Foraminiferal proxies for pollution monitoring in moderately polluted harbors

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Abstract

Benthic foraminifera are increasingly used as environmental bio-indicators, especially in polluted environments where their sensitivity to pollutants may be expressed by a modification of the assemblages. Eighteen sediment samples were collected in September 2000 in five harbors located in moderately polluted estuaries on the coast of Vendée (France) for the study of foraminiferal assemblages. Ten heavy metals and 13 PAH have been analyzed from the sediments. The marine to continental estuarine gradient has a prevalent influence on the foraminiferal distribution. However, the results show that foraminiferal density and species richness of the assemblages decrease with an increase in heavy metal and PAH concentration, and therefore may be used as pollution indicators. Moreover, the more polluted areas are dominated by the tolerant pioneer species Haynesina germanica that may be used as bio-indicator of pollution, mainly in the uppermost areas.

Introduction

Foraminifera (class Foraminifera, phylum Granuloreticulata) are among the more abundant and most conspicuous protozoa in most marine and brackish water habitats. Many species have durable shells (tests), which are an important component of marine and paralic sediments (Lee et al., 2000). Because foraminifers have a short life cycle, they react quickly and can be used as an early warning indicator, as defined by Kramer and Botterweg (1991). Even if their growth follows a yearly cycle, their mineralized tests remain in the sediments and give an average picture of the assemblages, and therefore of the ecologic conditions, despite the great annual or even inter annual variability. Foraminiferal assemblages are easy to collect, and are often found in high-density populations, providing an adequate statistical base (even in small volume samples). Salinity-tolerant coastal and paralic foraminifera have first been used as salinity indicators in the standard estuarine classifications based on the spatial distribution of salinity from oceanic to continental end members (e.g., Nichols, 1974), but they are also well adapted for environmental monitoring (Scott et al., 2001). When pollution is suspected, they may serve as a means of judging whether costly comprehensive studies should be initiated.

Studies of pollution effects on benthic foraminifera and of the possible use of these organisms as proxies were initiated by Resig, 1960, Watkins, 1961, although pollution effects on foraminifera had been mentioned earlier by other workers (e.g. Zalesny, 1959). Throughout the last few years, many studies dealing with benthic foraminifera as bioindicators of coastal pollution have been carried out (review in Boltovskoy et al., 1991, Alve, 1995, Yanko et al., 1999, Scott et al., 2001). An entire volume of the Journal of Foraminiferal Research was devoted to this problem in 1995. Most of the papers focussed on the effects of pollution upon foraminiferal distribution and morphology and dealt with domestic and agricultural wastes (Watkins, 1961, Seiglie, 1968, Schafer, 1970, Schafer, 1973, Schafer and Cole, 1974, Seiglie, 1975, Setty, 1976, Bates and Spencer, 1979, Setty and Nigam, 1984, Alve and Nagy, 1986, Ellison et al., 1986, Nagy and Alve, 1987, Alve, 1991, Yanko and Flexer, 1992, Yanko et al., 1994), paper mill effluent (Schafer, 1970, Schafer, 1973, Buckley et al., 1974), oil spill (Seiglie, 1968, Alve, 1995, Alve, 1991), or trace metals in sediments (Alve, 1991, Sharifi et al., 1991, Alve and Olsgard, 1999, Debanay et al., 2001). They concluded that foraminifera are very sensitive to pollution. They should be applied as part of integrated programs of pollution monitoring, including chemical analysis of the contaminants. Their uses should include routine long-term surveillance programs, hazard assessment at specific discharge sites, and monitoring of the effectiveness of remedial actions (Debenay et al., 2000).

Most of this literature described highly polluted environments but, despite their large distribution, little attention has been paid to the widely distributed moderately polluted estuarine environments, probably because of their very high complexity due to the double effect of natural and anthropogenic stresses.

