Temporal variability of live (stained) benthic foraminiferal faunas in a river-dominated shelf – Faunal response to rapid changes of the river influence (Rhône prodelta, NW Mediterranean)

[摘要] In the context of the French research project CHACCRA (Climate andHuman-induced Alterations in Carbon Cycling at the River-seA connection),living (rose Bengal-stained) benthic foraminifera were investigated at twostations (24 and 67 m depth) in the Rhône prodelta (NW Mediterranean,Gulf of Lions). The aim of this study was to precise the response of benthicforaminiferal faunas to temporal changes of the Rhône River inputs (e.g. organic and terrigeneous material). Each site was sampled in April 2007,September 2007, May 2008 and December 2008, permitting to observeforaminiferal faunas of the 63–150 and >150 μm size fractions undera wide range of environmental conditions. Obvious variations inforaminiferal faunal composition were observed during the four investigatedperiods at the shallowest Station A located in the close vicinity of theRhône River mouth. After major Rhône River flood events, differentcolonisation stages were observed with foraminiferal faunas responding withan opportunistic strategy few days to weeks after the creation of a peculiarsedimentary environment (Leptohalysis scottii, May 2008) or high organic matter supplies(Ammonia tepida, December 2008). Under more stable conditions, relatively diverse andequilibrated faunas grew in the sediments. Species benefited from noticeableinput of riverine phytodetritus to the sediment during spring bloomconditions (April 2007; e.g. Bolivina dilatata, Nonionella stella, Stainforthia fusiformis),or high amounts of still bio-availableorganic matter under more oligotrophic conditions (September 2007; e.g. Ammonia tepida,Psammosphaera fusca). The reduced influence of the Rhône River input at the fartherStation N led to less contrasted environmental conditions during the foursampling periods, and so to less obvious variations in foraminiferal faunalcomposition. During reduced riverine influence (i.e. low Rhônedischarge), species able to feed on fresh phytodetritus (e.g. Clavulina cylindrica, Hopkinsina atlantica,Nonionella iridea and Nonionella turgida) benefited from eutrophic conditions of the spring bloom (April 2007,May 2008). Conversely, the occurrence of Nouria polymorphinoides under oligotrophic conditions(September 2007, December 2008) was indicative of a benthic environmentpotentially disturbed by bottom currents. This study put into evidence theextremely rapid response of benthic foraminiferal faunas to strongvariations in environmental conditions mostly induced by the Rhônedynamics.

[学科分类] 地球化学与岩石