Two-dimensional distribution of living benthic foraminifera in anoxic sediment layers of an estuarine mudflat (Loire estuary, France)

Abstract

Abstract. We present a new rapid and accurate protocol to simultaneously sample benthic living foraminifera in two dimensions in a centimetre-scale vertical grid and dissolved iron and phosphorus in two dimensions at high resolution (200 μm). Such an approach appears crucial for the study of foraminiferal ecology in highly dynamic and heterogeneous sedimentary systems, where dissolved iron shows a strong variability at the centimetre scale. On the studied intertidal mudflat of the Loire estuary, foraminiferal faunas are dominated by Ammonia tepida, which accounts for 92 % of the living (CellTracker Green(CTG)-labelled) assemblage. The vertical distribution shows a maximum density in the oxygenated 0–0.4 cm surface layer. A sharp decrease is observed in the next 2 cm, followed by a second, well-defined maximum in the suboxic sediment layer (3–8 cm depth). The presented method yields new information concerning the 2-D distribution of living A. tepida in suboxic layers. First, the identification of recent burrows by visual observation of the sediment cross section and the burrowing activity as deduced from the dissolved iron spatial distribution show no direct relation to the distribution of A. tepida at the centimetre scale. This lack of relation appears contradictory to previous studies (Aller and Aller, 1986; Berkeley et al., 2007). Next, the heterogeneity of A. tepida in the 3–8 cm depth layer was quantified by means of Moran's index to identify the scale of parameters controlling the A. tepida distribution. The results reveal horizontal patches with a characteristic length of 1–2 cm. These patches correspond to areas enriched in dissolved iron likely generated by anaerobic degradation of labile organic matter. These results suggest that the routine application of our new sampling strategy could yield important new insights about foraminiferal life strategies, improving our understanding of the role of these organisms in coastal marine ecosystems.

Effect of light on photosynthetic efficiency of sequestered chloroplasts in intertidal benthic foraminifera (<i>Haynesina germanica</i> and <i>Ammonia tepida</i>)

“…mainly from diatoms (Lopez, 1979;Knight and Mantoura, 1985;Grzymski et al, 2002;Goldstein, 2004). This was confirmed by experimental feeding studies (Correia and Lee, 2002a;Austin et al, 2005) and by molecular analysis of kleptoplastic foraminifera from different environments (Pillet et al, 2011;Tsuchiya et al, 2015). Foraminifera from intertidal mudflat environments (e.g.…”

“…estuaries, bays, lagoons) A. tepida and H. germanica are usually the dominant meiofauna species in West Atlantic French coast mudflats (Debenay et al, 2000Morvan et al, 2006;Bouchet et al, 2009;Pascal et al, 2009;Thibault de Chanvalon et al, 2015). Their vertical distribution in the sediment is characterised by a clear maximum density at the surface (Alve and Murray, 2001;Bouchet et al, 2009;Thibault de Chanvalon et al, 2015) with access to light, followed by a sharp decrease in the next two centimetres (Thibault de Chanvalon et al, 2015).…”

“…They may play a relevant role in the carbon cycle in sediments from deep sea (Moodley et al, 2002) to brackish environments (Thibault de Chanvalon et al, 2015). Their minor role in organic carbon cycling in aerobic sediments, compared to bacteria, contrasts with their strong contribution to anaerobic organic matter mineralisation (Geslin et al, 2011) and they can be responsible for up to 80 % of benthic denitrification (Pina-Ochoa et al, 2010;Risgaard-Petersen et al, 2006).…”

Ultrastructure and distribution of kleptoplasts in benthic foraminifera from shallow-water (photic) habitats

“…In photic shallow-water habitats (e.g., estuaries, bays, lagoons and other intertidal or shallow-water subtidal areas), kleptoplastic benthic foraminiferal species, such as Haynesina germanica, Elphidium williamsoni, the "excavatum" species complex (e.g., E. oceanense, E. selseyense, see Darling et al (2016)), or Ammonia spp., are often the dominant mudflat foraminiferal taxa (Debenay et al, 2000;Debenay et al, 2006;Morvan et al, 2006;Bouchet et al, 2009;Pascal et al, 2009;Thibault de Chanvalon et al, 2015;Cesbron et al, 2016). Their vertical distribution is characterized by a clear maximum density in the upper oxygenated millimeters of the sediment (Alve and Murray, 2001;Bouchet et al, 2009;Thibault de Chanvalon et al, 2015;Cesbron et al, 2016), where light can also penetrate (Kuhl et al, 1994;Cartaxana et al, 2011).…”

“…Their vertical distribution is characterized by a clear maximum density in the upper oxygenated millimeters of the sediment (Alve and Murray, 2001;Bouchet et al, 2009;Thibault de Chanvalon et al, 2015;Cesbron et al, 2016), where light can also penetrate (Kuhl et al, 1994;Cartaxana et al, 2011). However, in some kleptoplastic species (e.g., the morphospecies A. tepida and E. excavatum) kleptoplasts lack photosynthetic activity (Lopez, 1979;Jauffrais et al, 2016), and in many other kleptoplastic species, the photosynthetic activity has not yet been assessed and/or quantified.…”