| Summary: | Coralline algae create reef habitat, induce larvae settlement, and are primary producers in ecosystems worldwide. Ocean acidification (OA) and changes in the grazing behavior of the purple urchin Strongylocentrotus purpuratus are threatening coralline algae. The susceptibility of coralline algae to grazing could be worsened by a weakening of the skeletal under OA conditions. Coralline algae are unique among macro-algae in that they are photosynthesizing and calcium carbonate forming organisms, allowing them to create a diffusion boundary layer (DBL) that could influence their biomineralization. It has been hypothesized that the DBL could ameliorate the effects of OA, but the extent to which this is true is largely unknown, as is the importance of water flow on the physiology of coralline algae. Two species of articulated coralline algae (ACA) (Calliarthron tuberculosum and Bossiella orbigniana), and cobbles with crustose coralline algae (CCA) were grown for one month. Two levels of pH, and flow were crossed to create a fully factorial mesocosm experiment. Coralline algae were exposed to S. purpuratus for 24-48 hours. Under the low flow treatment, the DBL was on average 1.7mm thick compared to the fast flow treatment where no measurable DBL was detected. The two ACA species had lowered net calcification and linear area extension under low pH and flow treatments. Most notably, C. tuberculosum had a 40.6% decrease in proportional buoyant weight under low pH treatments and a 13.5% decrease in proportional buoyant weight under low flow treatments. CCA had a 67.11% decrease in surface area under low flow treatments, and a 271% increase in grazed surface area under low pH treatments. With no interactions between flow and pH it does not appear that a DBL can ameliorate the effects of OA; however, these results do indicate that low pH and low flow negatively impact coralline algae physiology and those impacts translate to the ecological consequence of increased grazing susceptibility. ...
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