Impact of organic matter source and quality on living benthic foraminiferal distribution on a river‐dominated continental margin: A study of the Portuguese margin

Abstract

Living (rose Bengal stained) benthic foraminifera were investigated on surface sediments from 23 stations from the river-dominated northwestern Portuguese margin. Samples were collected in March 2011, following the period of the maximum rainfall over the Iberian Peninsula, between 20 and 2000 m water depth along five cross-margin transects. Four of them are located off the Douro, Mondego, Tagus, and Sado Rivers and one off the Estremadura coast. The major objectives of this study are (1) to assess the impact of organic matter of various origin and quality on the benthic foraminifera and (2) to investigate the spatial differences of faunal distribution from coastal waters to the deep sea under river influences. To do this, sedimentological and biogeochemical characteristics of the sediments were identified by measuring grain size, oxygen penetration depth, total organic carbon (TOC) content, stable carbon isotopic composition of TOC (δ 13 C TOC ) and concentration of pigments and amino acids. Based on the principal component and cluster analyses of the environmental data, three major geographical groups are identified: (1) deep stations, (2) coastal and middle slope stations, and (3) shelf stations under river influence. At the deepest stations, species are associated with high organic matter (OM) quantity but low OM quality, where Uvigerina mediterranea, Hoeglundina elegans, and agglutinated species such as Reophax scorpiurus or Bigenerina nodosaria are dominant. All stations off the Sado River, which is the most affected area by the anthropogenic influence, are also characterized by high quantity but low quality of OM with the minimum faunal density and diversity within the study area. Middle slope stations are associated with low OM content and coarse sediments (Q 50 ) with the predominance of N. scaphum. Shallow shelf stations close to the Douro and Tagus River mouths show a dominance of taxa (e.g., Ammonia beccarii, Bulimina aculeata, Eggerelloides scaber, Nonion scaphum, Cancris auriculus, and Quinqueloculina seminula) adapted to environments characterized by high OM quality (high fresh chlorophyll (Chl a/Phaeo)) and available amino acids (enzymatically hydrolyzable amino acid (EHAA)/total enzymatically hydrolyzable amino acid (THAA)). The Biotic and Environmental linking analysis suggests that the benthic foraminiferal distribution is mostly controlled by three environmental parameters, i.e., TOC (quantity), EHAA/THAA (quality), and δ 13 C TOC (source). Hence, this study clearly highlights that the quantitative and qualitative inputs of OM and its source are the most important factors controlling the living benthic foraminiferal distribution with clear influences between the different rivers. This study also suggests a good tolerance of several species for river discharges where the OM quality is high.

Benthic Foraminifera in Arctic Methane Hydrate Bearing Sediments

“…Microhabitat controls faunal distribution (density and diversity) mainly through its physical and chemical properties like grain size, oxygen depletion, and source, quantity, and quality of the organic matter stocked in the sediment (e.g., Jorissen et al, 1995;Schönfeld, 2002;Dessandier et al, 2016). The impact of organic content on benthic foraminiferal density and diversity is unclear at the locations of this study.…”

Living (stained) foraminifera in the Lesser Syrtis (Tunisia): influence of pollution and substratum

“…Bouchet (2007) suggests that the sedimentary detrital organic matter represents the main food resource for this species. Dessandier et al (2016) studied the distribution of E. scaber along the Portuguese coast, highlighting its preference for environments characterized by high-quality of the organic matter (e.g., high chlorophyll-a to phaeopigment ratio and available amino acids). Murray (2013) reported that E. scaber is sensitive to salinity not exceeding 24 psu and oxygen depletion (<0.5 ml/L), and that it is tolerant to fluctuations in temperature and heavy metal pollution.…”

Variations in benthic foraminiferal assemblages in the Tagus mud belt during the last 5700 years: Implications for Tagus River discharge

“…This is particularly true for river-dominated shelves where living benthic foraminifera have been identified to respond to river inputs (e.g., Mendes et al, 2004;Mojtahid et al, 2009;Goineau et al, 2011). Dessandier et al (2016Dessandier et al ( , 2018 have shown through studies on living and dead faunas that some species could be used as bio-indicators of river discharge and upwelling intensity in the Portuguese Margin. During the last millennia, these environmental conditions were influenced by rapid climatic changes (Abrantes et al, 2005.…”