Arcachon Bay is one of the most important spat-supplying area for the French oyster production. During the last 16 years, the local economic activity of spat production has been threatened recurrently by a high interannual variability of seed harvesting. This threatened spat production was attributed to reproductive problems highlighted by both a delay in oyster spawning and a decrease of small larvae abundances since 1995. Recent works showed that these two reproduction indices seem closely related to climate change that might controls the phytoplankton communities of the Bay. Food availability in term of quantity and quality appears thus as a serious track to understand the reproductive problems of oyster in the Arcachon Bay. The aim of this work was thus to understand how phytoplankton might control the growth and reproductive effort of oysters in Arcachon Bay through microphytoplankton diversity and composition.