Etude de la qualité environnementale de la Baie de Toliara à travers Marphysa mossambica, espèce indicatrice de pollution et bioaccumulatrice de certains métaux lourds toxiques.

Abstract

An environmental study of the bay of Toliara has been realized during 6 months from November 2009 to
April 2010. The polychaeta annelid Marphysa mossambica was the main biological material used for the
evaluation of the health state of the studied zone, as a pollution indicator and toxic heavy metals
bioconcentrator.
Various parameters were studied so as to estimate the causes of the benthic population’s distribution,
especially that one of the species Marphysa mossambica. It concerns the physico-chemical parameters of the
water and the sediment, the bio-ecological parameters of the populations, and the rates of chemical
elements such as nutrients and heavy metals, in the water, in the sediment and in the annelids.
It was shown by the results that the Toliara Bay is for the moment, not yet threatened by the pollution.
Nevertheless, the stations were Marphysa mossambica dominate (station 4, near COPEFRITO where it
occupy 94,21% of the total density and stations 5 and 6, near the harbor with respectively 46,16 and 62,12%
of the total density) seem to tend to a slight disturbance of the communities.
The analysis of nutrients and heavy metals in the water, in the sediment and in the worms prove that this
species is a pollution indicator. Indeed, the rates of ammonium, nitrite and phosphate are higher in this zone
of Marphysa mossambica, and the variation of the contamination index of the heavy metals indicate high
values in this part of the studied zone.
It was also highlighted that the studied species can concentrate certain toxic heavy metals such as cadmium,
copper and zinc, with respectively a concentration factor of Fc=13,75 , Fc=4,62 and Fc=2,00. Another
polychaeta species Lumbrinereis latreilli can also concentrate those elements too, but in contrast to
Marphysa mossambica, this species does not exists on the most contaminated stations. That confirms the
indicator role of Marphysa mossambica in this Bay where the tendency of the pollution is about organic
matters and chemical contaminants.