Phytoplankton dynamics in coastal oceans is a major component of the global biogeochemical carbon cycle, and is currently affected by global change through modifications in levels of primary productivity and composition of phytoplankton communities. Despite many attempts, no straightforward geochemical proxy has been found yet in marine biogenic carbonates for reconstruction of past phytoplankton dynamics with high temporal resolution. Here, we report on sub-weekly variations of lithium-to-calcium ratios (Li/Cashell) along the axis of maximum growth of great scallop shells (Pecten maximus) collected alive between 1999 and 2007 in the bay of Brest, northwest France. Inter-individual variability of Li/Cashell time series was very low, suggesting an environmental control on the incorporation of Li within shells. Conversely, inter-annual variability of Li/Cashell was high, with limited seasonal Li/Cashell variations in 2001 and 2007, and the presence of Li enrichments from May to July in 1999 and 2004. Comparison of these results with shell growth measurements (increment width) and environmental parameters suggests (i) that shell calcification rate is likely the main factor controlling incorporation of Li in Pecten maximus shell calcite, (ii) that seawater temperature has only a weak positive influence on Li/Cashell of this species over the range 8-18 °C, and (iii) that during diatom blooms, additional amounts of Li may be trapped in the shell following dissolution of Li-rich frustules of edible species in the digestive tract of scallops, being responsible for Li/Cashell peaks. Therefore, we suggest that Li/Cashell ratio may be a novel proxy for timing and magnitude of diatom blooms in coastal ecosystems. Analysis of ancient shells may thus provide useful information on past phytoplankton dynamics and on the importance of recent shifts observed from diatoms to non-siliceous phytoplankton in coastal areas affected by anthropogenic activities.