Deciphering the potential of Ba/Ca, Mo/Ca and Li/Ca profiles in the bivalve shell Pecten maximus as proxies for the reconstruction of phytoplankton dynamics.

• Time lag of 8 to 12 days between environmental signal and geochemical response. • Ba/Ca shell peaks are linked to specific diatom, dinoflagellate and flagellate blooms. • Mo/Ca shell chronologies reveal similarities with Gymnodinium spp. time-series. • Li/Ca shell peaks could be related to neurotoxin producing diatom blooms. Shells of the fast-growing bivalve Pecten maximus serve as a geochemical archive for the reconstruction of past phytoplankton dynamics. Specifically, high-resolution, temporally accurately aligned molar barium-to-calcium (Ba/Ca shell), molybdenum-to-calcium (Mo/Ca shell) and lithium-to-calcium ratios (Li/Ca shell) of the shell calcite revealed distinct peaks which are closely linked to phytoplankton dynamics. Yet, the development and applicability of these geochemical proxies is still at an early stage and needs further calibration. In this study, we examined the relationship between the timing and magnitude of Ba/Ca shell , Mo/Ca shell and Li/Ca shell peaks of P. maximus and the occurrence of various phytoplankton species (diatoms and dinoflagellates) from a statistical perspective. Studied shell samples (three specimens per calendar year) as well as detailed phytoplankton observation data were derived from the well-studied costal ecosystem of the Bay of Brest (France) over three years (2011, 2012 and 2019). An algorithm-based pseudo-random sampling simulation technique was established that analyzed the complex phytoplankton datasets with respect to the profiles of Ba/Ca shell , Mo/Ca shell and Li/Ca shell to identify potential patterns between phytoplankton and trace element time-series. The simulation results indicate that the timing and magnitude of Ba/Ca shell , Mo/Ca shell and Li/Ca shell peaks agreed best with the occurrence of specific phytoplankton blooms that developed ca. one to two weeks earlier. The data suggest that the formation of transient Ba/Ca shell peaks had a plurispecific origin, i.e., potentially linked to blooms of ingestible diatom, dinoflagellate and flagellate species enriched in Ba that occurred 8 to 12 days earlier. Observed peaks in Mo/Ca shell profiles demonstrably followed the timing and intensity of blooms of the dominant dinoflagellate genus Gymnodinium spp. after a short time lag of around 8 days, potentially linked to an enhanced enzyme activity of nitrate reductase that requires the presence of Mo in the dinoflagellate cells. In addition, Mo/Ca shell peaks agreed with periods of diatom aggregate formation which were hypothesized to induce the formation of Mo/Ca shell peaks in scallop shells. Li/Ca shell profiles revealed similar patterns as large blooms of the diatom Chaetoceros spp. as well as to neurotoxin producing diatoms of the genus Pseudo-nitzschia when considering a short time lag of 8 to 12 days. These findings highlight the great potential of using Ba/Ca shell , Mo/Ca shell and Li/Ca shell chronologies in P. maximus shells as proxies of past phytoplankton dynamics. [ABSTRACT FROM AUTHOR]