Ba/Ca profiles in shells of Pecten maximus – A proxy for specific primary producers rather than bulk phytoplankton

Molar barium-to‑calcium ratios in bivalve shells (Ba/Ca shell ) have been proposed in a variety of studies to serve as a potential proxy for the reconstruction of phytoplankton dynamics. However, the link between phytoplankton and Ba/Ca shell profiles remains unclear and needs to be deciphered more accurately. In this study, we analyzed the relationship between Ba/Ca shell peaks and specific phytoplankton species, and assessed the applicability of Ba/Ca shell ratios as a species-specific phytoplankton proxy. The timing of peaks in highly resolved Ba/Ca shell time-series in Pecten maximus shells (Bay of Brest, France) from two years (2011, 2012) were compared to the chlorophyll a concentration and the occurrence of individual diatom and dinoflagellate species. In addition, Monte Carlo simulations were used to approximate a species-specific contribution to the measured Ba/Ca shell peaks. The results clearly demonstrated that the Ba/Ca shell profiles cannot be explained either by the chlorophyll a concentration nor by the total diatom or dinoflagellate abundance. Instead, time-series of specific phytoplankton, especially diatoms, revealed a high degree of synchronicity with Ba/Ca shell peaks when temporally lagged by 8 to 13 days (depending on species). The Monte Carlo simulations suggested that the Ba/Ca shell peak heights cannot be fully explained by the observed phytoplankton cell concentration, but rather by individually weighted phytoplankton time-series, most likely caused by inter-species differences such as cell size and chemical ability to adsorb Ba. Moreover, the approximated species-specific weighting factors agreed well between the studied years. According to our findings, Ba/Ca shell peaks are likely associated with blooms of specific phytoplankton taxa, with a time lag of ca. one to two weeks, and the amount of cell-associated Ba varies between phytoplankton species. These conclusions provide further insights into the formation of Ba enrichments in bivalve shells and improves the applicability of Ba/Ca shell profiles as a species-specific proxy of past phytoplankton dynamics.