Articles | Volume 12, issue 20
Biogeosciences, 12, 6219–6234, 2015

Special issue: Low oxygen environments in marine, fresh and estuarine...

Biogeosciences, 12, 6219–6234, 2015

Research article 29 Oct 2015

Research article | 29 Oct 2015

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

A. Thibault de Chanvalon1,2, E. Metzger1, A. Mouret1, F. Cesbron1, J. Knoery2, E. Rozuel2, P. Launeau1, M. P. Nardelli1, F. J. Jorissen1, and E. Geslin1 A. Thibault de Chanvalon et al.
  • 1Université d'Angers, Université de Nantes, LPG-BIAF, UMR CNRS 6112, 49045 Angers Cedex, France
  • 2Ifremer, LBCM, Rue de l'Ile d'Yeu, 44300 Nantes, France

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.

Short summary
We present a new rapid and accurate protocol to simultaneously sample, in two dimensions, benthic living foraminifera at the centimetre scale and dissolved iron and phosphorus at the submillimetre scale. It was applied to a highly bioturbated site in a mudflat of the Loire estuary and showed that, in the suboxic zone, foraminifera are less affected by active burrows (i.e. reoxygenated) than by iron reactive hotspots. This unexpected result calls for a generalization of this new protocol.
Final-revised paper