We have investigated the environmental controls of seasonally resolved records of oxygen and carbon isotopes of modern Mercenaria mercenaria bivalves collected live from five coastal sites along the east coast of North America. Seasonal profiles of δ18O and δ13C obtained by subsampling the incremental growth layers of aragonite were compared with in situ historical records of temperature and salinity. We show that M. mercenaria δ18O profiles track accurately the seasonal variations of water temperature and that variations in growth rates do not affect the shell δ18O values. Growth rates are strongly reduced or interrupted below water temperatures ranging from 8 to 10¿C, implying that only bivalves sampled in tropical areas track the full amplitude of seasonal variations. Although further measurements of water δ18O and a better understanding of seasonal variations of the water δ18O-salinity relationships are necessary, Mercenaria mercenaria bivalves appear to precipitate their shell close to isotopic equilibrium. Whereas the amplitude of the seasonal profiles of δ18O reflects variations in water temperature, the annual average δ18O decreases with latitude, reflecting the trend of rainfall and riverwater δ18O with latitude over the Northern American continent.