Les foraminifères benthiques : bio-indicateurs d’eutrophisation naturelle et anthropique en milieu marin franc

The aim of this work is to evaluate the potential utilisation of benthic foraminifera as bioindicators of eutrophication phenomena in open marine environments. More precisely, we have compared different types of eutrophicated environments, in natural (sediments of the Rhône prodelta) and anthropogenic conditions (oil drill cuttings off Congo and Gabon; sewage sludge in the Firth of Clyde in Scotland). In this context, we studied the spatial distribution of benthic foraminifera, and compared their distributional patterns with the available physico-chemical parameters. Foraminifera are amongst the most abundant protists in benthic marine environments. Because of their high biodiversity, their short life cycles and the specific ecological requirements of the various species, foraminiferal faunas react quickly to the environmental disturbances. Our study reveals a typical faunal succession in response to an increasing gradient of organic enrichment. When approaching the source of organic enrichment, a diminution of the species characteristic of stable environments is observed; these species are replaced gradually by more opportunistic taxa in the highly enriched area. If the environmental degradation is persistent, only species very resistant to the environmental stress are present, generally with low densities. Thus, benthic foraminifera represent a very good tool to evaluate the impact of ecosystem eutrophication. These bio-markers can be used successfully in environmental monitoring and in survey networks of marine environments in order to evaluate the impact on the ecosystem of environmental disturbance

Living (Rose-Bengal-stained) benthic foraminiferal faunas along a strong bottom-water oxygen gradient on the Indian margin (Arabian Sea)

Rose-Bengal-stained foraminiferal assemblages (> 150 μm) were analysed along a five-station bathymetric transect across the core and the lower part of the oxygen minimum zone (OMZ) on the Indian margin of the Arabian Sea. Sediment cores were collected using the manned submersible Shinkai 6500 during the RV Yokosuka cruise YK08-11 in the post-monsoon season (October 2008) at water depths ranging from 535 to 2000 m, along a gradient from almost anoxic to well-oxygenated (0.3 to 108 μM) bottom waters. Stained benthic foraminifera were investigated from two different size fractions (150–300 μm and > 300 μm). Stained foraminiferal densities were very high in the core of the OMZ (at 535 and 649 m) and decreased at deeper sites. The faunas (> 150 μm) were dominated (40–80 %) by non-calcareous taxa at all stations. These were mainly species of Reophax and Lagenammina but also included delicate monothalamous taxa (organic-walled "allogromiids", agglutinated saccamminids, psammosphaerids and tubular forms). These new data from the Indian margin are compared to previous studies from the Murray Ridge, the Pakistan margin and the Oman margin. The fact that similar species were found at sites with comparable bottom-water oxygen concentrations but with very different surface water productivity suggests that, within the strongly developed Arabian Sea OMZ, bottom-water oxygen concentration, and not the organic flux to the sea floor, is the main factor controlling the species composition of the foraminiferal communities. Several foraminiferal species (e.g. Praeglobobulimina sp. 1, Ammodiscus sp. 1, Bolivina aff. dilatata) were confined to the core of the OMZ. These species are presently known only from the Arabian Sea. Because of their association with extremely low oxygen concentrations, these species may be good markers for very low oxygen concentrations, and could be used to reconstruct past OMZ variability in the Arabian Sea

Towards an operational use of benthic foraminifera for organic pollution monitoring in open and enclosed marine environments: case histories from the outer shelf off Congo and the Firth of Clyde in Scotland.

