From electrical cable bacteria acidification to eelgrass colonisation: seasonal monitoring of foraminiferal ecology and shell preservation on estuarine mudflats.

This study presents the seasonal monitoring of sediment acidification in two intertidal mudflat stations in the Auray river estuary (Atlantic coast, France). Sediment geochemistry and living benthic foraminifera and the preservation of their shells were investigated from April 2022 to July 2023. The development of eelgrass meadows was observed in both mudflats during Summer, something that had not happened for over ten years. Before these sprouts, the mudflats were bare, with seasonal algal deposits, and colonised by cable bacteria. Cable bacteria activity is characterised by electrogenic sulphide oxidation (e-SO_x) measured by O₂, H₂S and pH microprofilings. e-SO_x redesigns diagenetic processes generating strong pH gradients within the first few centimetres of sediment. The upstream mudflat showed seasonal dynamics of e-SO_x. Cable bacteria appeared to be inactive in Winter (∆pH = 0.4) and led to intense pore water acidification during Fall (∆pH = 1.9) under meadow senescence. In the downstream mudflat, e-SO_x remained continuous through the year with ∆pH from 0.9 in Winter to 2.3 in Fall. At both stations, the Ω_calc decreased from supersaturated to values well below 1 in the first few millimetres of sediment, excepted in Winter when Ω_calc was undersaturated due to freshwater flow. All year long, calcareous specimens, mostly dominated by Ammonia morphocomplex tepida and Haynesina germanica, showed test dissolution below the sedimentary oxic layer. During Fall, at both stations, calcareous specimens dwindled and tests were extremely corroded. In the meantime, the agglutinated species Ammobaculites balkwilli dominated the assemblage. During Spring, the upstream station was the setting for a H. germanica bloom after the cable bacteria seemed no longer active in Winter. During Summer, the upstream station showed a well-developed eelgrass meadow together with e-SO_x (ΔpH = 1.3). Agglutinated species dominated the foraminiferal assemblage with A. balkwilli in the upper 5-mm and Eggerelloides scaber deeper down. The eelgrass colonisation has seemed to be beneficial to the foraminiferal community and stimulates its dynamism by encouraging a new species equilibrium in the assemblage. The most impacted species seemed to be A. morphocomplex tepida as between Summer 2022 and 2023 their density and relative abundance felt sharply in favour of Elphidium spp., Quiqueloculina spp. and A. balkwilli. These summery observations were quite different from those at the downstream station where cable bacteria were active all year long. Surprisingly, agglutinated species remained in minor proportions and A. morphocomplex tepida more or less constant. Moreover, dead assemblages showed important losses of calcareous tests where cable bacteria were active conducting to an organic lining enrichment with depth. To summarize, our study shows that foraminiferal ecology responds quickly to environmental changes in coastal sediments making them suitable for biomonitoring while the loss of their tests in acidic environments weakens their applicability for reconstructing temporal environmental chronicles.