ABSTRACT: Whether naturally present or reared on specific structures, bivalves play a key role in the functioning of coastal ecosystems, especially in the food web. Although they are known to be omnivores, they feed mainly on phytoplankton and resuspended phytobenthos, whose communities are affected by short- and long-term dynamics. In situ bivalve food preferences remain poorly understood: which taxa are preferentially selected, which are avoided, and why? To investigate these questions, we examined 10 yr of monitoring data in 5 Atlantic bays, recording phytoplankton abundance and growth rates of the reared Pacific oyster Crassostrea gigas. Correlations between oyster growth and phytoplankton abundance were studied using a dual approach, coupling taxonomy and trait analysis, to summarise phytoplankton diversity. Phytoplankton biological traits were chosen to reflect their ability to resist grazing and their importance to oyster feeding physiology. Our results showed that Skeletonema was a key positive genus associated with oyster growth, like Asterionellopsis glacialis and Thalassiosiraceae, to a lesser extent. Conversely, Leptocylindrus, Alexandrium genera and Gymnodiniaceae were associated with lower oyster growth. The trait-based approach highlighted the importance of phytoplankton cell cover, the presence of spicules and the harmful nature of phytoplankton. The highest oyster growth values were obtained for low abundances of phytoplankton cells with organic cover and spicules. However, cell shape, cell size, the capacity to form colonies and the habitat of phytoplankton appeared to be secondary traits. This original study corroborates existing knowledge of the potential in situ phytoplankton prey of C. gigas and provides new avenues for research.