Heavy metal concentrations in the bivalve Corbicula fluminalis shells from Shatt Al-Arab River

This study was to investigate the contamination in the Shatt Al-Arab River by determining the heavy metals in the limnic bivalve Corbicula fluminalis, a sentinel species. The results showed that the values of Ba, Zn, Pb, Ni, Co, Cr, Sr, Cu, Mn and Fe in Corbicula fluminalis shells increased significantly from the beginning point toward the central part of Shatt Al-Arab River. This increase may be explained from the high incidence of pollution from sewage, development, and industry in the central part of Shatt Al-Arab River compared to the northern parts.


Introduction
The tolerance and adaptability of the bivalve molluscs have made them a preferred organism as indicator of the quality of ecosystems (Conti and Cecchetti, 2003). Trace metal contamination level in a given organism results from the net balance between the processes of metal uptake and metal loss (Goodfriend and Magaritz, 1989). Chemical analysis of hard body parts can yield vital information on the processes involved in biomineralisation and the interaction between the organism and the surrounding environment.
Trace elements are incorporated into bioic systems at significantly lower quantities (~0.1wt%) than elements in the surrounding environment (Dalbeck, 2008). Even though these elements do not form crystal phases, their presence can be used to provide a better understanding of biomineralisation processes.
Trace element composition can allow insight to the conditions in which mineralisation occurred, how these 'foreign' elements can affect growth of hard body parts, and growth of the individual, as well. Concentration of trace elements in hard body parts depends on the acceptability of the elements (Dodd, 1965;Afaj and Al-Dabbas, 1998).
In the Shatt Al-Arab River, there is high concentration of population and industries. Most of domestic sewage and industrial effluents with high organic matter are discharged untreated. This extra organic matter has caused several biogeochemical changes, both in small side streams and in the river proper, which is a source of heavy metals in these water bodies. A number of authors have studied the heavy metals in Corbicula fluminalis shells as a pollution indicator, such as the work of Mustafa (1989) and Salman (2007) in the Shatt Al-Arab River, Al-Bassam (1999) in the Euphrates River, and the study of Al-Saady (2008) in the Hor Al-Chekka River. The main objective of the present study was to determine the concentration of metals in specimens of the limnic bivalve Corbicula fluminalis shells.

Study Area
The study area was located on the banks of Shatt Al-Arab River north of and within the area of Basrah city, south Iraq ( Figure 1).

Materials and Methods
Eight Corbicula fluminalis shells were chosen for analysis. These shells were tested by scanning electron microscope-EDAX, Inductively Coupled Plasma-Atomic emission spectrometry (ICP-AES), and Inductively Coupled Plasma-Mass spectrometry (ICP-MS).
Shell samples were first separated from living soft tissues, washed with distilled water, exposed to ultrasonic International Journal of Marine Science 2015, Vol.5, No.39, 1-4 http://ijms.biopublisher.ca 2 Figure 1 Study area with location of sampling sites radiation for 2 h to remove possible foreign materials, and then air-dried, crushed to get particles of size less than 2 microns. Processed shells were assayed for Ca, Ba, Zn, Pb, Ni, Co, Cr, Sr, Cu, Mn and Fe elements.
Subsequently, graphite coated the bulk of shells prior to scanning electron microscope (SEM-EDAX) examination.

Results and Discussion
Bivalves were chosen for this study because they meet many of the requirements of a good biological sentinel (Phillips, 1980). They are somewhat sedentary, regionally abundant, and are long lived. They readily accumulate many metals and bioaccumulate contaminants.
Consequently, such organisms have been largely used in both salt and fresh water monitoring programs (Farrington, 1983). Chemical analyses of molluscs shells provide more information about the environment in which these organisms lived.
Ba, Zn, Pb, Ni, Co, Cr, Sr, Cu, Mn and Fe are the main trace elements in Corbicula fluminalis shells (Figure 2 and 3). These elements in our target organism increased significantly towards the central parts of Shatt Al-Arab River (Table 1)   untreated sewage discharge in the river from industrial facilities and hospitals that spread in the north and center part of Basrah city. This demonstrates that C. fluminalis may be used locally as an indicator of contamination by trace metals.