of Experimental Marine Biology and Ecology 454:1-8. Article (subscription required).