Effects of food supply on northern bay scallops Argopecten irradians reared under two pCO₂ conditions

For calcifying organisms such as bivalves, short-term exposure to increased ocean acidification (OA; elevated pCO₂) may reduce growth rate, increase mortality, and disrupt shell formation. A growing body of research suggests that clearance rates and what particles bivalves select may change under high pCO₂ exposure; however, these experiments are acute, ranging from days to weeks. The effects of food supply on bivalves under long-term OA exposure remain incompletely understood. In this study, juvenile northern bay scallops Argopecten irradians (Lamarck) that had been reared since 4 h post-fertilization under one of 2 OA conditions (~500–600 or ~750–850 μatm pCO₂; ~1.37–1.5 or ~1.0–1.2 Ω_aragonite), were subjected to 2 food levels for 42 d (low food: ~400, high food: ~1400 chlorophyll cells ml^–1). Standard metabolic rate (SMR) and clearance rate (CR) were measured on Day 0, and SMR, CR, growth, and survivorship were measured at 14 and 42 days of exposure to 2 food levels for each of the OA treatments. Juveniles under food scarcity had reduced survivorship and growth independent of OA treatment. We found no effect of OA treatment or an OA × food interaction for these metrics. There was only a food-level effect for SMR and no OA treatment effect; however, there was an interaction between food and OA for CR. Under elevated pCO₂ concentrations, scallops cleared Chaetoceros neogracile (strain Chaet-B) over Tetraselmis chui (strain PLY429) and natural seston. Altogether, these data suggest that tolerance to OA mediated by food may depend on food quality or other characteristics that influence particle selection under short-term experimental challenges.