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As part of the HIPPO (HIgh-resolution Primary Production multi-prOxy archives) project, environmental monitoring was carried out between March and October 2021 in the Bay of Brest. The aim of this survey was to better understand the processes which drive the incorporation of chemical elements into scallop shells and their links with phytoplankton dynamics. For this purpose, biological samples (scallops and phytoplankton) as well as water samples were collected in order to analyze various environmental parameters (element chemical properties, nutrients, chlorophyll a , etc.). Given the large number of parameters that were measured, only the major results are presented and discussed here. However, the whole dataset, which has been made available, is much larger and can potentially be very useful for other scientists performing sclerochronological investigations, studying biogeochemical cycles or conducting various ecological research projects. The dataset is available at 10.17882/92043 (Siebert et al., 2023). [ABSTRACT FROM AUTHOR]

Primary producers are essential organisms for marine ecosystems because they form the basis of food webs, produce half of atmospheric oxygen and are involved in various biogeochemical cycles. At the end of a bloom event, phytoplankton cells are known to produce organic compounds that act as a 'cement', allowing the cells to stick together and form large sinking structures called aggregates. These aggregates are microenvironments with chemical properties that are very different from the surrounding water. The main objective of this study was to determine how the temporal variations in cell assemblages over time and the formation of aggregates following a bloom affect the concentrations of molybdenum (Mo) and barium (Ba) in the water column, which are elements typically measured within accretionary hard tissues (e.g., mollusc shells) to track phytoplankton dynamics in the environment. To do so, we performed an environmental monitoring from March to October 2021 at Lanvéoc in the Bay of Brest (France) during which several biological (e.g., variations in phytoplankton assemblages) and chemical (e.g., chemical properties of the water column) parameters were measured once to twice per week. Our results show that spring and summer blooms of Gymnodinium , known to be enriched in Mo, could be one of the reasons explaining the particulate Mo enrichments in the water column. In addition, large phytoplankton aggregates transported a significant amount of Mo to the seafloor and associated suspension feeders. In contrast, the temporal variations in dissolved and particulate Ba concentration were strongly influenced by the formation of diatom blooms. Interestingly, there was a significant shift in Ba from the dissolved to the particulate fraction during the largest diatom bloom in late spring, associated with a significant Ba transport to the seafloor, which may be explained by the adsorption of this element onto diatom frustules. This study therefore highlights the impacts of phytoplankton on the dynamics of these elements in coastal ecosystems. [ABSTRACT FROM AUTHOR]

• 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]

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. • Ba/Ca shell profiles in Pecten maximus are correlated with blooms of specific phytoplankton species, especially diatoms. • Ba/Ca shell peak follows phytoplankton bloom with temporal lag. • Ba/Ca shell profiles can serve as a proxy for dynamics of certain phytoplankton species. [ABSTRACT FROM AUTHOR]