Lateral and vertical distributions of living benthic foraminifera off the Douro River (western Iberian margin): Impact of the organic matter quality

•Living benthic foraminifera (> 150 μm) off the Douro river are studied on a shelf-slope transect.•Faunal density is higher on the shelf stations.•Living faunas found in deep microhabitats below a massive sandy deposit was observed.•Organic matter quantity and quality are determinant for faunal distribution.•δ13CTOC, Chl-a, EHAA/THAA, TOC and Chl-a/Phaeo. are well correlated with microhabitats.

In this study, we assess the impact of the quantity and quality of the organic matter on the distribution of live benthic foraminifera on a cross-margin transect off the Douro River (western Iberian margin). Surface sediments from five stations ranging from 50 to 2000 m water depth were collected in March 2011 at the end of the period of maximal river runoff, in order to better evaluate the importance of terrestrial input. Living (stained) benthic foraminiferal assemblages (> 150 μm) and environmental parameters (grain size, total organic carbon (TOC), total nitrogen (TN), C/N ratio, δ13CTOC, pigments and amino acids) were investigated. Organic matter amounts follow the sediment grain size pattern with higher concentration in fine sediments and seem to control faunal density, except at the shallowest station where TOC is low and faunal density high. At this coastal site, a massive sandy deposit is identified and associated with very low faunal diversity. Labile organic matter is richest at the shelf stations where benthic foraminifera standing stocks are high. This is particularly evident on the Douro mud patch where both density and diversity are high. Foraminiferal faunas living at the shelf stations are dominated by species (e.g., Nonion scaphum, Ammonia beccari,i Bulimina aculeata and Eggerella scabra) characteristic of the rich trophic conditions, with organic matter of mixed estuarine and marine origin. Very low total standing stocks on the upper slope are associated with coarse sediments, probably resulting from intense bottom currents at this depth resulting in poor trophic conditions. Deeper environments are characterized by species (e.g., Hoeglundina elegans, Uvigerina mediterranea and Reophax scorpiurus) indicative of fairly abundant but low-quality organic matter. Finally, the faunal differences between the shelf stations may reflect different successional stages: an early stage at 50 m water depth where unstable conditions are observed, with faunas dominated by opportunistic species and a more mature fauna in the mud patch where more stable conditions prevail.