Impacts of ocean acidification on intertidal macroalgae and algivore preference

Ocean acidification, a facet of global climate change, has the potential to induce changes in marine macroalgae that modify their existing interactions with algivorous invertebrates. In this study, I examined the effects of elevated carbon dioxide (pCO2) on several species of intertidal macroalgae (Phaeophyta, Rhodophyta) and evaluated the present-day and predicted future preferences of algivores (Pugettia producta and Tegula funebralis) by assessing grazing rates on untreated algal tissue and on algae exposed to high-pCO2 seawater. Both red and brown algae grew faster in elevated pCO2 than in ambient seawater, and algae in intermediate pCO2 generated more new growth overall than those in highly elevated pCO2. The effect of pCO2 on the carbon and nitrogen contents of algae depended on species identity, and C:N ratios decreased slightly with increasing pCO2 for four of the five species studied. Total phenolic content in each alga was unaffected by pCO2 treatment, although similar (distinct) levels between untreated species became distinct (similar) when those same species were compared after highpCO2 treatment. Algivores demonstrated contrasting responses to changes in their food sources; P. producta, a specialist crab grazer, did not modify its preference for the brown alga Egregia menziesii when offered high-pCO2 treated individuals, but the generalist snail T. funebralis adjusted its feeding behavior to choose algae with low phenolic contents, which created different patterns of preference for untreated and pCO2-treated algae. C:N ratios of algae did not appear to be a strong driver of preference for either grazer in feeding experiments. These results indicate that algae may be well-equipped to benefit from moderate increases in seawater pCO2, but they exhibit species-specific rates of growth and phenolic production, which in turn affect their appeal to a generalist algivore. Intertidal algal communities will therefore face altered patterns of predation under future ocean acidification conditions as generalist algivores adjust to new variation in algal palatability.

Fieber A. M., 2020. Impacts of ocean acidification on intertidal macroalgae and algivore preference. MSc thesis, Humboldt State University. Article.

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