Foraminiferal responses to polluted sediments in the Montevideo coastal zone, Uruguay
Introduction
Several strategies can be adopted to characterize, evaluate and monitor environmental quality. In polluted areas, the determination of physical–chemical variables by analytic methods has been prioritized historically. Nevertheless, there are several organisms that can be used as environmental indicators and the success of evaluation plans or environmental monitoring depends mainly on the correct choice of the bioindicator that will be used. Among the benthic organisms used for assessing anthropogenic effects, benthic foraminifera are one of the preferred groups because they are very sensitive to environmental stress. Studies dealing with benthic foraminifera as bioindicators of pollution have been increasing over the last decades. In this way, the first studies investigating the relationship between foraminifers and pollution were carried out by Zalesny, 1959, Resig, 1960 working in California Southern coastal region. Since that time, pollution studies using these organisms have been expanded to include bays, harbors, and costal margins worldwide (see Alve, 1991, Alve, 1995, Boltovskoy et al., 1991, Yanko et al., 1999, Angel et al., 2000, Debenay et al., 2001a, among others). These studies have shown that the distribution of benthic foraminifers is affected by several anthropogenic contamination factors, like organic enrichment of the sediments, heavy metal loading and petroleum hydrocarbons. Foraminiferal response to these conditions can include shifts in abundance, in species composition and the presence of test abnormalities.
This study aims to document the benthic foraminiferal assemblages in the Uruguayan coast (an impacted area) and to relate these assemblages to the environmental conditions. As far as we know, this is the first study on the Uruguayan coast concerning foraminiferal distribution in a polluted area.
The Montevideo coastal zone is situated in the middle Río de la Plata basin (Ayup, 1986), on the east coast of South America between 34°50′–34°56′S and 56°05′–56°25′W (Fig. 1). It covers an area of about 38,000 km2 and drains a 3,170,000 km2 basin, the second largest in South America. The Río de la Plata is a coastal-plain tidal river, with a semi-enclosed shelf sea at the mouth. The mean annual river flow is 25,000 m3 s−1, with a maximum in June and a minimum in January (Nagy et al., 1997). Tides are semi-diurnal with amplitude of about 40 cm on the Uruguayan coast. Features such as salinity, depth of the halocline and vertical mixing vary with astronomic tidal oscillations on an hourly basis, while axial winds influence water height and salinity variations on a daily basis. The outer Río de la Plata and the adjacent continental shelf are covered with sand, while silt clay, clayed silt and silt are confined to the upper and middle Río de la Plata basin (López-Laborde, 1997).
The Montevideo Bay lies in the Montevideo coastal zone with an approximate area of 10 km2 and a mean depth of 5 m. It is geologically characterized by Precambrian outcrops and, in some regions, by more recent materials derived from these belonging to the Geological Formation Libertad-North Zone (Cardellino and Ferrando, 1969). The modern sediments are mainly fine grained, as clay and silt (Ayup, 1986). Predominant winds are from NE and W-SW, and they are very important in determining water circulation at shallow depths (Moresco and Dol, 1996), which is mainly clockwise.
The study area is affected by different pollutants that are derived from different sources. Thus, Montevideo Bay harbors the ANCAP refinery (Uruguayan Petroleum Company), the Batlle stream water plant (UTE, electrician Uruguayan company) and the Port of Montevideo. As can be seen from Fig. 1 that three streams flow into the bay: the Miguelete Stream, Pantanoso Stream and the Seco Stream, the latter flows through a pipe. These streams carry wastes from many different industries and urban centers, as well as from a great number of sewage sources. The entrance to the port is by a 9.3 km width channel and the port’s main structure is in the southern area (La Teja Dock) between the mouths of the Pantanoso and Miguelete streams, where oil tankers are loaded and unloaded. In the opposite side, eastwards of the Montevideo costal zone (Punta Carretas region), lies the most important sewage pipe of Uruguay. Authorities of Montevideo are planning the construction of another one, of similar characteristics, westwards in the Punta Yeguas zone, that will concentrate the sewage of the Pantanoso and Miguelete streams. At present these sewage are discharged into Montevideo Bay.
Moyano et al., 1993, Moresco and Dol, 1996 studied the hydrocarbon and heavy metal concentrations in intertidal sediments along the coast of the Montevideo area. They found sediments at the mouth of the Pantanoso stream to be severely contaminated, particularly by chromium, a metal released with the untreated wastewaters of numerous tanneries located along its shore. More recently, Muniz et al. (2002) studied lead and chromium in Montevideo coastal zone sediments, and classified the inner region of the bay as highly polluted, and the adjacent coastal zone as moderately polluted. Danulat et al. (2002) defined the Montevideo Port as a hyper-eutrophic system that receives considerable nutrients and organic loads. Muniz et al. (2004) also detected an important enrichment of copper in sediments when compared with previous studies in the area.
Section snippets
Materials and methods
Samples were collected from 24 stations in January 1998 (Fig. 1). At each station seven sediment samples were taken with a corer (5 cm internal diameter) for the analysis of the following variables: granulometric parameters, photosynthetic pigment content of surface sediments, redox potential, organic matter content, Cr, Pb and petroleum hydrocarbon concentrations. The detailed methodology used for the analysis of each variable can be found in Muniz et al. (2002). Bottom water samples were
Results
A total of 18 species and 18,341 individuals of benthic foraminifera were recorded at the 24 sampling stations of the Montevideo coastal zone (Table 3) belonging to the suborders Rotaliina (13 species) and Textulariina (five species).
Discussion
As have been shown by Muniz et al. (2002), the Montevideo coastal zone has at least two different regions clearly related to their environmental characteristics and degree of anthropogenic impact. In the present work, it was possible to understand how the different features of these regions are reflected in the benthic foraminiferal assemblages.
Conclusion
In this work, we have studied the effect of pollution on foraminiferal fauna in an estuarine region of the Río de la Plata. Through the foraminiferal assemblages presented in the study area, it was possible to distinguish three different sub-environments. On one hand, Montevideo Bay, the inner bay particularly, which showed an extremely poor foraminiferal fauna—including a totally azoic station—evidencing the high degree of local contamination. On the other hand, Punta Carretas and Punta Yeguas
Acknowledgements
We are grateful to Fondo Clemente Estable to provide a pos-doctoral fellowship (No. 8268) to LB. The Uruguayan Army is acknowledged for the use of its facilities during the field work. Comisión Sectorial de Investigación Científica (CSIC) of Uruguay and the Government of Montevideo City (IMM) provided financial support for this study. LB expresses her gratitude to Carlos Martinez that kindly allowed the use of the laboratory of the Sección Oceanología of the Facultad de Ciencias, Uruguay.
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