Evaluation of the potential toxicity of sediment and pesticides on the basin of Marennes-Oléron using a bivalve bioassay

• Main Author: Klein, Benoît
• Format: Report
• Language: English
• Published: 2003
• Subjects: Crassostrea gigas
• Online Access: https://archimer.ifremer.fr/doc/00451/56303/57891.pdf; https://archimer.ifremer.fr/doc/00451/56303/

The department of Charentes-Maritime has a maritime facade of 440 Km and a surface a rea of 6,383 Km2. The type of soil and the large maritime facade has allowed the development of two different activities: agriculture and shellfish farming. This shellfish farming takes place for a large part in the Marennes-Oléron basin. It regroups the production of oysters (50,000 tons per year) and the production of mussels. The shellfish farming creates 5,000 direct jobs. The agriculture, for its part, is divided into two main activitics: cattle farming and plant production, such as the culture of cereals, oleaginous, wine and other vegetables. The intensive culture induces the farmers to use a lot of pesticides, su ch as insecticides and herbicides in order to comply with the required efficiency. Nevertheless, the culture on marhlands gives sorne constraints and draining is necessary. Thus the products spread by the farmers will partly flow away in the effluent downstream. Thus in the department of Charentes-Maritime, four slope beds have been defined: The Charente, the Seudre, the short coastal rivers and the coastal municipality effluents. Ali of those feed the Marennes-Oléron basin with freshwater. ln 1997,2,700 tons of pesticides were used by agriculture and other activities in those slope beds; the main period of spreading occurs during the spring time. Regarding to their chemical and physical parameters, pesticides will be more or less able to be diffuse in the environment and become bio-available. Eight pesticides considered to be the most frequently met in the environment have been studied: (Alachore, Atrazine, Carbaryl, Diuron, Glyphoste, Fosetyl-Al, Metolachlore and Terbuthylazine). The pesticides could be adsorbed on suspended matter or on sediment, leached in the soil and found back in the aquifer, evaporated or transported by volatilisation. Few models of the quantification and the dispersion of pesticides in the environ ment allow to assess the impact of it. In 2001 , the concentrations analysed in the rivers of Seudre and Charente do not exceed one miligram of pesticides per litre. Until the pesticides are carried to the river estuary, they will undergo a series of transformations that affect their toxicity to the marine organisms and then they will be adsorbed on the sediment of the Marennes-Oléron basin. Since the end ofthe nineties, the oyster farmers have encountered problems of mortality and especially in the Marennes-Oléron basin during the summer. A challenge « Défi MOREST » has been designed to gather laboratories ofresearch on different themes around the oyster. It will try to find the reasons of those mortalities. An experimental study site « Dynamor » situated in the south of the Marennes-Oléron basin is the device for monitoring the oyster behaviour along the year and moreover during the mortalities. The laboratory of ecotoxicology of DEL/PC Ifremer Brest, has focused on the role of the sediment and the pesticides on the oyster mortality, using a bivalve bioassay on Crassostrea gigas. The spatiotemporal study of the sediment from the Marennes-Oléron basin shows th at the potential toxicity of the sediment increase at the end of May (more than 50% of abnormal D larvae whereas it was 25 % in the middle of May). Furthermore, the experimental study site « Dynamor » is potentially more toxic regarding the sediment than the site Agnas situated in the middle of the basin. Oysters have then been taken out from the « Dynamor » site to assess their capacity to produce normal D larvae and their sensirivity to CuS04. Thanks to those oysters, two environmental sites and two livestock have been studied. The proximity of the oyster to the sediment influences the sensitivity of its larvae to CuS04: EC50-24h = 2.62 µg CuSO4/L at 15 cm whereas EC50-24h = 14.4 µg CuS04 /L at 70 cm. Moreover, oysters at 70 cm above the sediment produce more normal D larvae than the ones at 15 cm (73 %versus 57 %). Furthermore, it has to be noticed that oysters genetically selectcd to be resistant are less sensitive that the ones from the natural spat. The study of the effect of pesticides on the embryonic development has shown that three of the eight pesticides studied induce a significant response of toxicity (from 12 to 32% of abnormal D larvae for 1 µg of pesticides per litre) : respectively for Carbaryl, Metolachlore and Fosetyl-Al. But the effect of the mixing of the eight substances leads to 35 % of abnormal D larvae at concentrations met in the environment.: This results indicates that it exists a synergy effect of the pesticides between themselves. The massive spreading of pesticides during the spring could be put into re lation with the sudden increase of the potential toxicity of the sediment. Moreover, pesticides are hazard for the oysters, and its potential adsorption on sediments induces different effects regarding to the proximity of the oysters to it. A hypothesis could be advanced: the energy spent to face pesticides stress during the ripeness may reduce the sum of energy allocated for the growth, the feeding and the pathogens protection. This could induce its death.