Foraminifera are among the most abundant protists in marine benthic environments. Because of their short life cycles, high biodiversity and specific ecological requirements of individual species, foraminifera react quickly to environmental disturbance, and can be successfully employed as bio-indicators of environmental change, such as those brought about by anthropogenic pollution. In the last decennia, foraminifera have been increasingly used to monitor pollution in a wide range of marine environments, such as intertidal mudflats impacted by oil spillages, harbours affected by heavy metal pollution, or eutrophicated continental shelves. Our best examples of anthropogenic eutrophication are 1) a drill cutting disposal sites at the outer continental shelf off Congo, where we observed a zonation of foraminiferal faunas in the 750 m around the discharge point. In the immediate vicinity of the discharge points (within 70 m), faunas are characterised by low foraminiferal densities. Faunas between 70 m and 250 m of the disposal sites have very high foraminiferal densities, with high percentages of opportunistic taxa such as B. aculeata and B. marginata. Between 250 and 750 m, foraminiferal densities decrease, and the percentages of opportunistic species are lower; 2) a sewage sludge disposal on the sea floor in the Firth of Clyde (Scotland) where we used benthic foraminifera and macrofaunal/meiofaunal assemblages to evaluate the impact. These two communities present a very similar distributional pattern around the disposal site. In its immediate vicinity, both groups show impoverished faunas composed exclusively of species tolerant to strong oxygen depletion. This area is surrounded by an aureole of high density faunas dominated by opportunistic species. Still farther away, faunal density decreases, and equilibrium taxa gradually replace opportunistic species. At about 3 Km of the disposal site, both foraminiferal and macro-/meiofaunal taxa become comparable to those found at the reference station. We used these data to develop a quantitative pollution index, values of which are strongly correlated with the distance to the disposal site. This foraminiferal index offers the possibility to quantify the impact of anthropogenic eutrophication in marine environments, but its validity must be tested in wider range of naturally and anthropogenetically impacted marine environments.Foraminifera are among the most abundant protists in marine benthic environments. Because of their short life cycles, high biodiversity and specific ecological requirements of individual species, foraminifera react quickly to environmental disturbance, and can be successfully employed as bio-indicators of environmental change, such as those brought about by anthropogenic pollution. In the last decennia, foraminifera have been increasingly used to monitor pollution in a wide range of marine environments, such as intertidal mudflats impacted by oil spillages, harbours affected by heavy metal pollution, or eutrophicated continental shelves. Our best examples of anthropogenic eutrophication are 1) a drill cutting disposal sites at the outer continental shelf off Congo, where we observed a zonation of foraminiferal faunas in the 750 m around the discharge point. In the immediate vicinity of the discharge points (within 70 m), faunas are characterised by low foraminiferal densities. Faunas between 70 m and 250 m of the disposal sites have very high foraminiferal densities, with high percentages of opportunistic taxa such as B. aculeata and B. marginata. Between 250 and 750 m, foraminiferal densities decrease, and the percentages of opportunistic species are lower; 2) a sewage sludge disposal on the sea floor in the Firth of Clyde (Scotland) where we used benthic foraminifera and macrofaunal/meiofaunal assemblages to evaluate the impact. These two communities present a very similar distributional pattern around the disposal site. In its immediate vicinity, both groups show impoverished faunas composed exclusively of species tolerant to strong oxygen depletion. This area is surrounded by an aureole of high density faunas dominated by opportunistic species. Still farther away, faunal density decreases, and equilibrium taxa gradually replace opportunistic species. At about 3 Km of the disposal site, both foraminiferal and macro-/meiofaunal taxa become comparable to those found at the reference station. We used these data to develop a quantitative pollution index, values of which are strongly correlated with the distance to the disposal site. This foraminiferal index offers the possibility to quantify the impact of anthropogenic eutrophication in marine environments, but its validity must be tested in wider range of naturally and anthropogenetically impacted marine environments

High-resolution study of planktic foraminifera from the eastern Mediterranean over the last 13 cal ka BP

A unique high-resolution record from the Nile prodelta has been investigated in order to study past hydrological and climatic changes in the southeastern Levantine region over the last 13 cal ka BP. To this end, we used planktic foraminifera (accumulation rates, diversity, assemblages and size properties) as bioindicators of the ecological characteristics of the water column (temperature, salinity, primary production and hydrology). These characteristics were mainly connected to Nile discharges and thermohaline circulation which in turn were controlled by various global and regional climatic forcing factors (e.g., orbital forcing, African and Indian Monsoon, North Atlantic Oscillation (NAO)). Our data showed seven main climatic periods: 1) from 13.0 to 11.5 cal ka BP encompassing the Younger Dryas and characterized by rather cold productive and mixed waters; 2) from 11.5 to 10.1 cal ka BP matching the start of the Holocene and the onset of the African Humid Period (AHP). This period was defined by surface water warming and increasing stratification due to increased river outflow; 3) from 10.1 to 6.4 cal ka BP encompassing the Sapropel deposit (S1) and matching the maximum of the AHP with drastic ecological conditions and maximum water stratification. During this period, the dominant warm taxon Globigerinoides ruber increased significantly in size and accumulation rate marking an opportunistic behavior and a total adaptation to the less saline and stratified waters. After 8.8 cal ka BP, the increase in diversity marked a progressive return to normal conditions; 4) from 6.4 to 2.9 cal ka BP, a progressive aridification period was recorded and the planktic ecosystem returned progressively to equilibrium conditions due to the recovery of thermohaline circulation after S1 and the decrease in Nile runoff; 5) from 2.9 to 1.1 cal ka BP, particular dry conditions were recorded leading to a severe drop in planktic diversity. These conditions seemed to be connected to a negative state of the NAO marking the Roman Humid Period in the western Mediterranean and being anti-phased with the southeastern Mediterranean; 6) from 1.1 to 0.54 cal ka BP, a humid period was recorded matching the Medieval Warm Anomaly and this time connected to a positive NAO. The highest foraminiferal diversity was recorded and the increase in proportions of deep dwellers and eutrophic taxa marked highly productive and mixed waters; 7) from 0.54 cal ka BP to modern time encompassing the Little Ice Age and recorded in our data by a general aridity and surface water warming