Seasonal variability of living benthic foraminifera from the West-Gironde mud patch (Bay of Biscay, NE Atlantic): Three contrasted periods under the stereomicroscope

Abstract

Living continental shelf foraminifera were studied at three stations along a shore to open ocean transect between 39 and 69 m depth in the West-Gironde Mud Patch (WGMP) (Bay of Biscay, NE Atlantic). The aim of this work was to understand how the complex temporal variability of the environmental conditions (e.g., hydrosedimentary processes, sedimentary organic matter, oxygenation levels) controls foraminiferal ecological patterns (i.e., diversity, faunal composition, standing stock, and microhabitats). The WGMP was sampled during three different seasons (boreal summer – August 2017; winter – February 2018 and spring – April 2018), with very different meteorological patterns and benthic environmental conditions. The sedimentary facies at the shallowest station (Station 1, 39 m) varies significantly due to hydrometeorological constraints (strong storms and swells), which are extremely marked in late autumn and during the winter. The erosion of the sandy substrate by strong bottom currents and the deposition of a silty surface layer leads to the recorded spectacular drop in foraminiferal diversity and density recorded in February and April 2018. All foraminiferal species were affected by this hydrosedimentary instability, likely due to the partial destruction of their microhabitat by intense erosional and depositional processes. At the middle WGMP station (Station 2, 47 m), benthic fauna changed much more gradually. The sedimentary imprint of the spring phytoplankton bloom is clearly recorded in April 2018 with an increase in fresh and altered phytopigment content in surface sediments. Eggerelloides scaber, a deposit feeder and hypoxia-tolerant species, dominated the 2017 summer foraminiferal fauna but was gradually replaced by Ammonia falsobeccarii, a phytophagous taxon considered quite reactive to spring bloom inputs. At the distal WGMP (Station 4, 69 m), E. scaber and A. falsobeccarii were outcompeted and gradually replaced by Nonion faba and Nonionoides turgidus, both highly adaptable species able to settle down in surface and subsurface sediments during the spring bloom periods. Able to endure a range of microhabitats and food availability, there they rely on both fresh and altered phytodetritus. We propose a conceptual scheme summarizing the putative interconnexion between foraminiferal faunas, geochemistry and physics in the WGMP.