While ocean acidification has different effects on herbivores and autotrophs, how acidification may influence herbivory is poorly understood. This study examined how grazing by the gastropod Lacuna vincta (hereafter Lacuna) on the macroalgae Ulva spp. (hereafter Ulva) is influenced by ocean acidification. Herbivory by Lacuna was significantly reduced under elevated partial pressure of carbon dioxide ( pCO2; 1500-2000 µatm) relative to ambient pCO2 (400 µatm). This significant decrease in herbivory was unrelated to the physiological status of Ulva but rather was specifically elicited when Lacuna was exposed to elevated pCO2 in the absence of food for 18 to 24 h prior to grazing Ulva. The negative effects of elevated pCO2 on Lacuna were absent at 400 to 800 µatm pCO2 or when fed but persisted for up to 72 h following a 24 h exposure to elevated pCO2 without food. Depressed respiration rates in Lacuna following exposure to high pCO2 without food indicated these conditions produced metabolic suppression potentially associated with acidosis. Collectively, the lasting (72 h) nature of grazing inhibition of Lacuna following brief exposure (18 h) to moderate pCO2 levels (>1500 µatm) when food was not available suggests this process could have broad effects on the dynamics of macroalgae in estuaries where Lacuna is a dominant grazer; these effects will be amplified as climate change progresses.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2019) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2020-02-17.