Seasonal evolution of the diatom and foraminifer fluxes in the Northwest Mediterranean based on the study of a 12-year-long sediment trap record

We studied the annual evolution of the planktonic foraminifer assemblages through the analysis of two sediment trap records from the Gulf of Lions (Northwest Mediterranean). Sea surface circulation in this region is the result of the interaction between the Northern Current and wind-driven currents along the continental shelf. Particle fluxes to the sea floor were controlled by changes in sea surface circulation and primary productivity, which underwent strong annual cycles. To investigate seasonal and annual changes in particle fluxes over the last decade two sediment traps were deployed in the middle part of the Planier and Lacaze-Duthiers canyons and samples were taken fortnightly or monthly from 1993 to 2006. More than 120 diatom species were identified in the samples. Thalassionema frauenfeldii was the dominant taxa during the study period for both traps. Planier station was the trap with less influence of the continental shelf and, among others, exhibited the classical succession Skeletonema-Chaetoceros-Rhizosolenia through the spring bloom. Lacaze-Duthiers station showed important differences with Planier, as maximum diatom productivity peaks in different moments or the dominance of the summer season by Chaetoceros RS (resting spores), caused by the influence of the Rhône river and the general oceanic circulation along the shelf. Eleven planktonic foraminifer species were recognized in our study, however, the assemblage was generally dominated by Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia inflata and Globorotalia truncatulinoides. The foraminifer assemblages in both sediment traps show a clear relationship with the seasonal changes of stratification of the water column and the spring phytoplankton bloom. Globigerina bulloides is the dominant species in both records and its distribution is closely related with the primary productivity cycles. The seasonal distribution of Neogloboquadrina pachyderma and Globorotalia inflata seems to be mainly controlled by water temperature, and secondarily by the primary productivity. Globorotalia truncatulinoides is an abundant species at times of strong vertical water mixing, which usually occurred in winter in this region. Temperature and primary productivity seem to play a secondary role on the distribution of this species.