The samples used for this study come from five harbors located in estuaries on the coast of Vendée, (Atlantic coast of France, Fig. 1). Four of the harbors are located within the bay of Bourgneuf (Brochet, Epoids, Pont Neuf and Champ harbors: respectively B, E, P, and C in Fig. 1), and the last one, southward, is located in the estuary of the Lay River (Aiguillon-sur-Mer-la-Faute-sur-Mer: A in Fig. 1). The estuary of the Lay River, wide open to the ocean, is subject to a strong marine influence, contrary to what happens in the other four small estuaries. In all of the five harbors, a sluice controls the flow of fresh water. Hence, the salinity, a predominant parameter to foraminifera distribution in marginal marine environments (e.g., review in Murray, 1991, Hayward and Hollis, 1994, De Rijk, 1995), is highly variable, not only depending on tide cycles and on natural river flow, but also on the opening of the sluices.

Industrial activities are poorly developed along the coast of Vendée. As a consequence, the area is not subjected to strong pollution stresses. Heavy metal values measured in the study area are much lower than those measured in Restronguet Creek (UK) (Stubbles, 1993, Stubbles et al., 1996) and are similar to the values measured in Goro lagoon (Coccioni, 2000) where the pollution was considered by the author as moderate. Pollution throughout the study area, based on these considerations, will also be considered as moderate. The pollution originates mainly from boat careening and painting that release pollutants including heavy metals, and the release of hydrocarbons containing PAHs (polycyclic aromatic hydrocarbons) from fishing boat engines. In such small estuaries, a small amount of contamination may disrupt the environment. Since December 1999, the bay of Bourgneuf has been affected by the oil slick caused by the wreck of the oil tanker Erika and a small amount of hydrocarbon was deposited in the harbors.

The main objectives of this study are to investigate the distribution and abundance of benthic foraminifera in five estuaries moderately polluted by small harbor activities, and to determine species which may be used as indicators of pollution in this kind of environment.

Section snippets

Material and methods

The study was carried out on bottom sediment collected in September 2000 by means of a Van Veen grab sampler that collects sediment over a surface of about 400 cm2. In the boat, the grab is carefully opened in a container where the sediment is deposited in its initial position. Generally, a diatom film covers the surface of the sediment and indicates the absence of disturbance during collection. In such shallow muddy environments, the surface of the sediment is rarely washed away during

Sediment and chemical analysis

The silt and clay sedimentary fraction (<63 μm) is always dominant, making up more than 61% of all the samples and even more than 90% of ten samples (Table 1). No general tendency may be established for the distribution of organic matter throughout the whole studied area. In the harbors of Epoids, Pont Neuf and Aiguillon-sur-Mer-la-Faute-sur-Mer, organic matter content is at its highest upstream and at its lowest towards the mouth of the estuary (Table 1). Sample P5 taken out from the main

Discussion

Lower density and species richness in the most polluted areas of the studied harbors are in agreement with most of the studies carried out in polluted environments. These studies often consider that a lowering in density and species richness (or diversity) are a response to pollution (e.g., Schafer, 1973). When the source of pollution is well identified (e.g., sewage outfall, effluent source from an industry), a barren zone with very low diversity and density often occurs around it. In this

Conclusion

This study shows that even if the main factor that determines the distribution of foraminiferal species in the estuarine harbors on the coast of Vendée is the position of the sample inside the estuary, related to the balance between fresh water and sea water influence, heavy metal and PAHs concentrations modify the foraminiferal habitat. Density and species richness of the assemblages decrease with an increase in heavy metal and PAH concentration and may be used as pollution indicators. The

Acknowledgements

This study was part of contract: Conseil Général de la Vendée/IFREMER n° 97/5 211 264. The authors thank DDE for providing samples and chemical analysis and the Government of Vendée for financial support. They also thank F. Redois for his contribution in the analysis of foraminiferal assemblages, C. Hardouineau for his technical assistance and R. Adams for his help in improving the manuscript. The authors are grateful to D.B. Scott and to the anonymous reviewers for the thorough revision of the

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