Bivalve shell-based, high-resolution multi-proxy reconstruction of marine primary production – Pecten maximus, Bay of Brest – HIPPO

The project relies on a triple approach combining observations in the field, experimentations in the lab, and reconstruction using museum shells. Firstly, a high-frequency environmental survey of physical, chemical and biological parameters of the water mass will be conducted in a natural population of Pecten maximus (Lanvéoc, southern shore of the bay of Brest). During this survey, live specimens will be regularly collected (twice weekly) in order to measure the elemental and isotopic composition of their soft tissues. At the end of the survey, another batch of live scallops will be sampled and their shells will be analyzed for their elemental, isotopic and organic (pigments) compositions. Comparisons will be done between shells from the sediment/water interface and scallops which grew in a cage 1 m above the sediment (in order to check whether the sediment influences the shell geochemistry).
The lab experiments will be carried out with a Plexiglas column filled with seawater, with a rolling tank on top of it. The latter will allow the continuous production of phytoplankton agregates (such as in the bay of Brest) that could eventually settle on scallops located on the floor of the column. This device will also be used to test the influence of particular material (red, green, brown macroalgae fragments, different phytoplankton species and biovolumes) on Pecten shell geochemistry.
Once calibrated, our proxies will be applied on shells curated at the University of Western Brittany (past 30 years) and on specimens from archeological middens (Middle Age and Neolithic).

The Covid-19 pandemics strongly affected the project. Consequently, we are not yet able to present significant results.

Results of this study will significantly advance marine sciences including paleoecology, paleoclimatology and fisheries sciences.

No published papers on 01/09/2021

Primary producers form the basis of marine food webs, control population sizes at higher trophic levels and fish stock recruitment. Marine photoautotrophic organisms are also responsible for nearly half of the global net primary production, i.e., they replenish the ocean (and atmosphere) with oxygen and fix substantial amounts of carbon. Despite its outstanding relevance for the functioning of marine ecosystems and global climate, past primary production dynamics and mechanisms controlling them are not well characterized. This is particularly true for nearshore coastal environments and for times prior to significant human perturbation of biogeochemical cycles. Available data sources for changes in marine primary production (i) do not provide the necessary temporal resolution to resolve short-lived and spatially restricted phytoplankton blooms, specifically in shallow waters, (ii) are too short to distinguish trends from low-frequency cycles of primary production and (iii) do not cover the entirety of photoautotroph taxa which include more than just phytoplankton and cyanobacteria, i.e. microphytobenthos and macroalgae.

Therefore, this project will develop an innovative technique that can provide reliable, temporally well-constrained, seasonally to inter-annually resolved data on past primary production dynamics in coastal nearshore environments based on shells of bivalve mollusks. For this purpose, we will test and refine existing proxies (surrogates) for primary production, and develop new proxies and integrate them in a multiproxy approach. In order to obtain a mechanistic understanding of how information on the species composition and number of marine photoautotrophs is recorded in chemical properties (Ba/Ca, Mo/Ca, Li/Ca, stable isotopes of carbon and nitrogen, triple isotope composition of oxygen, pigments) and color (hue and saturation index) of the shells, field and tank experiments will be conducted during which environmental variables can be closely monitored and manipulated. Since the study involves experiments with living bivalves we chose the fast-growing species, Pecten maximus, and an ecosystem that has been studied in great detail, the Bay of Brest, France.

The multiproxy approach will subsequently by applied to subfossil shells collected from an archaeological site to determine the human impact on primary production dynamics of the Bay of Brest including (i) the seasonal occurrence of photoautotrophs as well as the intensity, frequency and seasonal timing of phytoplankton blooms, (ii) shifts in species composition and biomass through time and (iii) changes in the link between organisms inhabiting the sea floor and those living near sea surface. Results of this study will significantly advance marine sciences including paleoecology, paleoclimatology and fisheries sciences.