The influence of seasonal processes on geochemical profiles and foraminiferal assemblages on the outer shelf of the Bay of Biscay

Benthic foraminifera are widely used tools to reconstruct paleoenvironments. They reflect bottom water oxygen concentrations and organic carbon flux. To increase their accuracy as proxies, seasonal variability of foraminiferal populations from the outer shelf of the Bay of Biscay (150 m water depth) was followed over a 3-year period. The selected area is characterized by seasonal variation in organic matter deposition. A strong interaction between organic detritus input, macrofaunal bioturbational activity, the degree of separation of redox zones and the reproduction, growth and migrational response of benthic foraminifera was found. During periods of high labile organic matter input, as occurs during spring bloom conditions, the redox zonation was not well established. Different organic degradation pathways occurred at the same depth intervals in the sediment. Shallow infaunal species reacted in response to the high nutrient content with rapid growth (Bolivina subaenariensis) or reproduction (Epistominella vitrea, Bolivina seminuda, Bolivina dilatata/spathulata). Infaunally living foraminifera, Chilostomella oolina and to lesser extent Nonion scaphum, migrated to the sediment surface in reaction the increase organic matter input to the seafloor. This migration could have been induced by the more intensive overlapping of redox zones that were perhaps better separated vertically in winter. However, this migration could also have been triggered by the occurrence of fresh phytodetritus in the sediment, becoming even available in slightly deeper sediment level due to the increased bioturbation activity.