a un developpement d'opportunistes necrophages comme le suggere Chasse 

 (1978) puisque la composition faunistique reste tres comparable a celle 

 du temoin. La chute des densites observee ulterieurement dans le bac 

 le plus pollue est conforme aux resultats obtenus experimentalement 

 in situ par Bakke et al. (1980) ou en mesocosmes par Elmgren et al. 

 (1980 b). Elle s'accompagne d'un changement tres perceptible de la com- 

 position faunistique avec reduction du nombre d'especes et diminution 

 de la biomasse. 



Contrairement a 1 ' idee generalement admise, le groupe des Nema- 

 todes peut done constituer un indicateur biologique fiable des modifi- 

 cations de l'ecosysteme (Piatt & Warwick, 1980) puisque leurs possibili- 

 tes adaptatives permettent a certaines especes de se maintenir quelles 

 que soient les conditions de milieu, a d'autres de proliferer en quel- 

 ques semaines pour occuper la niche laissee vide. La determination ex- 

 perimental de groupes de Nematodes a comportement similaire vis-a-vis 

 de 1 'eutrophisation ou des pollutions, apparait done comme une voie de 

 recherche prometteuse pour caracteriser 1'etat ou la dynamique des eco- 

 systemes perturbes. 



SUMMARY 



Experimental study of hydrocarbon pollution in a sand microecosystem 

 I. Effect of the sediment contamination on meiofauna. 



The effects of hydrocarbon pollution, at two different intensi- 

 ties with respect to a control, on the microecosystems were studied 

 using recirculating experimental tanks containing 100 liters of subti- 

 dal fine sand. Changes in the population characteristics of meiofauna 

 (nematodes and copepods) were chosen to follow the effects of oil pol- 

 lution. Irrespective of the intensity of pollution, the density of ne- 

 matodes in the experimental tanks increased at a significantly higher 

 rate than in the control tank during the first two months after pollu- 

 tion and then decreased slowly. The density of harpacticoid copepods 

 was negatively related to the intensity of oil pollution. It appears 

 that the nematodes /copepods ratio would be an useful indicator of the 

 degree of oil pollution. 



After 3 months of experimental duration, the faunal composition 

 of the nematodes in the highly polluted tank was drastically modified. 

 This change is evident from a sharp fall in biomass and species diver- 

 sity; small opportunistic nematode species, known for their associa- 

 tion with eutrophicated environment, became dominant. Changes in the 

 meiofauna population parameters in the slightly polluted experimental 

 tank did not show any significant variation from those in the control 

 tank. 



REMERCIEMENTS 



L'ensemble des tris de la meiofaune a ete realise par Melle L. 

 Cras, technicienne CNRS que nous tenons plus particulierement a remer- 

 cier avec toutes les personnes ayant collabore a ce travail. 



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