Oribatid mites (Acari, Oribatida) - bioindicators of forest soils pollution with heavy metals and fluorine

The present study analyzes the effects of pollution with heavy metals and fluorine on the oribatid mite communities populating the forest soils, on the basis of the researches carried on in three oak-type forests, situated at different distances from the Phosphoric Fertilizers Plant of Valea C lug reasc (the Prahova county, Romania).In the forest strongly affected by pollution, the heavy metals concentrations were 2-9 times higher than the maximum allowable limits (MAL). In the perimeter with medium level of pollution, the content of Pb, Cr and Ni were over the MAL, while Co and Cd concentrations are closed to these limits. Two years after closing ofthis industrial unit, a decrease of soil loading with heavy metals was to be found, mostly in the surface sub-horizon. In the control perimeter, the oribatids constitute a complex community with a large specific diversity. The characteristic species for this zone (South-Eastern of Romania) are frequent and/or abundant, having a high ecological significance. In the affected forests, the oribatid mites' densities are 6-476 times lower than in the control perimeter. Their communities are constituted of a small number of tolerant species (euryplastic, unspecific fauna), being characterized by a low specific diversity and a marked structural instability. The analysis of the oribatid species distribution in the control and polluted ecosystems has evidenced that certain elements can be considered bioindicators for this type of pollution. Our researches carried out two years after the production stopping, have not evidenced afavourable evolution of the oribatid mites communities. It is probably that the recovery of the decomposers' trophic chains requires a longer time.